Sea Trout: Biology, Conservation and Management

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SEA TROUT: BIOLOGY, CONSERVATION AND MANAGEMENT Proceedings of the First International Sea Trout Symposium, Cardiff, July 2004 Edited by

Graeme Harris and Nigel Milner

Central Fisheries Board

Blackwell Publishing

SEA TROUT: BIOLOGY, CONSERVATION AND MANAGEMENT

SEA TROUT: BIOLOGY, CONSERVATION AND MANAGEMENT Proceedings of the First International Sea Trout Symposium, Cardiff, July 2004 Edited by

Graeme Harris and Nigel Milner

Central Fisheries Board

Blackwell Publishing

© 2006 by Blackwell Publishing Ltd Editorial Ofﬁces: Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK Tel: +44 (0)1865 776868 Blackwell Publishing Professional, 2121 State Avenue, Ames, Iowa 50014-8300, USA Tel: +1 515 292 0140 Blackwell Publishing Asia Pty Ltd, 550 Swanston Street, Carlton, Victoria 3053, Australia Tel: +61 (0)3 8359 1011 The right of the Author to be identiﬁed as the Author of this Work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. First published 2006 by Blackwell Publishing Ltd ISBN-13: 978-1-4051-2991-6 ISBN-10: 1-4051-2991-3 Library of Congress Cataloging-in-Publication Data International Sea Trout Symposium (1st : 2004 : Cardiff, Wales) Sea Trout: Biology, Conservation, and Management : Proceedings of First International Sea Trout Symposium, Cardiff, July 2004/editors, Graeme Harris and Nigel Milner. p. cm. Includes bibliographical references and index. ISBN-13: 978-1-4051-2991-6 (hardback : alk. paper) ISBN-10: 1-4051-2991-3 (hardback : alk. paper) 1. Sea-run brown trout–Congresses. I. Harris, Graeme. II. Milner, Nigel. III. Title. QL638.S2I484 2004 639.3’757–dc22 2006014213 A catalogue record for this title is available from the British Library. Set in 10/13pt Times by Newgen Imaging Systems (P) Ltd., Chennai, India Printed and bound in Singapore by Markono Print Media Pte Ltd The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp processed using acid-free and elementary chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. For further information on Blackwell Publishing, visit our website: www.blackwellpublishing.com

Contents

Foreword

ix

Preface

xi

Opening Address 1. Sea Trout: A Welsh Perspective Carwyn Jones AM

xiii

Opening Address 2. Sea Trout and the Environment Agency Helen Phillips

xvii

1

Setting the Scene – Sea Trout in England and Wales – A Personal Perspective G. Harris and N. Milner

1

Section 1 STOCKS AND FISHERIES 2

Patterns of Anadromy and Migrations of Paciﬁc Salmon and Trout at Sea T.P. Quinn and K.W. Myers

11

3

A Review of the Status of Irish Sea Trout Stocks P. Gargan, R. Poole and G. Forde

25

4

Characteristics of the Sea Trout Salmo trutta (L.) Stock Collapse in the River Ewe (Wester Ross, Scotland), in 1988–2001 J.R.A. Butler and A.F. Walker

5

6

Characteristics of the Sea Trout (Salmo trutta L.) Stocks from the Owengowla and Invermore Fisheries, Connemara, Western Ireland, and Recent Trends in Marine Survival P.G. Gargan, W.K. Roche, G.P. Forde and A. Ferguson Annual Variation in Age Composition, Growth and Abundance of Adult Sea Trout Returning to the River Dee at Chester, 1991–2003 I.C. Davidson, R.J. Cove and M.S. Hazlewood

45

60

76

7

Sea Trout Stock Descriptions in England and Wales G. Harris

88

8

The Rod and Net Sea Trout Fisheries of England and Wales R. Evans and V. Greest

107

9

General Overview of Turkish Sea Trout (Salmo trutta L.) Populations I. Okumu¸s, I.Z. Kurtoglu and S¸ . Atasaral

115 v

vi

Contents

10 The Status and Exploitation of Sea Trout on the Finnish Coast of the Gulf of Bothnia in the Baltic Sea E. Jutila, A. Saura, I. Kallio-Nyberg, A. Huhmarniemi and A. Romakkaniemi 11 Sea Trout (Salmo trutta L.) in European Salmon (Salmo salar L.) Rivers N.J. Milner, L. Karlsson, E. Degerman, A. Johlander, J.C. MacLean and L-P. Hansen

128

139

Section 2 GENETICS AND LIFE HISTORY 12 Genetics of Sea Trout, with Particular Reference to Britain and Ireland A. Ferguson 13 The Genetic Basis of Smoltiﬁcation: Functional Genomics Tools Facilitate the Search for the Needle in the Haystack T. Giger, U. Amstutz, L. Excofﬁer, A. Champigneulle, P.J.R. Day, R. Powell and C.R. Largiadèr

157

183

14 Life History of the Anadromous Trout Salmo trutta B. Jonsson and N. Jonsson

196

15 Migration as a Life-History Strategy for the Sea Trout D.J. Solomon

224

16 Life History of a Sea Trout (Salmo trutta L.) Population from the North-West Iberian Peninsula (River Ulla, Galicia, Spain) P. Caballero, F. Cobo and M.A. González

234

17 Review and Perspectives on Molecular Genetic Approaches to Sea Trout Biology M.W. Bruford

248

Section 3 POPULATION DYNAMICS, ECOLOGY AND BEHAVIOUR 18 A 35-Year Study of Stock–Recruitment Relationships in a Small Population of Sea Trout: Assumptions, Implications and Limitations for Predicting Targets J.M. Elliott and J.A. Elliott

257

19 Characteristics of the Burrishoole Sea Trout Population: Census, Marine Survival, Enhancement and Stock–Recruitment Relationship, 1971–2003 W.R. Poole, M. Dillane, E. DeEyto, G. Rogan, P. McGinnity and K. Whelan

279

20 Population Dynamics and Stock–Recruitment Relationship of Sea Trout in the River Bresle, Upper Normandy, France G. Euzenat, F. Fournel and J-L. Fagard

307

Contents

vii

Section 4 MANAGING STOCKS AND FISHERIES 21 The Spawning Habitat Requirements of Sea Trout: A Multi-Scale Approach A.M. Walker and B.D. Bayliss

327

22 Research Activities and Management of Brown Trout and Sea Trout (Salmo trutta L.) in Denmark G.H. Rasmussen

342

23 Stocking Sea Trout (Salmo trutta L.) in the River Shieldaig, Scotland D.W. Hay and M. Hatton-Ellis

349

24 Is Stocking with Sea Trout Compatible with the Conservation of Wild Trout (Salmo trutta L.)? H. Lundqvist, S.M. McKinnell, S. Jonsson and J. Östergren

356

25 Sea Lice Lepeophtheirus salmonis Infestations of Post-Smolt Sea Trout in Loch Shieldaig, Wester Ross, 1999–2003 M. Hatton-Ellis, D.W. Hay, A.F. Walker and S.J. Northcott

372

26 Comparison of Survival, Migration and Growth in Wild, Offspring from Wild (F1) and Domesticated Sea-Run Trout (Salmo trutta L.) S. Pedersen, R. Christiansen and H. Glüsing

377

27 The Rapid Establishment of a Resident Brown Trout Population from Sea Trout Progeny Stocked in a Fishless Stream A.F. Walker

389

28 Predicted Growth of Juvenile Trout and Salmon in Four Rivers in England and Wales Based on Past and Possible Future Temperature Regimes Linked to Climate Change I.C. Davidson, M.S. Hazlewood and R.J. Cove 29 Sea Trout (Salmo trutta L.) Exploitation in Five Rivers in England and Wales B.A. Shields, M.W. Aprahamian, B.D. Bayliss, I.C. Davidson, P. Elsmere and R. Evans 30 Catch and Release, Net Fishing and Sea Trout Fisheries Management D.J. Solomon and M. Czerwinski

401

417

434

31 A Review of the Statutory Regulations to Conserve Sea Trout Stocks in England and Wales G. Harris

441

32 An Appreciation of the Social and Economic Values of Sea Trout in England and Wales P. O’Reilly and G.W. Mawle

457

viii

Contents

33 Sea Trout Fisheries Management: Should We Follow the Salmon? A.M. Walker, M.G. Pawson and E.C.E. Potter

466

34 Perspectives on Sea Trout Science and Management N.J. Milner, G.S. Harris, P. Gargan, M. Beveridge, M.G. Pawson, A. Walker and K. Whelan

480

Declaration

491

Index

493

Foreword

Fish that spawn in stony rivers tend not to leave many fossils so perhaps we should not be too surprised that the earliest salmonid we know of for certain died a mere ﬁve million years ago. Primitive anatomical features, like the wide separation of the pectoral and pelvic girdles and the physostomous swim bladder, suggest that the origins of the group to which Atlantic salmon and sea trout belong lie tens of millions of years earlier when the Atlantic Ocean was much narrower than it is today. Cyto-geneticists tell us that at some point in the early history of the salmonids a ‘mistake’ in cell division led to a doubling up of chromosome numbers. Although there has since been some rearrangement of chromosome arms, especially in the more advanced members of the family, all members of the group still carry clear evidence of their tetraploid ancestry. It is interesting to reﬂect that, among living ﬁshes closely related to the Salmonidae, the smelt, Osmerus eperlanus (L.), carries a normal diploid complement of chromosomes. This observation tells us that the osmerids and the salmonids separated before the doubling of chromosome numbers characteristic of the latter and it is tempting to speculate that tetraploidy persisted because it had adaptive value in estuarine ﬁshes making increasing use of an environment, the sea, where rapid growth to a large size is possible. We have no direct way of knowing what the life cycles of the ﬁrst salmonids were like but perhaps among those followed by Salmo trutta (L.), especially the sea running populations, we may see reﬂections of an archaic life style (also followed by sea running lampreys) in which the beneﬁts of using the comparative safety of fresh water for reproduction, but exploiting the dangerous but much more productive world of estuarine waters and the sea for growth, ﬁrst established themselves in yet another family of aquatic vertebrates. If fuelling the idle speculations of aged zoologists was the sole justiﬁcation for studying sea trout, it is highly unlikely that the First International Sea Trout Symposium, of which this book is such a signal celebration, would ever have come about. The fact is that the sea trout is a ﬁsh that demands to be studied. Markedly superior to any salmon at table and pound for pound its sporting equal, its high unit value on both counts effortlessly earns it a place in the ﬁrst division of top quality European ﬁshery resources. Perhaps surprisingly for a commodity which is so highly prized, it is but lightly exploited by directed ﬁshing (apart from certain interception ﬁsheries which also target salmon). Thus, most of the variation we see in the abundance and structure of sea trout resources is driven, not by ﬁshing pressure, but by changes in the growth and survival opportunities provided by the principal environments through which they pass namely, freshwater lakes and rivers, estuaries and coastal waters. Similar to salmon and other anadromous ﬁshes, sea trout are highly sensitive to deterioration in the quality of any of these habitats. However, sea migratory trout have a survival trick up their sleeves. They are often part of more broadly based trout population complexes, some of whose members complete their entire development in their tributaries of origin, some in the main stems and estuaries of rivers and others, especially females, follow the archetypal life cycle by achieving large size and high fecundity at sea. This ix

x

Foreword

ﬂexibility in the life cycle options available to S. trutta has enabled it to survive insults to coastal waters and estuaries which have led to the total loss of salmon populations in many parts of their traditional range. The good news is that, when estuarine conditions once more permit the passage of sea migratory trout, their numbers build up rapidly sometimes, as appears to have happened in the Aberdeenshire Don, outcompeting the river resident trout that hitherto had dominated the system. How lucky we Europeans are still to have the sea trout and how fortunate also that its importance as a biological entity and as a priceless socio-economic resource is at last being accorded the academic recognition (of which this excellent book is the most recent expression) it richly deserves. Richard Shelton

Preface

This volume contains the proceedings of the ‘First International Symposium on the Biology, Conservation & Management of Sea Trout’, held in Cardiff in July 2004. The aim of the Symposium was to assemble and discuss new knowledge and understanding of the biology of sea trout and the science and management of its ﬁsheries. This was required because much has changed since the last scientiﬁc workshops on sea trout in Wales and in Scotland in the late 1980s and since ICES started progressing sea trout work on an international level in the mid-1990s. The Symposium attracted contributions from 12 different countries, revealing a wide range of ﬁshery problems and a variety of opportunities and circumstances within which management and science are carried out. The chapters in this book convey this and we have tried through the editorial process to retain the variety of styles and approaches rather than try to apply overly prescriptive structures. The diversity of approaches and data reﬂects the subject itself. The book structure is straightforward. An introductory section sets the scene historically, identiﬁes some key features of sea trout and raises some of the major topics to be dealt with. The main body of chapters is divided into the four themes of the Symposium: (1) Stocks and Fisheries; (2) Genetics and Life History; (3) Population Dynamics, Ecology and Behaviour and (4) Managing Stocks and Fisheries. A concluding chapter brings together the common threads with recommendations for the future science and management. Finally, the Symposium produced a ‘Declaration’ (always a risky activity) which was drafted by the organising committee and widely circulated at the time. We hope this will offer some milestones against which to judge progress at future Symposia and specialised workshops on sea trout.

Nomenclature In common English parlance, ‘sea trout’ is the name usually given to the adult anadromous (migratory, sea-going) form of Salmo trutta (L). Amongst taxonomists, S. trutta is usually known as the brown trout. However, although by no means universal, common usage has adopted the term ‘brown trout’ as representing the non-migratory form and this can cause confusion. Much time can be spent debating the classiﬁcation of the phenotypes representing the life history continuum in S. trutta, and the accompanying nomenclature and synonyms (e.g. resident, non-migratory, freshwater, anadromous, brown trout, slob trout, sea trout, sea-trout). We have not attempted to prescribe terminology in this volume, because there is no consensus across the board, but we recognise its desirability for the future. Nigel Milner and Graeme Harris January 2006 xi

xii

Preface

Acknowledgements We wish to thank our colleagues on the Organising Committee, who have in so many ways provided practical ideas, support and encouragement. Alphabetically they are: Steve Barnard (Environment Agency Wales) Mike Bruford (School of Biosciences, Cardiff University) Paddy Gargan (Central Fisheries Board) Graeme Harris (FishSkill Consultancy Services) Tim Hoggarth (Atlantic Salmon Trust) Paul Knight (Salmon and Trout Association) Nigel Milner (Environment Agency) Pat O’Reilly (Environment Agency Wales) Mike Pawson (Centre for Ecology, Fisheries and Aquatic Sciences) Andy Walker (Fisheries Research Services Ken Whelan (Marine Institute) The following comprised the editorial committee: Mike Bruford (School of Biosciences, Cardiff University Paddy Gargan (Central Fisheries Board) Graeme Harris (FishSkill Consultancy Services) Nigel Milner (Environment Agency) Mike Pawson (Centre for Ecology, Fisheries and Aquatic Sciences) Andy Walker (Fisheries Research Services Ken Whelan (Marine Institute) and we thank numerous referees who helped with this task. We are also grateful to Samantha Emmott of the University of Cardiff Conference Ofﬁce for the overall smooth running of the Symposium and to Bernie Barron, National Fisheries Technical Team, Environment Agency, for her efﬁcient administrative and organisational support.

Sponsorship We gratefully thank the organisations that provided ﬁnancial support for the Symposium. Such sponsorship demonstrates their commitment to achieve improved understanding and better management of our sea trout resource and we hope that they get value for money from the programme and its outcomes. They are, in alphabetical order: the Atlantic Salmon Trust UK), the Central Fisheries Board (Republic of Ireland), the Centre for Ecology, Fisheries & Aquaculture Sciences (E&W), the Environment Agency (E&W), the Salmon and Trout Association (UK), the Scottish Executive (Scotland) and the Welsh Assembly Government (Wales).

Opening Address 1 Sea Trout: A Welsh Perspective Carwyn Jones AM Minister for Environment, Planning & Countryside, Welsh Assembly Government, Cathays Park, Cardiff CF10 3NQ, Wales, UK

Ladies and gentlemen, I take great pleasure in opening the proceedings of this International Symposium and extend a warm welcome to delegates, especially those who have travelled from overseas. It is encouraging that so many of you are here today. It is appropriate that Wales was selected as the venue for this symposium on ‘The Biology, Conservation and Management of the Sea Trout’ as the sea trout has always been regarded as very special to Wales; where it is still widely referred to as the ‘Sewin’: an old Welsh name that means ‘the silver or shining one’ when loosely translated into English. Indeed Cardiff is one of the few capital cities where, not 400 yards from this venue, sea trout can be caught. It is almost 20 years since the last major scientiﬁc meeting on sea trout was held in the British Isles. Since that time there has been an important constitutional change within the UK. We no longer have a centralised structure of Government centred in London. The National Assembly for Wales was set up in 1999. Its executive, the Welsh Assembly Government, now exercises an extensive portfolio. Importantly, matters relating to the management of the coastal and inland waters of Wales are the responsibility of the Welsh Assembly Government. What this means is that we can, and are, shaping policy and actions directly relevant to the needs of Wales and Welsh ﬁsheries. There have also been important changes in England and Wales in recognising the need to ensure that ﬁshery management strategies are under-pinned by robust science. There are many questions yet to be asked, and answered, in relation to the sea trout and the time for this International Symposium is right in bringing these to the fore. The sea trout is of great importance to Wales for several reasons: In terms of its geographical area, Wales is a small part of the British Isles, but it has more than its fair share of sea trout. Following in part from Welsh Assembly funded work, the recovery of the once polluted, over-abstracted and obstructed rivers of south-east Wales from the effects of the Industrial Revolution is well advanced. Almost every river and stream that enters the sea around our 1200 km (750 miles) of Welsh coastline now contains a natural and self-sustaining run of migratory trout. Many of these rivers support productive rod ﬁsheries and, in some instances, commercial net ﬁsheries. In a typical year more than half of the 40 000 or so sea trout caught by anglers xiii

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Opening Address, Carwyn Jones AM

in England and Wales come from Welsh rivers. The Tywi, Teiﬁ, Dyﬁ and Mawddach, for example, consistently yield rod catches that are amongst the best in Britain. Compared with the sea trout in most other parts of the British Isles, Welsh sea trout grow faster, live longer and attain a much larger size. Sea trout in excess of 15 lb weight (6.8 kg) are not unusual in most rivers and some, such as the Dyﬁ, have produced several specimens exceeding 20 lb weight (9.1 kg) in recent years. Unlike other areas in Britain, large amounts of the ﬁshing in Wales are owned or controlled by locally based angling clubs. In effect, they are owned by the community and provide readily affordable ﬁshing for both local and visiting anglers alike. The sea trout has been described as an ‘enigmatic’ and ‘neglected’ species that was often considered to be less worthy of sporting endeavour than its more prestigious cousins – the brown trout and the Atlantic salmon. Sea trout ﬁsheries have not had the attention they deserve. Throughout the British Isles, scientiﬁc endeavour and pro-active management action has concentrated on the salmon and brown trout. However, the recent and widespread general decline in salmon stocks has only served to highlight the underlying importance of the sea trout as the mainstay of many ﬁsheries and attract the attention of ﬁshermen in many parts of England and Wales and beyond. Although our sea trout ﬁsheries in Wales appear to be reasonably healthy, we must not be complacent and take our sea trout stocks for granted on the basis of the historical catch record. Recent history shows us that ﬁsheries can be overexploited and that collapsing stocks can lead to both social and economic deprivation. One of the most signiﬁcant developments in recent years has been the shift towards managing Welsh ﬁsheries in ways that seek to maximise their social and economic value to the community as a whole. This is a goal that is well recognised in Wales and one that is incorporated in our ﬁshing strategies. Studies on the socio-economic value of ﬁsheries have been a recent development in the UK compared with North America and other parts of Europe. However, there have been some, and one of the most recent was the ‘Study into Inland & Sea Fisheries in Wales’ commissioned by the Welsh Assembly Government. This was prepared by Nautilus Consultants Ltd. and published in 2000. It concluded that recreational sport ﬁshing in all its different forms generated an income of some £100 million a year to the economy of Wales. It is clear that angling is an important source of enjoyment, employment and income – especially in those rural areas where most of the ﬁsheries are located. In the context of this Symposium, the ‘Nautilus Report’ also recommended that the sea trout ﬁsheries of Wales should be more actively nurtured and promoted in order to increase the social and economic beneﬁts of angling tourism in Wales. This is also one of our goals. The report of the ‘Salmon and Freshwater Fisheries Review’ published in 2000 also stressed the importance of managing our ﬁsheries to enhance social and economic beneﬁts to the community as a whole: and it further underlined the importance of sea trout in this context. The ﬁsheries legislation in England and Wales was largely formulated in the 1860s when the pressures on our ﬁsheries and their management needs were very different. It has long

Opening Address, Carwyn Jones AM

xv

been recognised that this legislation needs to be strengthened and updated to address the very different environmental pressures and management needs of today. This seminal Review, commissioned by Parliament in 1998, examined the policies and legislation relating to the management of salmon and freshwater ﬁsheries in England and Wales and made 195 broad and far-reaching recommendations for change. The Welsh Assembly Government has considered these. It is supportive of the need for change along the lines set out in the Review and work is ongoing. As a response to the central recommendations of these two major reports, the Welsh Assembly Government in partnership with the Environment Agency Wales launched the ‘Sustainable Fisheries Programme’ and increased funding by an additional £2.4 million (e3.9 million) over a 3-year period. Early results from the initiative are extremely encouraging with aquatic environments being improved, barriers to ﬁsh movement removed or eased and angling participation increased. Furthermore, Wales has been successful in obtaining European Objective 1 funding of £5.3 million (e8.5 million) which has been invested in this Programme and in the ‘Fishing Wales’ Project. The latter is aimed at encouraging angling tourism within Wales and will initially promote sea trout, wild brown trout and bass ﬁshing. These programmes, led by the Environment Agency Wales, involve both other agencies and the angling community. This increased investment in our ﬁsheries is not merely to provide more ﬁsh for anglers to catch: although that should be a natural consequence. Instead, the overall objective is to maximise the social and economic value of the ﬁsheries in Wales by making the ﬁshing more attractive to local and visiting anglers alike. The ultimate aim is to increase employment opportunities and incomes within the community as a whole. Another development of potentially enormous signiﬁcance for the future well being of our migratory ﬁsheries in England and Wales has been the overall reduction in the commercial ﬁshing effort for salmon and sea trout over recent years. Commercial ﬁshing for migratory salmonids is now carefully controlled and regulated to protect and maintain adequate spawning populations. The number of commercial ﬁshing instruments licensed in Wales each year to ﬁsh for salmon and sea trout in tidal waters has decreased progressively from 187 in 1981 to the more sustainable level of just 65 in 2003. Most signiﬁcantly, Wales has now phased out all commercial net ﬁshing in coastal waters that exploited known ‘mixed stocks’ of salmon and sea trout returning to Welsh rivers. Other signiﬁcant initiatives promoted by the private sector have resulted in the buy-out of the few remaining drift nets on the Usk, Clwyd and Dee. This means that there are now no interceptory ﬁsheries operating off the coast of Wales. Thus, we can say that we have put our own house in order in this particular respect. Although much of the reduction in commercial ﬁshing effort has been driven by the need to protect and conserve Atlantic salmon, these measures do much to increase both the numbers and average size of the sea trout now returning to Welsh rivers. This is making a major contribution to increasing the economic value of our ﬁsheries. Another salutary development over recent years has been the growing awareness within the angling community of the need to conserve ﬁsh stocks for the beneﬁt of future generations. Self-imposed ‘Rules and Regulations’ combined with the adoption of voluntary

xvi

Opening Address, Carwyn Jones AM

‘Codes of Conduct’ by the angling community, have all helped to reduce the number of ﬁsh killed and so served to increase the size of the spawning population. Most impressive has been the rapid adoption of catch-and-release as a conservation ethic by sea trout anglers. Whereas less than 10% of the rod catch of sea trout was returned alive to the river after capture in 1993, the corresponding ﬁgure across Wales in 2002 was more than 50%; and on some rivers it approached as much as 90% of the total rod catch. In practical terms this has meant that some 13 000 sea trout were voluntarily released to increase the spawning population in 2002 and so strengthen our sea trout stocks in future years. This is a measure that I welcome and would wish to see increase; not only in respect of the sea trout. It is encouraging to think that one of the important outcomes of this International Symposium will be to raise the proﬁle of the sea trout. Given the complex and seemingly opportunistic life history of the ﬁsh and the many gaps that appear to exist in our knowledge about its biology, ecology, migration behaviour and genetic status, it is clear that enormous challenges are still faced in attempting to manage this valuable natural resource. Setting ‘egg-deposition targets’ and ‘conservation limits’ for the management of our salmon stocks is difﬁcult. How much more difﬁcult then will it be to manage a species where the proportion of juveniles that may or may not migrate to sea to become sea trout in any year is unknown and can vary widely from river to river and from year to year? This is one of the questions that need to be addressed. The effective management of our sea trout ﬁsheries in the future will require answers to this and many other fundamental questions if this amazing natural resource is to be managed in a sustainable way for the beneﬁt of future generations. That then is the challenge! I wish you all an enjoyable and fruitful meeting. I look forward to seeing a positive outcome from your deliberations over the next 3 days. It is quite clear from the impressive and packed programme that you have a great deal to consider.

Opening Address 2 Sea Trout and the Environment Agency Helen Phillips Regional Director, Environment Agency Wales, Cambria House, 29 Newport Road, Cardiff CF24 0TP, Wales, UK

Ladies and gentlemen, On behalf of the Environment Agency Wales, it is a pleasure to welcome delegates to Cardiff and to address this Symposium which is considering a topic that has substantial signiﬁcance for the Environment Agency in Wales and in England. Amongst its many other roles in environmental protection, the Environment Agency also has a statutory duty ‘to maintain, improve and develop’ ﬁsheries for salmonids, freshwater ﬁsh and eels on behalf of the two nations: and that duty includes sea trout. Fisheries and their successful management are important for two main reasons: ﬁrst, because ﬁsh are icons of environmental quality, people are reassured by their presence in rivers, particularly the big, visible migratory salmonids like sea trout and salmon. The status and well-being of ﬁsh stocks is a key bio-indicator of the water quality, water quantity and general well-being of our rivers. The role of ﬁsh as bio-indicators is why they will play an important part in helping to deﬁne good ecological status in the forthcoming Water Framework Directive. Second, because ﬁsh stocks are the basic resource for recreational and commercial ﬁshing, and ﬁshing is a major activity that generates sustainable beneﬁts to rural and urban communities. We have just heard from the Minister the importance that the Welsh Assembly Government attaches to sustainable ﬁsheries management as part of its National strategy for the development of social and economic beneﬁts within the Welsh community as a whole. These two facets of ﬁsh stocks, as bio-indicators and in supporting commercial and recreational ﬁshing, determines the Agency’s principal policy aims with respect to ﬁsheries. These are: •

to promote the conservation and maintain the diversity of freshwater ﬁsh, salmon, sea trout and eels and to conserve their aquatic environment; • to enhance the contribution that salmon and freshwater ﬁsheries make to the economy, particularly on remote rural areas and in areas with low levels of income; • to enhance the social value of ﬁshing as a widely available and healthy form of recreation. The sea trout has a special place in Welsh ﬁsheries. It has now overtaken salmon as a ﬁsh of importance to anglers. Since 1992, when rod catches of both species were the same (at xvii

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Opening Address, Helen Phillips

about 14 500 ﬁsh each year), the total English and Welsh sea trout catch has increased to 45 000 and is now about four times the salmon catch. Of this total approximately 50% come from Welsh rivers; and so we see Wales as a focal point for research and its translation into better management practice. Let me give you a little background to the Agency and its duties. It is a big organisation, having some 2500 employees across England and Wales, with a regulatory and management role across the environment. For example, discharges and waste disposal from industry to air, land and water, are licensed and regulated by the Agency. We manage water resources and ﬂood risk management. We advise planning consultations on most environmental matters and we are the ‘competent authority’ charged with delivering many of the European Directives: such as ‘Nitrates’, ‘Fish’, ‘Urban Waste Water’ and the ‘Water Framework Directive’. Fisheries are just one small, but high proﬁle, part of this activity (with an annual spend of £28 million (e45m) of a revenue budget of £650m (e1 billion). The multifunctional brief of the Agency ensures that ﬁsheries and conservation needs are met within an integrated approach to environmental management and protection. This is a real beneﬁt that facilitates the difﬁcult balancing act of managing conﬂicting demands on the environment of our rivers, streams and coastal waters where sea trout live. With respect to ﬁsheries, we licence net and rod ﬁsheries, we administer and enforce regulations, we monitor and report on catches, ﬁsh stocks, ﬁshing effort and demand, we control ﬁsh movements and, most importantly, we facilitate and encourage ﬁshery development in order to promote the high level aim of good ﬁshing that is accessible to all. Sound science is a keystone of the Agency’s business. Modern ﬁsheries management has a strong scientiﬁc basis and the ﬁsheries service in Wales has played a leading role in developing and then applying that science. The scientiﬁc basis of salmon management has changed considerably over the last 10 years. For example, the methods for carrying out and reporting stock assessment using conservation limits for salmon have introduced a quantitative rigour to the process that was missing before. More effective and better-targeted stock monitoring and assessment methodologies have accompanied this change. Practical methods to protect, enhance and restore ﬁsheries and their aquatic environment have been developed through improved understanding of ﬁsh ecology and behaviour. For example, the opening of barriers to ﬁsh passage and the protection of bankside habitat by riparian corridor protection schemes will lead to increased production of salmonids. Developments in the techniques and application of ﬁsheries economics and social sciences have enabled better targeting of development and needs to the beneﬁts of management. The science behind sea trout ﬁsheries needs to tackle some tricky problems. During the Symposium we shall hear of the complex links between the non-migratory brown trout and the migratory sea trout: and we shall ponder over the balance between ‘nature and nurture’ in determining migratory behaviour in anadromous ﬁsh. The scientiﬁc understanding of these contrasting life histories is a formidable scientiﬁc challenge. But it will be necessary to meet this challenge, if we are able to develop ‘biological reference points’ for sea trout as has been explicitly required of the Agency by the Governments’ recent ‘Review of Salmon and Freshwater Fisheries’.

Opening Address, Helen Phillips

xix

To meet many of these recommendations, the Agency has recently launched its ‘National Trout & Grayling Strategy’ for England & Wales, which naturally includes the interests of migratory sea trout. There has not been a scientiﬁc meeting speciﬁcally about sea trout in Britain for about 20 years. But, as we shall hear, there have been big changes in our knowledge and understanding and we need to review and harness this knowledge and then to reappraise it in the light of a changing regulatory and management framework. The Agency needs to have this information so that it can more effectively discharge its duties with respect to sea trout ﬁsheries and its stakeholders. We see it as one of our roles to support and encourage such events as this Symposium. The Welsh Assembly Government (WAG) were quick to realise the value of Welsh ﬁsheries, and the contribution to that value made by sea trout. In 2002, it provided the Agency with an additional investment of £2.4 million (e3.8 million) to support the ‘Sustainable Fisheries Programme’ over a 3-year period with which it could undertake a range of enhanced ﬁshery management actions including habitat improvements, ﬁsh pass construction and angling participation. Stimulated by the WAG Grant-in-Aid funding for sustainable ﬁsheries, we have secured a further e3.8 million of Objective 1 money over 3 years and, with EU Structural funds, the Agency and its partners have been able to generate a direct investment in our ﬁsheries of over e15.8 million. This investment is crucially important in meeting our strategic aims for the local economy, quality of life and the environment in Wales. It has helped to reduce the impact of barriers to migration, so that upstream access has been created or improved to over 250 km of river in Wales. It has also restored 90 km of degraded ﬁsheries habitat in the past year alone. Much of this work was made possible by the cooperation and support of the landowners and angling clubs, resulting in not only ecological beneﬁts but also in providing ‘exemplar’ demonstration sites that will raise public awareness of the problems being addressed and the practical solutions to those problems. Investment in training for our voluntary and private ﬁshery managers throughout Wales is playing a crucial part in building skills and capability within the ﬁsheries community and so increasing their capacity to identify, fund and deliver projects for themselves. None of this ﬁsheries science, management or development occurs without close partnerships and collaboration between many organisations. I am particularly glad to acknowledge, and pay tribute to, the substantial and wide-ranging works of the ﬁshery associations and angling clubs, the nature conservation bodies (English Nature and The Countryside Council for Wales), the universities and the local authorities. In Wales we are blessed with active and motivated individuals who have had the foresight to take the lead in establishing Angling Federations and Rivers Trusts. The Wye-Usk Foundation and the Pembrokeshire Rivers Trust being two of the best examples having both evolved to a position where they are now able to access £5 million (e8 million) of EU funds for the beneﬁt of their local rivers and communities. So, what of the future? In Wales at least, it seems that sea trout stocks are mostly at historically high levels. But we must not be complacent. The historical problems of gross pollution from domestic and industrial point-sources have been largely overcome: but there are new problems that are more insidious, more complex and with the ability to do as much, if not more, harm to the aquatic environment and its associated ﬂora and fauna. New

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challenges lie in the effects of low level diffuse organic pollutants, such as pesticides and low level industrial contaminants. Britain is a small Island and intensive land use has been an increasing problem leading to the degradation and loss of habitat from intensive land use and water demand. These issues will not go away and so we have to repair the damage of the past and ﬁnd ways to manage them in the future. In Wales we have a landscape dominated by uplands where the issues of high grazing pressure and forestry can exacerbate problems of acidiﬁcation, soil erosion, siltation and water loss. These are still signiﬁcant issues that require scientiﬁc understanding to develop practical management solutions. Demands for access to water-space for recreational use and the emergence of waterside development in urban regeneration programmes are reasonable expectations by society, but they can bring real problems for ﬁsheries. In Cardiff we have the biggest estuarine barrage development in Europe and another similar development in neighbouring Swansea. The problems of ﬁsh passage through a total exclusion tidal barrier, such as that in Cardiff Bay, with a 13 m tidal rise and fall on the downstream side are immense. We hope they have been solved; but this has only been achieved at considerable expense and by using the latest technology and understanding of ﬁsh behaviour through ﬁsh-passes. All these issues are set in the context of climate change: and its effects on our seas and fresh waters are likely to be considerable. Sea trout use both environments and so we can expect some changes. There may be gains and losses, but more knowledge is needed and the Agency, along with may others, is active in this area. Through our work, such as the Sustainable Fisheries Programme, we will continue to encourage working in partnership with others, developing awareness within our communities of the social and economic value of their ﬁsheries resource, and in enhancing the capacity of the community to manage it in a sustainable way for future generations – of both ﬁsh and people. I hope you have a successful Symposium and I look forward to being able to apply its results to better management of ﬁsheries in the future.

Chapter 1

Setting the Scene – Sea Trout in England and Wales – A Personal Perspective G. Harris and N. Milner Symposium Convenors

Introduction Before starting the formal proceedings of this Symposium, we thought that it might help to set the scene by presenting a broad perspective on the sea trout based largely on our experience of the situation in England and Wales. In doing so we are aware that the importance and management needs of the sea trout may vary widely throughout its natural and introduced ranges and that different perspectives may apply in other geographical regions. However, as the aims of this Symposium are to consider ‘perceptions’, to debate ‘conventional wisdom’ and to challenge ‘assumptions’ relating to the sea trout, we make no apologies if this preamble stimulates discussions during the course of the programme. Indeed, it is our intention that it should do so. Keywords: Sea trout, historical neglect, importance, management advantages, uncertainties.

Historical neglect The epithets ‘neglected’, ‘overlooked’, ‘enigmatic’ and ‘taken-for-granted’ have been variously applied to describe the sea trout by commentators in the past. While these descriptions may not be wholly accurate today, they were certainly apposite to sea trout until quite recently compared with the amount of attention given to its more prestigious relatives – the Atlantic salmon and the non-migratory brown trout. The traditional view that sea trout are less worthy of attention is exempliﬁed by ‘Jock Scott’, the pen-name of a well-known Scottish angling author who published Sea Trout Fishing (Scott, 1969) at the end of a long and distinguished career as a salmon angler. He wrote: The sea trout is the Cinderella of the game ﬁsh world: authors, government department, ﬁshing owners and all other interested parties pass him by. This is really not so surprising when one considers . . . that a good sea trout river frequently accommodates salmon also, and naturally the latter ﬁsh steals all the limelight, attention and money.

When that was written almost 40 years ago, the angling literature contained several hundred books on ﬁshing for salmon and brown trout but only ﬁve books had ever been published that were dedicated solely to the sea trout: and almost the only single source of any technical information about the ﬁsh itself for managers and scientists was George Herbert Nall’s The Life of the Sea Trout (Nall, 1930) based on his prodigious scale-reading studies throughout the British Isles. Any scientiﬁc work up to that time had been largely 1

2

Sea Trout

opportunistic and incidental to work on the salmon and the general attitude of ﬁshery managers appears to have been based on the assumption that if everything was ‘all right’ for salmon and for brown trout then it would be ‘all right’ for the sea trout also. This somewhat complacent approach is illustrated by the fact that while catch records for salmon had been routinely collected on most English and Welsh rivers since at least 1952, it was not until 1976 that sea trout rod catch records began to match those for salmon (Milner et al., 2001). The 1950s and 1960s are now remembered as the ‘golden age’ of salmon angling in the British Isles. Salmon were then abundant almost everywhere and so it is not surprising that most anglers were not particularly interested in sea trout as their main target species: except at a few notable venues where sea trout ﬁshing had become established as a minor cult – such as the Loch Maree in Scotland, the Delphi in Ireland and the Dyﬁ in Wales. However, things started to change in the late 1960s and early 1970s with the outbreak of the disease ulcerative dermal necrosis (UDN). This pandemic ravaged salmon and sea trout stocks in most rivers throughout the British Isles (and elsewhere) causing massive mortalities among returning adult ﬁsh for several years. However, whereas sea trout stocks steadily recovered over the next decade or so to something like their former levels of abundance, salmon stocks continued to decline to their present parlous state on many rivers in England and Wales. While this decline in salmon stocks helped to raise the proﬁle of sea trout as a natural alternative on many ﬁsheries, the growing interest in sea trout by anglers and ﬁshery managers was further increased by three other separate developments that have occurred since UDN. Angler awareness The ﬁrst development was in the angling literature. In 1962 Hugh Falkus published the ﬁrst edition of Sea Trout Fishing. It was a slim volume that had very little impact within the angling community at that time because salmon stocks were plentiful (Falkus, 1962). However, its re-publication in a much revised and expanded format in 1975 coincided with the time when salmon stocks were at their nadir following the UDN outbreak and when there was widespread gloom and despondency within the salmon ﬁshing community (Falkus, 1975). As a result of his persuasive writing about the special delights and opportunities of sea trout ﬁshing (especially at night), many new anglers were recruited into the sport and the demand for good sea trout ﬁshing steadily increased in many regions. Subsequent angling authors have developed this theme and have drawn attention to the practical advantages of sea trout as a sport ﬁsh when compared with either salmon or brown trout. These are, in brief: •

•

They can be caught in both the largest rivers and in the smallest streams, in lakes with open access to the sea, in estuaries and even in the sea itself. Many good sea trout ﬁsheries have yet to be ‘discovered’ by the angling community. Although generally smaller on average than salmon they are frequently more abundant, and there are probably more salmon-sized sea trout than there are salmon in many of the smaller rivers.

Setting the Scene •

•

•

• •

3

They can be caught using a more extensive repertoire of angling techniques covering ﬂy ﬁshing with the dry ﬂy, wet ﬂy, surface and deep-sunk lures and then by spinning and bait ﬁshing. Not only can they be caught during the day, but they also have the added attraction in that they can also be caught at night. Indeed, throughout Wales and in some parts of England, by far the greatest proportion of the rod catch of sea trout is taken by ﬂy ﬁshing between the hours of dusk and dawn. They will continue to enter the river and move upstream on very low summer ﬂows – when salmon ﬁshing is a normally complete waste of time: and it is often under such conditions that some of the best ﬂy ﬁshing, especially at night, can occur. Access to sea trout ﬁshing is generally less costly than salmon ﬁshing – particularly on those smaller, spate rivers where salmon runs do not appear until late in the season. Quite apart from the popular view that sea trout ﬁght harder and more spectacularly than either salmon or brown trout when compared for size, they can normally be caught in larger numbers than salmon, they come ‘naturally packed’ in a range of more convenient sizes for the dinner table – from about 10 ounces (0.4 kg) to as much as 10 lb (4.5 kg) or more on some rivers and, according to many, they taste better than either salmon or brown trout.

Socio-economic importance The second development stemmed from the late 1970s when the ﬁrst socio-economic investigations into the value of commercial and recreational ﬁsheries were commissioned in England and Wales. Notwithstanding the decline in salmon stocks, these added to the growing awareness that the sea trout was the mainstay of the rod and net ﬁsheries on many rivers and had a social and economic value equal to or even greater than that of the salmon in many regions (Harris & Winstone, 1990). One of the more important outcomes from these studies was to show that the sea trout was of particular importance in providing recreational opportunities throughout the season on the many minor rivers with small runs of salmon, predominantly grilse, which did not appear until late summer or early autumn. When viewed overall, the value of these minor ﬁsheries was cumulatively greater than that of the smaller number of better-known salmon rivers that had traditionally dominated and driven our management approach and priorities over the years. Events elsewhere The third, and perhaps most important, event that did much to redress the long history of complacency about the active management of the sea trout and ensured that they were no longer taken-for-granted in England and Wales, was the very loud wake-up call provided by the sudden and dramatic collapse of many important and valuable sea trout ﬁsheries in the west of Ireland from the late 1980s and then in some regions of Scotland from the early 1990s. Although the reasons for this collapse were little understood and hotly contested at the time, they generated grave concern that something similar might happen

4

Sea Trout

in England and Wales. This concern triggered a concerted effort to draw together all the disparate information available from past sea trout studies in England and Wales and led to the implementation of the National Sea Trout R&D Programme in a structured endeavour to identify the urgent gaps in our knowledge about the status and well-being of our ﬁsheries and how best to manage them should such a collapse occur. Thankfully it never did, for reasons that eventually became apparent, but it has now placed us in a far better position to manage our sea trout ﬁsheries in a proactive and sustainable way, although there is still much to learn.

Management advantages In addition to its many attributes as a sport ﬁsh, the sea trout has certain characteristics that provide several practical management advantages when compared with the salmon and the brown trout. The chief characteristics among these are discussed in the following sections.

Robust life history The pattern of divided smolt migration to the sea when combined with the pattern of divided adult return to fresh water to spawn for the ﬁrst time as maiden ﬁsh and then again as repeat spawners is far more robust than that of the salmon because it is better able to spread the risks to survival across a greater number of year classes and cohorts of ﬁsh. It is therefore potentially better able to withstand and recover from any short-term factors affecting survival in the river and in the sea.

Size-range and in-river distribution Sea trout are better able to make use of a greater range of the available spawning and nursery habitat within a catchment than either salmon or brown trout by virtue of the wide range of adult sizes that are likely to occur. The smaller sea trout can penetrate and utilise the smaller tributaries and smaller spawning gravels that would not be suitable for the bigger salmon, whereas the bigger sea trout can utilise the larger spawning gravels that cannot be used by the smaller brown trout.

Total lifetime fecundity Although sea trout are generally much smaller than salmon on their ﬁrst return to the river as maiden ﬁsh, their potential to live to a greater age and to survive to make several spawning trips to fresh water means that the total, or cumulative, fecundity of these multiple spawning sea trout over their lifetime may be several times greater than that of each individual salmon – where repeat spawning, while not unknown, is now a very rare occurrence. Some British sea trout may survive to make as many as 11 (or more) separate spawning visits to fresh water, increasing in size after each visit to the sea.

Setting the Scene

5

Marine movements and ecology Although our information on the marine phase in the life history of the sea trout is very sparse and incomplete for the British Isles, the general picture that emerges is that it is more coastal in its sea feeding habits than salmon. It is therefore less vulnerable to exploitation and interception by high seas ﬁsheries on its return migrations to fresh water and it is less likely to be affected by those factors oceanic affecting the marine survival of the salmon. Return to the river The sea trout is less dependent than the salmon on the occurrence of natural ﬂoods and spates to trigger its migration from the sea, through the estuary and into fresh water. Indeed, some ﬁsh will migrate upstream on even the lowest drought ﬂows. This characteristic means that sea trout are less exposed to problems of illegal ﬁshing and poor water quality that can affect accumulations of salmon in tidal waters waiting for the next ﬂood to trigger movement into fresh water.

Unknowns and uncertainties So what are the gaps in our knowledge that limit our ability to manage our sea trout ﬁsheries efﬁciently and effectively? It is difﬁcult to know where to start, so let us begin with the big question. What is a sea trout? This central question was ﬁrst posed by Lamond (1916). It has yet to receive a deﬁnitive answer. Until quite recently, the conventional wisdom was based largely on the conclusions of Regan (1911) that were subsequently reconﬁrmed and popularised by Trewavas (1953). It was that the sea trout and the brown trout were freely interbreeding fractions of the same single species, Salmo trutta (as ﬁrst described ‘page-and-line’ by Linnaeus in 1758), and that there were no genetic differences between the many different forms of migratory and non-migratory trout in the British Isles that had previously been accorded species status by assorted naturalists and taxonomists during the nineteenth century on the basis of differences in their external appearance, morphology and anatomy. In essence the popular view was that S. trutta was a highly polymorphic and ‘plastic’ species that was able to exist in many different forms in response to differences in its local environment. However, advances in science can change conventional wisdom: and there is now growing evidence to show the existence of sympatric and reproductively isolated populations of S. trutta that may qualify for recognition as genetically different races or subspecies. It seems that the scientiﬁc debate over the relative importance of genetics (nature) or the environment (nurture) in explaining the variable life history of S. trutta, and the occurrence of so many different forms of brown trout and sea trout, has now swung back in favour of accepting that there may well be an important measure of genetic control over the extent to which migratory and non-migratory forms of trout are expressed in different situations.

6

Sea Trout

Natural regeneration Once salmon runs have become locally extinct, for whatever reason, their restoration depends on some form of artiﬁcial restocking to kick-start the regeneration of a new founder stock of adult ﬁsh. Such stocking programmes are enormously costly. They also pose genetic risks and other practical problems. However, it may be that stocking is not an inevitable consequence when seeking to regenerate sea trout stocks that have been lost or which have been seriously impoverished if, as has been postulated by some workers, the existence of a healthy resident brown trout population within the upstream catchment has the residual potential to produce a proportion of juvenile parr that become smolts and migrate to sea to become sea trout. If this is so, and therein resides the fundamental question, it means that runs of locally adapted sea trout will never be lost permanently provided the local stock of brown trout has not been lost also. All that is required is patience to let the natural sequence of events take place. Is stocking with sea trout a good idea? What are the roles and risks of artiﬁcial stocking in sea trout management? Salmon stocking has been a popular management technique for over a hundred years, although it brings risks that need to be managed (Aprahamian et al., 2003; McGinnity et al., 2003) and its efﬁcacy has been regularly questioned (Harris, 1974, 1994). However, sea trout stocking has been much less extensive, partly because of uncertainty over its outcome, attributable to the ﬂexible resident–migratory life-history pattern of S. trutta and partly because of limited demand in England and Wales. Furthermore, the production of sea trout from residual ‘resident’ trout populations in recovering rivers (Champion, 1991) suggests that the natural regeneration capability of sea trout is higher than salmon once limiting factors have been removed. Nevertheless, sea trout stocking is extensive in other countries, but as in the case of salmon, research into its beneﬁts and the consequences for wild ﬁsh has been sparse. Given the emergence of sea trout as a popular ﬁshery species, demand for stocking can be expected to increase and so these questions need to be addressed. Climate change If it is genetics rather than environment that determines what proportion of juvenile trout may become either sea trout or brown trout, then what are the implications of climate change likely to be on the abundance and composition of sea trout stocks in the future? Could it be that better conditions for the feeding and subsequent growth of juvenile ‘trout’ parr will occur in fresh water so that fewer sea trout and more brown trout are likely to be produced in accordance with ‘conventional wisdom’? Other issues There are many other issues and concerns about the biology, conservation and management of the sea trout. Some of the more immediate questions that are likely to emerge during the

Setting the Scene

7

Symposium are: •

•

• • • • • • • • •

•

Do any reliable ﬁsh counters exist that operate across the range of sizes encountered with most sea trout stocks and can they distinguish reliably between salmon and sea trout of the same sizes? Do the ofﬁcial catch statistics and catch records currently published by various agencies have any meaning as indicators of the strength and structure of our sea trout stocks or the quality of our ﬁsheries? What needs to be done to improve their accuracy and reliability so that valid spatial and temporal comparisons can be made within and across different rivers? What are the rates of exploitation, and the impacts of selective ﬁshing by rod and net ﬁsheries, on sea trout during the marine and freshwater phases of their life history? When will we be in a position to set appropriate conservation limits (CLs) (or their equivalent) for sea trout, and how might this be done? How extensive are sea trout migrations in the sea, and do different stocks and different components of the stock behave differently in this respect? Are there any mixed stock ﬁsheries for sea trout? If so, how vulnerable are their stock components to selective exploitation? Do local races and sympatric sub-stocks of sea trout exist that might require different management approaches for their conservation and development? Does the life-history variation in sea trout have a special biodiversity and conservation value that require stronger protection? Do sea trout tend to smolt as they migrate towards the sea and do a signiﬁcant proportion of sea trout migrate to sea in autumn? Do sea trout compete with salmon to any signiﬁcant extent? Could in-stream habitat improvement work to beneﬁt juvenile salmon be detrimental to sea trout by reducing the amount of favourable habitat suitable for juvenile trout and the production of sea trout smolts? How will the recent and emerging European legislation, such as the Habitats and Water Framework Directives, inﬂuence future strategies for the protection and sustainable management of sea trout stocks?

The sea trout is a fascinating species that presents a signiﬁcant challenge to ﬁshermen, managers and scientists alike. Some of the questions raised here will be addressed during the course of the Symposium – and some new ones may emerge. Two key features need to be borne in mind throughout these proceedings. The ﬁrst relates to the continuum of the migratory habit in S. trutta because recognising and understanding this will ensure that we connect properly across the full range of environmental and ecosystem processes acting on ‘sea trout’. The second is that sea trout management and science are of unusually wide interest and application to a range of stakeholder activities from recreational and commercial ﬁshing, through conservation, to environmental protection. Just as S. trutta is diverse and ﬂexible in its different life-history strategies in response to its widely differing environments, we too must adopt a similarly integrated, cross-cutting and collaborative approach to understanding and managing it for future generations.

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Sea Trout

References Aprahamian, M.W., Martin Smith, K., McGinnity, P., McKelvey, S. & Taylor, J. (2003). Restocking of salmonids – opportunities and limitations. Fisheries Research, 62, 211–27. Champion, A.S. (1991). Managing a recovering salmon river – the river Tyne. In: Strategies for the Rehabilitation of Salmon Rivers (Mills, D., Ed.). Proceedings of the Linnaean Society Joint Conference, November 1990, pp. 63–72. Falkus, H. (1962). Sea Trout Fishing. Witherby, London, 185 pp. Falkus, H. (1975). Sea Trout Fishing, 2nd edn. Witherby, London, 445 pp. Haris, G.S. (1974). Salmon propagation in England and Wales. A Report by the Association of River Authorities/National Water Council Working Party. National Water Council, London, 62 pp. Harris, G.S. (1994). The identiﬁcation of cost-effective stocking strategies for migratory salmonids. National Rivers Authority, R&D Note 353, Bristol, 150 pp. Harris, G.S. & Winstone A. (1990). The sea trout ﬁsheries of Wales. In: The Sea Trout in Scotland (Picken, M.J. & Shearer, W.M., Eds). Proceedings of a Symposium, 18–19 July 1987, Dunstaffnage, pp. 25–33. Lamond, H. (1916). The Sea-Trout: A Study in Natural History. Sherratt & Hughes, Manchester, 219 pp. McGinnity, P., Prodöl, P., Ferguson, A. et al. (2003). Fitness reduction and potential extinction of Atlantic salmon, Salmo salar, as a result of interactions with escaped farm salmon. Proceedings of the Royal Society of London, B 270, 2443–50. Milner, N.J., Davidson, I.C., Evan, R., Locke, V. & Wyatt, R.J. (2001). The use of rod catches to estimate salmon runs in England and Wales. In: Proceedings of the Atlantic Salmon Trust Workshop (Shelton, R., Ed.). Lowestoft, November 2001, pp. 46–65. Nall, G.H. (1930). The Life of the Sea Trout: Especially in Scottish Waters. Seeley, London, 335 pp. Regan, C.T. (1911). The Freshwater Fishes of the British Isles. Methuen & Co. Ltd., London, pp. 54–72. Scott, J. (1969). Sea Trout Fishing. Seeley, Service & Co. Ltd., London, 216 pp. Trewavas, E. (1953). Sea-trout and brown-trout. Salmon & Trout Magazine, 139, 199–215.

Section 1

STOCKS AND FISHERIES

Chapter 2

Patterns of Anadromy and Migrations of Paciﬁc Salmon and Trout at Sea T.P. Quinn and K.W. Myers School of Aquatic and Fishery Sciences, Box 355020, University of Washington, Seattle, WA 98195, USA

Abstract: This chapter brieﬂy reviews the range of anadromy within the genus Oncorhynchus, using six criteria selected by Rounsefell (1958): (1) spatial extent of marine migrations; (2) duration of stay at sea; (3) state of maturity attained at sea; (4) spawning habitats; (5) post-spawning mortality and (6) occurrence of freshwater forms of the species. We provide updated information on anadromy and marine migration patterns, especially for the iteroparous cutthroat (O. clarki) and rainbow (O. mykiss) trout. These two species display a wide range of anadromy, including truly ‘landlocked’ populations, non-anadromous populations with access to the sea, coastal migrants and ﬁsh that migrate extensively at sea (in steelhead, the anadromous rainbow trout). We conclude, as did Rounsefell, that anadromy is best viewed as a suite of life-history traits that vary greatly among species and population. Seen in this light, the Paciﬁc species are not fundamentally different from Atlantic salmon and brown trout but rather all species are along a continuum of anadromy. Brown trout, Salmo trutta L., seem most similar to cutthroat trout in their limited native range, iteroparity, diversity of non-anadromous forms occupying streams, large rivers and lakes as well as the anadromous forms and the limited duration and spatial extent of migrations at sea. Keywords: Onchorhynchus spp., marine migrations, marine residence, maturation, spawning movements, spawning survival, freshwater forms.

Introduction Salmonid ﬁshes have three key life-history traits: (1) anadromy; (2) homing to the natal site for reproduction and (3) semelparity (Quinn, 2005). Homing seems to be essentially universal within the family, but anadromy and semelparity vary considerably among and within species, and these last two traits are often linked. Rounsefell (1958) argued that there are six components of anadromy: (1) extent of migrations at sea; (2) duration of stay at sea; (3) state of maturity attained at sea; (4) spawning habits and habitats; (5) postspawning mortality and (6) occurrence of freshwater forms of the species. This chapter revisits these criteria, providing a brief update on anadromy in Paciﬁc salmon and trout (genus Oncorhynchus) to help place the migration patterns of brown trout (Salmo trutta L.) in the broader context of the life-history variation within the salmonid family. Further details regarding many of these patterns for the Paciﬁc species can be found in Quinn and Myers (2004) and Quinn (2005); the large literature on brown trout will not be reviewed here, as extensive information is available (e.g. Jonsson & L’Abée-Lund, 1993; 11

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Sea Trout

Elliott, 1994; Klemetsen et al., 2003; Jonsson and Jonsson, Chapter 14 and other papers in this volume).

Extent of migration at sea The distribution and migration patterns of salmon in the Paciﬁc Ocean were documented by the International North Paciﬁc Fisheries Commission (coho, O. kisutch: Godfrey et al. [1975]; sockeye, O. nerka: French et al. [1976]; chum, O. keta: Neave et al. [1976]; chinook, O. tshawytscha: Major et al. [1978]; pink, O. gorbuscha: Takagi et al. [1981]; masu, O. masou: Machidori & Kato [1984]; steelhead [the term for anadromous rainbow trout], O. mykiss: Burgner et al. [1992]; juvenile salmonids: Hartt & Dell [1986]) and more recent results were reported by Myers et al. (1996). The most numerous species (pink, chum and sockeye) migrate to sea and feed in offshore epipelagic waters, within about 50 m of the surface. Chum salmon spend a few days or weeks in estuaries but the other species seem to move directly to coastal waters (e.g. sockeye in Bristol Bay: Straty & Jaenicke [1980] and the Fraser River: Groot et al. [1989]). North American populations migrate northward along the continental shelf, or westward along the Alaska Peninsula and across the eastern Bering Sea shelf, until in the fall when they move farther offshore, and they are seldom found in coastal waters until they return at maturity (Fig. 2.1). Variation in the early marine distribution, movements, growth and survival of these species is linked to physical (e.g. sea temperature, water currents) and biological factors, especially the distribution and abundance of their preferred prey (see reviews by Myers et al. [2000]; Beamish et al. [2003]; Brodeur et al. [2003]; Karpenko [2003]; Mayama & Ishida [2003]).

Arctic Ocean

N

Yana River

Lena River

70° Kotzebue Anadyr Sound River

Kolyma River

ALASKA (USA) Yukon River

Norton Sound

RUSSIA

Kuskokwim River Copper River

Bering Sea Bristol Bay

Kamchatka River

Sea of Okhotsk Amur River

Aleutian Islands

Prince William Sound Kodiak Queen Island Charlotte Islands Gulf of Alaska

Mackenzie River CANADA 60° Nass River Skeena River Fraser River 50°

Vancouver Island

CHINA Yalu River

Kuril Islands Sea of Japan

Columbia River

Kamchatka Peninsula North Pacific Ocean

Hokkaido

Sacramento River

Honshu KOREA

140°

40° USA

JAPAN

160°E

180°

160°W

140°

120°

Fig. 2.1 Paciﬁc Rim showing the region (shaded) from which Paciﬁc salmon and trout migrate to sea (from Quinn, 2005).

Anadromy and Migration Patterns

13

In contrast to these species, juvenile coho and chinook salmon seem to migrate more slowly along the coast, and are common in coastal and inland waters during their period at sea, though they also occur offshore. Coho and chinook salmon entering the ocean off California and Oregon are largely limited to the coastal zone where upwelling brings cool, nutrient rich water to the surface. In this region the offshore waters are warm, less productive and dominated by other ﬁshes. North American coho salmon in coastal waters tend to be caught (generally in their second summer at sea) near their area of origin (Weitkamp & Neely, 2002). Offshore, Asian and North American coho salmon overlap south of the central Aleutians (Myers et al., 1996). Interestingly, northern coho salmon stocks are found farther offshore (averaging four times as far from tag recovery sites), compared with the more coastal distribution of southern stocks (Walker et al., 1992). From south central Alaska to California, there are progressively fewer recoveries from offshore tag releases and more recoveries from releases in coastal waters. Travel rates also reﬂect this difference; Asian and western Alaska ﬁsh travel over 40 km/day compared with about 10 km/day for coho salmon from south-eastern Alaska and southwards. North American chinook salmon range across almost the entire Bering Sea (Myers et al., 1996), and in the North Paciﬁc from the coastal waters off California west to the central Aleutian Islands. Chinook salmon are classiﬁed as ‘ocean-type’ (migrating to sea in their ﬁrst year of life) and ‘stream-type’ (migrating to sea after a full year in fresh water; Healey [1991]). Ocean-type chinook, most prevalent towards the southern end of the species’ range, migrate downstream over a protracted period and then reside in estuaries much longer than stream-type chinook (Healey, 1991). As with coho, the tendency of chinook to migrate to the open ocean is not completely known, but stream-type chinook seem more inclined to do so than ocean-type. The vast majority of anadromous salmonids in the Paciﬁc Ocean are from one of the ﬁve semelparous Oncorhynchus species but interesting patterns are found in other species. In most North American populations of steelhead, the smolts migrate rapidly through estuaries (e.g. Dawley et al., 1986), leave coastal waters early in the summer and range at sea across almost the entire North Paciﬁc, in some cases over 5000 km from home. Steelhead from coastal Oregon and California may have more restricted westward migrations than more northern stocks, consistent with patterns in coho and chinook salmon. Masu salmon are exclusively Asian and their marine distribution is largely limited to the Sea of Japan and Sea of Okhotsk (Machidori & Kato, 1984). Cutthroat trout (O. clarki) are exclusively North American, and also have restricted (but poorly known) marine migrations. Presumably, the presence of these two species on only one continent (reminiscent of brown trout) results in part from their limited movements at sea, whereas steelhead or stream-type chinook salmon are most similar to Atlantic salmon (Salmo salar) in migration patterns.

Duration of stay in the sea and in fresh water The length of time that individual ﬁsh spend at sea, relative to time in fresh water, was Rounsefell’s second criterion for anadromy, and the information he presented needs little revision. Pink salmon, at one end of the continuum (Table 2.1), migrate seaward immediately

14

Sea Trout Table 2.1 Characteristic (++) and less common (+) ages at seaward migration (i.e. winters in fresh water after hatching) of different salmonids. Age Species Pink salmon Chum salmon Chinook salmon Coho salmon Sockeye salmon Masu salmon Steelhead trout Cutthroat trout Dolly Varden Brown trout Atlantic salmon

0 ++ ++ ++ + +

1

2

3

++ ++ ++ ++ + +

+ ++ ++ ++ ++ ++ + ++ ++

+ + + ++ ++ ++ ++ ++

+ +

4

5

6

7

8

+ + + +

+ + + +

+

+

+ + ++ ++ + ++

Source: From Randall et al., 1987; Quinn, 2005, and references therein.

Table 2.2 Generalised duration of marine residence (winters at sea) before ﬁrst maturation among salmonid species, expressed as characteristic (++) and less common (+) patterns. Age Species Chum salmon Chinook salmon Sockeye salmon Steelhead trout Masu salmon Coho salmon Pink salmon Cutthroat trout Dolly Varden Atlantic salmon Brown trout

0

+ + + ++ ++ ++

1 + + ++ ++ ++ ++ + + ++ ++

2 ++ ++ ++ ++ + +

3 ++ ++ ++ ++

4 ++ ++ + +

++ ++

++ +

+ +

5 or more + +

after emerging from redds, and some populations actually spawn within the intertidal zone, especially in south-east Alaska. Virtually without exception, pink salmon in their natural range return to spawn after one winter at sea (1SW), for a total age of 2 years (Table 2.2). Chum salmon show the next least reliance on fresh water for rearing, commonly spending a few days or weeks in streams, then rearing in estuaries before migrating through coastal waters and out to the sea where most spend 2–4 years. Seaward migration in the ﬁrst year of life also occurs in chinook, coho and sockeye salmon. At the southern end of the range and at lower elevations, chinook salmon typically migrate to sea as either newly emerged fry or after residing for a few months in rivers. Northern populations and many in the interior are yearling migrants (Healey, 1991), reﬂecting slower growing conditions. Chinook salmon also vary in duration of marine

Anadromy and Migration Patterns

15

residence, with 2–4 years being typical (Table 2.2; Roni & Quinn, 1995). Most sockeye salmon spend 1 or 2 years in a lake and then 2 or 3 years at sea but river-type juveniles reside for a year in a river and ocean-type sockeye migrate to sea in their ﬁrst year of life (Wood, 1995). The great majority of coho salmon smolts are also 1 or 2 years, with more 2-year-old ﬁsh in the north (Weitkamp et al., 1995). However, fry are often caught in downstream traps and in many coastal systems, they enter the ocean shortly after emergence; little is known about the fate of these fry. Regardless of freshwater age, most coho salmon spend one winter at sea but some males (jacks) spend only the summer at sea before returning. Japanese masu salmon smolts typically migrate in spring and the adults return the next spring. However, some populations in Miyagi and Fukushima prefectures migrate to sea in the fall and return the following spring (Masahide Kaeriyama, Hokkaido University, pers. comm.). In Russia, there are four life-history types of masu salmon (Tsiger et al., 1994; Anton Ulatov, KamchatNIRO, pers. comm.): (1) the typical anadromous form, most of which spend one winter at sea; (2) a neotenic form (almost always males but rarely females) that matures as parr in their ﬁrst or second fall of life; (3) males that mature once or twice as parr that then undergo smolt transformation, migrate to sea for 2–3 months, and then return to spawn and (4) fully resident populations. Paciﬁc salmon spawn in the fall (though this may be as early as July or as late as February, depending on species and region) whereas the Paciﬁc trout species spawn in spring. The evolutionary and ecological aspects of this are unclear; perhaps it reﬂects a niche shift to avoid competition. Atlantic salmon and brown trout spawn in the fall, as do charr, so this characterises the family in general. The Paciﬁc trout have somewhat smaller eggs than sympatric salmon with which they compete (notably coho). Perhaps because of these traits the trout tend to spend more time in fresh water before migrating to sea than the salmon. Most North American steelhead go to sea at age 2 or 3 (older smolts predominate in the northern end of the range, Busby et al. [1996]) and spend 2 or 3 full years at sea, but some (mostly males) spend only a single year at sea. Moreover, some populations from northern California and southern Oregon have ﬁsh that return after only a summer at sea and do not spawn. Owing to their small size, these ﬁsh are locally known as half-pounders (Kesner & Barnhart, 1972), and share some attributes with the sea trout, known as ﬁnnock or whitling, that spend only a summer at sea. Western Kamchatka steelhead are more diverse in life-history patterns than those in North America, showing four patterns: (1) typical anadromous ﬁsh that migrate far offshore to the North Paciﬁc Ocean to feed; (2) ﬁsh that stay near the coast and probably feed in the Sea of Okhotsk (including half-pounders); (3) a river-estuarine group, that enters saltwater lagoons and (4) river ﬁsh that do not migrate to the ocean, consisting mainly of males (Savvaitova, 1975; Savvaitova et al., 2003). The typical, anadromous pattern predominates in northern populations and especially in small rivers, whereas the coastal and river strategies are more common in the south and in larger rivers. Cutthroat trout may go to sea at ages 1–6 (Table 2.1) but 2, 3 and 4 are most common (Trotter, 1989; Johnson et al., 1999). Johnston (1982) noted that cutthroat smolts in protected waters such as Puget Sound are smaller and younger than those on the open coast. He hypothesised that in benign inland waters they forage in the littoral zone (where they are

16

Sea Trout

also caught by anglers) whereas the heavy surf of the ocean beaches forces them to forage farther from shore and so larger size is needed for survival. Unlike salmon or steelhead, cutthroat trout characteristically spend only a summer at sea. During this time, their growth is modest and they reach a much smaller ﬁnal size than steelhead.

State of maturity attained at sea The maturity state attained by salmonids at sea varies enormously. The great majority of pink and chum salmon return to fresh water in a nearly or completely mature state and spawn only a short distance inland within a few days or weeks, though some chums migrate far inland (e.g. in the Amur and Yukon rivers) and they enter in a much less advanced state of maturity. Coho salmon often enter fresh water in a less advanced state of maturity and spawn at least a month later, though they do not necessarily migrate very far inland. However, all of these species generally spawn in the same season in which they left the ocean but other salmonids show more complex patterns. ‘Fall’ chinook salmon enter fresh water in the fall and spawn within about a month but large rivers draining the interior plateau such as the Sacramento, Klamath, Columbia and Fraser rivers have ‘spring’ chinook that enter from March to June in a relatively immature state, migrate much of the way to their natal spawning grounds, hold during the summer and spawn in the fall (e.g. September). The Sacramento River also has unique ‘winter’ chinook that migrate in late winter and spawn in late spring (Fisher 1994). Some sockeye salmon populations also enter as early as March or April and then spawn in the fall. These are not populations with long migrations; rather, they are coastal populations in the southern end of the range. These sockeye avoid warm fall temperatures by entering early and remaining below the thermocline in lakes until they enter tributaries to spawn in the fall (Hodgson & Quinn, 2002). Even more extreme variation is seen in steelhead (Burgner et al., 1992; Busby et al., 1996). Ocean-maturing steelhead tend to occur in coastal rivers in the central and southern part of their range, entering fresh water in March or April and spawning in April or May and leaving thereafter. Stream maturing steelhead may enter large rivers such as the Columbia, Fraser and Skeena in late summer or early fall (Robards & Quinn, 2002), migrate part of the way home, then hold in suitable winter habitat and ascend to the spawning sites in spring. Some rivers have both forms of steelhead, and their migration timing is strikingly different. For example, the ocean-maturing steelhead migration into the Kalama River begins in late fall, peaks in April and most spawning is in mid-April (Leider et al., 1984). In contrast, the migration of stream-maturing ﬁsh peaks in July but continues into the winter, with a mean spawning date in early February. This migration pattern probably results where suitable areas for adults to spawn and juveniles to rear are inaccessible for reasons of ﬂow or temperature in the months shortly before spawning. Therefore, the only option is to leave the ocean early, get past the hazardous area and then minimise energy losses until it is time to spawn. Having said this, it is not always possible to identify the barrier to migration, and this subject needs further study. According to Johnston (1982), cutthroat from coastal Oregon and Washington rivers tend to be sexually mature at ﬁrst return to fresh water, whereas many from the Columbia River,

Anadromy and Migration Patterns

17

Puget Sound, British Columbia and Alaska do not spawn after their ﬁrst return to fresh water, and in this regard are similar to sea trout, much as Atlantic salmon seem more similar to steelhead and chinook salmon. In addition, the timing of cutthroat trout migration, and hence the state of maturity, varies among areas (Johnson et al., 1999). For example, in Eva Lake, Alaska, they migrated to sea in May and June and returned in September (Armstrong, 1971). Sand Creek, on the coast of Oregon, showed a longer period of marine residence, from April–May to October–December (Sumner, 1962). In Washington, there seem to be two patterns. In large rivers the cutthroat enter relatively early, in September and October (Johnston, 1982) but in smaller streams they enter later (January–March), shortly before spawning, and leave primarily in April (e.g. Big Beef Creek: Wenburg [1998]).

Spawning habits and habitat Rounsefell (1958) noted that spawning in streams is typical of all Oncorhynchus and Salvelinus species except lake trout, S. namaycush, but other habitats are also used. In terms of anadromy, intertidal spawning, chieﬂy displayed by pink and chum salmon in small streams of south-east Alaska, is the greatest shift from the stream habitats (Helle, 1970; Thorsteinson et al., 1971). The shortness of the streams in this area and the high densities at which these species typically spawn may have contributed to the evolution of intertidal spawning. On the other hand, many lakes support sockeye salmon populations that spawn on beaches. Sockeye fry typically rear in lakes, so this seems to be a natural expansion of the breeding habitat for them. The spawning beaches are commonly at the outwash of a river, or the margins of the lake where groundwater ﬂows down a hillside and wells up to irrigate the embryos (Wood, 1995). However, sockeye can spawn on lowlying islands with no groundwater. At these beaches the water is circulated through the gravel by wind-driven currents, and the substrate is very large (Kerns & Donaldson, 1968). Interestingly, coho salmon commonly rear in lakes and ponds, especially in winter, but we do not know of any beach spawning coho salmon populations. Indeed, it is noteworthy that salmonids as a group have taken little advantage of lakes for spawning. There have been many studies on the physical habitat features used by stream spawning salmonids. Some of these differences may arise from differences in size among the species (Kondolf & Wolman, 1993) and some from the rearing requirements of juveniles. However, most studies have been conducted on one species in a limited range of streams, so our holistic understanding of the roles of interspeciﬁc competition and species-speciﬁc habitat choice is still quite limited. There are species that are absent from streams that appear suitable and that support other species, and also cases of species with different habitat use patterns in different parts of their range. Perhaps the most puzzling is the distribution of pink salmon (Heard, 1991). They all spawn at 2 years of age, so even-year and odd-year lines are genetically distinct. At the southern end of their distribution, they are present almost exclusively on odd-numbered years, and many rivers have none at all or very few on evennumbered years. Streams in the middle of the range tend to have runs on both cycles but in the northern part of the range even-year runs tend to be more numerous. In addition to this very peculiar distribution pattern, pink salmon tend to use different habitats for spawning

18

Sea Trout

in the different parts of their range, and consequently different patterns of segregation and sympatry with other salmon species. This highlights the fact that there is still much to be learned before we can extrapolate from details of redd site features to explain the overall distribution of the species.

Mortality after spawning Mortality after spawning is not, strictly speaking, a component of anadromy but the two traits are linked to some extent. Rounsefell (1958) stated that all members of the genus Oncorhynchus (the Paciﬁc trout species were classiﬁed in the genus Salmo when his paper was written) die after spawning, and this is true with two exceptions. First, male masu salmon that mature in fresh water as parr are capable of surviving, migrating to sea and spawning in a subsequent season (Ivankov et al., 1977; Tsiger et al., 1994), though anadromous males and females are semelparous. Second, under experimental conditions male chinook salmon can mature as parr, survive spawning, grow and spawn again the following year and even a third year (Unwin et al., 1999). The mature parr had very large gonads and depleted energy reserves and so the likelihood of their surviving the winter in rivers would be low. Nevertheless, post-spawning survival in this species indicates that the separation between semelparous and iteroparous salmonids may not be as great as was once thought. Indeed, the life-history patterns of salmonids are much more variable than the discrete terms ‘semelparous’ and ‘iteroparous’ suggest. ‘Iteroparity’ refers more to the possibility rather than the likelihood of repeat breeding. The frequency of repeat spawning is higher in non-anadromous populations of both steelhead/rainbow trout and cutthroat trout (Fleming, 1998; Fleming & Reynolds, 2004). Steelhead are technically iteroparous but sampling indicates that most (sometimes nearly all) spawn only once. In 26 North American populations, ﬁrst time spawners comprised 92% of the adults in British Columbia and Washington, 94% in the Columbia River, 85% in Oregon and 81.5% in California (Busby et al., 1996). In addition, sampling on the high seas produced primarily steelhead that had never spawned (Burgner et al., 1992). The proportion of repeat spawners is both a natural attribute of populations and a consequence of ﬁshing; highly exploited populations would be expected to have a lower proportion of individuals surviving to spawn repeatedly. In addition, post-spawning survival is usually lower in males than in females, even though females have much larger gonads than males. In iteroparous species, females abandon their redd shortly after spawning whereas males remain on the spawning grounds longer, and this may deplete their energy and reduce survival. The morphological changes at maturity are also more extreme in males, and this energetic cost may be reﬂected in lower survival as well. Ocean-maturing steelhead should have a higher proportion of successful repeat spawners because they spend much less time in fresh water than the stream maturing ones, and data from the Kalama River (Leider et al., 1986) support this hypothesis. There has not been a comprehensive review of iteroparity in anadromous cutthroat trout, and the subject is complicated by the fact that some ﬁsh return to fresh water but do not spawn, especially on their ﬁrst time back, a trait that they share with sea trout. However,

Anadromy and Migration Patterns

19

data from four populations (Sand Creek, Oregon: Sumner [1962]; Snow and Salmon creeks and the Stillaguamish River, Washington: Michael [1989]; Big Beef Creek, Washington: Wenburg [1998]) indicated that few ﬁsh spawned more than once and most of the repeat spawners were females, consistent with the general pattern of higher post-breeding survival in females.

Occurrence of freshwater forms Paciﬁc salmonids vary widely in the occurrence of freshwater forms within their native range. Pink and chum salmon apparently have no natural non-anadromous populations, and this is consistent with their being the strongest anadromous salmonids. To our knowledge there are no naturally occurring non-anadromous populations of chinook salmon in their native range, though some have become established within this century. More fundamentally, some stream-type chinook salmon populations have mature male parr (Taylor, 1989; Myers et al., 1998). Coho salmon are essentially always anadromous, though there are a few reports of ‘residual’ populations (Sandercock, 1991). It is not clear whether these populations are truly self-sustaining (the bodies of water were all connected to the ocean, so the ﬁsh were not landlocked), and they have received little research attention. Interestingly, the residual coho salmon populations have been associated with lakes. Streams are the ‘classic’ coho salmon habitat but many populations spend a signiﬁcant amount of time in lakes. Given the abundance of non-anadromous sockeye salmon, it is unclear why non-anadromous coho salmon are so rare. Sockeye salmon commonly form non-anadromous populations called kokanee. Wood (1995) reviewed the ecological and evolutionary aspects of this life-history pattern and noted that anadromy prevails in systems with short migrations and unproductive rearing lakes whereas more productive lakes with more arduous migrations tend to have kokanee that are polyphyletic, having evolved from sockeye salmon on numerous independent occasions (Taylor et al., 1996), and the two forms can remain genetically distinct even in sympatry despite some interbreeding (Wood & Foote, 1996). Rainbow and cutthroat trout also commonly occur as non-anadromous forms, and in large parts of their range only this form exists (Behnke, 1992, 2002). Steelhead are rare or absent from Alaska, north of the Alaska Peninsula, despite numerous robust rainbow trout populations with easy access to the ocean. Non-anadromy in these trout seems to be determined more by opportunities for growth than difﬁculty of migration. At the southern end of their range, rainbow trout exist in northern Mexico, where migration to sea may be impossible or the conditions at sea may be unsuitable for trout, so they persist in fresh water. In between, there are examples of sympatric rainbow and steelhead trout populations, existing with some degree of genetic isolation (e.g. Docker & Heath, 2003). Resident trout tended to spawn later in the season and used shallower, lower velocity spawning sites (Zimmerman & Reeves, 2000) consistent with their smaller size than sympatric steelhead. Cutthroat trout have an even longer and more complex history of non-anadromy, as there are sub-species in drainages that do not connect with the Paciﬁc Ocean (Behnke, 1992, 2002). Some coastal cutthroat trout populations are truly landlocked by waterfalls but in other cases the non-anadromy is facultative.

20

Sea Trout

In addition to the non-anadromous populations of rainbow and cutthroat trout, a fraction of the males mature as parr in anadromous populations. This phenomenon has been closely studied in Atlantic salmon (Fleming, 1998; Hutchings & Jones, 1998) and brown trout (L’Abée-Lund et al., 1990); in these species rapid growth in fresh water is associated with a higher proportion of males maturing as parr. However, the reports for Paciﬁc trout species (e.g. Shapovalov & Taft, 1954; reviewed by Busby et al., 1996) are rather anecdotal and there does not seem to be a systematic survey of the proportion of male parr in steelhead or anadromous cutthroat trout populations. However, sea run cutthroat populations often show a predominance of females, and we might infer that the balance of the males remained in fresh water (Johnson et al., 1999).

Salmo trutta in comparison Scientists working on either Atlantic or Paciﬁc salmonids often seem to view the species in the other ocean as fundamentally different, sometimes ignoring those species when reviewing the pertinent literature for their papers. However, as Rounsefell (1958) noted, anadromy is really a series of interrelated traits, and salmonids can be arrayed along a continuum rather than falling into two discrete modes. Steelhead are more similar to Atlantic salmon than they are to pink, chum or sockeye salmon, and cutthroat trout are more similar to brown trout than they are to most species in their own genus. Cutthroat and brown trout share apparently limited migrations at sea in terms of distance (though very little is known about the distances travelled by cutthroat trout), and also in many cases spend only a summer at sea. Overwintering at sea (and especially for multiple years) is apparently much more common in sea trout (Jonsson & L’Abée-Lund, 1993; Knutsen et al., 2004) than in cutthroat trout. Sea trout commonly reach a larger maximum size (Jonsson & Jonsson, this volume) than is typical of anadromous cutthroat trout but have a wider range than is routinely seen in steelhead. Similar to cutthroat trout, brown trout show a wide range in smolt ages, a predominance of females among the smolts and return migration by immature as well as mature ﬁsh. Both cutthroat trout and brown trout are naturally distributed on only one side (in both cases, the eastern) of their respective oceans. Transplants have shown brown trout to be adaptable to many habitats, including those in eastern North America (MacCrimmon & Marshall, 1968), so we must infer that it was limited migration rather than narrow habitat suitability patterns that determined their distribution. Among the more pressing issues, from both the perspectives of basic science and also conservation, is the extent of genetic control over anadromy in the different salmonid species. Various lines of evidence have been presented indicating the plasticity of anadromy within polymorphic populations of brown trout (e.g. Jonsson, 1985; Jonsson & Jonsson, Chapter 14 this volume) and species of charr as well. On the other hand, genetically distinct anadromous and resident populations have been reported in sympatric populations of Atlantic salmon (Verspoor & Cole, 1989) and sockeye salmon (Wood & Foote, 1996). It would be very informative to learn whether changes in growing conditions (e.g. regional warming, increased forage ﬁsh populations, density-dependent growth, eutrophication, etc.) or selective ﬁsheries cause shifts in migration patterns. A thorough understanding of the

Anadromy and Migration Patterns

21

genetic basis for migratory patterns will also help determine whether the loss of one form in the presence of a healthy population of the other (e.g. declining sea trout but abundant resident brown trout) constitutes a transient and reversible or a permanent loss. Such information can help guide our efforts to conserve and restore populations of these ﬁshes, and especially the anadromous life-history pattern.

Acknowledgements The support provided by H. Mason Keeler Endowment to TQ during the preparation of this chapter, and by NOAA Contract 50-ABNF-1-0002, NPAFC Research Coordination to KM is gratefully acknowledged.

References Armstrong, R.H. (1971). Age, food and migration of sea-run cutthroat trout, Salmo clarki, at Eva Lake, Southeastern Alaska. Transactions of the American Fisheries Society, 100, 302–06. Beamish, R.J., Pearsall, I.A. & Healey, M.C. (2003). A history of the research on the early marine life of Paciﬁc salmon off Canada’s Paciﬁc coast. North Paciﬁc Anadromous Fish Commission Bulletin, 3, 1–40. Behnke, R.J. (1992). Native trout of western North America. American Fisheries Society Monograph 6, Bethesda, MD. Behnke, R.J. (2002). Trout and Salmon of North America. The Free Press, New York. Brodeur, R.D., Myers, K.W. & Helle, J.H. (2003). Research conducted by the United States on the early ocean life history of Paciﬁc salmon. North Paciﬁc Anadromous Fish Commission Bulletin, 3, 89–131. Burgner, R.L., Light, J.T., Margolis, L., Okazaki, T., Tautz, A. & Ito, S. (1992). Distribution and origins of steelhead trout (Oncorhynchus mykiss) in offshore waters of the North Paciﬁc Ocean. International North Paciﬁc Fisheries Commission Bulletin, 51, 92 pp. Busby, P.J., Wainwright, T.C., Bryant, G.J. et al. (1996). Status review of west coast steelhead from Washington, Idaho, Oregon and California. National Marine Fisheries Service Technical Memorandum NMFS-NWFSC-27, Seattle. Dawley, E.M., Ledgerwood, R.D., Blahm, T.H. et al. (1986). Migrational characteristics, biological observations and relative survival of juvenile salmonids entering the Columbia River estuary, 1966–1983. Final Report to the Bonneville Power Administration, Project 81-102, Portland, 256 pp. Docker, M.F. & Heath, D.D. (2003). Genetic comparison between sympatric anadromous steelhead and freshwater resident rainbow trout in British Columbia, Canada. Conservation Genetics, 4, 227–31. Elliott, J.M. (1994). Quantitative Ecology and the Brown Trout. Oxford University Press, Oxford. Fisher, F.W. (1994). Past and present status of Central Valley chinook salmon. Conservation Biology, 8, 870–3. Fleming, I.A. (1998). Pattern and variability in the breeding system of Atlantic salmon (Salmo salar), with comparisons to other salmonids. Canadian Journal of Fisheries and Aquatic Sciences, 55 (Suppl. 1), 59–76. Fleming, I.A. & Reynolds, J.D. (2004). Salmonid breeding systems. In: Evolution Illuminated: Salmon and Their Relatives (Henry, A.P. & Stearns, S.C., Eds). Oxford University Press, Oxford, pp. 264–94. French, R., Bilton, H., Osako, M. & Hartt, A. (1976). Distribution and origin of sockeye salmon (Oncorhynchus nerka) in offshore waters of the North Paciﬁc Ocean. International North Paciﬁc Fisheries Commission Bulletin, 34, 1–113. Godfrey, H., Henry, K.A. & Machidori, S. (1975). Distribution and abundance of coho salmon in offshore waters of the North Paciﬁc Ocean. International North Paciﬁc Fisheries Commission Bulletin, 31, 1–80. Groot, C., Bailey, R.E., Margolis, L. & Cooke, K. (1989). Migratory patterns of sockeye salmon (Oncorhynchus nerka) smolts in the Strait of Georgia, British Columbia, as determined by analysis of parasite assemblages. Canadian Journal of Zoology, 67, 1670–8.

22

Sea Trout

Hartt, A.C. & Dell, M.B. (1986). Early oceanic migrations and growth of juvenile Paciﬁc salmon and steelhead trout. International North Paciﬁc Fisheries Commission Bulletin, 46, 1–105. Healey, M.C. (1991). Life history of chinook salmon (Oncorhynchus tshawytscha). In: Paciﬁc Salmon Life Histories (Groot, C. & Margolis, L., Eds). University of British Columbia Press, Vancouver, pp. 311–93. Heard, W.R. (1991). Life history of pink salmon (Oncorhynchus gorbuscha). In: Paciﬁc Salmon Life Histories (Groot, C. & Margolis, L., Eds).University of British Columbia Press, Vancouver, pp. 119–230. Helle, J.H. (1970). Biological characteristics of intertidal and freshwater spawning pink salmon at Olsen Creek, Prince William Sound, Alaska, 1962–63. United States Fish and Wildlife Service, Special Scientiﬁc Report, Fisheries 602, Washington, DC, 1–19. Hodgson, S. & Quinn, T.P. (2002). The timing of adult sockeye salmon migration into fresh water: adaptations by populations to prevailing thermal regimes. Canadian Journal of Zoology, 80, 542–55. Hutchings, J.A. & Jones, M.E.B. (1998). Life history variation and growth rate thresholds for maturity in Atlantic salmon, Salmo salar. Canadian Journal of Fisheries and Aquatic Sciences, 55 (Suppl. 1), 22–47. Ivankov, V.N., Padetskiy, S.N. & Chikina, V.S. (1977). On the postspawning neotenic males of the masu, Oncorhynchus masu. Journal of Ichthyology, 15, 673–8. Johnson, O.W., Ruckelshaus, M.H., Grant, W.S. et al. (1999). Status review of coastal cutthroat from Washington, Oregon and California. NOAA Technical Memorandum NMFS-NWFSC-37, Seattle. Johnston, J.M. (1982). Life histories of anadromous cutthroat with emphasis on migratory behavior. In: Salmon and Trout Migratory Behavior Symposium (Brannon, E.L. & Salo, E.O., Eds). University of Washington, School of Fisheries, Seattle, pp. 123–7. Jonsson, B. (1985). Life history patterns of freshwater resident and sea-run brown trout in Norway. Transactions of the American Fisheries Society, 114, 182–94. Jonsson, B. & L’Abée-Lund, J.H. (1993). Latitudinal clines in life-history variables of anadromous brown trout in Europe. Journal of Fish Biology, 43 (Suppl. A), 1–16. Jonsson, B. & Jonsson, N. Life history of anadromous trout Salmo trutta. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the 1st International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 196–223. Karpenko, V.I. (2003). Review of Russian marine investigations of juvenile Paciﬁc salmon. North Paciﬁc Anadromous Fish Commission Bulletin, 3, 69–88. Kerns, O.E., Jr. & Donaldson, J.R. (1968). Behavior and distribution of spawning sockeye salmon on island beaches in Iliamna Lake, Alaska. Journal of the Fisheries Research Board of Canada, 24, 485–94. Kesner, W.D. & Barnhart, R.A. (1972). Characteristics of the fall-run steelhead trout (Salmo gairdneri gairdneri) of the Klamath River system with emphasis on the half-pounder. California Fish and Game, 58, 204–20. Klemetsen, A., Amundsen, P.-A., Dempson, J.B. et al. (2003). Atlantic salmon Salmo salar L., brown trout Salmo trutta L. and Arctic charr Salvelinus alpinus (L.): a review of aspects of their life histories. Ecology of Freshwater Fish, 12, 1–59. Knutsen, J.A., Knutsen, H., Olsen, E.M. & Jonsson, B. (2004). Marine feeding of anadromous Salmo trutta during winter. Journal of Fish Biology, 64, 89–99. Kondolf, G.M. & Wolman, M.G. (1993). The sizes of salmonid spawning gravels. Water Resources Research, 29, 2275–85. L’Abée-Lund, J.H., Jensen, A.J. & Johnsen, B.O. (1990). Interpopulation variation in male parr maturation of anadromous brown trout (Salmo trutta) in Norway. Canadian Journal of Zoology, 68, 1983–7. Leider, S.A., Chilcote, M.W. & Loch, J.J. (1984). Spawning characteristics of sympatric populations of steelhead trout (Salmo gairdneri): evidence for partial reproductive isolation. Canadian Journal of Fisheries and Aquatic Sciences, 41, 1454–62. Leider, S.A., Chilcote, M.W. & Loch, J.J. (1986). Comparative life history characteristics of hatchery and wild steelhead trout (Salmo gairdneri) of summer and winter races in the Kalama River, Washington. Canadian Journal of Fisheries and Aquatic Sciences, 43, 1398–409. MacCrimmon, H.R. & Marshall, T.C. (1968). World distribution of brown trout, Salmo trutta. Journal of the Fisheries Research Board of Canada, 25, 2527–48. Machidori, S. & Kato, F. (1984). Spawning populations and marine life of masu salmon (Oncorhynchus masou). International North Paciﬁc Fisheries Commission Bulletin, 43, 1–138. Major, R.L., Ito, J., Ito, S. & Godfrey, H. (1978). Distribution and origin of chinook salmon (Oncorhynchus tshawytscha) in offshore waters of the North Paciﬁc Ocean. International North Paciﬁc Fisheries Commission Bulletin, 38, 1–54.

Anadromy and Migration Patterns

23

Mayama, H. & Ishida, Y. (2003). Japanese studies on the early ocean life of juvenile salmon. North Paciﬁc Anadromous Fish Commission Bulletin, 3, 41–67. Michael, J.H., Jr. (1989). Life history of anadromous coastal cutthroat trout in Snow and Salmon creeks, Jefferson County, Washington, with implications for management. California Fish and Game, 75, 188–203. Myers, J.M., Kope, R.G., Bryant, G.J. et al. (1998). Status review of chinook salmon from Washington, Idaho, Oregon and California. National Marine Fisheries Service, NOAA Technical Memorandum NMFS-NWFSC35, Seattle. Myers, K.W., Aydin, K.Y., Walker, R.V., Fowler, S. & Dahlberg, M.L. (1996). Known ocean ranges of stocks of Paciﬁc salmon and steelhead as shown by tagging experiments, 1956–1995. North Paciﬁc Anadromous Fish Commission Document 192, School of Aquatic and Fishery Sciences, University of Washington, Seattle. Myers, K.W., Walker, R.V., Carlson, H.R. & Helle, J.H. (2000). Synthesis and review of U.S. research on the physical and biological factors affecting ocean production of salmon. North Paciﬁc Anadromous Fish Commission Bulletin, 2, 1–9. Neave, F., Yonemori, T. & Bakkala, R.G. (1976). Distribution and origin of chum salmon in offshore waters of the North Paciﬁc Ocean. International North Paciﬁc Fisheries Commission Bulletin, 35, 1–79. Quinn, T.P. (2005). The Behavior and Ecology of Paciﬁc Salmon and Trout. University of Washington Press, Seattle. Quinn, T.P. & Myers, K.W. (2004). Anadromy and the marine migrations of Paciﬁc salmon and trout. Reviews in Fish Biology and Fisheries, 14, 421–42. Randall, R.G., Healey, M.C. & Dempson, J.B. (1987). Variability in length of freshwater residence of salmon, trout, and char. American Fisheries Society Symposium, 1, 27–41. Robards, M.D. and Quinn, T.P. (2002). The migratory timing of adult summer-run steelhead trout (Oncorhynchus mykiss) in the Columbia River: six decades of environmental change. Transactions of the American Fisheries Society, 131, 523–36. Roni, P. & Quinn, T.P. (1995). Geographic variation in size and age of North American chinook salmon (Oncorhynchus tshawytscha). North American Journal of Fisheries Management, 15, 325–45. Rounsefell, G.A. (1958). Anadromy in North American Salmonidae. Fishery Bulletin, 131, 171–85. Sandercock, F.K. (1991). Life history of coho salmon (Oncorhynchus kisutch). In: Paciﬁc Salmon Life Histories (Groot, C. & Margolis, L., Eds). University of British Columbia Press, Vancouver, pp. 395–445. Savvaitova, K.A. (1975). The population structure of Salmo mykiss in Kamchatka. Journal of Ichthyology, 15, 876–88. Savvaitova, K.A., Kuzishchin, K.V., Gruzdeva, M.A., Pavlov, D.S., Stanford, J.A. & Ellis, B.K. (2003). Long-term and short-term variation in the population structure of Kamchatka steelhead Parasalmo mykiss from rivers of western Kamchatka. Journal of Ichthyology, 43, 757–68. Shapovalov, L. & Taft, A.C. (1954). The life histories of the steelhead rainbow trout (Salmo gairdneri gairdneri) and silver salmon (Oncorhynchus kisutch) with special reference to Waddell Creek, California and recommendations regarding their management. California Department of Fish and Game, Fish Bulletin, 98, 1–375. Straty, R.R. & Jaenicke, H.W. (1980). Estuarine inﬂuence of salinity, temperature and food on the behavior, growth and dynamics of Bristol Bay sockeye salmon. In: Salmonid Ecosystems of the North Paciﬁc (McNeil, W.J. & Himsworth, D.C., Eds). Oregon State University Press, Corvallis, pp. 247–65. Sumner, F.H. (1962). Migration and growth of the coastal cutthroat trout in Tillamook County, Oregon. Transactions of the American Fisheries Society, 91, 77–83. Takagi, K., Aro, K.V., Hartt, A.C. & Dell, M.B. (1981). Distribution and origin of pink salmon (Oncorhynchus gorbuscha) in offshore waters of the North Paciﬁc Ocean. International North Paciﬁc Fisheries Commission Bulletin, 40, 1–195. Taylor, E.B. (1989). Precocial male maturation in laboratory-reared populations of chinook salmon, Oncorhynchus tshawytscha. Canadian Journal of Zoology, 67, 1665–9. Taylor, E.B., Foote, C.J. & Wood, C.C. (1996). Molecular genetic evidence for parallel life-history evolution within a Paciﬁc salmon (sockeye salmon and kokanee, Oncorhynchus nerka). Evolution, 50, 401–16. Thorsteinson, F.V., Helle, J.H. & Birkholz, D.G. (1971). Salmon survival in intertidal zones of Prince William Sound streams in uplifted and subsided areas. In: The Great Alaska Earthquake of 1964: Biology. (Nybakker, J., Ed.). National Academy of Science Publication, vol. 1604, pp. 194–219. Trotter, P.C. (1989). Coastal cutthroat trout: a life history compendium. Transactions of the American Fisheries Society, 118, 463–73.

24

Sea Trout

Tsiger, V.V., Skirin, V.I., Krupyanko, N.I., Kashlin, K.A. & Semenchenko, A.Y. (1994). Life history form of male masu salmon (Oncorhynchus masou) in South Primoré, Russia. Canadian Journal of Fisheries and Aquatic Sciences, 51, 197–208. Unwin, M.J., Kinnison, M.T. & Quinn, T.P. (1999). Exceptions to semelparity: postmaturation survival, morphology and energetics of male chinook salmon (Oncorhynchus tshawytscha). Canadian Journal of Fisheries and Aquatic Sciences, 56, 1172–81. Verspoor, E. & Cole, L.J. (1989). Genetically distinct sympatric populations of resident and anadromous Atlantic salmon, Salmo salar. Canadian Journal of Zoology, 67, 1453–61. Walker, R.V., Davis, N.D. & Myers, K.W. (1992). High seas distribution of coho and chinook salmon. In: Proceedings of the 1992 Chinook and Coho Workshop, Boise, Idaho, September 28–30, 1992, American Fisheries Society, Bethesda, MD, pp. 120–34. Weitkamp, L. & Neely, K. (2002). Coho salmon (Oncorhynchus kisutch) ocean migration patterns: insight from marine coded-wire tag recoveries. Canadian Journal of Fisheries and Aquatic Sciences, 59, 1100–115. Weitkamp, L.A., Wainwright, T.C., Bryant, G.J. et al. (1995). Status review of coho salmon from Washington, Oregon and California. National Marine Fisheries Service, NOAA Technical Memorandum NMFS-NWFSC24, Seattle. Wenburg, J.K. (1998). Coastal cutthroat trout (Oncorhynchus clarki clarki): genetic population structure, migration patterns and life history traits. PhD Thesis, University of Washington, Seattle, WA. Wood, C.C. (1995). Life history variation and population structure in sockeye salmon. American Fisheries Society Symposium, 17, 195–216. Wood, C.C. & Foote, C.J. (1996). Evidence for sympatric genetic divergence of anadromous and nonanadromous morphs of sockeye salmon (Oncorhynchus nerka). Evolution, 50, 1265–79. Zimmerman, C.E. & Reeves, G.H. (2000). Population structure of sympatric anadromous and nonanadromous Oncorhynchus mykiss: evidence from spawning surveys and otolith microchemistry. Canadian Journal of Fisheries and Aquatic Sciences, 57, 2152–62.

Chapter 3

A Review of the Status of Irish Sea Trout Stocks P.G. Gargan1 , W.R. Poole2 and G.P. Forde3 1 2 3

Central Fisheries Board, Dublin, Ireland Marine Institute, Newport, Co. Mayo, Ireland The Western Regional Fisheries Board, Galway, Ireland

Abstract: The status of Irish sea trout stocks has been reviewed by the Sea Trout Working Group, 1991–94, and the Sea Trout Review Group, 2002, but little has been published since the collapse of the stocks in the mid-west region in 1989–90. This chapter presents the historical data, updates the national trap census and rod catch data from 1989 to 2003 and provides an assessment of the national current status of Irish sea trout stocks. Accurate sea trout rod catch statistics are available from the majority of the ﬁsheries in the midwestern zone (Connemara–South Mayo area) and rod catch per unit effort (CPUE) data for the Burrishoole, Owengowla, Invermore and Delphi ﬁsheries. Annual Connemara sea trout rod catch was about 10 000 ﬁsh between 1974 and 1986, a decline over the 1987–88 period followed by a collapse to 646 sea trout in 1989 and 240 sea trout in 1990. A progressive, but modest, improvement in rod catch occurred during the 1990s until 2001, after which catches decreased again. Catches have not recovered to the levels seen before 1988. Rod catches are presented for each of the ﬁsheries regions outside the mid-west from 1993 to 2003. Data from these regions indicate that appreciable numbers of sea trout have been captured on rod and line in the 1990s and indications are that stocks remain relatively good in most areas. Summary information is also presented on trap census of sea trout, including smolt output and marine survival from upstream and downstream trapping facilities on four key west of Ireland ﬁsheries, Burrishoole, Owengowla, Invermore and Tawnyard (Erriff catchment). Marine survival indices for mid-western ﬁsheries conﬁrm that the collapse in the rod catch was linked to a collapse in the stock and that unprecedented low marine survival of sea trout has been observed for most years since then. The chapter concludes that stock levels and marine survival in the majority of mid-western sea trout stocks are low, relative to historical records. Urgent management action is required if these sea trout stocks are not to be lost and the elimination of lice on and in the vicinity of marine salmon farms must be a constant priority of management and regulatory practice. Keywords: Sea trout, rod catch, commercial catch, stock census, marine survival.

Introduction Sea trout are to be found in most estuaries or bays around the Irish coast. The ancestors of our present sea trout stocks ﬁrst entered fresh water after the last Ice Age, some 14 000 years 25

26

Sea Trout

ago. Sea trout are essentially ﬁsh of acid, oligotrophic waters ﬂourishing where freshwater growth rates are poor, where survival in fresh water is difﬁcult and where there is easy access to the sea. Since the nineteenth century a modest, but locally important, tourism industry developed around the summer and autumn runs of these ﬁsh. This was particularly true in the Connemara, Ballinakill, Cork, Kerry and Donegal districts where there were extensive areas of blanket bog feed acid, nutrient-poor lough systems. Few accurate long-term data are available on Irish sea trout catches outside the Connemara region. During the 1970s and 1980s there was evidence of a slow decline in sea trout stock from the Burrishoole, north of Connemara, partly attributed to factors such as illegal ﬁshing, afforestation and hillside erosion because of overgrazing by sheep (Poole et al., 1996). Rod catch data from 15 Connemara ﬁsheries for the period 1974–86 showed no evidence of a decline, but catches had begun to decrease over the 1986–88 period. In 1989 both catch and stocks in many mid-western and Connemara sea trout ﬁsheries collapsed (Anon., 1992). The history of the sea trout stock collapse and subsequent events has been well documented (Whelan, 1993a, b; Poole et al., 1996; Gargan, 2000). In 1989, when sea trout stocks collapsed in western ﬁsheries, sea trout were observed in the lower pools of the Delphi ﬁshery in Connemara in late May with heavy infestations of juvenile sea lice (Lepeophtheirus salmonis). Sampling of rivers began in 1990 to determine whether this phenomenon was widespread and sea trout post-smolts and some sea trout kelts were recorded in all rivers sampled with infestations of sea lice, predominantly juvenile lice, indicating recent transmission (Tully et al., 1993). This has been linked with the development of marine salmon farming in the mid-west zone at that time (Gargan et al., 2003). Information on the status of Irish sea trout stocks was published by the Sea Trout Working Group in its annual reports for each year over the period 1991–94 (Anon., 1992, 1993, 1994a, 1995) and by the Sea Trout Task Force (Anon., 1994b). This chapter reviews the national sea trout catch and stock information as collected using rod catch statistics (Central and Regional Fisheries Boards) for 26 mid-western ﬁsheries from Clew Bay to Galway Bay (1985–2003), rod catch data from selected ﬁsheries in other areas around the coast (1993–2003) and summary data on stock from traps (Central and Regional Fisheries Boards, Marine Institute). Summarised sea trout stock numbers, including marine survival, are presented from four key west of Ireland ﬁsheries, Owengowla, Invermore, Burrishoole and the Tawnyard Lough trap. More detailed information on these ﬁsheries is presented elsewhere by Poole et al. (2006) and Gargan et al. (2006).

Sea trout life history Brown trout (Salmo trutta L.) occur in both fresh water (resident) and anadromous (sea trout) forms in Ireland. Sea trout smolts migrate to sea from March to June each year. Some of these ﬁsh return to fresh water in the summer following migration, and in Ireland these ﬁsh are known as ﬁnnock, harvesters, whitling, juniors or post-smolt. A further component may not return to the natal river to spawn for the ﬁrst time for at least 1 year; these are known as maidens and are an important component of the spawning stock. Sea trout migrate back to sea

Review of Irish Stock Status

27

after the winter, both as spawned kelts and ﬁsh which have overwintered without spawning. Sea trout are often multiple spawners, and there is a variation in smolt age and the time of ﬁrst return to fresh water. Many different life-history strategies have been observed (Fahy, 1985).

Materials and methods Rod-ﬁshery statistics Sea trout rod catch statistics were available for 26 mid-western ﬁsheries (Connemara– South Mayo area) for the period 1985–2003, along with historical data for 16 Connemara ﬁsheries since 1974, as individual ﬁshery owners and/or ﬁshery managers were in place on most ﬁsheries (Fig. 3.1). Rod catch data were divided into Category 1 and Category 2 data depending on the source and accuracy of the information. Fisheries included in Category 1 report accurate catch data collected by ﬁshery owners or ﬁshery managers in their own ﬁsheries. Data for Category 2 rivers are less accurate, having only reports by district ﬁshery inspectors gathered from individuals or angling club records. Rod catch data from areas in Ireland other than the mid-western zone represent Category 2 data. Since 1990, a bye-law prohibiting the retention of rod caught sea trout has been in place in the mid-western zone from Galway Bay to Achill. Numbers reported for the 1990–2003 period refer to ﬁsh captured and returned to the water. These may be overestimates in some ﬁsheries as sea trout below the 1990 size limit of 25 cm and resident brown trout may have been included and there is also the possibility that some ﬁsh may have been captured on more than one occasion. Rod effort has also declined in many sea trout ﬁsheries over the period.

Sea trout CPUE rod catch data Catch per unit effort (CPUE) data were available for the Burrishoole, Owengowla, Invermore and Delphi ﬁsheries. Effort was calculated in rod days (∼8 h) and CPUE data are calculated as number of trout caught per rod day.

Commercial catches Sea trout are taken commercially in all ﬁshery districts, both by drift and draft nets, normally as a by-catch in the salmon ﬁshery, although in the mid-western region ﬁshermen are obliged, since 1990, under the commercial bye-law to return all trout to the water. The Eastern Regional Fisheries Board collected the most complete annual data on total commercial sea trout catch. The collection of this data has been superseded since 1 January 2001 by a wild salmon and sea trout tagging and logbook scheme (Fisheries [Amendment] Act, 1999 [Number 35 of 1999]) and therefore the total catch data was only available for the period 1990–99. Fish trapping facilities Information was available on various aspects of the biology of sea trout from upstream and downstream traps on the Burrishoole, Owengowla, Invermore and Erriff (Tawnyard) systems (Fig. 3.1).

28

Sea Trout

1

2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

3

4 5

Killary 6 8

7

9

12

10 11

13

Burrishoole Newport Belclare Bunowen Carrowniskey Delphi Erriff Culfin Kylemore Clifden Ardbear Doohulla Ballinahinch Gowla Carna Invermore Inverbeg Screebe Furnace Lettermuckoo Costello Crumlin

14 16

17

18

Bertraghboy Bay 15

19 20

Kilkieran Bay

Connemara Fishery District Boundary

21 22

Fig. 3.1

Mid-western sea trout ﬁsheries and trapping location (•).

The Burrishoole system discharges to the north-east corner of Clew Bay on the midwestern coast of Ireland. Fish trapping facilities in Burrishoole in operation since 1958, with full trapping on both rivers ﬂowing out of lake Feeagh since 1970, have enabled a full census to be made of all migratory ﬁsh movements upstream and downstream since 1970 (see Poole et al., 1996; Poole et al., Chapter 19). The Owengowla system discharges into Bertraghboy Bay in Connemara. A downstream smolt and kelt trap and an upstream trap were installed by the Fisheries Board in 1991 and upgraded in 1994 and 2002 (see Gargan et al., Chapter 5).

Review of Irish Stock Status

29

The Invermore river discharges into Kilkerrin Bay in Connemara. In 1992 an upstream trap was installed on the Invermore system approximately 500 m from the sea and a smolt and kelt trap was installed in 1993 and upgraded in 2002 (see Gargan et al., Chapter 5). A downstream Wolf-type trap has operated since 1985 on the Black River downstream of Tawnyard Lough, a sub-catchment of the Erriff ﬁshery, which discharges into Killary Harbour. This consists of a ﬁsh fence barrier which diverts downstream migrants, smolts and kelts, into a wolf trap with sloped grids with 12-mm spacing leading into a box trap. Traps were generally monitored at least daily and more frequently during runs of ﬁsh or periods of high water. Fish in the Tawnyard traps were ﬁn-clipped and pan-jet marked. Assessment of the number of downstream migrating kelts was made on the basis of previous clips, or on ﬁsh length and condition. Both spawned kelts and unspawned overwintered ﬁnnock were recorded as sea trout kelts. Upstream migrating ﬁsh judged to be less than 32 cm were classiﬁed as ﬁnnock.

Results National rod catch statistics

Mid-western zone Annual sea trout rod catches for the period 1985–2003 for 26 mid-western ﬁsheries (Fig. 3.1, Table 3.1a,b) display an overall trend for the period of catches decreasing until 1988, followed by a collapse in 1989–90. Some improvement in catches was seen in the Delphi, Erriff and Kylemore ﬁsheries in 1991 and 1992. A notable increase in sea trout catches was recorded in the Costello ﬁshery in the 1993–94 period while in 1994 the Delphi and Erriff sea trout rod catch continued to improve. These ﬁsheries recorded a reduced catch in 1995 while the catches on the Ballynahinch ﬁshery showed an increase. Over the 1991–95 period the total sea trout rod catch for Category 1 ﬁsheries remained low in comparison with catches recorded before the stock collapses of 1989–90 (Table 3.1a). In 1995, the overall catch for Category 1 ﬁsheries decreased below that recorded for the previous 2 years. The overall catch rose in 1996 and decreased back again in 1997. Highest overall recorded sea trout catches since the 1989–90 collapse were recorded over the 1998– 2000 period, largely reﬂected by good catches in the Delphi, Erriff, Kylemore and Costello ﬁsheries. By the end of the 2003 season, catches had decreased considerably in these four ﬁsheries and very poor sea trout rod catches continued to be recorded in the Burrishoole, Newport, Screebe and Crumlin ﬁsheries. Sea trout catches in Category 2 ﬁsheries have closely (correlation coefﬁcient = 0.856, d.f. = 16) followed those in Category 1 ﬁsheries (Fig 3.2), have ﬂuctuated since the 1989–90 collapse and still remain low in comparison with pre-1988 data (Fig. 3.2, Table 3.1b).

Long-term trends in Connemara sea trout catches Accurate rod catch data were available from 16 Connemara ﬁsheries covering the period 1974–2003 (Fig. 3.3). Data show a sea trout rod catch of about 10 000 ﬁsh over the

Table 3.1a Fishery Burrishoole Newport Delphi Erriff Kylemore Ballynahinch B’hinch Up. B’hinch Mid B’hinch Lr. Inagh Athry Gowla Invermore Inverbeg Screebe Costello Crumlin Total

Rod catch ﬁgures for Category 1 rivers in the mid-western region of Ireland. 1985

1986

1987

1988

1989

1990a 1991a 1992a 1993a 1994a 1995a 1996a 1997a 1998a 1999a 2000a 2001a 2002a 2003a

497 1155 2150 770 2411

614 1485 1281 433 1099

237 783 832 450 543

245 1049 675 308 1116

41 135 309 120 198

39 N/A 112 60 10

106 N/A 437 219 450

24 30 494 293 200

159 109 660 217 320

166 112 709 318 362

200 47 181 202 181

125 90 412 263 675

136 61 446 466 296

150 58 753 520 862

47 48 653 637 862

40 52 346 321 826

48 36 519 359 676

12 50 568 282 732

16 61 124 142 549

378 202 2300 2316 218 1035 1481 254 665 2745 328

398 150 2000 1104 283 867 1345 220 337 2316 222

306 224 1500 1369 153 266 325 67 346 1698 261

173 75 850 824 89 210 199 18 396 1851 26

10 5 20 29 0 0 48 0 55 462 0

N/A 0 90 10 0 0 0 0 0 140 N/A

N/A 30 200 7 0 0 0 10 0 234 N/A

N/A 45 50 45 N/F N/F 0 50 2 375 25

N/F N/A 100 N/F N/F N/F N/F N/A 10 1041 20

15 10 208 185 N/F N/F N/F 5 40 1064 N/A

59 40 334 297 N/F N/F N/F N/A 118 634 N/A

5 0 304 650 N/F N/F N/F 0 0 1281 0

25 13 59 203 N/F N/F N/F 10 0 679 13

10 36 91 87 N/F N/F N/F 0 20 1744 0

14 15 123 195 N/F N/F N/F 0 30 1381 0

14 0 350 411 N/F N/F N/F 0 45 1604 0

105 105 300 600 N/F N/F N/F 0 56 1778 0

19 0 166 256 N/F N/F N/F 0 47 598 0

6 0 256 245 N/F N/F N/F 0 35 693 0

18 905

14 154

9360

8104

1432

461

1693

1633

2636

3194

2293

3805

2407

4331

4005

4009

4582

2730

2127

Category 1: Statistics collected from local sources by private ﬁshery owners. N/A = Not Available; N/F = Not Fished. a post catch & release bye-law; Burrishoole – (Only L.Furnace ﬁshed, 1999–2003).

Table 3.1b Fishery

Rod catch ﬁgures for Category 2 rivers in the mid-western region of Ireland. 1985

Carowniskey N/A Bunowen 475 Belclare 95 Culﬁn 298 Clifden 95 Ardbear 86 Doohulla 200 Carna 60 L’muckoo 74 Furnace 426 Total Cat. 1 & 2 combined

1986

1987

1988

1989

1990a 1991a 1992a 1993a 1994a 1995a 1996a 1997a 1998a 1999a 2000a 2001a 2002a 2003a

97 110 70 173 70 75 150 180 50 525

90 146 98 222 98 36 100 100 100 165

160 340 400 235 20 27 20 60 30 200

45 88 0 36 4 5 1 3 2 6

N/A 10 6 8 N/A N/A N/A N/A 0 0

65 120 60 175 6 15 N/A N/A 0 0

10 25 20 60 N/A N/A N/A N/A N/A 0

60 100 76 120 N/A 30 N/A 15 40 0

42 65 25 47 62 N/A N/A 45 2 3

28 55 35 20 22 25 N/A 26 0 30

21 42 35 26 25 0 0 0 0 0

28 70 40 0 17 0 50 20 0 10

65 170 90 76 130 0 0 0 0 0

70 120 35 90 35 0 0 0 3 6

90 150 110 156 73 0 0 0 0 0

18 19 12 102 70 0 0 0 0 0

60 98 96 65 83 0 10 0 0 0

0 50 63 70 71 0 40 0 0 0

1809

1500

1155

1492

190

24

441

115

441

291

241

149

235

531

359

579

221

412

294

20 714

15 654

10 515

9596

1622

485

2134

1748

3077

3485

2534

3954

2642

4862

4364

4588

4803

3142

2421

Category 2: Statistics collected from local sources by District Fishery Inspectors. N/A = Not Available; N/F = Not Fished. a post catch & release bye-law.

32

Sea Trout

Standardised annual rod catch

4

Category 1 rivers

Category 2 rivers

3

2

1

2003

2002

2001

2000

1999

1998

1997

1996

1995

1994

1993

1992

1991

1990

1989

1988

1987

1986

1985

0

Year Fig. 3.2 Annual rod catches for Category 1 and Category 2 rivers for the mid-western region, standardised to their long-term mean, 1985–2003.

Connemara sea trout rod catch 1974–2003 14 000

Number of fish

12 000 10 000 8000 6000 4000 2000 2002

2000

1998

1996

1994

1992

1990

1988

1986

1984

1982

1980

1978

1976

1974

0

Year Fig. 3.3 Annual sea trout rod catch from Connemara District between 1974 and 2003, with a 3-year moving average.

period 1974–86, a decline over the 1987–88 period followed by a rod catch collapse to 646 sea trout in 1989 and 240 sea trout in 1990. While there has been a progressive small improvement in rod catch since 1990, largely contributed to by an improvement in Costello– Fermoyle, Ballynahinch Lower and Inagh between 1998 and 2001, catches decreased again in 2002 and 2003 and have not recovered to the levels before the collapse in 1988 and 1989.

Review of Irish Stock Status

33

Northern region

North-western region

Western region

Eastern region

Shannon region Southern region

South-western region

Fig. 3.4

Location of the Irish Regional Fisheries Boards.

Other areas in Ireland Rod catch data for each of the six other ﬁsheries board regions (Fig. 3.4) are shown in Table 3.2 and summarised in Fig. 3.5.

Eastern region The reported sea trout catch has ranged from 6125 to 11 492 over the 1993–2003 period. Good catches were made in most years on the Castletown, Fane, Boyne, Slaney and Dee. A reduced sea trout rod catch was reported for the Slaney in 2000. Available information does not suggest the same sea trout stock collapse experienced in mid-western ﬁsheries, although there appears to be some decline in catches over time (r 2 = 61; P = 0.005), possibly linked to poor water quality and low summer ﬂows.

Southern region Information collected from angling clubs indicates that sea trout angling has been good on the Colligan and Bride rivers over the 1993–2003 period. Angling on the Colligan was

34

Sea Trout

Table 3.2

Estimated sea trout rod catches by ﬁsheries region, 1993–2003. 1993

1994

1995

1996

1997

1998

1999

2000

2001a

2002

2003

Northern Erne Estuary Murvagh Eske Eany Clonmany Glen Glengannon Owenea Gwebarra Clady Ray Tullaghobegley Lackagh Swilly Leannan Trawbreaga Bay Crana Total

1400 N/A N/A N/A N/A N/A N/A N/A N/A 350 N/A N/A 800 N/A N/A N/A N/A 2550

1200 134 190 469 N/A 202 N/A 656 705 100 383 N/A 440 N/A N/A N/A 70 4549

1200 250 200 430 N/A 550 N/A 183 181 490 380 N/A 78 N/A N/A N/A 180 4122

N/A N/A N/A 100 N/A 168 N/A 60 746 307 505 N/A 400 89 153 N/A 400 2928

N/A N/A N/A 100 N/A N/A 77 105 N/A N/A N/A 195 N/A 122 172 195 154 1120

1300 N/A 20 100 71 300 93 100 800 350 N/A 230 400 452 176 230 350 4972

2000 N/A 20 105 68 150 114 115 N/A N/A 52 421 336 558 80 421 344 4784

600 140 75 80 50 140 52 100 200 200 50 100 300 150 69 176 74 2556

1000 150 100 150 50 85 40 200 600 200 50 100 1000 130 100 200 255 4410

800 100 200 80 20 70 18 50 250 80 20 40 400 100 60 150 120 2558

2000 50 180 50 N/A 140 N/A 70 200 50 N/A N/A 1000 80 110 140 80 4150

Eastern Castletown Fane Boyne Ballymascanlon Dargle Vartry Slaney Dee South Wicklow Glyde Total

500 400 3250 300 700 200 3000 N/A N/A N/A 8350

400 400 3500 350 258 200 1800 N/A N/A N/A 6908

350 250 3800 200 489 250 3500 300 N/A N/A 9139

300 350 2500 250 250 150 2250 400 N/A N/A 6450

400 300 2500 250 150 125 1800 600 N/A N/A 6125

500 500 2000 150 330 150 2000 500 N/A N/A 6130

400 500 3000 200 242 150 4000 3000 N/A N/A 11 492

500 500 1900 300 180 80 1200 3000 N/A N/A 7660

1000 750 1300 400 50 70 1300 2000 N/A N/A 6870

200 200 2000 100 150 60 1500 300 300 50 4860

200 250 1200 100 150 70 1200 150 200 N/A 3520

Southern Colligan Bride Total

1800 1800 3600

1700 2000 3700

1600 1500 3100

1700 1650 3350

1550 1600 3150

2500 2000 4500

2100 1500 3600

1500 1100 2600

1600 1000 2600

1800 1200 3000

1850 1230 3080

South-Western Bandon Argideen Ilen Currane Inny Owenmore Total

N/A 573 224 345 120 500 1762

986 530 375 1655 N/A 470 4016

1450 265 388 5410 N/A 250 7763

1800 400 185 6899 100 100 9484

600 150 142 3820 20 60 4792

1015 200 215 4583 110 200 6323

2000 700 350 6073 120 250 9493

2000 700 200 4440 125 260 7725

600 220 85 3500 100 150 4655

1400 1200 400 3300 170 150 6620

1200 650 80 2500 400 200 5030

North-Western Newport Burrishoole Owenduff Owenmore

109 159 528 1062

112 166 800 850

47 200 273 1180

90 125 163 370

61 136 363 2211

58 150 634 889

48 47 277 805

52 40 362 968

36 18 357 494

50 12b 470 608

61 16 343 267c

Continued

Review of Irish Stock Status Table 3.2

35

Continued. 1993

1994

1995

1996

1997

1998

1999

2000

2001a

2002

2003

Glenamoy 300 Palmerstown 150 Moy Estuary 3036 Easky 40 Drumcﬁffe 221 Total 5605

115 150 2800 N/A 392 5385

225 30 1100 50 500 3605

42 25 796 350 88 2049

34 70 2852 200 26 5953

150 270 1980 100 650 4881

37 150 4000 110 254 5728

30 115 3450 240 300 5557

150 50 1200 90 350 2745

65 30 800 280 400 2703

60 0 3500 302 350 4899

a Reduced angling effort due to Foot & Mouth Restrictions. b Burrishoole Fishery closed in August 2002 (only L. Furnace ﬁshed since 1999). c Carrowmore lake section of Owenmore closed.

2001

2002

2003

2002

2003

2000

North-western region

4000

5000 Rod catch

6000

3000 2000 1000

4000 3000 2000

Year

2000

1999

1998

1997

1996

1995

1994

0

1993

2003

2002

2001

2000

1999

1998

1997

1996

1995

1994

1000 1993

Rod catch

Southern region

Fig. 3.5

1999

Year

5000

0

2001

Year

1998

2003

2002

2001

2000

1999

1998

1997

1996

1995

1994

0

1993

2000

1997

4000

1996

6000

1995

Rod catch

Rod catch

8000

1994

South-western region 8000 7000 6000 5000 4000 3000 2000 1000 0

1993

Eastern region 10 000

Year

Regional rod catches for ﬁsheries with complete data (see Table 3.2).

particularly good over the 1998–99 period with sea trout up to 6 lb taken regularly and some individual catches of up to 30 sea trout per day being taken. An electro-ﬁshing survey of sea trout stocks in the Colligan carried out in 1999 by the Central Fisheries Board indicated a good stock of sea trout with a full representation of age classes in the population structure.

South-western region A very low sea trout rod catch was recorded for Lough Currane in 1993 and 1994. The recorded catch rose signiﬁcantly in 1995 and has ranged from 3820 to 6899 over the 1996– 2000 period. Good sea trout rod catches have been recorded from the Bandon in recent years, and recent sea trout catches from the Argideen have been among the highest over the time period.

36

Sea Trout

Shannon region No accurate sea trout rod catch data were available from the River Feale, the most important sea trout ﬁshery in the region. However, anecdotal information indicates that good catches are taken annually. In the Feale estuary over 1000 sea trout have been taken annually in the draft-net ﬁshery in recent years. Good angling is also reported from a number of small rivers in the Clare area.

North-western region Sea trout catches have remained low in the Burrishoole and Newport systems, the two main ﬁsheries entering Clew Bay in the north-western region, since the stock collapse experienced in 1989–99. Catches in 1999 and 2000 were particularly poor and the Burrishoole ﬁshery has been closed to angling for the latter half of the previous two seasons. Sea trout catches in the Owenmore and Owenduff, situated north of Clew Bay, have ﬂuctuated over the 1993–2003 period but there is no indication that the stock is in decline and sea trout in all the age classes previously recorded are still represented in the recent catches. The Owenmore catch was low in 2003, partly because of an intense algal bloom in Carrowmore lake which resulted in angling being suspended that year. Sea trout angling has been very good in the Moy estuary in recent years and a bag limit of six sea trout per day has been imposed. Despite this daily bag limit, an estimate of 4000 sea trout was recorded for 1999 and 3500 sea trout recorded in 2003.

Northern region The availability of rod catch data is variable from year to year and it is difﬁcult to follow deﬁnite catch trends in many ﬁsheries. The Eske, Eany, Ray, Gweebarra, Swilly and Crana have all recorded poorer catches in recent years. Unlike other regions outside the midwestern zone, it is difﬁcult to generalise from the catch data available regarding the state of sea trout stocks in the northern region.

Sea trout CPUE effort data It is acknowledged that the Catch and Release Bye-Law introduced in the mid-1990s may have affected angling effort on some ﬁsheries and has made the collection of accurate data more difﬁcult. Sea trout CPUE data available for the Burrishoole, Owengowla, Invermore and Delphi ﬁsheries (Fig. 3.6) demonstrates that catch collapse between 1988 and 1990 was not related to reduced angling effort but to an actual collapse in stock, indexed by CPUE.

Sea trout commercial catch data Sea trout were taken commercially in all ﬁshery districts, both by drift and draft nets, normally as a by-catch in the salmon ﬁshery. The most accurate data on commercial sea trout catches came from the eastern region where detailed data on number and weight

Review of Irish Stock Status

37

6 Delphi Burrishoole

5

Owengowla Invermore

Rod CPUE

4 3 2 1

2003

2001

1999

1997

1995

1993

1991

1989

1987

1985

1983

1981

1979

1977

1975

1973

1971

1969

1967

1965

1963

1961

1959

1957

1955

0

Year

Fig. 3.6 Catch per unit effort (CPUE) for rod ﬁshery data for the Delphi, Invermore, Owengowla and Burrishoole ﬁsheries. Table 3.3 Total numbers of sea trout captured over the 1990–1999 period in commercial ﬁsheries in the Eastern Region. Eastern

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

Dublin Wexford Drogheda Dundalk Total

2605 975 710 80 4370

1134 864 254 179 2431

2274 1214 511 142 4141

2829 2553 1034 668 7084

2893 5670 1697 2964 1084 1246 320 413 5994 10293

4982 1533 1068 56 7639

4154 1394 982 281 6811

2552 1792 2032 426 6802

2158 3090 1646 956 7850

of sea trout taken per district are recorded annually. The total number of sea trout taken commercially in the eastern region has ranged from 2431 in 1991 to 10 293 in 1996 with an overall increasing trend for the period (r 2 = 0.45; P < 0.05) (Table 3.3). Catches recorded over the 1996–99 period were substantial and have averaged over 6000 ﬁsh annually. Since the introduction of the logbook and tagging scheme for salmon and sea trout (>40 cm) in 2001, returns for sea trout greater than 40 cm are available for all ﬁshery districts (Table 3.4). The total declared catch in 2001 was 4225 trout and this has decreased to 1945 in 2003 (Anon., 2003). The majority of large trout were taken on the east and south coasts with fewer ﬁsh being taken north of Limerick on the west coast. No sea trout were declared in Galway, Connemara, Ballinakill and relatively few were declared in Ballina, Bangor and Sligo. Table 3.4 also shows the number of draft and drift net licences granted in each ﬁshery district. There is a close association between the distribution of draft-net licences and the catches of sea trout. It is notable that the draft nets in Ballinakill have not recorded sea trout over 40 cm in the past 3 years where traditionally a large proportion of trout would have been taken in, for example, Killary Harbour. The sea trout conservation bye-law may also have affected catch returns in these mid-western districts.

38

Sea Trout

Table 3.4 Commercial sea trout catch for 2001–03 determined from logbook returns and presented by district, and number of commercial licences (drift net and draft net). District

2001 Total

Dundalk Drogheda Dublin Wexford Waterford Lismore Cork Kerry Limerick Galway Connemara Ballinakill Ballina Bangor Sligo Ballyshannon Letterkenny Total

374 180 609 574 787 365 653 220 285 0 0 0 9 34 3 61 71 4225

% by district 8.9 4.3 14.4 13.6 18.6 8.6 15.5 5.2 6.7 0 0 0 0.2 0.8 0.1 1.4 1.7 100

2002 Total 280 86 362 233 376 195 340 65 84 0 0 0 1 10 0 8 43 2083

% by district 13.4 4.1 17.4 11.2 18.1 9.4 16.3 3.1 4 0 0 0 0 0.5 0 0.4 2.1 100

Total 134 88 213 310 482 168 243 100 51 0 0 0 4 0 0 41 111 1945

2003

Number of licences

% by district

Draft net Drift net

6.89 4.52 10.95 15.94 24.78 8.64 12.49 5.14 2.62 0 0 0 0.21 0 0 2.11 5.71 100

42 51 11 75 3 6 33 50 95 4 0 17 3 31 1 84 43

0 0 16 0 172 80 106 39 86 37 29 39 83 25 10 28 127

549

877

National trap census data

Smolt output Summary trap data up to 2003 are presented for the Burrishoole, Owengowla, Invermore and Erriff traps in Table 3.5. In spite of wide variation, there was no signiﬁcant change in smolt output from the Burrishoole between 1970 and 1989. However, after the collapse in spawning stock, smolt output has decreased signiﬁcantly (P < 0.005). The highest smolt counts in the Owengowla were in 1991 (7540) and 1992 (5999) and in the Invermore in 1993 (4837) and 1996 (4643), with a signiﬁcant downward trend for Owengowla (P < 0.05) but not for Invermore. Smolt numbers in the Erriff trap declined over the 1991–93 period, with the recorded run for 1993 being the lowest over the entire time series (Table 3.5). These smolts, primarily 2-year-old ﬁsh, would have been derived from the 1991 kelt run of 78 sea trout. Sea trout smolt numbers improved steadily over the 1994–96 period and ranged from 2659 to 4149 over the 1997–2003 period. The release of sea trout fry into streams entering Tawnyard Lough since 1994 may have contributed to the sea trout smolt run recorded from 1996 onwards, along with an unknown contribution from resident trout.

Adult migrations Upstream migrations of trout were low since 1989, although a recovery was seen in the Tawnyard sub-catchment of the Erriff, as demonstrated by the downstream kelt numbers

Review of Irish Stock Status

39

Table 3.5 Sea trout smolt numbers, total upstream counts and downstream kelts (Erriff), and proportions of ﬁnnock (0+ sea age) at the four trapping stations in the west of Ireland. Data summarised from Poole et al. (this meeting) & Gargan et al. (this meeting). Year

Burrishoole

Owengowla

Invermore

Smolt Total % 0+ Smolt Total % 0+ Smolt Total output upstream sea age output upstream sea age output upstream 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

3228 2961 5465 6071 4527 3587 5207 3889 3167 5676 2337 6710 3907 4852 2383 4238 3454 3371 4290 3719 2063 2520 1936 1720 1127 1821 1289 817 1608 1260 769 530 1272 787

1244 1407 2225 2844 2929 3348 3302 2212 1830 2430 2004 1896 1624 1572 1501 1465 1387 950 863 224 155 342 151 173 210 180 197 137 106 264 111 89 115 78

50.0 50.0 50.0 50.0 50.0 33.0 44.9 32.7 21.0 26.6 31.0 44.8 48.4 56.5 36.9 46.8 50.7 48.8 42.4 24.6 73.5 62.3 45.7 68.8 49.5 48.3 58.9 55.5 70.8 76.5 59.5 49.4 69.6 75.6

Erriff–Tawnyard % 0+ Smolt Total sea age output kelts

2877

7540 5999 4090 3962 3517 4800 3045 4831 2762 3614 F&M 4027 Flood

13 1 6 637 322 117 24 16 117 57 489 61 157

84.6 0.0 0.0 98.1 58.4 14.5 62.5 75.0 99.1 57.9 100.0 68.9 89.7

— 31 4837 87 4332 53 1570 137 4643 76 2262 13 3527 23 2654 32 — 48 F&M 152 3249 19 1391 69

80.6 44.8 20.8 56.2 100.0 84.6 47.8 100.0 70.8 100.0 100.0 94.2

2448 3534 1841 416 1475 2900 3468 3020 3339 3915 2659 2270 4149 3481

412 510 489 633 no trap 60 78 313 332 354 518 478 582 690 740 640 370 647 962

Data excludes premature returning ﬁnnock before 1st June. Incomplete trapping periods for Owengowla & Invermore 1991–93. F&M: Foot & Mouth Restrictions affected access.

with numbers since 1995 similar or higher than those recorded between 1985 and 1988 (Table 3.5). It is also evident that there has been a change in the proportion of ﬁnnock (0+ sea age) returning with fewer older ﬁsh in the stock. The proportion of ﬁnnock in the Burrishoole system before 1989 averaged between 32% and about 50%; but after 1989 the proportion increased to more than 60%, although the numbers returning had decreased by

Sea Trout 1985–88

80 70 60 50 40 30 20 10 0

No. of kelts

No. of kelts

40

80 70 60 50 40 30 20 10 0

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 Length (cm)

1990–91

No. of kelts

80 70 60 50 40 30 20 10 0

1996–97

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58

Length (cm)

Length (cm)

1992–93

80 70 60 50 40 30 20 10 0

No. of kelts

No. of kelts

No. of kelts

Length (cm) 80 70 60 50 40 30 20 10 0

1994–95

80 70 60 50 40 30 20 10 0

1998–99

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58

Length (cm)

Length (cm)

Fig. 3.7 Length–frequency distribution of sea trout kelts in the Tawnyard trap on the Errif ﬁshery, 1985–99.

a factor of about 20. This is because of a marked reduction in the abundance of older age classes. Over the 1985–88 period, the Erriff Tawnyard sea trout kelt population structure comprised a ﬁnnock peak (small sea trout kelts), a peak of one and two sea winter (SW) maidens and some older previous spawners (Fig. 3.7). This would represent a normal population structure for a Connemara sea trout stock. The 1990 and 1991 kelt trapping data revealed a complete collapse in population structure with only 60 sea trout captured, the great majority being small ﬁsh. Over the 1992–95 period there was an increase in the number of small sea trout kelts recorded and some ﬁsh in the 35–45 cm length range were also present. Since 1995 there has been a progressive improvement in the numbers of small sea trout kelts, although their length distribution has not fully recovered to that recorded before 1989, with a dearth of sea trout larger than 35 cm.

Marine return In the Burrishoole system, the percentage of smolts that return as ﬁnnock in the same year historically ranged from 11.4% to 32.4% (Fig. 3.8) with a historical mean of 21%. In 1988 it decreased below the previous recorded minimum to 8.5% and in 1989 to a minimum of 1.5%. There has been a see-saw pattern of ﬁnnock return rates in the 1990s increasing to

Review of Irish Stock Status Burrishoole

Percentage marine survival

35

Gowla

41

Invermore

30 25 20 15 10 5

01

99

03 20

20

19

95

93

91

89

97 19

19

19

19

19

85

83

81

79

87 19

19

19

19

19

75

77 19

73

19

19

19

71

0

Fig. 3.8 Trends in smolt to ﬁnnock survival for three Irish ﬁsheries: Burrishoole, Gowla and Invermore.

16.7% in 1999 – the highest return rate since 1986. The mean for the 1990s, excluding 1999, was 6.8%, three times lower than the historical average. Poor sea trout ﬁnnock returns were also recorded for the Owengowla since 1991, with the exception of 1994 (when whole bay spring fallowing of marine salmon farms took place in Bertraghboy Bay, into which the Owengowla discharges) (Fig. 3.8). Returns of ﬁnnock ranged from zero in 1992 and 1993 to a maximum of 625 in 1994. The marine return rate as ﬁnnock was equal to or below 1% for 8 of the 11 years examined. There was a marked increase in return rates in 1994 to 15.8%. The marine return rate to Invermore as ﬁnnock was also low since 1992 with a highest recorded value of 4.9% in 1995 (Fig. 3.8). The higher return observed in the Owengowla in 1994 was not observed in the Invermore.

Discussion Few accurate data on the status of Irish sea trout stocks have been published since their collapse in the mid-west region in 1989–90. It is now 15 years since the collapse and all available data have been compiled here to assess current stock status nationally. Rod catch ﬁgures alone may not be a useful indicator of stock in individual years because of the many variables inﬂuencing the catch, particularly ﬁshing effort (Mills et al., 1986) and the methods of collection of the statistics. However, if rod catch ﬁgures are collected systematically and consistently over a number of years in each ﬁshery they can be useful in indicating possible trends in the population rather than absolute stock numbers (Anon., 1995). The most reliable rod catch estimates in Ireland are for those ﬁsheries in the midwest and these may be treated as indicative of the trend in that region (Anon., 1994b). Detailed marine survival and stock data from Burrishoole and the Erriff before 1989 and

42

Sea Trout

Owengowla and Invermore after 1991 support the trends observed in rod catch. The overall picture presented here showed that catch, CPUE and trap data all indicated a stock collapse in sea trout stocks in the mid-west in 1988 and 1989. Over the 1993–2001 period, rod catch data indicated a gradual increase in numbers of trout captured in some ﬁsheries, such as Delphi, Erriff, Kylemore and Costello. However, rod catches never approached the levels recorded before the 1989–90 collapse and catches decreased again in recent years. Marine survival data from three ﬁsheries in the mid-west also demonstrate that sea trout stocks remain in a much depleted state with a change in population structure. Sea trout length–frequency data for the Tawnyard sub-catchment of the Erriff ﬁshery (this chapter) and the Burrishoole system (Poole et al., 1996) demonstrate that a breakdown of population structure can occur over a very short period and that consistent improvements in marine survival are required over a number of years in order to rebuild the stock structure. Data from the ﬁsheries regions outside the mid-west zone indicate that appreciable numbers of sea trout have been captured on rod and line in the 1990s and the sea trout stock collapse documented in the mid-west was not generally apparent elsewhere. While appreciable numbers of sea trout were taken on rod and line in the eastern region, a downward trend in catch may be linked to deterioration in water quality (McGinnity et al., 2003). In the period 1990–99, there has been a signiﬁcant increase in the commercial catch in the same region. However, without basic information on the origins of the stocks available to the commercial ﬁshery it is difﬁcult to draw ﬁrm conclusions on its impacts. A period of poor angling was observed in the south-west, particularly the Currane ﬁshery in the early 1990s, but since that time there has been a marked improvement in rod catch. A signiﬁcant increase in tourist angling has emerged for sea trout ﬁshing in estuarine and sea areas in recent years, particularly in the Moy and Erne estuaries. This may have implications for the management of neighbouring sea trout stocks. Tully et al. (1999) have demonstrated that the presence of marine salmon farms signiﬁcantly increased the level of sea lice infestation on sea trout post smolts in those areas in Ireland. Similar ﬁndings have been reported from Norway (Grimnes et al., 2000) and Scotland (Mackenzie et al., 1998; Butler, 2002). Gargan et al. (2003) demonstrated a statistical relationship between lice infestation on sea trout in Ireland and the distance to the nearest salmon farm over a 10-year-period with highest infestations seen close to ﬁsh farms and concluded that sea lice from marine salmon farms were a major contributory factor in the sea trout stock collapses observed in aquaculture areas in western Ireland. This study has demonstrated a sea trout stock collapse in the mid-west region. This coincided with the introduction and location of marine salmon farms. Stocks have not recovered in these areas, and ﬁsheries remain vulnerable, if present trends in marine survival continue in the short term. As stated by the Sea Trout Task Force (Anon., 1994b): ‘if these stocks are to be saved and restored, both the elimination of lice on and in the vicinity of sea farms, … as the factor most closely associated with the marked incidence of adverse pressure on sea trout stocks …, must be a constant priority of management and regulatory practice’. This must

Review of Irish Stock Status

43

be achieved on a consistent annual basis for successful recovery of sea trout population structure. This should also be accompanied by protection and improvement of the freshwater environment and habitat. The sea trout life-cycle is complex and the indications from unpublished electro-ﬁshing surveys are that in some systems juvenile salmon may have now ﬁlled the niche previously occupied by trout. Thus, the time period for a full recovery of the population structure of these stocks may be considerable and this will only be achieved through sustained improvements in marine survival. Sea trout stocks in the remainder of the country are under increasing pressure and, here also, protection of the freshwater environment is paramount. As noted above, there is increasing interest in the marine exploitation of sea trout by anglers and the effects of these ﬁsheries need to be quantiﬁed and management action needs to be taken, if damage to individual local stocks is to be avoided. This review conﬁrms earlier conclusions that statistics on sea trout stocks and catch outside the mid-western region still require improvement if the status of those stocks is to be accurately monitored and protected on a countrywide basis.

Acknowledgements The authors are indebted to the staff of the Central and Regional Fisheries Boards for providing information for this report. The provision of sea trout CPUE data for the Delphi ﬁshery is gratefully acknowledged. The authors would also like to thank the staff of the Western Regional Fisheries Board and the Marine Institute, Newport, for the dedicated work in the various ﬁsh trapping facilities.

References Anon. (1992). Report of the Sea Trout Working Group, 1991. Department of the Marine, Dublin, 49 pp. Anon. (1993). Report of the Sea Trout Working Group, 1992. Department of the Marine, Dublin, 109 pp. Anon. (1994a). Report of the Sea Trout Working Group, 1993. Department of the Marine, Dublin, 127 pp. Anon. (1994b). Report of the Sea Trout Task Force, Department of the Marine, Dublin, 80 pp. Anon. (1995). Report of the Sea Trout Working Group, 1994. Department of the Marine, Dublin, 254 pp. Anon. (2003). Wild Salmon And Sea Trout Tagging Scheme; Fisheries Statistics Report 2001–2003. Central Fisheries Board, Dublin. Butler, J.R.A. (2002). Wild salmonids and sea louse infestations on the west coast of Scotland: sources of infection and implications for the management of marine salmon farms. Pest Management Science, 58, 595–608. Fahy, E. (1985). The Child of the Tides. The Glendale Press, Dublin, 188 pp. Gargan, P.G. (2000). The impact of the salmon louse (Lepeophtheirus salmonis) on wild salmonids in Europe and recommendations for effective management of sea lice on marine salmon farms. In: Aquaculture and the Protection of Wild Salmon (Gallaugher, P. & Orr, C., Eds). Workshop Proceedings, July 2000. Simon Fraser University, Vancouver, British Columbia, Canada, pp. 37–46. Gargan, P.G., Tully, O. & Poole, W.R. (2003). The relationship between sea lice infestation, sea lice production and sea trout survival in Ireland, 1992–2001. In: Salmon at the Edge (Mills, D., Ed.). Proceedings of the Sixth International Atlantic Salmon Symposium, July 2002, Edinburgh, UK, Chapter 10. Atlantic Salmon Trust/Atlantic Salmon Federation, pp. 119–35. Gargan, P.G., Roche, W.K., Forde, G.P. & Ferguson, A. (2006). Characteristics of sea trout (Salmo trutta L.) stocks from the Owengowla and Invermore ﬁsheries, Connemara, western Ireland, and recent trends in marine survival. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 60–75.

44

Sea Trout

Grimnes, A., Finstad, B. & Bjorn, P.A. (2000). Registrations of salmon lice on Atlantic salmon, sea trout and charr in 1999. NINA Oppdragsmelding (In Norwegian with English abstract), 634, 1–34. Mackenzie, K., Longshaw, M., Begg, G.S. & McVicar, A.H. (1998). Sea lice (Copepoda: Caligidae) on wild sea trout (Salmo trutta L.) in Scotland. ICES Journal of Marine Science, 55, 151–62. McGinnity, P., Gargan, P., Roche, W., Mills, P. & McGarrigle, M. (2003). Quantiﬁcation of the freshwater salmon habitat asset in Ireland using data interpreted in a GIS platform. Irish Freshwater Fisheries Ecology and Management Series Number 3, Central Fisheries Board, Dublin, Ireland, 132 pp. Mills, C.P.R., Piggins, D.J. & Cross, T.F. (1986). Inﬂuence of stock levels, ﬁshing effort and environmental factors on anglers’ catches of Atlantic salmon, Salmo salar L. and sea trout, Salmo trutta L. Aquaculture and Fisheries Management, 17, 289–97. Poole, W.R., Whelan, K.F., Dillane, M.G., Cooke, D.J. & Matthews, M. (1996). The performance of sea trout, Salmo trutta L., stocks from the Burrishoole system western Ireland, 1970–1994. Fisheries Management and Ecology, 3(1), 73–92. Poole, W.R., Dillane, M., deEyto, E., Rogan, G., McGinnity, P. & Whelan, K. (2006). Characteristics of the Burrishoole sea trout population: census, marine survival, enhancement and stock recruitment, 1971–2003. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 279–306. Tully, O., Poole, W.R. & Whelan, K.F. (1993). Infestation parameters for Lepeophtheirus salmonis (Krøyer) (Copepoda: Caligidae) parasitic on sea trout (Salmo trutta L.) post smolts on the west coast of Ireland during 1990 and 1991. Aquaculture and Fisheries Management, 24, 545–57. Tully, O., Gargan, P., Poole, W.R. & Whelan, K.F. (1999). Spatial and temporal variation in the infestation of sea trout (Salmo trutta L.) by the Caligid Copepod Lepeophtheirus salmonis (Krøyer) in relation to sources of infection in Ireland. Parasitology, 119, 41–51. Whelan, K.F. (1993a). Historic overview of the sea trout collapse in the west of Ireland. In: Aquaculture in Ireland – Towards Sustainability (Meldon, J., Ed.). Proceedings of a Conference held at Furbo, Co. Galway. 30th April–1st May, 1993, An Taisce, Dublin, pp. 51–3. Whelan, K.F. (1993b). Decline of sea trout in the west of Ireland: an indication of forthcoming marine problems for salmon? In: Salmon in the Sea and New Enhancement Strategies (Mills, D., Ed.). Proceedings of the Fourth International Atlantic Salmon Symposium, June 1992. St. Andrews, N.B. Canada, Chapter 9, Fishing News Books.

Chapter 4

Characteristics of the Sea Trout Salmo trutta (L.) Stock Collapse in the River Ewe (Wester Ross, Scotland), in 1988–2001 J.R.A. Butler1 and A.F. Walker2 1

CSIRO Sustainable Ecosystems, c/o Faculty of Science, Engineering & IT, James Cook University, PO Box 6811, Cairns, QLD 4870, Cairns, Australia 2 Fisheries Research Services Freshwater Laboratory, Faskally, Pitlochry, PH16 5LB, UK

Abstract: Rod catches of sea trout Salmo trutta L. from the River Ewe system collapsed in 1988 and have not recovered since. Data from the Ewe ﬁshery, including Loch Maree, collected before and after the collapse demonstrated changes in the sea trout population structure. Between 1980 and 1997–2001 maximum sea age decreased from 11 to 5 years, and marine growth rates declined. This was reﬂected in River Ewe rod catches, with changes in the weight distribution of ﬁsh between 1971–80 and 1992– 2001, and mean weights decreased from 0.54 to 0.34 kg. The mean frequency of spawning migrations also declined from 2.3 in 1980 to 1.3 in 1997–2001. These changes coincided with shifts towards earlier run timing: in 1971–80, 53% of the River Ewe catch was taken in July, but in 1992–2001 61% were caught in June. The mean date of ﬁrst entry of ﬁsh <0.5 kg (‘ﬁnnock’) also changed, from 12.2 weeks post-May 1st in 1974–86, to 5.3 weeks in 1987–2001. Mean smolt age decreased from 3.2 years in 1980 to 2.9 in 1997–2001. Combined with an increased prevalence of resident brown trout in catches, this suggested reduced juvenile recruitment. The characteristics of the stock collapse are consistent with low smolt-ﬁnnock marine survival, estimated to be 0.8–8.1% in 1999–2001 for the neighbouring River Tournaig, and 1.0–4.6% for the River Shieldaig, Loch Torridon. Links to sea lice Lepeophtheirus salmonis (Krøyer) epizootics following the establishment of marine salmon Salmo salar (L.) farms near the river mouth in 1987 are discussed. Keywords: Lepeophtheirus salmonis, marine survival, Salmo trutta, stock collapse, salmon aquaculture.

Introduction In the late 1980s unprecedented declines in sea trout Salmo trutta L. rod ﬁshery catches occurred throughout the western coast of Scotland (Walker, 1994a; Northcott & Walker, 1996), and there has been little evidence of recovery since (Butler, 2002a). Studies were undertaken to examine the cause of the suspected collapse in stocks, focusing on disease (McVicar et al., 1993) and parasites (Sharp et al., 1994; Mackenzie et al., 1998) possibly related to the rapid expansion of the Atlantic salmon Salmo salar (L.) aquaculture industry in the region (McVicar, 1997). However, beyond rod catches no detailed long-term information 45

46

Sea Trout

is available to describe the changes in sea trout population structure that occurred during the collapse in ﬁsheries. This chapter describes the biological features of the sea trout stock collapse in the River Ewe catchment, including Loch Maree, one of the largest river systems on the Scottish west coast (Butler & Watt, 2003). Preliminary data on marine age, growth rates and spawning ages have previously been presented by Walker (1994a, b). This chapter consolidates these data with more recent observations from the late 1990s, in addition to information on runtiming collected before and after the collapse. Estimates of sea trout marine survival are also presented for the River Tournaig, a small catchment neighbouring the Ewe. Taken together these data allow a detailed examination of the symptoms and possible causes of the population’s decline.

Materials and methods Study site The Ewe catchment is located in the district of Wester Ross in western Scotland (Fig. 4.1). The river drains a catchment of 441 km2 and ﬂows into Loch Ewe, an enclosed sea loch. The wetted area accessible to sea trout is 489 517 m2 of ﬂuvial habitat, and 31 005 504 m2 of lacustrine habitat, 91% of which consists of Loch Maree, an oligotrophic glacial ribbon lake. Land cover in the catchment is dominated by heather moorland (68%). Atlantic salmon, European minnows Phoxinus phoxinus (L.), European eels Anguilla anguilla (L.), Arctic charr Salvelinus alpinus (L.) and three-spined sticklebacks Gasterosteus aculeatus (L.) also occur in the accessible area (Butler, 2002b). Little Loch Broom

Loch Ewe

R. Tournaig

R. Ewe Loch Tollaidh Loch Maree Key Freshwater cage Marine cage Fish trap

Loch Torridon Loch Clair N

0

6

12 Kilometers

Fig. 4.1 The River Ewe catchment, relative to fresh water and marine salmon farm cage sites and the River Tournaig ﬁsh trap.

Stock Collapse in the River Ewe

47

Sea trout and salmon ﬁsheries within the catchment and Loch Ewe are controlled by the Ewe District Salmon Fishery Board (DSFB), a statutory body representing owners of local ﬁshing rights. Since the early 1970s exploitation has been limited to six recreational rod ﬁsheries within the river system. The sea trout ﬁshing season extends from March to October. In 1986 two commercial salmon smolt cage sites were established in the freshwater catchment, in Loch Clair and Loch Tollaidh (Fig. 4.1). The Loch Tollaidh site has a consented maximum biomass of 4.5 tonnes and remains active. The Loch Clair site was abandoned for salmon production in 1992. In 1996 the Fisheries Research Services (FRS) Freshwater Laboratory began an experimental stock restoration programme at the site, involving the on-growing of wild broodstock for the supply of ova to a hatchery. This site has a maximum consented biomass of 1.5 tonnes. In 1987 two commercial marine cage sites were established in Loch Ewe, 4 and 7 km from the river mouth. Both have been in production annually with maximum consented biomasses of 919 and 950 tonnes, respectively. Other than these sites the nearest active marine farms were 40 km by sea to the north in Little Loch Broom and 55 km to the south in Loch Torridon (Fig. 4.1). Sea trout abundance The longest time series of reliable rod catches are for the Loch Maree Hotel ﬁshery in 1969–2001. Over this period the ﬁshery was consistently based on boat angling in daylight, to a maximum of nine boats per day, but this decreased during the 1990s following the collapse in catches. The ﬁshery only recorded trout ≥0.5 kg, and therefore these catch data were used as an index of the abundance of sea trout and brown trout of this size in the River Ewe catchment. Sea trout weights As the Loch Maree Hotel rod catches only recorded ﬁsh ≥0.5 kg this ﬁshery did not accurately reﬂect the weights of all ﬁsh caught. To investigate the weight distribution of rod-caught ﬁsh before and after the collapse, data were used from the River Ewe rod ﬁshery, where all catches were recorded from 1971. Marine growth rates Scales and fork length measurements were taken from rod-caught sea trout sampled in the six ﬁsheries within the catchment in 1926 by Nall (1928), the FRS Freshwater Laboratory in 1980 (Walker, 1980) and 1989–99 and the Wester Ross Fisheries Trust in 1997–2001 (Butler, 2002b). In 1996–2001 sample sizes were augmented by ﬁsh taken during autumn broodstock collection. To estimate marine growth the mean length of ﬁsh at total sea age (years) was calculated, based on annuli on scales. Where sample sizes were limited successive years’ data were pooled.

48

Sea Trout

Marine growth rates were derived by ﬁtting von Bertalanffy curves to the sets of data expressed as: LA = L∞ (1 − e−K(A−A0 ) ), where L is length (cm), A is age (years), L∞ is asymptotic length, K is related to the rate of growth and A0 is the hypothetical age at zero length. Linear growth curves were calculated to compare slopes and intercepts, using the equation: L = a + bA, where L is length (cm) and A is age (years), a is the intercept and b is slope. Spawning migrations and smolt age Scale samples were also analysed for the presence of spawning marks, which indicated the number of previous spawning migrations completed by mature ﬁsh. Smolt age was assessed by the total number of annuli during the freshwater phase of each ﬁsh. Run timing Changes in the timing of freshwater entry of sea trout were derived from catches in the River Ewe rod ﬁshery. Because this ﬁshery is located immediately upstream from the estuary, the catches provided the most sensitive indication of sea trout migration into the Ewe catchment. The week of the ﬁrst catch of ﬁsh <0.5 kg was assumed to represent the start of the run of 0 sea year trout (‘ﬁnnock’), measured from 1st May annually. However, because of the decline in growth rates in the 1990s (see Section Results) ﬁsh of this size may also have included individuals of 1 and 2 sea years of age, and hence the term ‘ﬁnnock’ is used loosely. Marine survival In 1999 a ﬁsh trap was established in the 9 km2 River Tournaig catchment, which adjoins the Ewe catchment and also drains into Loch Ewe (Fig. 4.1). The trap is located in a ﬁsh ladder at the seaward extremity of the catchment, and was designed to monitor marine survival of sea trout by capturing the majority of emigrating smolts and returning ﬁnnock and adults during March–December annually. Rod catches and weights of trapped ﬁsh suggest that a collapse of sea trout stocks also occurred at Tournaig during the late 1980s (Butler & Starr, 2001; Cunningham et al., 2002).

Results Sea trout abundance In 1971–99 the 5-year average sea trout catch from the Loch Maree Hotel ﬁshery ranged between 81 and 1400, with the minimum recorded in 1999. Until 1987 annual catches

Stock Collapse in the River Ewe

49

1800 Sea trout Brown trout 5-year sea trout av.

1600 1400

Marine salmon farms start

Trout catch

1200 1000 800 600 400 200

1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

0

Year

Fig. 4.2 Annual rod catches of sea trout and brown trout from the Loch Maree Hotel, 1969–2001, relative to the establishment of marine salmon farms in Loch Ewe.

ranged between 546 and 1575, but from 1988 ranged between 35 and 342, with the minimum recorded in 2001. Before the collapse of the sea trout ﬁshery, brown trout were generally small and rarely reported caught. However, after 1988, they began to appear regularly in the catches and in 1997–2001 they outnumbered sea trout (Fig. 4.2).

Sea trout weights The weight distribution of sea trout caught in the River Ewe in 1971–80 differed signiﬁcantly from that in 1992–2001 (χ 2 = 753.1, d.f. = 10, P < 0.001), with an increase in the proportion of smaller ﬁsh (Fig. 4.3). This was reﬂected in the mean weight, which declined from 0.54 kg (n = 3305, ±0.01 SE, range 0.25–5.06) in 1971–80, before the collapse, to 0.34 kg (n = 7094, ±0.01 SE, range 0.25–2.0) after the collapse in 1992–2001.

Marine growth rates Von Bertalanffy growth curves showed changes in marine growth rates and maximum sea ages (Table 4.1, Fig. 4.4). In 1926 and 1980 the maximum sea age recorded was 11 years, and mean ﬁtted length at age was similar for ﬁsh of 0–3 years, but greater for ﬁsh ≥4 years in 1926 than in 1980. Between 1980 and 1989–90 mean length at all ages declined, and maximum sea age decreased to 8 years. Between 1989–90 and 1992–93 mean length at all ages declined further, and the maximum sea age decreased to 6 years. Between 1992–93 and 1997–2001 mean length at 0, 4 and 5 years declined further, and the maximum sea age decreased to 5 years. To compare the data over a consistent set of sea ages linear functions were applied to curves for ﬁsh ≤5 sea years. Only mean lengths at sea age were available for 1926, therefore this growth curve was excluded. There was a signiﬁcant difference between the intercepts

50

Sea Trout (a) 100

1971–80

90 80

Percentage

70 60 50 30 20 10 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

(b) 100

1992–2001

90 80

Percentage

70 60 50 30 20 10 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Weight (kg) Fig. 4.3 Weight distribution of sea trout caught in the River Ewe rod ﬁshery in (a) 1971–80 and (b) 1992–2001.

(P < 0.001) and slopes (P < 0.05) of the 1980, 1989–90, 1992–93 and 1997–2001 curves (Table 4.2). Spawning migrations and smolt age In all years the largest proportion of mature sea trout had spawned once, with progressively fewer spawning more frequently. In 1926 and 1980 some ﬁsh had spawned up to 9 and 11 times, respectively. Following the ﬁshery collapse in 1988 the maximum spawning frequency declined from 7 in 1989–90 to 4 in 1992–93, and 2 in 1997–2001. This was

Stock Collapse in the River Ewe Table 4.1

51

Parameter estimates for von Bertalanffy growth curves of Ewe sea trout.

Year

n

1926 1980 1989–90 1992–93 1997–2001

L∞

2813 1122 369 317 382

K

A0

Estimate

±SE

Estimate

±SE

Estimate

±SE

Variance accounted for (%)

81.9 77.6 76.0 81.0 45.6

±3.2 ±2.5 ±6.3 ±9.6 ±2.0

0.16 0.16 0.14 0.11 0.42

±0.01 ±0.01 ±0.03 ±0.04 ±0.06

−2.3 −2.6 −3.0 −3.5 −1.7

±0.09 ±0.15 ±0.26 ±0.45 ±0.15

99.8 83.4 83.2 76.2 77.2

80 1926 1980

Length (cm)

70 1989–90

60 1992–93

50

1997–2001

40 30 20 0

1

2

3

4

5

6

7

8

9

10 11 12 13

Sea age (years) Fig. 4.4 Mean lengths at sea age from von Bertalanffy growth curves for Ewe sea trout sampled in 1926, 1980, 1989–90, 1992–93 and 1997–2001.

mirrored in statistically signiﬁcant declines in the mean frequency of spawning migrations, from 2.1 in 1989–90 to 1.6 in 1992–93 and 1.3 in 1997–2001. Prior to the collapse there was no signiﬁcant difference between the mean spawning frequency of 2.2 in 1926 and 2.3 in 1980 (Table 4.3). The majority of ﬁsh sampled in all years smolted after 3 years in fresh water. The mean varied between 3.2 and 3.3 years in 1980, 1989–90 and 1992–93, but these differences were not statistically signiﬁcant. However, the decline to 2.9 years in 1997–2001 was a signiﬁcant change. Prior to the ﬁshery collapse there was no signiﬁcant difference between the mean of 3.1 years in 1926 and 3.2 years in 1980 (Table 4.4).

Run timing In 1971–80 the primary sea trout run occurred in July, with 53% of ﬁsh caught this month (Fig. 4.5). In 1992–2001, after the ﬁshery collapse, the majority were caught in June (61%),

52

Sea Trout Table 4.2 Parameter estimates for intercept (a) and slope (b) for the linear relationships between sea age and length for Ewe sea trout aged 0–5 sea years. Year

n

1980 1989–90 1992–93 1997–2001

Table 4.3 Year

1926 1980 1989–90 1992–93 1997–01

a

1046 358 316 382

b

Estimate

±SE

Estimate

±SE

Variance accounted for (%)

28.8 27.2 25.1 24.1

±0.23 ±0.29 ±0.26 ±0.25

5.6 5.3 4.8 5.5

±0.87 ±0.14 ±0.17 ±0.17

79.5 79.6 71.8 72.9

Frequency distributions of spawning marks for mature Ewe sea trout. n

Mean ± SE

Spawning marks (%)

696 588 79 45 48

1

2

3

40.9 39.6 41.8 51.1 68.8

27.0 23.5 25.3 40.0 31.2

14.7 18.7 21.5 6.7

4

5

6

7

8

9

9.5 8.8 6.3 2.2

3.9 5.1 3.8

3.0 2.4

0.6 1.0 1.3

0.3 0.5

0.1 0.2

10

11

0.2

2.2 ± 0.05 2.3 ± 0.06ns 2.1 ± 0.14ns 1.6 ± 0.11∗∗ 1.3 ± 0.07∗

Statistical comparisons of means are made with each preceding year. t -test: ns = not signiﬁcant, ∗ P < 0.05, ∗∗ P = 0.01.

Table 4.4 Year

1926 1980 1989–90 1992–93 1997–2001

Frequency distributions of smolt ages for Ewe sea trout. n

1512 1163 187 315 376

Mean ± SE

Smolt age (years) 2

3

4

5

6

11.5 12.8 11.2 10.5 19.4

68.4 60.4 58.3 59.1 66.0

18.2 22.0 25.1 27.0 14.1

1.8 4.6 5.4 2.8 0.5

0.1 0.2 0.6

3.1 ± 0.02 3.2 ± 0.02ns 3.3 ± 0.05ns 3.2 ± 0.04ns 2.9 ± 0.03∗∗∗

Statistical comparisons of means are made with each preceding year. t -test: ns = not signiﬁcant; ∗∗∗ P < 0.001.

and the change in the monthly distribution of the rod catch was statistically signiﬁcant (χ 2 = 3884.5, d.f. = 4, P < 0.001). The mean number of weeks post-May 1st when the ﬁrst ﬁnnock was caught in 1974–86 was 12.2 weeks (n = 13, ±0.4 S.E., range 10.4–14.5) (Fig. 4.6). In 1987–2001 the mean decreased to 5.3 weeks (n = 15, ±0.7 SE, range 1.9–8.8), a signiﬁcant difference (t = 9.14, d.f. = 21, P < 0.001).

Stock Collapse in the River Ewe

53

80 1971–80 1992–2001

70

Percentage

60 50 40 30 20 10 0 May

June

July Month

Aug.

Sept.

Fig. 4.5 Monthly distribution of sea trout caught in the River Ewe rod ﬁshery in 1971–80 (n = 3305) and 1992–2001 (n = 7094).

Pre–1987

Post–1987

Date

Aug. 1st

July 1st

June 1st

1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

May 1st

Year

Fig. 4.6 Date of capture of the ﬁrst ﬁnnock in the River Ewe rod ﬁshery before and after the 1987 establishment of marine salmon farms in Loch Ewe.

Marine survival In 1999, 2000 and 2001 the smolt-ﬁnnock marine survival rates at the River Tournaig were 0.8%, 5.9% and 8.1%, respectively (Butler & Starr, 2001).

54

Sea Trout

Discussion The collapse in the sea trout rod ﬁshery beginning in 1988 was characterised by three coincidental alterations in the sea trout stock. First, the abundance of ﬁsh declined sharply to levels not previously recorded. Second, there was an increased proportion of smaller ﬁsh, and their mean weight decreased. Scale analysis showed that this was because of signiﬁcant declines in marine growth rates between 1980 and 1997–2001, in addition to the reduced longevity of ﬁsh from a maximum of 11 to 5 sea years for the same period. As a consequence, the frequency of spawning migrations by mature ﬁsh also declined. Third, the timing of the primary sea trout freshwater migration advanced by approximately one month from July to June, and ﬁnnock returned earlier in May and June. Sea trout smolt runs at Tournaig peak in late April and early May (Butler & Starr, 2001; Cunningham, 2004), suggesting that the 0 sea year ﬁsh among the ﬁnnock were post-smolts which had spent less than 1 month at sea. Sea trout rod catches on the Scottish west coast have varied substantially during the twentieth century implying changes in abundance, although rod catches are inﬂuenced by weather conditions and changes in effort (Picken, 1990; Walker, 1994a). One factor driving regional abundance may be the availability of marine feeding. Finnock and older trout feed on a variety of marine ﬁsh in coastal waters including sandeels Ammodytes spp. and herring Clupea harengus (Pemberton, 1976a, b), and stocks of these species ﬂuctuate widely between years (Morrison, 1979; Tingley et al., 1992; Wright & Reeves, 1994; Whitmarsh et al., 1995), potentially inﬂuencing marine growth and survival. Climatic extremes such as droughts and ﬂoods may also affect smolt output from the freshwater environment, and hence the abundance of returning adults (Whelan, 1993; Butler, 1999, 2000a). While these factors may have contributed to the variations in Ewe catches prior to the 1988 collapse, the relative consistency of longevity, marine growth rates, spawning frequencies and smolt ages between 1926 and 1980 suggest that there was little fundamental change in sea trout stock structure during this period. Instead, the changes that occurred from 1988 are symptomatic of an unprecedented reduction in marine growth and survival. The coincidence of sea trout stock collapses soon after the establishment of marine salmon farms in their vicinity, such as that which occurred in the Loch Maree ﬁshery raised speculation of a causal link in western Ireland, western Scotland and Norway (Tully et al., 1993a, b; Walker, 1994a; Northcott & Walker, 1996; Anon., 1997). Initial research in Scotland indicated no cause–effect relationship between a number of pathogens and sea trout decline, though high infestations of sea lice Lepeophtheirus salmonis (Krøyer) and related physical damage were found in areas of marine aquaculture (McVicar et al., 1993). Subsequent studies have concluded that lice epizootics on wild and farmed salmonids originate from the large numbers of farmed salmon in aquaculture zones of Scotland (Butler, 2002a), Ireland (Tully & Whelan, 1993; Tully et al., 1999; Gargan et al., 2003) and Norway (Heuch & Mo, 2001). Elevated infestations of lice cause physical damage (Dawson et al., 1998) and osmoregulatory breakdown (Bjorn & Finstad, 1997) and infected sea trout of all ages can return prematurely to fresh water where they cease feeding, lose growth and condition and up to 20% die (Tully et al., 1993a, b;

Stock Collapse in the River Ewe

55

Birkeland, 1996; Birkeland & Jakobsen, 1997). Others do not return to fresh water and instead die at sea (Tully & Poole, 1999). As a consequence lice epizootics can cause the mortality of 30–50% of all emigrating sea trout smolts (Bjorn et al., 2001). The highest levels of sea lice infestation on sea trout in western Scotland occur within 25 km of active marine salmon farms (Butler & Watt, 2003) and also in Ireland (Tully et al., 1999; Gargan et al., 2003) and Norway (Bjorn et al., 2001). In 1987 marine salmon farms were established in Loch Ewe, 4 and 7 km from the river mouth. High levels of sea lice infestation have since been found on sea trout sampled in Loch Ewe (Sharp et al., 1994; Mackenzie et al., 1998; Butler et al., 2001; Cunningham et al. 2002; Cunningham, 2004) and prematurely returning, lice-infested ﬁsh have been observed in the River Ewe in May and June (Butler, 1998, 2000a, 2002b; Cunningham, 2004). Spring lice epizootics have occurred every second year, coinciding with the development of ovigerous lice in the local farms (Butler, 2002a). This pattern has been conﬁrmed in Loch Torridon, where ovigerous lice abundance on local farms correlates with spring peaks of infectious larval lice near river mouths (McKibben & Hay, 2004a). In turn, premature-returning lice-infested sea trout were found in the River Shieldaig, and these were most abundant in years when larval lice prevalence was highest (McKibben & Hay, 2004b). In 1999–2001 the range of smolt-ﬁnnock survival at the River Tournaig was 0.8–8.1%, and this population also collapsed in the late 1980s (Butler & Starr, 2001). Survival rates recorded for 1999–2001 at Shieldaig in Loch Torridon were also within this range (1.0– 4.6%), and the wild sea trout stock has also collapsed (McKibben & Hay, 2004b). In Ireland such low levels of survival are correlated with high lice infestations on sea trout, while survival rates of greater than 10% are only found in areas with low infestation levels (Gargan et al., 2003). No marine survival data exist for Scottish west coast rivers before the sea trout collapse in the late 1980s. However, long-term monitoring at the Burrishoole system in western Ireland shows that in 1971–88 smolt-ﬁnnock survival was 11.4–32.4%, but decreased to 1.5–10.0% from 1989 (Poole et al., 1996). Taken together this evidence suggests that the changes in Ewe sea trout stock structure are related to declines in marine growth and survival, which have been at least partly caused by lice epizootics emanating from salmon farms in Loch Ewe. This is corroborated by the fact that on the east coast of Scotland, outside the salmon farming zone, sea trout stock structure in monitored streams has remained stable over the same period (Walker, 2006). The extent of the stock collapse can be quantiﬁed in terms of sea trout egg deposition. In the Ewe system 57% of sea trout are females (Walker, 1994a; Butler, 2002b), and because of their larger size and numbers relative to resident female brown trout they drive juvenile recruitment (Walker, 1994b). The combination of reduced abundance, size, longevity and hence frequency of spawnings has probably had a major inﬂuence on total egg deposition by sea trout. Walker (1994b) estimated that the potential lifetime egg deposition of a typical female decreased from 30 000 in 1980 to 6000 in 1993. Given that the abundance, maximum sea age and mean spawning frequency of Ewe sea trout decreased again between 1992–93 and 1997–2001, total egg deposition has probably further reduced. No measures of smolt output have been made for the Ewe system. However, monitoring at Burrishoole showed that after marine survival declined to less than 10% in 1989, the

56

Sea Trout

abundance of female sea trout was insufﬁcient to maintain juvenile recruitment, and by 1992 smolt output declined to unprecedented levels, further limiting numbers of returning adults (Poole et al., 1996). Similarly, trapped wild smolt runs at Tournaig and Shieldaig have declined to negligible numbers following marine survival rates of less than 10% (Butler & Starr, 2001; Cunningham et al., 2002; Cunningham, 2004; McKibben & Hay, 2004b). Hence it is highly likely that smolt output in the Ewe system is being restricted by sea trout egg deposition. One indication of this may be the decreasing mean smolt age from 3.2 years in 1980 to 2.9 years in 1997–2001, perhaps caused by reduced competition amongst juveniles resulting in increased freshwater growth rates (e.g. Nordwall et al., 2001). A further symptom of stock collapse may be the increased abundance of brown trout, as suggested by Loch Maree Hotel rod catches. Scale samples taken from putative brown trout conﬁrmed total freshwater residency of these ﬁsh, and trapping of a Loch Maree spawning stream in 1997 showed that 88% of spawners were brown trout, while 12% were sea trout, mirroring their proportions in rod catches (Butler, 2002b). Similar increases in brown trout catches and growth rates following collapses in traditional sea trout ﬁsheries have been observed elsewhere in western Scotland during the 1990s, including the Rivers Balgy (Butler, 2000b), Gruinard (Butler, 2001) and Loch Eilt (Walker, 1994a). Sea trout progeny may adopt variable life-history tactics according to environmental conditions (Walker, 1994a, 2006), and increased freshwater growth rates reduce the tendency for juvenile sea trout to smolt (Morgan & Paveley, 1993). Therefore, it is possible that the lack of competition and related improvements in freshwater growth rates are leading to a greater prevalence of freshwater-resident trout. Another possible cause of increased brown trout residency is the nutrient enriching effect of waste from the freshwater cages in Loch Tollaidh and Loch Clair, as observed elsewhere (e.g. Gabrielsen, 1999). However, this seems unlikely since growth rates of brown trout from Loch Tollaidh are similar to those from Loch Maree (Butler, 2002b). Furthermore, Loch Tollaidh discharges into the River Ewe, and therefore any enriching effect is not likely to affect Loch Maree or the major tributaries upstream. The commercial site in Loch Clair was only operational during 1986–92, and was re-activated in 1996 for a small biomass of sea trout broodstock. Therefore this site is also unlikely to have had a major impact on the catchment’s ecosystem and overall trout growth rates. Although a switch to freshwater residency by sea trout progeny may partially reverse the stock collapse, it is unlikely to result in the rapid recovery of the anadromous morph because only 13–19% of Ewe system brown trout are females (Walker, 1994b; Butler, 2002b). The characteristics of the collapse in the sea trout stock are indicative of poor marine growth and survival, suggesting that if abnormal marine conditions were reversed, sea trout juvenile recruitment, smolt output and stock recovery would quickly follow. The presence of sea lice epizootics in Loch Ewe related to farm production cycles, and the resulting premature return of post-smolts, ﬁnnock and older sea trout suggests that this is the primary anthropogenic factor concerned. While other environmental issues may also be inﬂuential, a resolution of the sea lice problem is likely to contribute signiﬁcantly to the restoration of the Ewe sea trout stock.

Stock Collapse in the River Ewe

57

Acknowledgements This work would not have been possible without the cooperation and support of the Ewe DSFB and the Wester Ross Fisheries Trust. Scale samples were generously collected by Coulin, Kinlochewe, Gairloch, Grudie, Letterewe and Inveran Estates. Mark Vincent of the Loch Maree Hotel also provided valuable information. Lady June Horlick of Tournaig Estate and the National Trust for Scotland’s Inverewe Gardens allowed the establishment of the Tournaig trap, which was operated by Jackie Mackenzie, Ben Rushbrooke and Jonathon Davis. The trap was part-funded by the Highland Council and Ross and Cromarty Enterprise. Aileen Shanks (Fisheries Research Services) provided statistical analyses of the growth curves.

References Anon. (1997). Report of the Workshop on the Interactions between Salmon Lice and Salmonids, Edinburgh, Scotland, UK, 11–15 November 1996. ICES CM 1997/M: 4, Ref.: F. Birkeland, K. (1996). Consequences of premature return by sea trout (Salmo trutta) infested with the salmon louse (Lepeophtheirus salmonis Krøyer): migration, growth and mortality. Canadian Journal of Fisheries and Aquatic Sciences, 53, 2808–13. Birkeland, K. & Jakobsen, P.J. (1997). Salmon lice, Lepeophtheirus salmonis, infestations as a causal agent of premature return to rivers and estuaries by sea trout, Salmo trutta, juveniles. Environmental Biology of Fishes, 49, 129–37. Bjorn, P.A. & Finstad, B. (1997). The physiological effects of salmon lice infection on sea trout post-smolts. Nordic Journal of Freshwater Research, 73, 60–72. Bjorn, P.A., Finstad, B. & Kristoffersen, K. (2001). Salmon lice infection of wild sea trout and Arctic charr in marine and freshwaters: the effect of salmon farms. Aquaculture Research, 32, 1–17. Butler, J.R.A. (1998). Wester Ross Fisheries Trust Annual Review, 1997. Wester Ross Fisheries Trust, Gairloch, Ross-shire. Bulter, J.R.A. (1999). Wester Ross Fisheries Trust Annual Review 1998–1999. Wester Ross Fisheries Trust, Gairloch, Ross-shire, UK. Butler, J.R.A. (2000a). Wester Ross Fisheries Trust Annual Review, 1999–2000. Wester Ross Fisheries Trust, Gairloch, Ross-shire. Butler, J.R.A. (2000b). River Balgy Fishery Management Plan, 2000–2005. Wester Ross Fisheries Trust, Gairloch, Ross-shire. Butler, J.R.A. (2001). River Gruinard Fishery Management Plan, 2001–2006. Wester Ross Fisheries Trust, Gairloch, Ross-shire. Butler, J.R.A. (2002a). Wild salmonids and sea lice infestations on the west coast of Scotland: sources of infection and implications for the management of marine salmon farms. Pest Management Science, 58, 595–608. Butler, J.R.A. (2002b). River Ewe Fishery Management Plan, 2002–2006. Wester Ross Fisheries Trust, Gairloch, Ross-shire. Butler, J.R.A. & Starr, K. (2001). Tournaig Trap Report, 1999–2001. Wester Ross Fisheries Trust, Gairloch, Ross-shire. Butler, J.R.A. & Watt, J. (2003). Assessing and managing the impacts of marine salmon farms on wild Atlantic salmon in western Scotland: identifying priority rivers for conservation. In: Salmon at the Edge (Mills, D.H., Ed.). Blackwell Science Ltd, Oxford, pp. 93–118. Butler, J.R.A., Marshall, S., Watt, J. et al. (2001). Patterns of Sea Lice Infestations on Scottish West Coast Sea Trout: Survey Results, 1997–2000. Association of West Coast Fisheries Trusts, Gairloch, Ross-shire. Cunningham, P.D. (2004). Wester Ross Fisheries Trust Review April 2004. Wester Ross Fisheries Trust, Ross-shire. Cunningham, P.D., Starr, K. & Butler, J.R.A. (2002). Wester Ross Fisheries Trust Review, June 2002. Wester Ross Fisheries Trust, Gairloch, Ross-shire.

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Dawson, L.H.J., Pike, A.W., Houlihan, D.F. & McVicar, A.H. (1998). Effects of salmon lice Lepeophtheirus salmonis on sea trout Salmo trutta at different times after seawater transfer. Diseases of Aquatic Organisms, 33, 179–86. Gabrielsen, S.-E. (1999). Effects of ﬁsh farm activity on the limnetic community structure of brown trout, Salmo trutta and Arctic charr, Salvelinus alpinus. Environmental Biology of Fishes, 55, 321–32. Gargan, P.G., Tully, O. & Poole, W.R. (2003). Relationship between sea lice infestation, sea lice production and sea trout survival in Ireland, 1992–2001. In: Salmon at the Edge (Mills, D.H., Ed.). Blackwell Science Ltd., Oxford, pp. 119–35. Heuch, P.A. & Mo, T.A. (2001). A model of salmon louse production in Norway: effects of increasing salmon production and public management measures. Diseases of Aquatic Organisms, 45, 145–52. Mackenzie, K., Longshaw, M., Begg, G.S. & McVicar, A.H. (1998). Sea lice (Copepoda: Caligidae) on wild sea trout (Salmo trutta L.) in Scotland. ICES Journal of Marine Science, 55, 151–62. McKibben, M.A. & Hay, D.W. (2004a). Distributions of planktonic sea lice larvae Lepeophtheirus salmonis in the inter-tidal zone in Loch Torridon, western Scotland in relation to salmon farm production cycles. Aquaculture Research, 35, 742–750. McKibben, M.A. & Hay, D.W. (2004b). Shieldaig Project Review: June 2002–June 2003. Fisheries Research Services Freshwater Fisheries Laboratory, Pitlochry, Perthshire. McVicar, A.H. (1997). Interaction of pathogens in aquaculture with wild ﬁsh populations. Bulletin of the European Association of Fish Pathology, 17, 197–200. McVicar, A.H., Sharp, L.A., Walker, A.F. & Pike, A.W. (1993). Diseases of wild sea trout in Scotland in relation to ﬁsh population decline. Fisheries Research, 17, 175–85. Morgan, R.I. & Paveley, D.S. (1993). Sea trout rearing – the food connection. Fish Farmer, 28, 58–9. Morrison, J.A. (1979). 0-Group herring surveys on the west coast of Scotland 1975–1978. International Council for the Exploration of the Sea, CM 1979/H:64. Nall, G.H. (1928). The Sea Trout of the River Ewe and Loch Maree, Part 2, 1926–1927. Fishery Board for Scotland, No. 11: HM Stationery Ofﬁce, Edinburgh. Nordwall, F., Naslund, I. & Degerman, E. (2001). Intercohort competition effects on survival, movement and growth of brown trout in Swedish streams. Canadian Journal of Fisheries and Aquatic Sciences, 58, 2298–308. Northcott, S.J. & Walker, A.F. (1996). Farming salmon, saving sea trout; a cool look at a hot issue. In: Aquaculture and Sea Lochs, Proceedings of a Joint Meeting of the Scottish Association for Marine Science and the Challenger Society at Dunstaffnage Marine Laboratory, June 1996, pp. 72–81. Pemberton, R. (1976a). Sea trout in North Argyll sea lochs, population, distribution and movements. Journal of Fish Biology, 9, 157–79. Pemberton, R. (1976b). Sea trout in North Argyll sea lochs: II. Diet. Journal of Fish Biology, 9, 195–208. Picken, M.J. (1990). The history of west coast sea trout catches. In: The Sea Trout in Scotland (Picken, M.J. & Shearer, W.M., Eds). Proceedings of a Symposium held at Dunstaffnage Marine Research Laboratory, 18–19 June 1987. Scottish Marine Biological Association, Oban, pp. 53–9. Poole, W.R., Whelan, K.F., Dillane, M.G., Cooke, D.J. & Matthews, M. (1996). The performance of sea trout, Salmo trutta L., stocks from the Burrishoole system western Ireland, 1970–1994. Fisheries Management and Ecology, 3, 73–92. Sharp, L., Pike, A.W. & McVicar, A.H. (1994). Parameters of infection and morphometric analysis of sea lice from sea trout (Salmo trutta L.) in Scottish waters. In: Parasitic Diseases of Fish (Pike, A.W. & Lewis, J.W., Eds). Samara Publishing Limited, Dyfed, Tresaith, pp. 151–70. Tingley, G.A., Trenkel, V.M. & Beddington, J.R. (1992). Northwest Scottish Salmonid Catches and the West Coast Sandeel Fishery. MRAG Ltd, Edinburgh. Tully, O. & Poole, W.R. (1999). Parameters and Impacts of Sea Lice Infestation of Sea Trout along a Salinity Gradient in Clew Bay, Ireland. Report to the Atlantic Salmon Trust, Pitlochry, and the Salmon Research Agency, Newport, Ireland. Tully, O. & Whelan, K.F. (1993). Production of nauplii of Lepeophtheirus salmonis (Krøyer) (Copepoda: Caligidae) from farmed and wild salmon and its relation to the infestation of wild sea trout (Salmo trutta L.) off the west coast of Ireland in 1991. Fisheries Research, 17, 187–200. Tully, O., Poole, W.R. & Whelan, K.F. (1993a). Infestation parameters for Lepeophtheirus salmonis (Krøyer) (Copepoda: Caligidae) parasitic on sea trout, Salmo trutta L., off the west coast of Ireland during 1990 and 1991. Aquaculture and Fisheries Management, 24, 545–55.

Stock Collapse in the River Ewe

59

Tully, O., Poole, W.R., Whelan, K.F. & Merigoux, S. (1993b). Parameters and possible causes of epizootics of Lepeophtheirus salmonis (Krøyer) infesting sea trout (Salmo trutta L.) off the west coast of Ireland. In: Pathogens of Wild and Farmed Fish: Sea Lice (Boxshall, G.A. & Defaye, D., Eds). Ellis Horwood, New York, pp. 202–13. Tully, O., Gargan, P., Poole, W.R. & Whelan, K.F. (1999). Spatial and temporal variations in the infestation of sea trout (Salmo trutta L.) by the caligid copepod Lepeophtheirus salmonis (Krøyer) in relation to sources of infection in Ireland. Parasitology, 119, 41–51. Walker, A.F. (1980). A report on the growth, size and age composition of sea trout caught by anglers ﬁshing Loch Maree, Clair and Coulin in 1980. Freshwater Fisheries Laboratory, Pitlochry, Scotland. Walker, A.F. (1994a). Sea trout and salmon stocks in the western Highlands. In: Problems with Sea Trout and Salmon in the Western Highlands. Atlantic Salmon Trust, Pitlochry, Perthshire, pp. 6–18. Walker, A.F. (1994b). Fecundity in relation to variation in life history of Salmo trutta L. in Scotland. PhD Thesis, University of Aberdeen. Walker, A.F. (2006). The rapid establishment of a resident brown trout population from sea trout progeny stocked in a ﬁshless Stream. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 389–400. Whelen, K.F. (1993). Decline of sea trout in the west of Ireland: an indication of forthcoming marine problems for salmon? In: Salmon in the Sea and New Enhancement Strategies (Mills, D., Ed.). Finishing News Books, Oxford, pp. 171–83. Whitmarsh, D.J., Reid, C.A., Gulvin, C. & Dunn, M.R. (1995). Natural resource exploitation and the role of new technology: a case history of the UK herring industry. Environmental Conservation, 22, 103–10. Wright, P.J. & Reeves, S.A. (1994). Sandeel availability to salmonids in Scottish waters. In: Problems with Sea Trout and Salmon in the Western Highlands. Atlantic Salmon Trust, Pitlochry, Perthshire, pp. 42–7.

Chapter 5

Characteristics of the Sea Trout (Salmo trutta L.) Stocks from the Owengowla and Invermore Fisheries, Connemara, Western Ireland, and Recent Trends in Marine Survival P.G. Gargan1 , W.K. Roche1 , G.P. Forde2 and A. Ferguson3 1 Central

Fisheries Board, Dublin, Ireland Regional Fisheries Board, Galway, Ireland 3 School of Biology and Biochemistry, Queen’s University, Belfast, Northern Ireland 2 Western

Abstract: The Rivers Owengowla and Invermore, situated in Connemara, western Ireland, have historically supported important sea trout angling ﬁsheries. Similar to other mid-western sea trout ﬁsheries, both suffered a sea trout stock collapse in 1989 and have failed to recover substantially since then. Upstream and downstream traps were installed on both rivers and data on sea trout smolt and kelt runs, age and length frequency of migrants, and marine survival over the 1991–2003 period are presented. The trapping data from both systems indicate that substantial runs of smolts were derived from an extremely small spawning escapement of sea trout, implying that the freshwater trout stocks contributed signiﬁcantly to sea trout smolt runs. Thus the potential for smolting had remained in the freshwater stock, in keeping with the hypothesis that anadromy is a threshold quantitative trait. Exceptionally low ﬁnnock (0 sea winter [SW]) marine survival rates were recorded annually for the Owengowla ﬁshery for most of a 10-year period. Exceptions of good survival were associated with whole-bay fallowing by adjacent marine salmon farms. These data suggest that the practice of wholebay spring fallowing has a positive effect on sea trout marine survival in ﬁsheries within such bays. The data also strongly indicate that infestations by sea lice from marine Atlantic salmon farms made an important contribution to the sea trout stock collapse on Ireland’s west coast. Keywords: Sea trout, west of Ireland, marine survival, anadromy, sea lice.

Introduction The Owengowla and Invermore sea trout ﬁsheries are located within the Western Regional Fisheries Board (WRFB) area in Connemara, Co. Galway on the west coast of Ireland (Fig. 5.1). Sea trout angling primarily took place from boats on lakes on both ﬁsheries while angling on rivers downstream of the lowest lake on both systems also occurred. The sea trout stock on both ﬁsheries comprised a high proportion of ﬁnnock (83.3% 0 sea age for 60

Characteristics of Sea Trout Stocks

Invermore System

Owengowla System

Upstream trap

Bertraghboy Bay

Fig. 5.1

61

Upstream / downstream trap

Upstream / downstream trap

Kilkieran Bay

Location of catchments and trapping sites.

the Owengowla) and a smaller stock of 1 sea winter (SW) maiden ﬁsh and multi-spawners (Went, 1949). The 25-year mean sea trout rod catch (1963–87) for the Owengowla and Invermore was 690 and 900 ﬁsh, respectively. Sea trout catches decreased in both ﬁsheries in 1988. In 1989, no sea trout were recorded in the Owengowla ﬁshery and no catch was recorded from the Invermore ﬁshery in 1990. These events were mirrored in many other western sea trout ﬁsheries and a population collapse undoubtedly occurred in 1989 (Anon., 1992; Gargan et al., 2006). Poole et al. (1996) observed a virtual failure in marine survival of smolt to the ﬁnnock stage at the Burrishoole system, near Newport, in 1989, the only location in Ireland with total trapping facilities. In 1990, on many ﬁsheries in the Connemara region, sea trout post-smolts and some kelts were observed returning prematurely to fresh water in May, with a heavy juvenile sea lice (Lepeophtheirus salmonis Krøyer) burden. Prematurely returned sea trout taken from the lower tidal reaches of the Owengowla system were among the highest liceinfested sea trout examined in 1991 (Anon., 1992). In the subsequent decade, sea trout post-smolts have returned prematurely to both ﬁsheries annually with varying degrees of sea lice infestation (Tully et al., 1999). Both ﬁsheries have remained closed to angling since the early 1990s as a stock conservation measure. Downstream and upstream traps were installed in the Owengowla system in 1991 to determine the numbers of sea trout smolts and kelts produced and to determine upstream

62

Sea Trout

escapement and marine survival. In 1992, an upstream trap was installed on the Invermore system and a downstream smolt and kelt trap was installed in 1993. Altantic salmon farming activities began in Kilkieran Bay and Bertraghboy Bay (Fig. 5.1) in 1982 and 1985, respectively. Production of farmed salmon increased considerably in the late 1980s and by 1989 reached 2400 tonnes in Kilkieran Bay and 1600 tonnes in Bertraghboy Bay (Anon., 1992). Heavy infestations of the sea louse Lepeophtheirus salmonis (Krøyer) have been recorded on sea trout in salmon aquaculture areas in Ireland, Scotland and Norway (Anon., 1997). Sea lice epizootics have followed the development of marine salmon aquaculture in all three countries (Butler, 2002). Continuous year-round farmed salmon production took place in Bertraghboy Bay until December 1993. Whole-bay fallowing, practised to control disease and sea lice infestations in farm stock, took place from December 1993 for a prolonged period until the re-introduction of salmon smolts in April 1994. This was the only year over the study period in which prolonged spring fallowing took place. Whole-bay spring fallowing was not practised in Kilkieran Bay. Owengowla catchment The Owengowla catchment encompasses an area of 45.3 km2 and has a freshwater surface area of 157.6 ha upstream of the smolt trap (Fig. 5.1). The catchment is low-lying and the principal soil type is low-level blanket peat overlying mica and hornblende schists. Some areas of granite and felsite also occur. The lower eastern part of the catchment has lithosols, with rock outcrops and peat, the parent material being granite and sandstone. There is no limestone in the catchment and the water has a pH of about 5.8. There is very little afforestation in the catchment and the main agricultural practice is small-scale sheep farming. Rainfall in the area is heavy with more than 254 cm per annum (Whittow, 1975). Non-native unfed sea trout fry (0+) originating from Burrishoole sea trout (some 60 km north) were stocked into the Owengowla catchment over the 1994–2000 period, as part of a sea trout rehabilitation programme (Poole et al., 2002). Invermore catchment The Invermore catchment comprises an area of 38 km2 and has a freshwater surface area of 236.0 ha upstream of the smolt trap (Fig. 5.1). This catchment borders the Owengowla catchment to the east and has similar characteristics, being low-lying and predominated by blanket peat. A small area of high-level blanket peat is found in the middle of the catchment. Schist and gneiss predominate in the upper catchment while granite is the principal rock type in the lower part. There is no limestone and the water has a pH of about 5.8. The majority of the upper catchment has been afforested in the past 20 years (27.5% of total catchment area) and afforestation represents the major agricultural practice. Cattle and sheep-grazing occurs on a small scale in the lower catchment. The rainfall pattern is similar to that of the Owengowla. Sea trout fry (0+) originating from the adjacent Costello stock, some 20 km to the east (Fig. 5.1) were stocked into the Invermore catchment over the 1997–2000 period as part of a sea trout rehabilitation programme.

Characteristics of Sea Trout Stocks

63

Materials and methods Owengowla trapping facilities One downstream and two upstream traps were operated on the Owengowla over the study period (Fig. 5.1). The downstream trap is located on the lower Owengowla River some 250 m from the sea. Fish are diverted into the trap by a line of vertical steel bar screens (bar spacing 6 mm) which were placed across the main channel to prevent further downstream migration. A sloped wooden grid (4 m in length) leading to the trap prevents any upstream movement of ﬁsh. The trap measures 2.5 m (l) × 1.5 m (w) × 1.2 m (h) and has a water regulating sluice at the head of the wooden grid. This trap operated for monitoring sea trout smolt and kelt production over the 1991–2002 period. In 1991, an upstream sea trap was constructed close to the downstream trap consisting of a metal tank, measuring 2 m × 2 m × 2 m. Fish entered the trap through in-scales (10 cm aperture). Bar screens (6 mm spacing) placed on the upstream side of the trap prevented further upstream migration. The channel was cordoned off with vertical bar screens (6 mm bar spacing) to ensure that upstream migrating ﬁsh could only enter the system through the trap. This sea trap was not operated in 1992 or 1993. In 1994, the sea trap was further modiﬁed and increased in size (3 m [l], 2.85 m [w] and 1.3 m [h]). An upstream lake trap, of similar design, situated at the outfall from Gowla lake (Fig. 5.1) was operated in 1992 and 1993. Invermore trapping facilities An upstream trap was constructed on the Invermore River, approximately 500 m from the sea, in 1992. The holding box measured (3.2 m [l], 2.5 m [w] and 1.2 m [h] and the trap was of similar design and operation to the Owengowla sea trap. A downstream trap was constructed at the same location in 1993. This trap (holding box 2.5 m [l], 1.95 m [w] and 0.8 m [h]) had a down-sloping wooden grid intake. The holding box had two overﬂow panels with metal screens (6 mm bar spacing) and was of similar design to the Owengowla downstream trap. Monitoring of trap facilities The downstream and upstream traps in both ﬁsheries were monitored twice daily, morning and evening, during normal ﬂow conditions. During ﬂood events, the traps were monitored more frequently. Sea trout smolts and kelts were taken from the traps in small batches using a ﬁne-meshed hand-net, counted and released downstream. On a number of occasions during smolt runs, ﬁsh were anaesthetised, measured (nearest mm) and a scale sample was taken for age analysis. Assessment of the number of downstream migrating kelts was made on the basis of appearance, length (>25 cm) and condition. Upstream migrating sea trout were classiﬁed as 0SW ﬁnnock (<0.45 kg and <32 cm) or 1SW adult ﬁsh (>0.45 kg and >32 cm). This length separation of sea age was based on scale-reading information. For the calculation of marine survival for both ﬁsheries, all adult sea trout were classiﬁed as 1SW ﬁsh although a small number of previous spawners may have been included.

64

Sea Trout

Upstream trap data Fahy (1985) notes that sea trout post-smolts in Ireland may make a ﬁrst return to fresh water within 2 months of migrating to sea. Trapping data from Burrishoole supports this comment (Poole pers. comm.) Traditionally, the opening date for angling on the Owengowla and Invermore ﬁsheries was 20th June. Based on the local smolt run timing it is reasonable to expect that sea trout post-smolts would normally return as ﬁnnock from the beginning of June at the earliest. Upstream ﬁsh returns have therefore been divided into returns before and after 1st June to differentiate between premature returning post-smolts and normal returns as lice-infested sea trout post-smolts returned primarily in May, in this study.

Results Sea trout rod catch statistics Sea trout rod catch per unit effort (CPUE) data from the Owengowla, covering the periods 1955–89, and from the Invermore, covering the periods 1955–73 and 1985–92 are presented elsewhere (Gargan et al., 2006). CPUE generally ranged from 2 to 4 ﬁsh until 1987. Rod effort was calculated in rod days (8 h) and CPUE is calculated as number of trout caught per rod day. Catch per unit effort decreased from 1987 on both ﬁsheries and reduced to zero on the Owengowla in 1989 and the Invermore over the 1990–92 period, reﬂecting the sea trout stock collapse observed in the region in 1989 (Anon., 1992).

Downstream trap data

Owengowla Data on sea trout smolt output indicate a steady decline over the study period from 1991 to 2003 (Table 5.1) with a signiﬁcant negative trend evident (P = 0.005, F = 12.5, d.f. = 11). Based on a freshwater surface area of 157.6 ha upstream of the sea trap, a mean annual sea trout smolt production of 27.8/ha (range 17.5–47.8) was recorded for the Owengowla. Smolt runs commence in late March, peak in mid-April and fall off by early May (Fig. 5.2). This migration pattern was evident over the 12-year monitoring period, although the smolt run continued throughout May in 1997, 1998 and 1999. The counts from 1991 to 1994 are probably underestimates – particularly 1991, when the trap was not operated until April. From 1991 to 1994, the smolts ranged in length from 15 to 27 cm. Smolts of 2 and 3 years predominated, averaging 62.6% and 36.4% of the runs, respectively. Larger sea trout smolts predominated in the earlier portion of the smolt run each year, with a general tendency towards smaller sizes as the migration progressed. With the exception of 1995, 1996 and 2002, the recorded numbers of sea trout kelts over the 1995–2003 period were low (range 23–486) (Table 5.1). The kelt run during the 1991–94 period is likely to be an underestimate as trapping did not begin until March–early April.

Characteristics of Sea Trout Stocks

65

Table 5.1 Downstream trapping data for sea trout smolts and kelts from the Owengowla and Invermore ﬁsheries. Year

Owengowla

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 Mean

Invermore

Smolt count

Smolt/ha

Kelt count

Trapping duration

7540 5999 4090 3962 3517 4800 3045 4831 2762 3614

47.8 38.1 26.0 25.1 22.3 30.5 19.3 30.7 17.5 22.9

2 2 3 33 416 197 79 23 45 34

4/4–8/6 19/3–29/5 18/3–28/5 3/3–29/5 1/1–30/6 1/1–29/12 1/1–13/6 1/1–13/6 1/1–27/6 1/1–3/7

b

Smolt count

Smolt/ha

Kelt count

Trapping duration

4837 4332 1570 4643 2262 3527 2654

20.5 18.3 6.6 19.7 9.6 14.9 11.2

9 119 24 201 42 15 20

16/3–31/5 16/3–1/6 1/1–24/9 1/1–29/12 1/1–1/8 1/1–31/7 1/1–31/7

13.7 5.9 13.2

65 14

1/1–31/7 1/1–31/7

a b

4027 143c 4381

25.6

486 67

1/1–4/7 1/1–4/7

27.8

3249 1391 3162

a Trap modiﬁcations. b Foot and Mouth restrictions. c Flood event.

8000

7000

1991 n = 7540 1992 n = 5999 1993 n = 4090

6000

1994 n = 3962 1995 n = 3517

Number of fish

5000

1996 n = 4800 1997 n = 3045 4000

1998 n = 4831 1999 n = 2762

3000

2000 n = 3614 2002 n = 4027 2003 n = 143

2000

1000

ar 5Ap r 12 -A pr 19 -A pr 26 -A pr 3M ay 10 -M ay 17 -M ay 24 -M ay 31 -M ay 7Ju n

ar

29 -M

ar

22 -M

M ar 8-

15 -M

eb

M ar

-F 22

1-

b

eb -F

15

b

8Fe

1Fe

e at

25 -J

D

Fig. 5.2

an

0

Cumulative sea trout smolt counts from the Owengowla downstream trap 1991–2003.

Invermore Sea trout smolt runs exceeding 4000 ﬁsh were recorded in 1993, 1994 and 1996 (Table 5.1). There was a signiﬁcant decreasing trend in smolt output over the period (P = 0.026,

66

Sea Trout 6000 1993 n = 4837 5000

1994 n = 4332 1995 n = 1570

4000

1996 n = 4643

Number of fish

1997 n = 2262 1998 n = 3527 3000 1999 n = 2654 2000 no count 2000

2001 no count 2002 n = 3249

1000

2003 n = 1391

20 -J an 27 -J an 3Fe b 10 -F eb 17 -F eb 24 -F eb 3M ar 10 -M ar 17 -M ar 24 -M ar 31 -M ar 7Ap r 14 -A pr 21 -A pr 28 -A pr 5M ay 12 -M ay 19 -M ay 26 -M ay 2Ju n 9Ju n

0

Fig. 5.3

Cumulative sea trout smolt counts from the Invermore downstream trap 1993–2003.

F = 7.3, d.f. = 9). A mean ﬁgure of 13.2 smolts/ha (range = 5.9 − 20.5) was recorded for the Invermore over the period based on a freshwater surface area of 236.0 ha upstream of the Invermore trap. Typically, Invermore sea trout smolt runs begin in late March and continue up to early May, with the peak of the run occurring during April (Fig. 5.3). Smolt length frequency ranged from 17 to 27 cm with ﬁsh in the 19–23 cm range predominant. Data for 1993 and 1994 reveal 66% smolts of 2 years and 32.3% smolts of 3 years. Fewer sea trout smolts <19 cm length were recorded compared with the Owengowla. Sea trout kelt numbers were low, ranging from 9 to 201 ﬁsh (Table 5.1). The 1993 and 1994 data are likely to be an underestimate as trapping did not begin until mid-March in those years. Upstream trap data Sea trout recorded through the upstream traps on the Owengowla system are shown in Table 5.2. Upstream runs varied considerably over the period, with the largest run of 637 ﬁsh being recorded in 1994, coincidental with the fallowing of the salmon cages in Bertraghboy Bay earlier that year. Small numbers of prematurely returning post-smolt sea trout were recorded over the 1998–2000 period. Total returns for the Invermore ﬁshery were low in all years between 1992 and 2002, with the largest number of sea trout recorded in 1994, when the majority of the ﬁsh returned prematurely (Table 5.2). Between 1993 and 2000, a high proportion of the sea trout returned to the Invermore trap before 1st June.

Contribution of freshwater and stocked trout to sea trout smolt runs The relationship among sea trout returns, as shown by rod catches, or upstream trapping data and the subsequent smolt production from the ﬁsheries is intriguing, as large smolt runs

Characteristics of Sea Trout Stocks Table 5.2

Upstream trapping data for sea trout from the Owengowla and Invermore ﬁsheries.

Year

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

67

Owengowla

Invermore

Sea trout total return

Post smolt return before 1st June

Trapping duration

8 (5) (1)a (6)a 637 (367) 322 117 24 48 150 66 489 61 155

0 — — — 0 0 0 32 33 9 0 6 0

14/5–12/9 (12/9–14/10) (5/6–19/9) (14/5–2/10) 1/6–31/12 (1/6–31/12) 7/5–31/12 7/4–31/12 7/4–31/12 14/4–31/12 8/4–31/12 14/4–31/12 14/4–31/12 8/2–31/12 14/4–31/12

Sea trout total return

Post smolt return before 1st June

Trapping duration

31 203 232 163 108 26 55 113 113a 152 19 69

0 116 179 26 32 13 32 81 65a 0 0 3

28/5–22/10 1/4–13/9 1/4–31/12 1/1–31/12 1/1–24/12 1/1–31/12 1/5–31/12 1/4–31/12 1/4–31/12 1/4–31/12 1/4–31/12 1/4–31/12

Entries in bold are upstream sea trap data only. ( ) Upstream lake trap data. a Minimum ﬁgure, trap maintenance.

Table 5.3 Year

Estimation of expected smolt run.

Rod catch

Estimated Total run

Escapement

1+ Stock

0+ Stock

Owengowla 1990 0 1989 0 1988 127 1987 261

155a 155a 969 1992

155 155 842 1731

26a 26a 141 289

129 129 702 1442

Invermore 1991 0 1990 0 1989 26

62b 62b 198

62 62 172

16b 16b 29

46b 46b 144

a Mean returns 1991–2003. b Mean returns 1992–2003.

were recorded after very low numbers of upstream migrants were recorded. Smolt counting has been in place since 1991 in Gowla and 1993 in Invermore. In order to predict the likely smolt output from adult sea trout contributing to these smolt runs in the years preceding smolt trapping, an estimate of expected smolt output was calculated using rod catch data and average upstream returns to traps (Table 5.3). As no rod catches were recorded in the Owengowla in 1989 and 1990, or in the Invermore in 1990 and 1991, for the purpose of the estimate, the mean upstream run recorded over the entire 12-year period in the Owengowla

68

Sea Trout Table 5.4

Smolt estimate parameters.

Smolt estimate parameters Rod exploitation (O’Farrell & Whelan, 1991) Prop of 0+ sea trout, 83.3% (Went, 1949) Prop of 1+ and older, 16.7% (Went, 1949) Finnock sex ratio (F : M) (Poole et al., 2006) 1+ and older sex ratio, (F : M) (Poole et al., 2006) Finnock maturity (Poole et al., 2006) No. eggs/0+ ﬁnnock (Poole et al., 2006) 1+ and older maturity (Poole et al., 2006) No. eggs/1+ sea trout (Poole et al., 2006) Ova to smolt survival (Poole et al., 2006) 2+ smolts (this study) 3+ smolts (this study) Stocked fry survival to smolt (Anon., 2002)

13% 83.3% 16.7% 55.5% 62.5% 30% 754 100% 1027 1.5% 63% 36% 2%

Table 5.5 Numbers of 0+ sea trout fry released in the Owengowla and Invermore ﬁsheries between 1994 and 2000. Year

Owengowla

Invermore

1994 1995 1996 1997 1998 1999 2000

28 844 64 599 54 804 119 062 120 000 108 000 108 604

0 0 0 41 497 150 000 83 854 47 843

and Invermore traps is used. These data were combined with the smolt estimate parameters (Table 5.4) to predict smolt runs over the 1991–93 period for the Owengowla and the 1991–94 period for Invermore. Estimation of smolt output for later years are based on actual upstream trap data. Sea trout unfed fry were stocked out into both ﬁsheries over the period 1994–2000 (Table 5.5) and should have contributed to smolt production. The estimated smolt output for the Owengowla, before the inﬂuence of stocking indicated that the recorded smolt run was much greater than expected (Fig. 5.4). Even after augmentation by planted fry, larger smolt runs were recorded over the 1996–98 period than could be accounted for by upstream migrants and stocking, although in 1999 the observed smolt run was close to that expected. A similar pattern was observed for the Invermore data, with bigger smolt runs being recorded over the 1993–99 period than could have been expected (Fig. 5.4). The large sea trout smolt runs recorded from both ﬁsheries, before the inﬂuence of stocking, coincident with a substantially reduced sea trout spawning stock, suggests that the freshwater trout populations made a signiﬁcant contribution to the sea trout smolt output.

Characteristics of Sea Trout Stocks

69

Owengowla 8000 Recorded Smolt Output Expected Smolt Output

7000

Potential Stocking Contribution

No. of sea trout smolts

6000 5000 4000 3000 2000 1000 0 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Invermore 6000 Recorded Smolt Output Expected Smolt Output

No. of sea trout smolts

5000

Potential Stocking Contribution

4000

3000

2000

1000

0 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Fig. 5.4 Comparison of recorded and expected smolt output and estimate of smolt output from stocking.

Sea trout marine survival Very poor smolt to ﬁnnock survival was recorded for the Owengowla over the time period (0.25–5.34%) with the exception of 1994 (15.77%, Table 5.6). The pattern was similar for total marine survival. Recorded ﬁnnock survival rates for 1991, 1992 and 1993 may be underestimated, as trapping did not continue to the end of each year. However, in 1994,

70

Sea Trout Table 5.6 Sea trout marine survival to the Owengowla system, 1991–2002. Total return from annual smolt run includes ﬁnnock return in year x and ≥1SW return in year x+1. Year

Sea trout smolt run

Finnock return

Finnock % return

Sea trout ≥1SW return

Total return from annual smolt run

Total survival from annual smolt run (%)

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

7540 5999 4090 3962 3517 4800 3045 4831 2762 3614

11 0 0 625 188 17 15 12 116 33 489 42 139

0.14a 0a 0a 15.77 5.34 0.35 0.49 0.24 4.19 0.91

2 1 6 12 134 100 9 4 1 24 0 19 19

12a 6a 12a 759 288 26 19 13 140 33 508 61

0.16a 0.10a 0.29a 19.15 8.18 0.54 0.62 0.26 5.06 0.91

b

4027 143c

1.04

1.51

a Based on upstream lake trap data only and shortened trapping period. b Trap not monitored because of Foot and Mouth restrictions. c Underestimate because of ﬂood event.

trapping was carried out up to the end of December and 94% of all ﬁnnock had entered the sea trap by the end of August. As trapping was carried out until at least mid-September, it is very likely that almost all ﬁnnock were counted during 1991, 1992 and 1993. The ﬁgures from the Owengowla upstream lake trap might also have been underestimates, because this trap is located 6 km from the sea and some ﬁsh might not have migrated up so far. However, 62% and 58% of all sea trout that entered the sea trap during 1991 and 1994 passed through the lake trap. If these percentages are taken as normal, even a doubling of the recorded returns still represents an almost complete failure of the ﬁnnock runs over the 1991–93 period. The longest period of whole-bay fallowing of salmon farms in Bertraghboy Bay over the 1991–2002 period took place from December 1993 to April 1994 and coincided with the largest subsequent recorded ﬁnnock marine survival for the Owengowla. In 1995 and 1996, harvesting of 1SW farmed salmon began in late February and was completed in March before cages were restocked in late March–early April. This much-reduced period of spring fallowing coincided with poorer sea trout marine survival in both years. Finnock survival to the Invermore system was very low for all years and below 1% for 5 of the 9 years examined (Table 5.7). Total marine survival was also very low over the period (range 0.25–4.9%). Again, ﬁnnock survival may also be underestimated in 1993 because upstream trapping was terminated on 11th September. However, any underestimation is likely to have been minimal because no sea trout were recorded in the upstream trap between 22nd August and 11th September and the data for 1994, with a full year trapping duration, showed that all upstream migrants entered the Invermore trap by 13th August 1994. The improved ﬁnnock survival seen in the Owengowla in 1994 was not reﬂected in the Invermore data, and whole bay spring fallowing had not taken place in Kilkieran Bay in spring 1994.

Characteristics of Sea Trout Stocks

71

Table 5.7 Sea trout survival to the Invermore System, 1992–2002. Total return from annual smolt run includes ﬁnnock return in year x and ≥1SW return in year x+1.

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Sea trout smolt run

Finnock return

Finnock % return

Sea trout ≥1SW return

Total return from annual smolt run

Total survival from annual smolt run (%)

— 4837 4332 1570 4643 2262 3527 2654

25a 39a 11 77 76 11 11 32 34 152 19 65

— 0.80a 0.25 4.90 1.63 0.48 0.31 1.20

6a 48a 42 60 0 2 12 0 14 0 0 4

73a 81a 71 77 78 23 11 46

— 1.67a 1.63 4.90 1.67 1.01 0.31 1.73

23

0.71

b c

3249 1391

0.58 4.7

a Based on incomplete trapping period. b Trap being modiﬁed. c Trap not monitored because of Foot and Mouth restrictions.

Gargan et al. (2006) compare the ﬁnnock survival data for the Owengowla and Invermore with long-term data for the Burrishoole ﬁshery for the period 1971–87, before the sea trout stock collapse. The annual ﬁnnock survival data for both ﬁsheries were extremely low when compared with the long-term Burrishoole data, with the exception of the Owengowla in 1994, when extended whole-bay fallowing had been practised locally during the previous late winter and spring.

Discussion It is evident from the collapse of the rod ﬁsheries and the poor upstream trapping counts that the sea trout run was substantially reduced in both ﬁsheries by 1990. The recorded sea trout smolt run from the Owengowla system then decreased signiﬁcantly over the 1991–93 period. A steady decline in sea trout smolt output was also recorded on the Burrishoole system after 1990 (Poole et al., 1996). A reduced smolt output from the Owengowla could be expected because of the severe drop and subsequent collapse of sea trout runs into the ﬁshery. It is highly probable that by 1991, when trapping ﬁrst commenced, sea trout smolt output had already decreased in the ﬁshery. However, substantial sea trout smolt runs continued to be recorded from both the Owengowla and Invermore ﬁsheries in the early 1990s, derived from very small sea trout adult runs and without the inﬂuence of fry stocking. O’Farrell and Whelan (1991) have shown that the larger adult (1 and 2SW) sea trout deposit 76% of the total ova in the Erriff system, another Connemara ﬁshery. The very low numbers of upstream migrating adult sea trout recorded entering both ﬁsheries in this study suggests that the freshwater trout population were major contributors to sea trout smolt output.

72

Sea Trout

There has been extensive debate on the relationship between anadromous and freshwater brown trout. Many authors have held the view that both are interbreeding components of a single spawning stock (Frost & Brown, 1967; Mills, 1971; Solomon, 1982; Jonsson, 1985, 1989; Cross et al., 1992). Elliott (1989) stated that any distinction between migratory anadromous and non-migratory brown trout is ‘probably meaningless’ as extensive interbreeding between the two forms is common. The fact that interbreeding between anadromous and freshwater forms occur does not exclude the possibility of a genetic component for anadromy. It has been hypothesised that anadromy in salmonids in general is a threshold quantitative trait (Hallerman, 2003). That is, the trait is governed by multiple genes together with an environmental component and an individual becomes anadromous whenever the combined genetic and environmental inﬂuences exceed a given threshold. It is thus possible for individuals to have the genetic pre-disposition to become sea trout without this being expressed, as may be the case among the freshwater trout in the two river systems considered here. The corollary is that it is possible to have sea trout smolts emanating from freshwater parents. Thrower et al. (2004) found that after 70 years a wild, physically isolated lake population of formerly anadromous rainbow trout (Oncorhynchus mykiss) had retained substantial genetic variability for smolt migration (heritability 0.56) in spite of complete selection against smolt migration. However, they also found that marine survival of the lake-derived smolts was poor relative to survival of smolts derived from anadromous parents. Further, Thrower et al. (2004) found that smolting and maturation were negatively genetically correlated and argue that balancing selection for male maturation age may have been responsible for the maintenance of genetic variability for smolting. Trend analysis indicates a reduction in sea trout smolt output from both ﬁsheries over the study period. This suggests that, although freshwater trout contribute signiﬁcantly to sea trout smolt runs, a reduction in smolt output can be expected after a relatively short period of very poor marine survival of sea trout. If the individuals which become anadromous have very low marine survival, perhaps because of elevated sea lice levels in the marine environment, there would be selection in favour of those which have a higher genetic propensity for freshwater residence. The declining numbers of smolts produced by the freshwater stock could be explained by this selection against anadromy. Eventually this would be expected to result in the loss of anadromy from the population. Given that Ireland was colonised by sea trout following the retreat of the last Ice Age, some 14 000 years ago, all current native freshwater trout must have been derived from sea trout ancestors. Moreover, this change from anadromous migratory behaviour to freshwater residence has occurred independently in each catchment. Thus, the results found in this study may not be applicable to other systems where because of differences in selection history there may be a different genetic propensity for anadromy. Calculation of sea trout marine survival has been possible on the Burrishoole ﬁshery since 1971. As the only long-term marine survival data available for west of Ireland sea trout ﬁsheries, these data serve as a means of comparison and evaluation of marine survival rates recorded in this study. The survival rate of smolt to return as ﬁnnock in any 1 year ranged from 11.4% to 32.4% for the Burrishoole ﬁshery for the period 1971–87, with a mean survival value of 21% (Poole et al., 1996). Data from this study show that ﬁnnock survival

Characteristics of Sea Trout Stocks

73

rates for the Owengowla and Invermore are signiﬁcantly below the 1971–87 recorded mean and lowest range values for the Burrishoole. Only the ﬁnnock marine survival data for the Owengowla for 1994 was within this previously recorded range. Whole-bay fallowing of Bertraghboy Bay, into which the Owengowla system discharges, took place from December 1993 until April 1994 and coincided with a signiﬁcant increase in ﬁnnock survival in the Owengowla ﬁshery that year. Whole-bay fallowing was not practised in Kilkieran Bay, into which the Invermore system discharges, and consistently very poor ﬁnnock marine survival rates were recorded. High sea lice intensities have been recorded on prematurely returned sea trout in Ireland, Scotland and Norway in areas with intensive salmon farming (Birkeland, 1996; Birkeland & Jakobsen, 1997; Butler & Watt, 2003; Gargan et al., 2003). Tully et al. (1999) suggest that there is substantial retention of larvae within the bay in which larvae are produced by selective tidal stream transport. Both Bjorn et al. (2001) and Anderson & Gordon (1982) note an excessive mortality of the heaviest infested ﬁsh. When salmon farms are fallowed over a prolonged spring period, lice infestation pressure is greatly reduced at the time wild sea trout smolt runs begin in late March. Studies in Ireland (Tully & Whelan, 1993; Gargan et al., 2003), Scotland (Butler, 2002) and Norway (Heuch & Mo, 2001) have indicated that in spring, the majority of nauplii arise from ovigerous lice infesting farmed salmon. The results from this study strongly suggest that the practice of whole-bay spring fallowing of marine salmon farms has a positive effect on sea trout ﬁnnock marine survival in ﬁsheries within such bays. They support the view that sea lice infestation from marine salmon farms was a major contributory factor in the sea trout stock collapse on Ireland’s west coast. The evidence from this study suggests that the restoration of sustainable sea trout populations in these ﬁsheries requires eradication of the cause of their excessively poor marine survival levels and utilisation of the potential of the residual stocks, while they still exist, to respond to the improved environmental conditions.

Acknowledgements The authors wish to acknowledge the time and effort of the staff of the Western Regional Fisheries Board, Connemara District, in constructing and operating the trapping facilities on both ﬁsheries. The contributions by Dr Philip McGinnity and Dr Russell Poole to the text are gratefully acknowledged.

References Anderson, R.M. & Gordon, D.M. (1982). Processes inﬂuencing the distribution of parasite numbers within host populations with special emphasis on parasite inﬂuenced host mortalities. Parasitology, 85, 373–98. Anon. (1992). Report of the Sea Trout Working Group, 1991, Department of the Marine, Dublin, 49 pp. Anon. (1997). Report of the Workshop on the Interactions between Salmon Lice and Salmonids, Edinburgh, Scotland, UK, 11–15 November 1996. ICES CM 1997/M: 4, Ref.:F. Anon. (2002). Shieldaig Project Review, June 2001–June 2002. Internal Report, Fisheries Research Services, Pitlochry, Scotland, 16 pp.

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Birkeland, K. (1996). Consequences of premature return by sea trout, Salmo trutta, infested with the salmon louse, Lepeophtheirus salmonis (Krøyer): migration, growth, and mortality. Canadian Journal of Fisheries and Aquatic Sciences, 53, 2808–13. Birkeland, K. & Jakobsen, P.J. (1997). Salmon lice, Lepeophtheirus salmonis, infestation as a causal agent of premature return to rivers and estuaries by sea trout, Salmo trutta, juveniles. Environmental Biology of Fisheries, 49, 129–37. Bjorn, P.A., Finstad, B. & Kristoffersen, R. (2001). Salmon lice infestation of wild sea trout and Arctic charr in marine and freshwater: the effects of salmon farms. Aquaculture Research, 32, 947–62. Butler, J.R.A. (2002). Wild salmonids and sea louse infestations on the west coast of Scotland: sources of infection and implications for the management of marine salmon farms. Pest Management Science, 58, 595–608. Butler, J.R.A. & Watt, J. (2003). Assessing and managing the impacts of marine salmon farms on wild Atlantic salmon in western Scotland: identifying priority rivers for conservation. In: Salmon at the Edge (Mills, D., Ed.). Blackwell Science, Oxford, UK, pp. 93–118. Cross, T.F., Mills, C.P.R. & de Courcy Williams, M. (1992). An intensive study of allozyme variation in freshwater resident and anadromous trout, Salmo trutta L., in western Ireland. Journal of Fish Biology, 40, 25–32. Elliott, J.M. (1989). Wild brown trout Salmo trutta: an important national and international resource. Freshwater Biology, 21, 1–5. Fahy, E. (1985). Child of the Tides: A Sea Trout Handbook. The Glendale Press, Dublin, 188 pp. Frost, W.E. & Brown, M.E. (1967). The Trout. Collins, London, 286 pp. Gargan, P.G., Tully, O. & Poole, W.R. (2003). The relationship between sea lice infestation, sea lice production and sea trout survival in Ireland, 1992–2001. In: Salmon at the Edge (Mills, D., Ed.). Blackwell Science, Oxford, UK, pp. 119–35. Gargan, P.G., Poole, W.R. & Forde, G.P. (2006). A review of the status of Irish sea trout stocks. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 25–44. Hallerman, E.M. (2003). Quantitative Genetics. In: Population Genetics: Principles and Applications for Fisheries Scientists (Hallerman, E.M., Ed.) American Fisheries Society. Bethesda, MD, 345 pp. Heuch, P.A. & Mo, T.A. (2001). A model of salmon louse production in Norway: effects of increasing salmon production and public management measures. Diseases of Aquatic Organisms, 45, 145–52. Jonnson, B. (1985). Life history pattern of freshwater resident and sea-run migrant brown trout in Norway. Transactions of the American Fisheries Society, 114, 182–94. Jonnson, B. (1989). Life history and habitat use of Norwegian brown trout (Salmo trutta). Freshwater Biology, 21, 71–86. Mills, D. (1971). Salmon and Trout: A Resource, its Ecology, Conservation and Management. Oliver and Boyd, Edinburgh, 351 pp. O’Farrell, M. & Whelan, K.F. (1991). Management of migratory trout (Salmo trutta L.) populations in the Erriff and other catchments in western Ireland. In: Irish Rivers: Biology and Management (Steer, M.W., Ed.), Royal Irish Academy, Dublin. pp. 99–112. Poole, W.R., Whelan, K.F., Dillane, M.G., Cooke, D.J. & Matthews, M. (1996). The performance of sea trout (Salmo trutta L.), stocks from the Burrishoole system, western Ireland, 1970–1994. Fisheries Management and Ecology, 3(1), 73–92. Poole, W.R., Byrne, C.J., Dillane, M.G., Whelan, K.F. & Gargan, P.G. (2002). The Irish sea trout enhancement programme: a review of the broodstock and ova production programmes. Fisheries Management and Ecology, 9, 315–28. Poole, W.R., Dillane, M., deEyto, E., Rogan, G., McGinnity, P. & Whelan, K. (2006). Characteristics of the Burrishoole sea trout population: census, marine survival, enhancement and stock recruitment, 1971–2003. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales UK. Blackwell Publishing, Oxford, pp. 279–306. Solomon, D.J. (1982). Migration and dispersion of juvenile brown and sea trout. In: Proceedings of the Salmon and Trout Migratory Behaviour Symposium (Brannon, E.L. & Salo, E.O., Eds). School of Fisheries, University of Washington, Seattle, pp. 135–45. Thrower, F.P., Hard, J.J. & Joyce, J.E. (2004). Genetic architecture of growth and early life-history transitions in anadromous and derived populations of steelhead. Journal of Fish Biology, 65 (Suppl. A), 286–307.

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Tully, O. & Whelan, K.F. (1993). Production of nauplii of Lepeophtheirus salmonis (Krøyer) (copepoda: caligidae) from farmed and wild Atlantic salmon (Salmo salar L.) on the west coast of Ireland during 1991 and its relation to infestation levels on wild sea trout (Salmo trutta L.). Fisheries Research, 17, 187–200. Tully, O., Gargan, P., Poole, W.R. & Whelan, K.F. (1999). Spatial and temporal variation in the infestation of sea trout (Salmo trutta L.) by the caligid copepod Lepeophtheirus salmonis (Krøyer) in relation to sources of infection in Ireland. Parasitology, 119, 41–51. Went, A.E.J. (1949). Sea trout of the Owengowla (Gowla River). Scientiﬁc Proceedings, Royal Dublin Society, 25, 55–64. Whittow, J.B. (1975). Geology and Scenery in Ireland. Pelican Books Ltd., Middlesex, 301 pp.

Chapter 6

Annual Variation in Age Composition, Growth and Abundance of Sea Trout Returning to the River Dee at Chester, 1991–2003 I.C. Davidson, R.J. Cove and M.S. Hazlewood Environment Agency, Chester Road, Buckley, Flintshire, CH7 3AJ, UK

Abstract: Partial trapping and tagging of adult sea trout (and salmon) have been carried out on the River Dee, at Chester Weir, since 1991 as part of a long-term programme to monitor changes in run size and composition (age, weight, length). Around 20 000 sea trout have been sampled to date. The Dee sea trout population, like many in the north-west of the country, is dominated by .0+ maiden ﬁsh (whitling). Annual run estimates for whitling and older sea trout (based on mark-recapture methods) have averaged 7207 (range = 2537–14 680) and 2069 (range = 1424–2776) respectively, over the period 1991–2002. The population is relatively long-lived and lightly exploited. For example, the combined extant exploitation rate for rod and net ﬁsheries has averaged only 3.0% over the past 12 years (based on declared catches) and multiple spawning ﬁsh are common (including one individual which had spawned on eight consecutive occasions). This chapter examines changes in growth and abundance of individual sea-age groups of sea trout and explores possible inﬂuencing factors. Fluctuations in mean smolt age (MSA), proportion of maturing whitling and post-spawner survival rates are also described. Keywords: Sea trout, long-term monitoring, stock structure, exploitation, growth, abundance.

Introduction In 1991, the National Rivers Authority (a predecessor organisation of the Environment Agency) began a long-term programme to monitor stocks of salmon and sea trout on the River Dee. This programme focuses on the upstream trapping and tagging of ﬁsh at a main river, head-of-tide trap at Chester Weir (SJ 408 658) (Fig. 6.1) and, for both species, provides annual estimates of run size (by mark-recapture) along with information on age, length, weight and other biological details. The purpose of this chapter is to review the results of 13 years of the Dee programme in which more than 36 000 sea trout have been caught at Chester Weir and over 20 000 have been tagged. It examines changes in run size, growth and age composition over that period and explores the factors which may have been inﬂuencing change. The ageing notation used in this account is based on that of Nall (1930) and Went (1962). 76

Variation in Age Composition and Growth N

77

50 Kilometres

CHESTER Dee ALYN

FARNDON

LLYN BRENIG

LLYN ALWEN

WREXHAM ALWEN

EITHA

CORWEN

LLYN CELYN

GLYNDYFRDWY

ERBISTOCK BANGOR

LLANGOLLEN

MELOCH TRYSTION

TRYWERYN

LLANDDERFEL

LLYNOR

CEIRIOG

CHIRK

N

BALA CEIDIOG

LLAFAR LLYN TEGID LLIW DYFRDWY

Fig. 6.1

GLYN

HIRNANT

TWRCH

SCALE 0

5

10 KMS

The Dee catchment.

Materials and methods Trapping and tagging Upstream trapping and tagging of sea trout (and salmon) began at Chester Weir in July 1991. From the outset, the trap has been operated in all months of the year but ﬁshed discontinuously (60–70% of the time). The partial nature of the trap means that, even if ﬁshing were continuous, a full count would not be possible as ﬁsh are able to ascend Chester Weir and by-pass the trap when the ﬂow and tidal state allow. Run estimates for sea trout (and salmon) are obtained indirectly using mark-recapture methods. In the case of salmon, run estimates are based on recaptures reported by anglers ﬁshing upstream of Chester Weir in the year of tagging. However, for sea trout, too few ﬁsh are caught by angling to adopt the same approach – despite greater numbers of tagged sea trout being available for capture (an average of 1768 sea trout per year from 1994 to 2003 compared with 1051 salmon). Instead, because of the higher post-spawner survival rates for sea trout, run estimates for this species are based on recaptures back at Chester Weir in the year following tagging. Run estimates for whitling (.0+) and older (>.0+) sea trout are made separately using the method of Chapman (1952), and are based on the ratio of tagged to untagged ﬁsh in the second year’s catch of .0+SM+ ﬁsh and other previous spawners, respectively.

78

Sea Trout

Tagging is carried out on sedated ﬁsh using large Visible Implant (VI) tags (www.nmt.us) inserted under clear tissue immediately posterior to the eye. Each tag carries a unique code and can be read through the skin. A second permanent mark – comprising removal of a small disc of tissue from the adipose ﬁn – is also applied to assess tag loss. Following tagging, fork-snout length is measured (to the nearest mm) and in a proportion of whitling and all older sea trout (i.e. >.0+), 3–6 scales are removed for age determination purposes. In addition, a lesser number of ﬁsh (approx. every third ﬁsh sampled) are weighed (to the nearest 25 g). After tagging and measuring, ﬁsh are allowed to fully recover in a holding pool before release. Further details on trapping, ﬁsh handling and other aspects of the Dee programme are given in Davidson et al. (1996). Note that since 1994, ﬁne mesh screens (20 mm × 20 mm) have been secured to the upstream bars of the trap in July and August in order to retain all sizes of whitling. Before 1994, sampling was biased towards larger whitling and so estimates of mean length or weight before 1994 are excluded from subsequent analyses. Sampling bias before 1994 may also have resulted in a tendency to underestimate the run of .0+ ﬁsh – although the degree to which this might have occurred is uncertain. Back-calculated freshwater growth Back-calculated lengths at each river annulus were derived from the scales of .0+ and .1+ maidens using the method described by Friedland et al. (2000). Measurements were obtained from cleaned scales viewed under magniﬁcation (×30 or ×50) with radii distances recorded from a single scale. In most years, scale measurements were taken from 150 to 200 ﬁsh. To avoid sub-sampling bias, ﬁsh were selected in proportion to the number available in each sea-age group per month.

Results Run size, timing and age composition Run estimates for .0+ (whitling) and older sea trout (>.0+) at Chester Weir for the past 12 years are shown in Fig. 6.2 (run estimates for salmon are also shown for comparison). Estimates for whitling showed an increasing trend over the time series, rising from a minimum run of 2537 in 1992 to a maximum of 14 680 in 2001 (overall mean = 7207). In contrast, run estimates for older (>.0+) sea trout remained relatively stable (mean = 2069; range = 1424–2776), as have those of salmon (Fig. 6.2). The combined average run for all sea trout at 9275 was more than 1.5 times that for salmon (5625) over the equivalent period. In all years, peak monthly trap catch rates for whitling occurred in July with an average of 90% of the total catch rate taken within a 3-month period centred on this month. The equivalent peak in catch rate for older sea trout occurred in June (except in 1994 when catch rates peaked in July). As with whitling, catch rates were intense around the peak month with 85% of the total catch rate taken in the peak month and one month before or after. Whitling sea trout were rarely caught at Chester before May but could still be present in

Variation in Age Composition and Growth

79

20 000 .0+ Sea trout >.0+ Sea trout Salmon (all ages)

Run size

15 000

10 000

5000

0 91

92

93

94

95

96

97

98

99

00

01

02

03

Return year

Fig. 6.2 Annual run estimates for sea trout (and salmon) at Chester Weir, 1991–2002 (error bars indicate 95% conﬁdence limits).

(a)

S4 0.2%

(b) 2+ 5.8%

Other 8.1%

0+SM+ 31.7%

S3 8.2%

S1 1.5%

1+SM+ 7.3%

0+2SM+ 6.3% 1+ 40.9%

S2 90.2%

Fig. 6.3 Age composition of Dee sea trout run, 1991–2003: (a) sea age (>.0+ ﬁsh only) and (b) smolt age (all adult ﬁsh).

December. Older ﬁsh appeared as early as February, but also persisted in catches to the end of the year. The sea and river age composition of the Dee sea trout run is summarised in Fig. 6.3. Maiden .1+ (41%) and .0+SM+ ﬁsh (32%) were the dominant age groups among the older sea trout (Fig. 6.3a). The remainder was made up of other previous spawners and a few maiden .2+ ﬁsh (6%). (Other previous spawners include individuals that appeared to have spawned for up to eight consecutive years.)

80

Sea Trout 2.4

Mean smolt age (years)

.0+ fish .1+ fish

2.2

2.0

1.8 89

90

91

92

93

94

95

96

97

98

99

00

01

02

03

Smolt year

Fig. 6.4 Annual variations in MSA (aligned by smolt year) for .0+ and .1+ sea trout 1990–2003 (error bars indicate 95% conﬁdence limits).

Based on readings from adult scales, around 90% of ﬁsh appeared to have emigrated as 2-year-old smolts (S2s) (Fig. 6.3b). S2s were present in similar proportions in both whitling and .1+ maiden sea trout groups (both close to 90%), but the proportion of S1s was larger than expected among .1+ ﬁsh (4%) and smaller than expected among whitling (1%), the opposite being true for S3s and S4s (a combined ﬁgure of 7% in .1+ ﬁsh and 10% in whitling) (χ 2 = 110.6; P < 0.001). Mean smolt age (MSA) of whitling and .1+ maiden sea trout declined over the period of the study (Fig. 6.4), signiﬁcantly so in the case of the latter (log10 transformed data: r = −0.583; P = 0.037). Annual estimates of sea-age composition among sea trout greater than .0+ have been used to assign the total run for this group to separate sea-age components. These are aligned by smolt year in Table 6.1 along with run estimates for whitling. Estimates of .1+ abundance from Table 6.1 show a signiﬁcant declining trend over the time series (r = −0.589; P = 0.044) – the opposite trend to that observed for whitling (r = 0.862; P = 0.003) (Fig. 6.5). Abundance of whitling was positively correlated with that of .0+SM+ ﬁsh from the same smolt year class (r = 0.766; P = 0.027). In contrast, no signiﬁcant correlation was evident between abundance of .1+ maidens and that of .1+SM+ ﬁsh (r = −0.002; P = 0.995) (Fig. 6.5). These relationships exclude .0+ run estimates before 1994 which are shown as open symbols in Fig. 6.5.

Post-spawner survival Post-spawner survival rates, estimated from Table 6.1, are shown in Fig. 6.6. Annual survival rates increase progressively across all maiden groups for the ﬁrst four post-smolt years (i.e. equivalent to sea ages .0+4SM+; .1+3SM+ and .2+2SM+), and then decline. (Survival

Table 6.1

Abundance estimates for individual sea-age groups aligned by smolt year class, 1982–2002. Smolt year class

Sea age

1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992

1993

1994

1995

1996

1997

1998

1998

2000

2001

2002

5202 2537

11 505

.0+ .0+SM+ .0+2SM+ .0+3SM+ .0+4SM+ .0+5SM+ .0+6SM+ .0+7SM+ .1+ .1+SM+ .1+2SM+ .1+3SM+ .1+4SM+ .1+5SM+ .1+6SM+ .1+7SM+ .1+8SM+ .2+ .2+SM+ .2+2SM+ .2+3SM+ .2+4SM+ .2+5SM+

0

0

0 0

0 0 0

0

0 0

0 0 0 0

0 0

9 0 0

0 0 0 0 0

9 4 3

9693

3897

6673

4645

5509

7877

6763

7502

14 680

89 89 14 14 2 0 0

406 106 23 7 6 0 0

457 74 13 9 8 3 0

483 59 28 16 6 0 3

576 146 107 23 25 7 8

520 222 15 11 13 4

983 132 63 36 8

597 222 108 46

758 210 34

774 92

1351

810

9 14 0 0 0

85 43 17 0 0 0

530 115 31 11 0 0 0

782 155 11 0 2 3 0 0 0

822 90 23 7 0 8 0 0 0

951 242 22 12 12 3 4 0 4

1391 106 44 36 12 4 0 0

588 159 79 3 4 7 0

676 238 18 39 10 15

744 64 53 30 11

469 194 85 46

695 73

43 0 3 3 0 0 0

1656 180 41 11 9 0 0 0 0

574 121 46

26 7 0 0 0 0

179 22 10 0 3 0 0 0

120 28 5 3 0 0

81 5 0 0 0 0

166 17 2 0 0 0

373 13 9 4 0 0

51 12 12 0 4 0

105 12 0 0 7 0

80 9 11 0 0

71 53 13 8

75 23 0

40 4

9 4 0 0

197 29 7 5 0 2

81

26 25 10 3 0

0 0 0 0

82

Sea Trout 10 000

10 000 r = 0.862 P = 0.003 1000 1990

1995

2000

.1+ abundance

.0+ abundance

100 000

1000

r = –0.589 P = 0.044 100 1985

2005

Smolt year

1990

1995

2000

2005

Smolt year 1000

1000

r = 0.766 P = 0.027 100 1000

.1+SM+ abundance

.0+SM+ abundance

10 000

100

r = –0.002 P = 0.995 10

10 000

.0+ abundance

100 000

100

1000

10 000

.1+ abundance

Fig. 6.5 Correlations between (i) abundance of .0+ and .1+ sea trout and smolt year and (ii) abundance of .0+ and .1+ ﬁsh and that of .0+SM+ and .1+SM+ ﬁsh of the same smolt year class (log10 scales used; trend lines indicated; open symbols indicate .0+ estimates from 1991, 1992 and 1993).

rates have been averaged across age groups older than four post-smolt years as very few individuals reach this age.) Average survival rates peaked between the third and fourth post-smolt years at 57% to age .0+4SM+, 52% to age .1+3SM+ and 41% to age .2+2SM+. Maturation of whitling Tag recoveries and scale readings indicate that, among the .0+ sea trout captured at Chester Weir, some will go on to spawn in the year of their ﬁrst return (i.e. are recaptured as .0+SM+ ﬁsh) while the rest remain maiden ﬁsh (most recaptured aged .1+ with a few aged .2+). It was not possible to judge the maturation state of whitling from external characteristics and no ﬁsh were killed for internal examination. For all years combined (1992–2003), 329.0+ ﬁsh were recaptured aged .0+SM+, 109 aged .1+ and 2 as .2+ ﬁsh (Table 6.2). These totals indicate a maturation rate in the ﬁrst year of return of 75%, although on an annual basis (1994–2002) this rate has varied from 58% to 95% and appears to have increased gradually over the time series (the latter ﬁgures exclude the ﬁrst 3 years when too few recaptures were made for meaningful comparison).

Variation in Age Composition and Growth

83

0.80

0.60

Survival rate

0.40

0.20

0.00 .2+3SM+ to .2+5SM+

.2+2SM+

.2+SM+

.1+4SM+ to .1+8SM+

.1+3SM+

.1+2SM+

.1+SM+

.0+5SM+ to .0+7SM+

.0+4SM+

.0+3SM+

.0+2SM+

–0.20

–0.40 Post-spawner age

Fig. 6.6 Average annual survival rates to post-spawner age for ﬁsh from the 1986–2002 smolt year classes (error bars indicate 95% conﬁdence limits).

Table 6.2 Estimated maturation rates for whitling sea trout based on recaptures of .0+SM+, .1+ and .2+ ﬁsh, 1991–2003. Recapture age

Tagging year 91

.0+SM+ .1+ .2+ Total % Maturation

92

93

94

95

96

97

98

99

00

01

02

All

4 0 0

8 4 0

5 9 0

42 30 0

34 11 1

38 16 0

47 19 1

35 7 0

28 4 0

18 5 0

40 2 0

30 2 0

329 109 2

4

12

14

72

46

54

67

42

32

23

42

32

440

66.7

35.7

58.3

73.9

70.4

70.1

83.3

87.5

78.3

95.2

93.8

100.0

74.8

These rates ignore any differential mortality between maturing and non-maturing groups; for example, they will underestimate the true maturation rate if mortality among previous spawners is greater than in maiden ﬁsh. Among the 440 recaptures identiﬁed in Table 6.2, 324 had retained their tags and so could be traced as individual .0+ ﬁsh. The remaining ﬁsh with missing tags were assigned to the tagging year based on the presence of a permanent mark made by the removal of a small disc of tissue from the adipose ﬁn at the time of tagging. The permanence of this ﬁn clip was conﬁrmed by its presence in all recaptured ﬁsh which had retained their tags. In the group of 324 ﬁsh which had retained their tags, 239 (74%) were recaptured as .0+SM+ ﬁsh, 83 (26%) as .1+ ﬁsh with only two ﬁsh recovered aged .2+. Based on these recaptures, whitling that returned as .0+SM+ ﬁsh were signiﬁcantly larger than those

84

Sea Trout

returning as .1+ maidens (mean lengths 322 and 309 mm, respectively; ANOVA: F = 8.95; P = 0.003) and arrived earlier in the year (mean capture date of the 20th July compared with the 14th August; ANOVA: F = 58.23; P = <0.001). Year-on-year, mean arrival times of maturing whitling were consistently earlier than those of non-maturing ﬁsh; however, size differences between these two groups have been more variable over the past 12 years (Fig. 6.7).

Size composition

450

350

400

300

350 300 250

maturing fish

Days from 1st January

Length (mm)

Time series of mean length data for .0+ and .1+ spawning groups are shown in Fig. 6.8 – up to sea ages .0+4SM+ and .1+3SM+, respectively. Estimates of mean ‘smolt’ length – that is, length at the last river annulus back-calculated from maiden .0+ and .1+ adult scales – also appear in Fig. 6.8 but showed no marked trends over the time series (r = −0.361; P = 0.340 and r = −0.393; P = 0.206, respectively). In contrast, the mean length of .1+ maidens increased almost year-on-year – producing a signiﬁcant trend overall (r = 0.864; P < 0.001) and representing an average yearly increment of 3.6 mm, while the mean length of whitling remained relatively stable over the equivalent period (r = 0.391; P = 0.264). Among all post-spawner age groups, trends in mean size were positive over the time series – signiﬁcantly so for .0+2SM+ ﬁsh (r = 0.607; P = 0.028); for .0+3SM+ ﬁsh (r = 0.613; P = 0.026) and for .1+SM+ ﬁsh (r = 00.834; P < 0.001). However, there was little evidence that growth patterns were strongly synchronous among spawning groups (i.e. that good or poor smolt year classes in terms of mean length tended to persist through successive age strata). Mean weight data showed similar patterns of inter-annual variation to those described for length, with condition (Fulton’s factor – see Bagenal, 1978) remaining relatively stable throughout for both .0+ and .1+ ﬁsh (annual mean ranges 1.202–1.283 and 1.131–1.243, respectively).

250 200 150 100

200

maturing fish non-maturing fish

non-maturing fish

50 91 92 93 94 95 96 97 98 99 00 01 02 Year tagged

91 92 93 94 95 96 97 98 99 00 01 02 Year tagged

Fig. 6.7 Differences in size and time of ﬁrst return for maturing and non-maturing whitling sea trout tagged at Chester Weir, 1991–2002 (error bars indicate 95% conﬁdence limits).

Variation in Age Composition and Growth .1+ Sea age groups

800

800

700

700

600

600 Length (mm)

Length (mm)

.0+ Sea age groups

500

400 300

0+

500 400 300

0+SM+

1+

0+2SM+

200

85

1+SM+

0+3SM+

1+2SM+

200

0+4SM+

1+3SM+

Smolt

Smolt

100

100 84 86 88 90 92 94 96 98 00 02 Smolt year

84

86

88

90

92

94

96

98

00

02

Smolt year

Fig. 6.8 Annual variation in mean length of .0+ and .1+ sea-age groups aligned by smolt year (error bars indicate 95% conﬁdence limits).

Discussion The River Dee is well known for its signiﬁcant net and rod ﬁsheries for salmon (declared 10-year mean catches for 1994–2003 of 1014 and 500, respectively). In comparison, few sea trout are caught by these ﬁsheries (59 and 239, respectively) – reﬂecting the low level of exploitation on this species and equivalent to mean extant rates for both ﬁsheries combined – only 3.0% over the past 12 years (see Shields et al., 2006). In contrast to net and rod catches, run estimates at Chester Weir highlight the numerical dominance of sea trout over salmon (average runs of 9275 and 5625, respectively). As on many west-coast rivers (Solomon, 1994; Harris, 2002), whitling make up the majority (59–87%) of the sea trout run on the Dee, and from this study it appears that their numbers have been increasing over the past 12 years. At the same time, while the abundance of ‘older’ (>.0+) sea trout has remained relatively stable, numbers of .1+ maiden ﬁsh (which normally comprise around 41% of this group) seem to have been in decline. Recaptures at Chester Weir indicate that most whitling entering the freshwater Dee spawn in their ﬁrst ‘post-smolt’ winter. These mature ﬁsh tend to be larger and arrive earlier than non-maturing contemporaries, which delay maturation for a further 1 or occasionally 2 years. There is no reliable information from this study on the sex composition of mature and immature whitling. It is also unclear as to whether immature whitling overwinter in fresh water or return to estuarine or coastal waters. However, as few whitling have been recaptured at Chester Weir (at head-of-tide) in the same year they were tagged it seems there is no tendency for ﬁsh to circulate between estuarine and fresh waters. Back-calculation studies (Davidson et al., 2001) suggest that mature whitling appear to be not only larger than immature ﬁsh on their return to fresh water but also show similar size

86

Sea Trout

log10 length increment (mm)

2.70 2.50 2.30

log10 Y = 15.4575 + 0.0090 X r = 0.789; P = 0.002

2.10 1.90

log10 Y = 11.520 + 0.0068 X r = 0.699; P = 0.036

1.70

.0 + fish .1+ fish

1.50 90

92

94

96

98

00

02

Smolt year

Fig. 6.9

Trends in post-smolt growth increment for .0+ and .1+ sea trout, 1990–2002 smolt years.

differences as 2-year-old smolts (although at river age 1 no size differences were apparent). These differences in smolt size are in keeping with observations on MSA – where all ﬁsh returning as whitling (a sample which includes immature as well as mature individuals) were also of consistently greater MSA than sea trout returning aged .1+ (see Fig. 6.4). A decline in MSA among .0+ and .1+ maidens over the period of this study (Fig. 6.4) suggests that ﬁsh may be growing faster to reach the size required to emigrate at a younger smolt age. However, back-calculated ‘smolt’ lengths (taken as the length when the previous river annulus was formed) for .0+ and .1+ ﬁsh of all smolt ages showed no evidence of increasing size over the time series (Fig. 6.8). This is consistent with the lack of a trend in trout growth rates predicted using the model of Elliott et al. (1995) and based on river temperature records for the Dee over the equivalent period (Davidson et al., 2006). It would seem then, that the increase in whitling abundance on the Dee has not arisen as a result of faster ‘pre-smolt’ growth. However, ‘post-smolt’ growth increment has increased signiﬁcantly for both .0+ and .1+ ﬁsh (Fig. 6.9) (where post-smolt growth increment is deﬁned here as the size difference between the back-calculated length at the previous river annulus and length at ﬁrst return for the same group of ﬁsh). This may be indicative of more favourable growth conditions in the marine environment, although the extent to which these two groups of ﬁsh coexist in the sea or estuarine waters is unknown. Improved growth rates in the post-smolt stage may be linked to an increased tendency for ﬁsh to return as whitling, although the size of whitling at return shows no signiﬁcant trend over time. If this were the case it might also explain the decline in .1+ abundance, presumably because a smaller proportion of outgoing smolts return as .1+ ﬁsh. However, the picture is only a partial one as no information is available on the numbers of smolts emigrating to sea each year or their subsequent survival to return as whitling or older maiden ﬁsh. Whatever mechanism is at work, if marine growth rates are increasing then inverse-weight hypotheses, such as that of Doubleday et al. (1979), suggest that survival rates should also

Variation in Age Composition and Growth

87

be improving – with larger ﬁsh being less susceptible to predation and other sources of mortality. This should beneﬁt smolt survival (e.g. Friedland et al. [2000] reported a positive correlation between post-smolt growth increment in 1SW salmon and smolt return rate), but may also improve the survival of post-spawners – assuming that larger individuals are better placed to withstand the rigours of spawning.

References Bagenal, T. (1978). Methods for Assessment of Fish Production in Fresh Waters. IBP Handbook No. 3. Blackwell Scientiﬁc Publications Ltd., Oxford. Chapman, D.G. (1952). Some properties of the hypergeometric distribution with applications to zoological sample censuses. University of California Publication on Statistics, 1, 131–60. Davidson, I.C., Cove, R.J., Milner, N.J. & Purvis, W.K. (1996). Estimation of Atlantic salmon (Salmo salar L.) and sea trout (Salmo trutta L.) run size and angling exploitation on the Welsh Dee using mark-recapture and trap indices. In: Stock Assessment in Inland Fisheries (Cowx, I.G., Ed.). Fishing News Books, Blackwell Science, Oxford, pp. 293–307. Davidson, I.C., Hazlewood, M.S., Cove, R.J. & McIlroy, J.T. (2001). Analysis of growth and survival of sea trout from the Welsh Dee, Atlantic Salmon Trust Project Ref 99/7. Environment Agency Wales (internal report). Davidson, I.C., Hazlewood, M.S. & Cove, R.J. (2006). Predicted growth of juvenile trout and salmon in four rivers in England and Wales based on past and possible future temperature regimes linked to climate change. Proceedings of Biology and Management of Sea Trout Conference, 6–8 July 2004, Cardiff, Wales. Doubleday, W.G., Rivard, D.R., Ritter, J.A. & Vickers, K.U. (1979). Natural mortality rate estimates for North Atlantic salmon in the sea. ICES C.M. 1979/M:26. Elliott, J.M., Hurley, M.A. & Fryer, R.J. (1995). A new, improved growth model for brown trout, Salmo trutta. Functional Ecology, 9, 290–98. Friedland, K.D., Hansen, L.P., Dunkley, D.A. & Maclean, J.C. (2000). Linkage between ocean climate, post-smolt growth and survival of Atlantic salmon (Salmo salar L.) in the North Sea area. ICES Journal of Marine Science, 57, 419–29. Nall, G.H. (1930). The Life of the Sea Trout. Seeley Service, London, 335 pp. Shields, B.A., Bayliss, B.D., Davidson, I.C., Elsmere, P. & Evans, R.E. (2006). Sea trout exploitation from ﬁve rivers in England and Wales. Proceedings of Biology and Management of Sea Trout Conference, 6–8 July 2004, Cardiff, Wales. Solomon, D.J. (1994). Sea trout investigations. Phase 1 – Final Report. R&D Note 318, National Rivers Authority. Went, A.E.J. (1962). Irish sea trout, a review of investigations to date. Scientiﬁc Proceedings of the Royal Dublin Society, 1A (10), 265–96.

Chapter 7

Sea Trout Stock Descriptions in England and Wales G. Harris FishSkill Consultancy and Resource Management Services, Greenacre, Cathedine, Bwlch, Brecon, Powys LD3 7PZ, Wales, UK

Abstract: The results of a major scale-reading investigation to provide baseline descriptions of the structure and composition of adult sea trout stocks from 16 rivers in four geographical regions of England and Wales from 1996 to 1998 are summarised. The sampling programme was synchronised over the same 3-year period with material collected only from rod-caught ﬁsh (15 rivers) or only from trap-caught ﬁsh (1 river). This standardised sampling procedure overcame many of the problems of selective sampling bias associated with previous investigations using samples obtained from multiple sources and provided results that were more directly comparable across rivers and regions. Potential sources of bias in the scale collections from the rod ﬁshery affecting the reliability of the stock descriptions are considered. A comparison of key life-history features (age at smolt migration, age at ﬁrst return as maiden ﬁsh and frequency of spawning) indicated the presence of at least three distinct groups of sea trout in different geographical regions. Some of the principal management applications of the investigation are discussed. Keywords: Salmo trutta L., sea trout, stock structure, smolt age, maiden sea age, spawning frequency, stock groupings.

Introduction and background The dramatic collapse of regional sea trout stocks that occurred in the west of Ireland from the late 1980s (Gargan et al., 2003) and then in some parts of Scotland from the early 1990s (Anon., 2005) raised serious concerns about the risks of a parallel situation developing in England and Wales. This led to the implementation of an R&D programme for sea trout in England and Wales to draw together all the available data on the structure and composition of sea trout stocks and to address any deﬁciencies in the nature and scope of those data so that the early symptoms of any existing or future stock collapse might be recognised and the appropriate management response identiﬁed. The programme had three phases. Phase 1 entailed a major desk-study to draw together all the available information about the biology, ecology, genetics and behaviour of sea trout in England and Wales from relevant sources, both published and unpublished. Much of that information related to disparate scale-reading investigations undertaken opportunistically by various workers between 1925 and 1995 that described the principal life-history characteristics of sea trout stocks in various 88

Stock Descriptions in England and Wales

89

rivers. The ﬁnal reports (Solomon, 1994, 1995) concluded that while past scale-reading studies provided a basic qualitative description of the structure and composition of sea trout stocks for 23 of the 100 or so sea trout rivers in England and Wales, minimal information of variable quality existed for some of those rivers, and for many others none was available. Phase 2 of the programme was commissioned to address deﬁciencies in the quality and scope of the historical database on adult stock structures identiﬁed by Solomon (1994). The overall objective was ‘to design a sampling programme to provide reliable data on sea trout stock characteristics in England and Wales and to allow those stocks to be monitored and managed in a cost-effective way’. The ﬁnal report (Harris, 1995) conﬁrmed many of the reservations expressed by Solomon (1994). It concluded that few of the previous studies were directly comparable, either spatially or temporally, because of (1) the multiplicity of different sampling strategies used to obtain scale samples; (2) the different and unknown extent of any selective sampling bias inherent within each of those methods; (3) the long period often taken to assemble the scale collections; (4) concern about the accuracy of the scale readings in some investigations; (5) doubts about the relevance to the present of stock descriptions obtained many years ago and (6) problems arising from the presentation of results in a variety of non-comparable formats. It was proposed that any future sampling strategy should accommodate four basic requirements: (1) generate robust and reliable information that allowed direct comparisons on a like-for-like basis among different rivers at any time and for any one river at different times; (2) provide as much information as practicable within a ﬁxed cost to describe the range of variability in different stocks; (3) allow the database to be expanded by the inclusion of directly comparable information from other rivers and (4) enable the database for any one river to be interrogated in future years by replication with the same sampling method so that any temporal changes in the stock structure and composition could be identiﬁed. It was accepted that the most complete, accurate and reliable stock descriptions were likely to be obtained with samples collected from ﬁxed trapping stations operating throughout the year on the lower reaches of rivers. However, the lack of such installations, both now and in the future, on all but a very few rivers effectively excludes this as a standard sampling strategy. The use of commercial net ﬁsheries licensed to ﬁsh for salmon and sea trout in tidal waters as a standard means for collecting samples was also discounted because (1) some net ﬁsheries were thought to exploit mixed stocks originating from an unknown number of different rivers; (2) the restricted netting season meant that ﬁsh entering a river before or after the end of the netting season could not be sampled; (3) the restrictions on the size of the mesh used in the construction of the nets would have resulted in the collection of unrepresentative samples biased against ﬁsh of less than 35 cm in length (Evans et al., 1995) and (4) net ﬁsheries operated on relatively few rivers and their number and distribution was likely to decrease even further in subsequent years. Therefore, because the only single source of material available on all rivers at that time, and likely to be so in the future, was from sea trout caught by the recreational rod-and-line ﬁshery, it was recommended that this source of material should form the basis of a future, long-term sampling strategy. This recommendation was accepted and the Phase 3 study was commissioned ‘to implement a sampling programme to provide a baseline of reliable data

90

Sea Trout

on sea trout stock characteristics in England and Wales which will enable sea trout stocks to be managed in a cost-effective way’. This chapter summarises the main ﬁndings of the ﬁnal report of that study (Harris, 2000).

Method and materials The sampling programme was based on material obtained from rod-caught ﬁsh on 15 rivers located in four different geographical regions, where scale samples were collected by volunteers recruited from within the local angling community (Fig. 7.1). The selection

Coquet North-east Esk Wear

Kent

Lune

North west

Ribble Clwyd Dwyfor Dee Wales Teifi

Dyfi

Tywi

Taw

Camel

Teign

South-west Tamar

Fig. 7.1

Names and geographical locations of the 16 rivers studied.

Stock Descriptions in England and Wales

91

of each river took into account an a priori assessment of whether the historical rod catch and the extent of cooperation by the angling community towards the sampling programme on each river were likely to yield the predetermined targets set for a statistically robust sample size of either 250 or 500 ﬁsh (Keating in Harris, 2000). Samples were collected also from ﬁsh captured in the permanent ﬁxed trap operating in the tideway of the River Dee, where there was an ongoing programme of monitoring runs of salmon and sea trout which commenced in 1992. This had established that although the declared annual catch of sea trout by the rod ﬁshery was insufﬁcient to yield an adequate sample of scales, very large numbers of sea trout (4000–10 000) entered the river each year (Davidson et al., 1996). The historically low level of exploitation by the rod and net ﬁsheries on this river, indicated by the declared average annual catches of only 149 for the rod ﬁshery and 75 for the net ﬁshery in the preceding years, suggested that this particular river was of special interest in establishing the composition of a relatively ‘pristine’ stock. Volunteers recruited from within the local angling community on each river by various means (Harris, 2000) undertook the collection of scale samples from rod-caught ﬁsh. Experienced ﬁshery workers engaged in the routine trapping programme undertook the collection of samples from the Dee. Every angler engaged in the scale collection programme was issued with specially designed scale packets for storing samples and recording the details of each individual ﬁsh. Clear instructions were given about the importance of taking a representative sample of their entire catch and, in particular, avoiding any non-random and selective sampling bias for a particular size of ﬁsh. The scale collection sampling programme on all 16 rivers was synchronised over a 3-year period from 1996 to 1998. The standard scale-reading procedures recommended by Shearer (1992) for salmon and by Elliott & Chambers (1996) for sea trout were adopted. For the immediate purposes of this investigation, life-history determination was restricted to the interpretation of (1) parr age in winters at the time of smolt migration; (2) maiden sea age in winters at the time of ﬁrst return to fresh water after smolt migration and (3) the number of spawning marks present.

Terminology The terminology used to describe the life-history characteristics of sea trout follows the International Standard Nomenclature proposed by Allan & Ritter (1997). The term ‘whitling’ refers to those sea trout that return to fresh water in the same year that they migrated to sea as smolts. It is synonymous with the terms ‘ﬁnnock’, ‘herling’ and ‘schoolpeal’ used elsewhere. The term ‘maiden’ refers to any adult ﬁsh that has yet to spawn for the ﬁrst time, and the term ‘previous spawner’ refers to any ﬁsh that has spawned on at least one previous occasion. The conventions used in the scale formulae to denote the smolt age, sea age and spawning history of sea trout are based on those adopted by both Nall (1930) and Went (1962) – with the notable exception of the whitling stage being shown as .0 (instead of .+) on the logic that everything after the decimal point denoting smolt migration in the formula should record the number of complete winters spent in the sea after smolt migration.

92

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Results Scale collections The number of scales collected from each river over the 3-year period is given in Table 7.1. This also shows the declared rod catch for each year of the study and the a priori target set for obtaining a statistically robust sample for each river. The number of samples obtained for each river exceeded or approximated the set target on all rivers with the notable exception of the Lune. However, the Lune data have been included here because they do not differ markedly from the results of other recent scale-reading investigations on this river given by Solomon (1995) and Harris (2000). Age at smolt migration A total of 6578 sets of readable scales were obtained, which showed the number of years each ﬁsh had spent in the rivers as a juvenile parr before migrating to the sea as a smolt. The results are given in Table 7.2. It should be noted that these data may not reﬂect the actual age composition of the smolt run at the time of migration because of differential rates of survival of different smolt age groups in the sea (Solomon, 1995). Table 7.1 The numbers of ﬁsh sampled in each river (1996–98) in relation to declared catches and target sample size. Region and river

Declared catch for rods and actual catch in the Dee trap each year

Total no. of ﬁsh in sample (1996–98)

Sample no. as percentage of catch (1996–98)

Target sample size (no. of ﬁsh)

Sample no. as percentage of set target

1996

1997

1998

Total

North-east England Wear 781 Coquet 417

914 423

1064 909

2759 1749

235 249

8.5 14.2

250 250

94.0 99.6

North-west England Border Esk 1357 Kent 450 Lune 1601 Ribble 686

1135 299 1701 952

1671 576 2730 1635

4163 1325 6032 3273

726 226 251 348

17.3 17.1 4.2 10.6

250 250 500 250

290.4 90.4 50.2 139.2

(1210) (1622) (1748) (4580) 528 717 1730 2975 775 389 1787 2951 752 1316 1337 3405 2325 2146 3605 8076 1838 2462 4539 8893

1579 176 235 658 314 570

34.4 5.9 8.0 19.3 3.9 6.4

500 250 250 500 250 500

315.6 70.4 94.0 131.6 125.6 114.0

206 659 411 612

10.3 26.0 33.0 26.9

250 250 250 250

82.4 259.6 164.4 248.8

Wales Dee – trapa Clwyd Dwyfor Dyﬁ Teiﬁ Tywi

South-west England Taw 510 Camel 818 Tamar 428 Teign 553

613 1077 344 705

869 658 475 1017

1992 2533 1247 2275

a Catches (in parenthesis) represent the actual number of ﬁsh trapped.

Stock Descriptions in England and Wales Table 7.2

Smolt age: the number and proportion of ﬁsh in each smolt age group.

Region and river

Age at smolt migration (winters) S1 No.

S2 %

No.

S3 %

No.

93

No. of ﬁsh sampled

S4 %

No.

%

North-east England Wear 6 Coquet 3

2.6 1.3

207 200

89.6 84.0

18 35

7.8 14.7

0 0

NR NR

231 238

North-west England Border Esk 20 Kent 5 Lune 2 Ribble 1

3.9 2.6 0.9 0.3

445 169 206 283

87.5 86.2 88.4 90.4

44 22 24 29

8.6 11.2 10.3 9.3

0 0 1 0

NR NR 0.4 NR

509 196 233 313

Wales Dee Clwyd Dwyfor Dyﬁ Teiﬁ Tywi

35 7 7 37 3 24

2.6 4.2 3.2 6.1 1.0 4.6

1191 152 201 542 267 452

88.8 91.0 92.6 88.5 92.1 85.5

112 8 9 33 20 51

8.4 4.8 4.1 5.4 6.9 9.7

2 0 0 0 0 1

0.2 NR NR NR NR 0.2

1340 167 217 612 290 528

South-west England Taw 3 Camel 6 Tamar 0 Teign 0

1.6 1.0 NR NR

176 418 283 347

92.2 71.6 75.9 62.4

12 156 88 209

6.2 26.7 23.6 37.6

0 4 2 0

NR 0.7 0.5 NR

191 584 373 556

NR = not relevant.

All smolts migrated after 1–4 years in the river, but there were variable proportions of ﬁsh in each smolt age group from different rivers. Between 93.9% and 100.0% of all ﬁsh became smolts at 2 and 3 years of age, with 2 year-old smolts forming the dominant age group in all rivers, at 62.4–92.6% of all age groups. Although they were present in all rivers, the proportion of 3-year-old smolts was relatively small, at 4.1–14.7%, in Wales and north-east England, but they represented between 23.6% and 37.6% of the sample in three of the four rivers in south-west England. Smolts aged 4 years were not found in 11 rivers and represented less than 0.7% of the sample in the other 5 rivers. Smolts aged 1 year were absent from two rivers and generally scarce in all other rivers, where they did not exceed more than 6.1% of the sample. Sea-age structure Adult sea trout stocks contain two distinct groups of ﬁsh based on their spawning history: (1) those ﬁsh that return to the river to spawn for the ﬁrst time (maidens) and (2) those other ﬁsh that have spawned on at least one previous occasion (previous spawners). A total of 7113 sets of scales were readable for both maiden sea age and previous spawning history (Table 7.3). Although the proportion of previous spawners exceeded that of

94

Sea Trout Table 7.3 Sea age: the number and proportion of maiden and previously spawned sea trout. Region and river

Sea-age group All maiden ﬁsh

All spawned ﬁsh

No.

No.

%

Total no. of ﬁsh

%

North-east England Wear 184 Coquet 209

79.3 83.9

48 40

20.7 16.1

232 249

North-west England Border Esk 385 Kent 125 Lune 148 Ribble 164

72.6 59.0 60.7 48.5

145 87 96 174

27.4 41.0 39.3 51.5

530 212 244 338

Wales Dee Clwyd Dwyfor Dyﬁ Teiﬁ Tywi

1066 135 214 485 250 410

67.5 80.4 93.5 75.1 82.0 73.6

513 33 15 161 55 147

32.5 19.6 6.5 24.9 18.0 26.4

1579 168 229 646 305 557

South-west England Taw 136 Camel 519 Tamar 242 Teign 367

68.3 82.7 61.0 60.7

63 109 155 238

31.7 17.3 39.0 39.3

199 628 397 605

maiden ﬁsh in the Ribble (51.5%), maiden ﬁsh were dominant on all other 15 rivers (range 59.0–93.5%). These data are broken down into separate maiden and previously spawned stock components for further analysis in the following sections. Maiden sea age Table 7.4 shows the number and proportion of ﬁsh in each maiden sea-age group. Although .3 sea-winter (SW) maidens were not recorded from any river, .2SW maidens, while scarce, were identiﬁed in nine rivers, where they represented 0.3–6.0% of the samples. At least 94% of all maiden ﬁsh in any single river were from the .0SW and .1SW sea-age groups. The .1SW maiden ﬁsh were the single most dominant group in seven rivers (range = 60.0–94.7%), while .0SW maidens were the most dominant maiden group in the other nine rivers. The relative importance of these two maiden groups varied across regions. The .1SW maiden group was dominant in the north-east and north-west of England but, with the single exception of the Dyﬁ, the .0SW maiden group dominated the samples from Wales and south-west England. The scarcity of .0SW ﬁsh in the two rivers of north-east England, at only 1.1% and 2.9% of the sample, is notable.

Stock Descriptions in England and Wales

95

Table 7.4 Maiden ﬁsh: the number and proportions of sea trout in each maiden sea-age group. Region and river

Maiden sea-age group (sea winters) .0SW No.

.1SW

%

Total no. of ﬁsh

.2SW

No.

%

No.

%

North-east England Wear 2 Coquet 6

1.1 2.9

171 198

92.9 94.7

11 5

6.0 2.4

184 209

North-west England Border Esk 152 Kent 48 Lune 49 Ribble 33

39.5 38.4 33.1 20.4

231 77 98 129

60.0 61.6 66.2 79.6

2 0 1 0

0.5 NR 0.7 NR

385 125 148 162

Wales Dee Clwyd Dwyfor Dyﬁ Teiﬁ Tywi

751 111 184 124 227 233

70.4 82.2 86.0 25.6 90.8 56.8

293 24 30 355 23 168

27.5 17.8 14.0 73.2 9.2 41.0

22 0 0 6 0 9

2.1 NR NR 1.2 NR 2.2

1066 135 214 485 250 410

South-west England Taw 83 Camel 464 Tamar 190 Teign 219

61.0 89.4 78.5 59.7

52 55 52 147

38.2 10.6 21.5 40.1

1 0 0 1

0.8 NR NR 0.3

136 519 242 367

NR = not relevant.

Spawning frequency Table 7.5 shows the number of spawning marks identiﬁed on the scales of 2076 ﬁsh that had spawned at least once. The maximum number of such marks recorded was x6, but this was limited to a single ﬁsh in each of the two rivers. The proportions of ﬁsh that had spawned on one or more previous occasions varied within wide limits for the individual rivers; the maximum number of spawning marks was x2 for 16 rivers, x3 for 15 rivers, x4 for 14 rivers, x5 for 8 rivers and x6 for 2 rivers. There were only four rivers where the proportion of ﬁsh that had spawned on at least two previous occasions exceeded 15% and only four rivers where more than 5% of the sample had spawned on at least three previous occasions. Adult life tables The data in Tables 7.3–7.5 were reproduced in a standard, readily comparable format as a series of adult life tables showing the structure and composition of each adult stock. These were supplemented with information on the length of each sea-age category (Harris, 2000). Table 7.6 for the River Dee and Table 7.7 for the River Dwyfor illustrate the extremes of variability encountered across the stock structures for the 16 rivers.

96

Sea Trout

Table 7.5 Spawning frequency: the number and proportion of sea trout in each group of previously spawned ﬁsh. Region and river

Number of spawning marks detected on scales x1 No.

x2 %

North-east England Wear 42 87.5 Coquet 34 85.0

No.

x3 %

No.

x4 %

x5

Total no. of ﬁsh

x6

No.

%

No.

%

No.

%

6 4

12.5 10.0

0 1

NR 2.5

0 1

NR 2.5

0 0

NR NR

0 0

NR NR

48 40

North-west England Border Esk 105 Kent 56 Lune 56 Ribble 112

72.4 64.3 60.4 65.5

32 16 19 37

22.1 18.4 19.8 21.6

6 8 12 18

4.1 9.2 12.5 10.5

1 5 8 3

0.7 5.8 8.3 1.8

0 2 1 0

NR 2.3 1.0 NR

1 0 0 1

0.7 NR NR 0.6

145 87 96 171

Wales Dee Clwyd Dwyfor Dyﬁ Teiﬁ Tywi

71.9 84.9 86.6 69.1 57.8 59.8

99 1 1 8 42 38

19.3 3.0 6.7 14.6 26.1 25.9

27 3 1 7 13 11

5.3 9.1 6.7 12.7 8.1 7.5

11 1 0 2 12 9

2.1 3.0 NR 3.6 7.5 6.1

7 0 0 0 1 1

1.4 NR NR NR 0.6 0.7

0 0 0 0 0 0

NR NR NR NR NR NR

513 33 15 55 161 147

60.3 74.3 79.4 71.0

15 18 23 48

23.8 16.5 14.8 20.2

6 5 7 12

9.5 4.6 4.5 5.0

3 3 2 7

4.8 2.8 1.3 2.9

1 2 0 2

1.6 1.8 0 0.8

0 0 0 0

NR NR NR NR

63 109 155 238

369 28 13 38 93 88

South-west England Taw 38 Camel 81 Tamar 123 Teign 169 NR = not relevant.

Key features summary The data in Tables 7.2, 7.4 and 7.5 were used to generate a series of arithmetic values representing key features of the age structure and spawning history of each stock, namely: • • • • •

mean smolt age (MSA) – the average number of years that juveniles spent in fresh water before migrating to sea as smolts; mean maiden age (MMA) – the average number of years (winters) that the post-smolts spent in the sea before returning to the river to spawn for the ﬁrst time as maiden ﬁsh; mean spawning frequency (MSF) – the average number of spawning marks detected on the scales of individual ﬁsh that had spawned on at least one previous occasion; mean adult age (MAA) – the average number of years (winters) as an adult after migrating to sea as a smolt (=MMA + MSF); mean total age (MTA) – the average age from birth to capture (=MSA + MAA).

The results of this summary are shown in Table 7.8.

Table 7.6

Adult life table: the structure and composition of the adult sea trout stock of the River Dee (sample size = 1579 ﬁsh). Maiden sea-age group

Sea age (SW)

First return as .0SW maidens Frequency

.0+ .1Sm+ .2Sm+ .3Sm+ .4Sm+ .5Sm+ .6Sm+ .7Sm+ NR = not relevant.

Sea age (SW)

Length (mm)

No.

%

Mean

Range

751 246 66 22 9 4

47.56 15.58 4.18 1.39 0.57 0.25

322 422 506 589 632 634

225–437 322–700 322–700 421–655 498–742 609–717

First return as .1SW maidens Frequency

.1+ .1 + 1Sm+ .1 + 2Sm+ .1 + 3Sm+ .1 + 4Sm+ .1 + 5Sm+ .1 + 6Sm+

Sea age (SW)

Length (mm)

No.

%

Mean

Range

293 113 29 5 1 2

18.56 7.16 1.84 0.32 0.06 0.13

468 539 614 673 677 722

323–624 422–671 489–777 623–734 NR 674–769

First return as .2SW maidens Frequency

.2+ .2 + 1Sm+ .2 + 2Sm+ .2 + 3Sm+ .2 + 4Sm+ .2 + 5Sm+

Length (mm)

No.

%

Mean

Range

22 10 4

1.39 0.63 0.25

619 671 687

533–768 623–761 642–757

1 1

0.06 0.06

825 793

NR NR

Table 7.7

Adult life table: the structure and composition of the adult sea trout stock in the Afon Dwyfor (sample size = 226 ﬁsh). Maiden sea-age category

Sea age (SW)

First return as .0SW maidens Frequency

.0+ .1Sm+ .2Sm+ .3Sm+ .4Sm+ .5Sm+ .6Sm+ .7Sm+ NR = not relevant.

Sea age (SW)

Length (mm)

No.

%

Mean

Range

184 9

81.42 3.98

329 451

267–483 368–660

First return as .1SW maidens Frequency

.1+ .1 + 1Sm+ .1 + 2Sm+ .1 + 3Sm+ .1 + 4Sm+ .1 + 5Sm+ .1 + 6sm+

Sea age (SW)

Length (mm)

No.

%

Mean

Range

30 4 1 1

13.3 1.8 0.4 0.4

490 584 775 768

343–584 559–635 NR NR

.2+ .2 + 1Sm+ .2 + 2Sm+ .2 + 3Sm+ .2 + 4Sm+ .2 + 5Sm+

First return as .2SW maidens Frequency

Length (mm)

No.

Mean

%

Range

Table 7.8 Key features summary: mean (±SD) of smolt age, maiden sea age, spawning frequency, total sea age and total age for each of the 16 stocks along with their derived length–weight relationship. Region and river

MSA Mean

MMA

MSY

MAA

MTA

Length–weight relationship SE

Slope

SE

R2

−8.91 −9.04

0.45 0.52

2.61 2.62

0.07 0.08

0.84 0.82

0.85 0.99 1.18 0.89

−8.87 −9.22 −10.12 −9.14

0.41 0.22 0.26 0.31

2.59 2.64 2.79 2.63

0.07 0.04 0.04 0.05

0.87 0.88 0.93 0.89

2.80 2.46 2.23 3.16 2.43 2.95

1.00 0.83 0.56 1.00 0.85 1.05

−10.50 −9.30 −8.95 −9.82 −9.16 −8.52

0.07 0.52 0.30 0.18 0.28 0.21

2.87 2.66 2.60 2.74 2.63 2.55

0.01 0.09 0.05 0.03 0.05 0.03

0.97 0.86 0.92 0.94 0.91 0.91

2.94 2.60 2.89 3.21

1.06 0.81 0.88 1.00

−10.35 −9.49 −9.94 −8.86

0.27 0.27 0.25 0.24

2.83 2.67 2.75 2.58

0.04 0.05 0.04 0.04

0.96 0.84 0.91 0.87

SD

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Intercept

North-east England Wear 2.13 Coquet 2.05

0.38 0.32

0.98 1.03

0.26 0.26

0.20 0.23

0.51 0.48

1.18 1.26

0.54 0.51

3.31 3.31

0.63 0.59

North-west England Border Esk 2.05 Kent 2.09 Lune 2.10 Ribble 2.09

0.35 0.36 0.34 0.30

0.57 0.52 0.65 0.58

0.51 0.51 0.53 0.51

0.37 0.67 0.68 0.77

0.71 1.04 1.07 0.96

0.94 1.19 1.33 1.35

0.84 1.08 1.20 0.94

2.98 3.21 3.41 3.38

Wales Dee Clwyd Dwyfor Dyﬁ Teiﬁ Tywi

2.06 2.01 2.01 1.99 2.06 2.05

0.33 0.30 0.27 0.34 0.28 0.38

0.33 0.20 0.16 0.75 0.12 0.48

0.52 0.40 0.36 0.46 0.34 0.53

0.46 0.26 0.08 0.42 0.27 0.43

0.81 0.62 0.33 0.87 0.68 0.85

0.79 0.46 0.24 1.17 0.40 0.91

0.97 0.80 0.54 0.99 0.83 1.05

South-west England Taw 2.05 Camel 2.27 Tamar 2.25 Teign 2.38

0.28 0.48 0.44 0.48

0.43 0.10 0.17 0.31

0.52 0.30 0.37 0.47

0.52 0.25 0.50 0.56

0.93 0.64 0.73 0.85

0.94 0.34 0.66 0.87

1.10 0.70 0.78 0.88

100

Sea Trout

North-east – Coquet North-east – Wear North-west – Ribble North-west – Kent North-west – Lune South-west – Taw Wales – Tywi North-west – Esk Wales – Dee Wales – Dwyfor Wales – Clwyd Wales – Teifi Wales – Dyfi South-west – Tamar South-west – Camel South-west – Teign

Fig. 7.2

Key feature analysis: dendrogram of stock groupings.

Stock groupings A cluster analysis was undertaken to compare the features in Table 7.8 that were thought most likely to indicate the biological similarities and differences between individual stocks that were least likely to be affected by local factors and were independent of any other feature. The three stock features used were (1) mean smolt age; (2) mean maiden sea age and (3) mean number of spawning years. The results of this analysis are shown in Fig. 7.2.

Discussion Stock descriptions for the 16 rivers were obtained using a single source of material for each river for the same time frame; scale readings were obtained by the same worker and the results were produced in a standard format. These constants have reduced many of the uncertainties associated with earlier studies (Solomon, 1994; Harris, 1995) and provided results that are now more directly comparable on a like-for-like basis than was possible hitherto. However, three criteria must be met before each stock description can be accepted as an accurate representation of the qualitative structure and quantitative composition of the total run of sea trout into any river: (1) the entire run of ﬁsh entering the river over any year should be sampled; (2) each stock component should be equally vulnerable to capture and (3) the ﬁsh caught should be sampled at random with no selective bias in terms of ﬁsh size. The large sample obtained from the ﬁxed trapping station in the tideway of the River Dee fulﬁlled the ﬁrst two criteria and, as far as was practicable and cost effective, broadly achieved the third. All stock components were equally liable to capture. The trapping programme operated regularly throughout each month of the year. All ﬁsh entering the trap were sampled at random by experienced ﬁshery workers. The spacing between the screens

Stock Descriptions in England and Wales

101

in the trap was such that 90% of all ﬁsh less than 35 cm in length were retained for sampling (Davidson, pers. comm.). It is therefore probable that the stock description for the River Dee is unprecedented (at least for England and Wales) in providing an accurate representation of both the qualitative structure and the quantitative composition of the annual runs of sea trout into that river. The accuracy of the stock descriptions for the 15 rivers where anglers collected the samples from their individual catches is less certain and more variable because of the extent to which these criteria might have been fulﬁlled on individual rivers. An important source of sampling bias will occur if different age groups of ﬁsh enter the river in signiﬁcant numbers after the end of the angling season. Such ﬁsh would be unavailable for capture and sampling by the rod ﬁshery. In broad terms, the timing and duration of the angling season coincide with the pattern of the adult runs into most rivers in north-west England, Wales and south-west England. However, there is evidence that a signiﬁcant proportion of sea trout entering the rivers of north-east England do so in November and December, after the end of the rod-ﬁshing season in late October (Champion in Le Cren, 1985; Solomon, 1995). As no sample could be obtained from these late-running ﬁsh in the Coquet and Wear, the qualitative stock descriptions for these two rivers should be viewed with some initial caution. Solomon (1995) drew attention to the general lack of good data on the rates of exploitation on sea trout stocks by rod-and-line ﬁshing. Shields et al. (2006) have since provided detailed information on total exploitation rates for ﬁve rivers and differential exploitation rates on sea trout above or below 1 lb weight (0.45 kg) from two rivers over a 14-year period. However, the results are contradictory in that the average rod exploitation rate (all methods) was positively correlated with stock size on two rivers but negatively correlated in two other rivers, and the level of exploitation was higher on smaller ﬁsh in one river but higher for larger ﬁsh in another river. Notwithstanding this confusion, there is as yet no published information to judge (1) whether some stock components are more vulnerable to capture by angling than others; (2) whether the different methods of angling (i.e. ﬂy, spin and bait ﬁshing) selectively exploit different stock components and (3) whether the often very different ﬁshing rules and regulations determining where, when and how anglers may ﬁsh within different sections of a river affect the relative rates of exploitation on different stock components for that river. It is therefore impossible to state whether samples obtained from the rod-and-line ﬁshery on any river provide a representative sample of the structure and composition of the actual stock that is potentially available for capture during the ﬁshing season. The adoption of catch-and-release by the angling community as a voluntary code of conduct to conserve adequate spawning stocks has increased in recent years to the point where some 50% of all sea trout taken by rod-and-line ﬁshing in England and Wales are now returned alive to the water immediately after capture (Anon., 2003). All anglers engaged in the sampling programme were clearly asked to ensure that their total catch on any occasion was sampled at random and that any selective sampling bias in terms of ﬁsh size was avoided. A signiﬁcant number of anglers expressed reluctance to increase the risks to survival of released ﬁsh because of the damage and increased handling stress caused by taking scale

102

Sea Trout

samples and measuring length and/or weight. As those sea trout most likely to be released are the smallest ﬁsh caught, it is likely that the relative proportions of .0SW whitling are underestimated in the stock descriptions. Notwithstanding these caveats, the stock descriptions obtained from the 15 rivers sampled by the rod ﬁshery represent a considerable improvement in the reliability and accuracy of the database when compared with previous studies. They now provide a reasonably robust characterisation of each stock in qualitative terms and a pragmatic description of each stock in semi-qualitative terms. It will be essential to monitor any changes in the ﬁshery rules and regulations on each river during the period between any initial survey and its replication on the same river in subsequent years to allow valid judgements about the signiﬁcance of any temporal changes in the stock structure. The analysis of key features (Table 7.8) for each of the 16 stocks suggests the occurrence of at least three different geographical groups of sea trout in England and Wales based on similarities and differences in their smolt age, maiden sea age and spawning frequency (Fig. 7.2). These groups cover (1) north-east England; (2) southwest England and (3) Wales and north-west England. This broadly corresponds to a more subjective and wider appraisal based on earlier scale-reading studies and supplementary information on run-timing and growth rates that indicated that the sea trout stocks of England and Wales could be grouped into four categories based on combinations of their longevity (long versus short living) and feeding conditions in the sea (fast versus slow growing). The sea trout of the rivers in north-east England (including the adjacent River Tweed) are characterised by stocks that contain very few .0SW whitling, which rarely survive to spawn more than once, run later in the year than most other stocks and attain a very large size by rapid growth in the sea over their short lifespan. In contrast, the sea trout stocks of south-west England are characterised by stocks that live longer, contain higher proportions of older smolts, .0SW whitling and ﬁsh that have spawned more than once, and grow more slowly in the sea and enter fresh water earlier in the season. The large and amorphous group of sea trout in Wales and north-west England was generally intermediate between these two extremes and exhibited a wide range of variability in most key features. They generally lived longer and spawned more frequently than the ﬁsh of north-east England and attained a large size because of their greater longevity and reasonably rapid rate of growth in the sea between multiple spawning visits to fresh water. The somewhat curious position of the Taw (south-west England) and Border Esk (northwest England) within the third group of 11 rivers around the west coast of England and Wales requires comment. While the Taw is grouped alongside the Tywi (South Wales), both rivers ﬂow into the Bristol Channel and have estuaries that are geographically close. The characteristics of sea trout in rivers along the north coast of Devon are different from those of sea trout in south Devon and Cornwall. The close similarity of the Taw ﬁsh to the longer living and faster growing sea trout of the Tywi may reﬂect similar marine feeding conditions and a common ancestry. The grouping of the Border Esk as remote from its neighbouring rivers in north-west England may be a reﬂection of the intensive commercial ﬁshing pressure

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in its near-coastal waters and estuary. The Border Esk exhibited an appreciably lower mean spawning age than the three other rivers in the north-west of England (Table 7.8), and this may reﬂect the impact of the commercial ﬁshery on the survival and abundance of repeat spawning ﬁsh. Nall (1930) was sceptical about attempts by Day (1887) and Regan (1911) to describe different ‘races’ of sea trout in the British Isles solely on differences in their morphology, anatomy and assumed migratory behaviour. However, he was able to draw on greater knowledge about their life history from his pioneer scale-reading investigations to propose that British sea trout were divided into two different forms (designated ‘east-coast’ and ‘west-coast’ types) based on their stock characteristics. While this study broadly conﬁrms that view, it is probable that this broad classiﬁcation contains further additional groups and sub-groups. In the absence of parallel information about the genetic composition of individual stocks of sea trout in England and Wales, it is not known whether the observed variability in the structure of individual stocks has an underlying genetic basis or whether the observed differences among regional stocks are merely a manifestation of differences in their local marine and freshwater environments and in how and how much they are exploited in those environments. However, recent signiﬁcant advances in our understanding of the genetic composition of different populations of migratory and non-migratory forms of Salmo trutta (Laikre, 1999; Fergusson, 2006) suggest that at least some of the differences may have a genetic basis linked to the postglacial recolonisation of the rivers in England and Wales by sea trout from one or more of four different evolutionary lineages (Fergusson, 2006). Most of the investigations to date on stock structure have concentrated on the larger, more important and valuable ﬁsheries. Further studies are now required to extend the scope of the database to cover the rivers of southern England and include adequate representation from the many other minor rivers that collectively represent a signiﬁcant part of the total resource in England and Wales. The stocks of many of these minor rivers frequently appear to have sea trout that differ in such features as run timing from the stocks of the larger rivers in their vicinity, which may reﬂect an important expression of the overall diversity exhibited within the sea trout/brown trout complex. The sea trout of southern England are of particular interest as they may form a further distinct group with characteristics that are different from or intermediate between those stocks in north-east and south-west England. Rivers from this region were excluded from this investigation because it was thought that an adequately sized sample could not be obtained from the rod-and-line ﬁsheries. Stock descriptions of the type produced by this study have a range of different management applications. First, they provide a snapshot of the structure and composition of each stock at a particular point in time. Their main value, and the original reason for this work, is that they provide a permanent historical baseline of information against which the results of replicate studies can be compared to determine the nature and extent of any changes that might have occurred over the intervening period. There are various reasons why managers might wish

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to interrogate stock descriptions of this type: • • • •

•

to undertake a periodic health check as part of a routine monitoring programme to determine the status and well-being of any stock; to determine the impact of changes in management practice intended to maintain, improve and develop a ﬁshery; to investigate the substance of any allegations about a decline in the quality of a ﬁshery by anglers, netsmen or ﬁshery owners; to monitor the impact of any developments within a catchment that may be detrimental to the quality of the ﬁshery (e.g. estuary barrage construction, hydropower generation, water abstraction schemes); to study long-term trends in stock composition and abundance.

The ability of any stock of sea trout to withstand and recover rapidly from any adverse factors in its freshwater and marine environments will depend initially upon its basic structure and composition. Sea trout stock structures are generally more robust and resilient than those of salmon because sea trout exhibit a less risk-averse life history in the pattern of divided smolt migration to the sea (1–4-year classes) and adult return to the river to spawn for the ﬁrst time (1–3-year classes) and because of the often high incidence of repeat spawning in some stocks. The potential signiﬁcance of this extended pattern of divided smolt migration and adult return is that it reduces the impact on future stock abundance of deleterious factors affecting survival in any 1 year because a proportion of the stock would remain unaffected, either in fresh water or in the sea, to cushion the stock from collapse and expedite its rate of recovery. Consequently, a stock that was ‘robust’ would contain a wide spread of smolt and maiden age groups and a large number of categories of previously spawned ﬁsh so that no single smolt age group or maiden sea-age group or year class was totally dominant. In contrast, a ‘fragile’ stock would be one where all ﬁsh from any spawning year class migrated to sea at the same smolt age, returned to spawn for the ﬁrst time at the same sea age and rarely survived to spawn more than once. The River Dee is an example of a robust stock (Table 7.6). It contains 17 different categories of ﬁsh derived from all three maiden sea-age groups. There are ﬁve different categories of previous spawners in each maiden group, of which 41% had spawned at least once and 6.6% at least twice. The maiden whitling component represented 47.5% of the sample. By contrast, the River Dwyfor is an example of a fragile stock (Table 7.7). It contains only six categories of ﬁsh derived from two maiden sea-age groups. There are only three categories of previous spawners, of which 6.6% had spawned at least once and 0.8% at least twice, .1SW maiden ﬁsh were uncommon and the entire stock was dominated by .0SW maiden ﬁsh, at 81.4% of the sample. The robust structure of the sea trout stock in the River Dee may reﬂect the low rates of exploitation recorded from the declared catches by the rod and net ﬁsheries and, therefore, the greater rate of escapement of .1SW and .2SW maidens and previous spawners. The fragile nature of the sea trout stock structure on the Dwyfor is a cause for concern. The fact

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that this stock is almost exclusively dominated by 2-year-old smolts (Table 7.3) derived from a single spawning year class that migrated to sea and returned to fresh water to spawn in the same year, rarely surviving to spawn a second time, suggests that it is seriously at risk of collapse if adverse conditions affecting the survival of the smolts or .0SW whitling occur at any stage over this short life history. Until quite recently the traditional management of sea trout stocks in England and Wales was very largely driven by the numbers of ﬁsh caught in the recreational and commercial ﬁsheries. However, much greater importance is now attached to managing both the quantitative and qualitative features of individual stocks (Harris, 2006). In practice, the broad aims of sea trout management should be to increase its strength in both ‘breadth’ (deﬁned as the number of maiden sea trout groups represented) and ‘depth’ (deﬁned as the number of categories of previous spawners within each maiden sea-age group). However, the wide variability in stock composition revealed by the 16 rivers in this study and the possibility that they represent at least three heterogeneous groups that may be genetically distinct suggests that a ‘one-size-ﬁts-all’ approach to the management of the resource in England and Wales is inappropriate.

References Allan, I.R.H. & Ritter, J.A. (1977). Salmon terminology. Part 2. A terminology list for migratory trout (Salmo trutta L.). Journal du Conseil International pour l’Exploration de la Mer, 37(3), 293–99. Anon. (2003). Salmonid and Freshwater Fisheries Statistics for England & Wales, 2002. Environment Agency, Bristol, 28 pp. Anon. (2005). Statistical Bulletin. Scottish Salmon and Sea Trout Catches, 2004. Fisheries Series No. Fish/2005/1. Davidson, I., Cove, R.J. & Milner, N.J. (1996). Dee Stock Assessment Programme. Annual Report 1994, Environment Agency, Bristol, 51 pp. Day, F. (1887). British and Irish Salmonidae. Williams & Norgate, London, 298 pp. Elliot, J.M. & Chambers, S. (1996). A Guide to the Interpretation of Sea Trout Scales. National Rivers Authority, Bristol, ISBN 1 873160 29 1. R&D report 22, 54 pp. Evans, D.M., Mee, D.M. & Clarke, D.R.K. (1995). Mesh selection in sea trout, Salmo trutta L., Seine net ﬁshery. Fisheries Management and Ecology, 2, 103–11. Gargan, P.G., Tully, O. & Poole, W.R. (2003). The relationship between sea lice infestations, sea lice production and sea trout survival in Ireland, 1992–2001. In: Salmon at the Edge (Mills, D., Ed.). Proceedings of the Sixth International Atlantic Salmon Symposium, Edinburgh, UK. Blackwell Science Ltd, Oxford, pp. 119–35. Harris, G.S. (1995). The Design of a Sea Trout Stock Description Sampling Programme. National Rivers Authority, Bristol, R&D Project 559, R&D Note 418, 94 pp. Harris, G.S. (2000). Sea Trout Stock Descriptions: The Structure and Composition of Sea Trout Stocks from 16 Rivers in England & Wales. Environment Agency, Bristol. R&D Technical Report W224, 93 pp. Harris, G.S. (2006). A review of the statutory regulations to conserve sea trout stocks in England & Wales. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales. Blackwell Publishing, Oxford, pp. 441–56. Laikre, I. (Ed.) (1999). Conservation and Genetic Management of Brown Trout (Salmo trutta) in Europe. Report of the concerted action on identiﬁcation, management and exploitation of genetic resources in the brown trout (Salmo trutta). (‘Trout Concert’: Eu Fair CT97-3882), 91 pp. Le Cren, E.D. (1985). The Biology of Sea Trout. Synopsis of a Symposium at Plas Menai, 24–26th October 1984. Atlantic Salmon Trust Ltd, Pitlochry, 42 pp. Nall, G.H. (1930). The Life of the Sea Trout: Especially in Scottish Waters. Seeley, Service & Co., London, 335 pp. Regan, C.T. (1911). The Freshwater Fisheries of the British Isles. Methuen & Co., London, 267 pp. Shearer, W.M. (1992). Atlantic Salmon Scale Reading Guidelines. International Council for the Exploration of the Sea, Cooperative Research Report No. 188, 446 pp.

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Shield, B.A., Aprahamian, M.W., Bayliss, B.D., Davidson, I., Elsmere, P. & Evans, R. (2006). Sea trout (Salmo trutta L.) exploitation of ﬁve rivers in England and Wales. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 417–33. Solomon, D.J. (1994). Sea trout investigations – phase 1. National Rivers Authority, R&D Note 318, Bristol, 104 + 62 pp. Solomon, D.J. (1995). Sea trout stocks in England and Wales. National Rivers Authority, R&D Report 25, Bristol, 102 pp. Went, A.E.J. (1962). Irish sea trout: a review of the investigations to date. Scientiﬁc Proceedings of the Royal Dublin Society, Series A, 10, 265–96.

Chapter 8

The Rod and Net Sea Trout Fisheries of England and Wales R. Evans and V. Greest National Fisheries Technical Team, Environment Agency, Cambria House, Cardiff, CF24 0TP, UK

Abstract: Socially and economically important sea trout (Salmo trutta L.) ﬁsheries exist in many of the rivers and coastal areas of England and Wales. Licensed ﬁshermen are required by law to submit a full and accurate catch return to the Environment Agency at the end of each ﬁshing season. Declared annual rod and net catches during the period investigated (1978–2003) ranged from 14 742 to 55 863 and 27 159 to 97 206, respectively. Since 1978, rod catches have increased signiﬁcantly (P < 0.05) on 29% of the main river ﬁsheries (n = 67), whilst signiﬁcant declines have been recorded on 21% of these rivers. Catch and release rates in the rod ﬁsheries have increased from 35% in 1994 to 55% in 2003. Differences in mean monthly catches and weights recorded in the ﬁsheries highlight the differences in sea trout life-history strategies around the country. Rod and net catches were not synchronous between 1978 and 2003 (F = 1.82; P = 0.074). Rod catches generally remained stable throughout the period investigated. In contrast, net catches have declined steadily since the late 1970s, partly in response to a decrease in the number of net licences issued (from 1060 in 1978 to 417 in 2003). Keywords: Sea trout, rod ﬁshing, net ﬁshing, catches, trends, regional variation, conservation limits.

Introduction The rivers and coastal areas of England and Wales support many sea trout (Salmo trutta L.) rod and net ﬁsheries, for which the Environment Agency collects data on the rod and net catches via licence holders’ catch returns (Environment Agency, 2003). In 2003, there were 110 rod and 38 net ﬁsheries for which sea trout catches were reported, although numbers were small in the Thames rod ﬁshery (<10/year), and net ﬁsheries in the Anglian region catch ﬁsh destined mainly for the rivers of north-east England and south-east Scotland (Russell et al., 1995). The largest net ﬁsheries are found along the north-east coast of England, where drift and T&J nets operate between March and August (2003 catch = 21 771 ﬁsh). Some net ﬁsheries, for example, the coracle ﬁsheries in Wales, have been in existence for hundreds of years and have local heritage value. The main rod ﬁsheries are found in Wales and the north and south-west of England. Fishing may commence as early as March in some areas and generally ceases in September or October (sea trout may begin spawning at the end of October). The Rivers Towy and Teiﬁ in south-west Wales and the River Dyﬁ in North Wales routinely record annual rod catches of several thousand ﬁsh, as does the River Lune in the north-west and the Rivers Wear 107

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and Tyne in the north-east. These ﬁsheries have signiﬁcant economic value, particularly in rural parts of Wales, from where approximately 50% of the national rod catch of sea trout is recorded annually (Nautilus Consultants, 2000). A recent study of the Teiﬁ, where the total migratory salmonid rod catch is dominated by sea trout, estimated the contribution of salmon and sea trout anglers to the local economy at £1 million (Radford et al., 2001). Locally important rod ﬁsheries are also found along the south coast of England where, although catch numbers are relatively small, average weights may be high. Salmon ﬁsheries have been in general decline throughout England and Wales in recent years (Anon., 2005), and this has highlighted the importance of the sea trout ﬁsheries. With the exception of a period of low stocks in the early- to mid-1990s, sea trout catches have remained stable since the late 1970s, and the 2002 rod catch (49 000 ﬁsh) was the second highest declared since 1978 (when national records commenced).

National catches It is recognised that catch data alone are not a reliable measure of stocks (Milner et al., 2002); however, when compiled in a consistent manner over a number of years, reported catches may be indicative of long-term trends in stock abundance (Russell et al., 1995). A statistical analysis of long-term sea trout catches undertaken by Elliott (1992) concluded that annual catches reﬂected the number of adult sea trout in a river and that such records could be used as the basis for temporal comparisons. However, the relationship between catch and stock size is not always straightforward, regardless of the accuracy of the data and great care must be taken therefore in interpretation. Declared annual rod and net catches of sea trout during the period 1978–2003 ranged from 14 742 (1992) to 55 863 (1987) and from 27 159 (1998) to 97 206 (1980), respectively (Figs 8.1 and 8.2). During the same period, the number of net licences issued annually decreased from 1060 to 417, a reduction of 60% in 25 years. Rod effort has also declined from 292 000 days ﬁshed in 1994 to 178 000 in 2003, a reduction of 40% in just under 10 years. Catch and release rates for rod-caught sea trout have increased from 35% in 1994 (when records were ﬁrst kept) to 55% in 2003 (Fig. 8.2). It is encouraging to note that release rates for larger ﬁsh have been increasing in recent years. A third of ﬁsh weighing more than 3.6 kg (8 lbs) were released following capture in 2003 compared with just 8% in 1994. Over the period 1978–2003, ﬂuctuations in sea trout rod and net catches were not synchronous (F = 1.82; P = 0.074), though they appear to have reﬂected similar changes in the availability of sea trout, mediated by the more consistent decline in netting effort (Figs 8.2 and 8.3). For this reason, catch per unit effort (CPUE) is considered to be a more accurate index of stock size than the catch itself. Unfortunately, reliable and consistent net effort data have only been recorded nationally in recent years. A more robust data set is available for the rod ﬁsheries (1994 onwards). CPUE in the sea trout rod ﬁsheries increased, particularly in recent years, in contrast to a relatively stable CPUE in the salmon rod ﬁsheries (Fig. 8.4).

Sea Trout Fisheries of England and Wales 60 000

109

350 000 300 000

50 000

250 000 200 000 30 000 150 000

Rod days

Rod catch

40 000

20 000 100 000 10 000

50 000

0

0 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 Caught and killed

Long-term average

Caught and relased

Rod days

Fig. 8.1 Total declared sea trout rod catch for England and Wales (1978–2003), the number of sea trout caught and released, and the number of days declared ﬁshed by salmon and sea trout anglers since 1994.

120 000

1200

1000 90 000

60 000

600

Net licences

Net catch

800

400 30 000 200

0

0 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 Net catch

Long-term average

Net licences

Fig. 8.2 Total declared sea trout net catch and number of netting licences for England and Wales (1978–2003).

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ln number of fish

11.5 11.0 10.5 10.0 9.5 1978

1983

1988 Sea trout nets

1993

1998

2003

Sea trout rods

Fig. 8.3 Sea trout rod and net catches, corrected for under-reporting and log10 transformed (1978–2003).

30

Catch/100 rod days

25 20 15 10 5 0 1994

1996

1998 Salmon

2000

2002

Sea trout

Fig. 8.4 National (England and Wales) salmon and sea trout catch per 100 rod-licence days (1994–2003).

Spatial variation in trends in the rod ﬁsheries Rod catch series (corrected for under-reporting and log10 transformed) for each of the 67 main river sea trout ﬁsheries (though excluding southern region rivers for which few records exist before 1990) were analysed using linear regression to assess the performance of each river over the 25-year period. Figure 8.5 indicates that catches decreased from 1974 to 2003 in 47% of these rivers, 21% signiﬁcantly (P < 0.05). Signiﬁcantly increased catches were recorded in 29% of rivers, particularly in rivers in the north of England and some of the South Wales rivers that have been recovering from pollution. In other areas, sea trout stocks

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Coquet Tyne Wear Tees Esk (Yorkshire) Avon (Hants) Piddle Frome Axe Exe Teign Dart Avon (Devon) Erme Yealm Plym Tavy Tamar Lynher Fowey Camel Torridge Taw Lyn Wye Usk Taff Ogmore Afan Neath Tawe Loughor Tywi Taf E/W Cleddau Teifi Aeron Ystwyth Rheidol Dyfi Dysynni Mawddach Artro Dwyryd Glaslyn Gwyrfai Seiont Ogwen Conwy Clwyd Dee Ribble Wyre Lune Kent Leven Duddon

NE

Esk (Cumbrian) Irt Calder

SW

Ehen Derwent

Wales

Ellen Eden

NW

Esk (Border) –0.3

–0.2

–0.1

0

0.1

0.2

Rate of change in rod catch 1974 to 2003

Fig. 8.5 River-by-river variation of trends in sea trout rod catches in England and Wales (1974–2003).

0.3

112

Sea Trout 7000 6000

Mean catch

5000 4000 3000 2000 1000 0 North-west

Wales Apr

Fig. 8.6

May

South-west Jun

Jul

Aug

Southern Sep

North-east

Oct

Mean monthly catches of sea trout for the rod ﬁsheries by region (1999–2003).

and ﬁsheries continue to be impacted by, for example, water-quality problems and barriers to migration.

Geographical variation Differences in sea trout life-history strategies around the country result in differences in the timing of runs and mean weights (Solomon, 1995; Harris, 2000). Sea trout rod catches peak in September and October in rivers in the north-eastern and southern regions (Fig. 8.6), whereas the peak in catch is reported in July and August for rivers in the south-west, the north-west and in Wales. In the latter rivers, large numbers of whitling (∼0.5 kg) are present in the rod catches, which results in small overall mean weights (Fig. 8.7), though large ﬁsh may be present in spring. The Towy, South Wales, in particular, is noted for large early running ﬁsh weighing up to 9 kg and highly prized by anglers. The mean weights of rod-caught sea trout in the north-east and southern regions are relatively high in all months.

Discussion Sea trout rod catches in England and Wales have generally remained stable throughout the period investigated, though poor catches were experienced in the mid to late 1990s. In contrast, net catches have declined steadily since the late 1970s, partly in response to reduced ﬁshing effort resulting from ﬁshery buy-outs, phase-outs and by the progressive shortening of netting seasons. Rod catches, therefore, are an increasingly important source of information on sea trout stock size and ﬁshery performance. The Fisheries Legislative

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2

Mean weight (kg)

1.5

1

0.5

0 North-west

Wales Apr

Fig. 8.7

M ay

South-west Jun

Jul

Aug

Southern Sep

North-east

Oct

Mean monthly weights of sea trout in England and Wales rod ﬁsheries (1999–2003).

Review (MAFF, 2000) recommended that the feasibility of setting conservation limits for sea trout in England and Wales should be investigated. A variety of Biological Reference Points can be envisaged, not all of which may use adult stock data. But it is likely that consistent and statistically robust estimates of stock size will be needed to compare against such limits as part of an overall assessment scheme. While ﬁshery-independent assessment methods such as counters and traps will be part of that activity, they are not available everywhere and are not without their own difﬁculties. Catch data are universally collected from all rivers with rod ﬁsheries. Their effective use will enable systematic and accurate catch recording, improved understanding of the relationships between catch and stocks and improvements to the measurement of ﬁshing effort. Improvements in these areas are being devised.

References Anon. (2005). Annual assessment of salmon stocks and ﬁsheries in England and Wales 2004. Report from the Environment Agency and CEFAS to ICES, 79 pp. Elliott, J.M. (1992). Analysis of sea trout catch statistics for England and Wales. Fisheries Technical Report No. 2, National Rivers Authority, 43 pp. Environment Agency (2003). Fisheries statistics: salmonid and freshwater ﬁsheries statistics for England and Wales, 2002. Environment Agency, Bristol, 52 pp. Harris, G.S. (2000). Sea Trout Stock Descriptions: The Structure and Composition of Sea Trout from 16 Rivers in England & Wales. Environment Agency, Bristol. R&D Technical Report W223, 93 pp. MAFF (2000). Salmon and freshwater ﬁsheries review. Ministry of Agriculture and Fisheries, London, 199 pp. Milner, N.J., Davidson, I.C., Evans, R., Locke, V. & Wyatt, R.J. (2002). The use of rod catch to estimate salmon runs in England and Wales. In: Interpretation of Rod and Net Catch Data. (Shelton, R., Ed.). Proceedings of Atlantic Salmon Trust Workshop, November 2001, Lowestoft, pp. 46–67.

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Nautilus Consultants Ltd; in association with EKOS Economic Consultants Ltd (2000). Study into inland and sea ﬁsheries in Wales, 125 pp. Radford, A.F., Riddlington, G. & Tingley, D. (2001). Economic evaluation of inland ﬁsheries. Environment Agency R&D Project W2-039/PR/1, 166 pp. Russell, I.C., Ives, M.J., Potter, E.C.E., Buckley, A.A. & Duckett, L. (1995). Salmon and migratory trout statistics for England and Wales, 1951–1990. Fisheries Research Data Report No. 38, Ministry of Agriculture, Fisheries and Food, Lowestoft, 252 pp. Solomon, D.J. (1995). Sea trout stocks in England and Wales. R&D Report 25, National Rivers Authority, 102 pp.

Chapter 9

General Overview of Turkish Sea Trout (Salmo trutta L.) Populations I. Okumu¸s1 , I.Z. Kurtoglu2 and S. ¸ Atasaral1 1 Karadeniz

Technical University, Faculty of Marine Sciences, Department of Fisheries, 61530 Camburnu, Trabzon, Turkey 2 Central Fisheries Research Institute, P.K: 129, 61001 Trabzon, Turkey

Abstract: This chapter reviews the limited studies conducted on the life history and ecology of Turkish sea trout populations in relation to conservation and management issues. A sea-going form of brown trout, called the Black Sea trout, Salmo trutta L., is found in only some Turkish and Georgian rivers of the Black Sea. There has been longstanding debate regarding its taxonomy, but on the basis of genetics, it is currently included in the ‘Danubian’ grouping. Historically, they were found in all rivers and streams ﬂowing into the eastern Black Sea, but now their distribution is mainly limited to the eastern Black Sea coast of Turkey. The Black Sea trout has a similar life cycle and biology to the European sea trout. Drastic declines in stocks have occurred through a combination of environmental degradation in fresh water and at sea, particularly the losses of ﬁsh passage facilities through the construction of hydroelectric dams and power plants. The ﬁsh require protection and ﬁshing has been prohibited since the early 1980s, but intensive illegal ﬁshing continues. There is an urgent need for an integrated and strategic approach to management across the Black Sea countries. Keywords: Sea trout, Salmo trutta, Black Sea, Turkey.

Introduction General The sea trout is an anadromous salmonid species of high ecological and commercial value, which is widely distributed in Europe, including the north-eastern Black Sea coast of Turkey and the Black, Azov and Caspian Sea basins. It occurs in rivers ﬂowing from Turkey and other coastal states into the Black Sea. Therefore, it has been called the Black Sea trout by Turkish authorities and Black Sea salmon in other Black Sea countries (Barach, 1962; Solomon, 2000). Earlier it has been described as a sub-species (Salmo trutta labrax) of brown trout (Geldiay & Balık, 1996; Solomon, 2000). A similar form, classiﬁed as S. trutta caspius, occurs in the Caspian basin (Berg, 1948). The brown trout (Salmo trutta L.) has been extensively studied in European rivers (e.g. Ferguson, 1989; Laikre, 1999; Bernatchez, 2001; Klemetsen et al., 2003) and its anadromous form, the sea trout, has received much attention (Dellefors & Faremo, 1988; 115

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Lelek, 1988; Hindar et al., 1991; Lund & Hansen, 1992; Johnstone et al., 1995; Solomon, 1995; Lyse et al., 1998; Euzenat et al., 1999; Pettersson et al., 2001; Snoj et al., 2002). Information on the Black Sea trout dates back to the nineteenth century (e.g. Pallas, 1811; Chernyavsky, 1882, cited by Goradze, R. & Gadaeva, M. [2004]. The Black Sea salmon in the past and present, development of the management strategy. Marine Ecology and Fisheries Research Institute of Georgia, Unpublished Report), but much of it lies in inaccessible Russian literature (e.g. Slastenenko, 1956; Barach, 1962). Accounts of its distribution, habitats and morphology came from Kosswig and Battalgil (1943, cited by Geldiay & Balık, 1996), Kuru (1971, 1975) and Aras (1974). In this chapter we review current knowledge on the status of their environment, life cycle, biology, genetic diversity and stock status and highlight the need for coordinated management and conservation. Distribution and habitats The Black Sea trout is present on all coasts of the Black Sea, from Turkey, through Georgia, Caucasus, Crimea, Azov Sea, Romania and Bulgaria (Edwards & Doroshov, 1989; Baglinière, 1999; Bushuev, S. [2000]. Information on Black Sea salmon population present in north-western part of sea. Unpublished notes.) (Fig. 9.1). Their current distribution in Turkey begins from Sürmene, 40 km east of Trabzon in the north-east, and crosses the Georgian border through the River Coruh (Chorokhi). The major rivers are the Firtina, Caglayan, Coruh, Kapisre, Findikli, Taslidere, Iyidere, Baltaci and Solakli (Fig. 9.2). In addition to the coastal waters of the Black Sea, they have been reported from the following rivers and lakes in Turkey: Çoruh, Aras River, Tortum River, Lake Cildir (Ardahan) (Geldiay & Balık, 1996). In neighbouring Georgia, the major part of their distribution occurs in the Coruh, Machakhela, Chakvistskali and Kintrishi rivers (Goradze, R. & Gadaeva, M. [2004]. The Black Sea Salmon in the past and present, development of the management strategy. Marine Ecology and Fisheries Research Institute of Georgia, Unpublished Report). No information is available on their current occurrence in Romania and Bulgaria. The eastern part of the Turkish Black Sea is fed by the water from many rivers, with a substantial total volume of discharge. In general the rivers are relatively short (<50 km) with

Black Sea

Fig. 9.1

Distribution of the Black Sea ‘trout’.

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Kapisre R.

Caglayan R. Black Sea

Coruh F.

Firtina R.

Rize

Trabzon

Iyidere R. Solakli R.

N Uzun Gb1

Fig. 9.2

0 5 10 G...

Black Sea

Georgia

Turket

Major sea trout rivers in Turkey.

high ﬂow variation, gravel-bedded, and are often channelised in their lower reaches. There is low human population density and tea and hazelnut are the predominant crops along the low-lying parts of the rivers. Almost one-third of the riverine discharge from Turkey into the Black Sea basin originates in the eastern Black Sea region. There are around 30 small rivers ﬂowing into the stretch from the Coruh to the Harsit basin, in 260 km of coastline. The major sea trout rivers along the Turkish coast of the Black Sea from east to west (Fig. 9.2) are the Coruh (Chorokhi), Firtina, Kapisre (Arhavi), Caglayan, Iyidere and the Solakli. Among other rivers, the Hemsindere and Taslidere have small sea trout stocks and limited numbers of ﬁsh may still enter the Baltaci River. The Degirmendere, which ﬂows through Trabzon city, also used to have a sea trout stock, but now is polluted and silted. Sea trout are rarely taken in the Harsit, which represents the western margin of the historical sea trout distribution in the Turkish Black Sea coast (Edwards & Doroshov, 1989). The rivers are mostly fed by melting snow and rain which, in spring, can cause ﬂooding. Water ﬂow is at its highest in May and June and minimum ﬂow occurs in February. For example, the approximate mean ﬂow in the River Coruh is 275 m3 s−1 , while the Firtina approaches maximum ﬂow of 100 m3 s−1 during May–June and minimum ﬂow of just over 20 m3 s−1 in February. Other major sea trout rivers, the Caglayan, Kapisre, Iyidere and Solaklı, have ﬂow rates of 17.5–42 m3 s−1 during the high and around 4–6 m3 s−1 during the low seasons. Water temperatures vary between 5◦ C and 6◦ C during January–February and 24–25◦ C in July–August. Dissolved oxygen levels may range from 7.0 to 12.5 mg l−1 , while pH values are around 6–9. Turbidity can be very high, particularly in the lower parts of the rivers. Lack of sheltered areas and a narrow coastal zone are two major geomorphological features of the south-eastern Black Sea. The climate is temperate and winter is the stormiest

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season. As a result of the high input of fresh water, the coastal surface water of the Black Sea is brackish and quite stable in salinity (around 17–18 ppt). Water temperature is highly variable, ranging 6–8◦ C in the southern waters, up to 26◦ C (Sahin et al., 1999). A summer thermocline forms at a depth of 30–40 m and temperatures at these depths vary between 10 and 20◦ C, while below 40 m the temperature is almost constant at around 8–10◦ C. Taxonomy The brown trout shows considerable variability and plasticity in many aspects of its morphology, ecology and behaviour. This variability led early workers to recognise numerous sub-species. Thus, in Turkey, as in Europe (Laikre, 1999), there is ongoing discussion and confusion regarding the taxonomy of the brown trout. Accordingly, the trout populations of Turkey, the Black and the Caspian Seas, have been classiﬁed into the following taxa (Berg, 1948; Geldiay & Balık, 1996): Salmo trutta macrostigma (Dumeril, 1855), S. trutta abanticus (Tortonese, 1954), S. trutta caspius (Kessler, 1877) and S. trutta labrax. Most of these classiﬁcations were based on minor morphological and/or lifehistory forms, and reﬂected mainly environmental and phenotypic plasticity. In recent studies (Bernatchez & Osinov, 1995; Osinov & Bernatchez, 1996; Togan et al., 1996), the combined results of allozyme and mitochondrial markers provided only weak support for this taxonomic distinction. In spite of these genetic ﬁndings this taxonomic distinction is still widely used (e.g. Solomon, 2000; Tabak et al., 2001; Kurtoglu, 2002). On the other hand the Black Sea trout has been divided into three functional forms by Slastenenko (1956): anadromous or marine, resident/fario and lacustris forms. According to Slasteneko, the major differences between the marine and resident forms are body size (e.g. the maximum size of the resident form is only 1–2 kg), the presence of red and white spots on the body and a thick caudal peduncle. Phylogeny and genetic diversity Studies on the genetic structures of brown trout populations in Turkey and the Black Sea basin have been very limited. Very few populations of brown trout in the eastern range, namely Turkey, the Black, Caspian and Aral Sea basins, have been investigated (Bernatchez & Osinov, 1995; Osinov & Bernatchez, 1996; Togan et al., 1996; Bernatchez, 2001). Within this region, three phylogeographical groupings, namely the Danubian, the Atlantic and the Adriatic, have been detected. However, a major part of the Black Sea populations that have been studied belongs exclusively to the Danubian grouping (Osinov & Bernatchez, 1996; Togan et al., 1996; Bernatchez, 2001). Togan et al. (1995) found ﬁxed differences at three allozyme loci between two populations of the Black and Mediterranean Sea basins. In their preliminary study, the authors suggested that the Black Sea brown trout population of Turkey belongs to an undescribed phylogenetic group lying between the Danubian and Mediterranean groupings. However, most of the Turkish brown trout populations have been included in the Danubian grouping (Bernatchez, 2001). The genetic and phylogenetic status of many populations in Turkey and its neighbouring

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Balkan and Black Sea countries, and Iran, remain to be conﬁrmed using molecular techniques. Togan et al. (1996) found four alleles speciﬁc to Turkish brown trout populations, but no speciﬁc allele or morph for the Black Sea trout. They concluded that, genetically, Turkish brown trout populations are still pure. There is also an ongoing study by Ciftci & Okumus (2004) that aims to reveal the genetic diversity of all brown trout populations, including sea trout, by nucleotide sequencing of the mitochondrial DNA (mtDNA) control regions, namely ND-1, ND-5/6, cytochrome b and D-loop. The study will determine the current genetic and phenotypic variability patterns before stock enhancement is undertaken.

Life cycle The life history of Black Sea trout (e.g. Berg, 1948; Slastenenko, 1956; Barach, 1962) is similar to that of north-western European sea trout populations. Anadromous and resident forms As elsewhere in its geographical range (Pettersson et al., 2001) sea trout in the Black Sea basin occur in fresh water together with resident forms and may represent a single freely interbreeding unit (Hindar et al., 1991), with large sea trout females mating with small, resident males. In the Black Sea basin, females predominate in the anadromous population, but during the ﬁrst year of the river life the sex ratio is almost 1 : 1. Some of these ﬁsh undergo the smoltiﬁcation process at age 1+ and then the sex ratio starts skewing. In particular, most of the males in a cohort mature as residents, and only a very small proportion migrates to sea. Nearly 75–85% of both the downstream migrating juveniles and the adults returning from the sea are female (Barach, 1962; Solomon, 2000; Tabak et al., 2001). Nearly all of the largest ﬁsh are females, indicating a higher mortality rate among mature males. However, the sex ratio may vary between rivers and years (Barach, 1962) and the precise mechanisms and possible genetic factors underlying this variation in life histories still remain largely unresolved (Laikre, 1999). Tabak et al. (2001) found no morphological difference between migratory and resident forms. Pettersson et al. (2001) reported that mtDNA and microsatellite markers revealed no genetic differentiation between coexisting resident and migratory females, whereas both groups were signiﬁcantly divergent from landlocked resident females. Sexual maturity and spawning Black Sea basin sea trout reach sexual maturity at over 35 cm, after age 2+ years, and return to rivers to spawn. It has been reported from other countries that sea trout can home accurately to their natal streams to spawn (e.g. Armstrong & Herbert, 1997; Bij De Vaate et al., 2003). According to Aarestrup & Jepsen (1998), in the Black Sea basin, the large freshwater discharge of some rivers may attract mature sea trout from other populations. Ascent of the rivers for spawning may start as early as March, but most ﬁsh

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enter in May and June (Tabak et al., 2001). At the start and end of the migratory period, the temperature in the coastal areas is 8–9◦ C and 20–22◦ C, while temperatures in the lower parts of the rivers range from 6–8◦ C to 12–15◦ C, respectively. Water ﬂows are at their maximum levels during the period of river ascent. Based on sea trout availability in markets in Tskal, Tsminda and Batumi, Solomon (2000) also concluded that the ﬁsh return to coastal waters and enter the rivers during March–May. They are capable of reaching the spawning grounds within a few days or weeks in most cases, but most continue to feed in the lower reaches. Black Sea trout still spawn in signiﬁcant numbers in the most easterly rivers, that is, Firtina, Caglayan and Kapisre and, in small numbers, possibly in Coruh, Taslidere, Iyidere, Baltaci and Solakli (Edwards & Doroshov, 1989; Aydin & Yandi, 2002; Goradze, R. & Gadaeva, M. [2004]. The Black Sea Salmon in the past and present, development of the management strategy. Marine Ecology and Fisheries Research Institute of Georgia, Unpublished Report). The substrate in parts of all these rivers is suitable for salmonid spawning, but in the second group of rivers siltation can cause mortalities of eggs and early fry. Tabak et al. (2001) surveyed the spawning grounds in the River Firtina and its tributaries. They reported that large ﬁsh of 5–16 kg spawn in the main stems, while smaller ﬁsh migrate into the upper tributaries. Redds are found in areas with a wide stream bed, clean water, slow but stable water ﬂow and coarse gravel and stone material. Spawning occurs from late October to the end of December, at a constant temperature of 8–10◦ C. The majority of females (>80%) spawn in November (Barach, 1962; Tabak et al., 2001; Goradze, R. & Gadaeva, M. [2004]. The Black Sea Salmon in the past and present, development of the management strategy. Marine Ecology and Fisheries Research Institute of Georgia, Unpublished Report). Eggs are laid in redds in suitable gravel in fast-ﬂowing, shallow areas. Fecundity varies between 2000 and over 3000 eggs per kilogram body weight, while the diameter of the eggs is around 4.8–7.2 mm (Tabak et al., 2001; Kurtoglu, 2002). With an incubation temperature of around 5–7◦ C, hatching starts after 60–80 days and fry emerge probably in April. Early development and smoltiﬁcation The early developmental stages of the Black Sea basin sea trout have not been studied extensively, but their development is thought to be similar to that in northern Europe. Smoltiﬁcation and migration of smolts starts in early spring (March–June) and may last until the beginning of winter in Turkey and Georgia (Barach, 1962; Solomon, 2000; Tabak et al., 2001). Most of the ﬁsh (around 80%) smoltify at age 1+. The size of the smolts increases with age and throughout the migration period (Barach, 1962). For example, mean lengths in spring and autumn migrating smolts were estimated as 18.4 and 24.3 cm in Turkish, and 16.8 and 20.6 cm in Georgian populations, respectively (Barach, 1962; Tabak et al., 2001). Apart from the preliminary studies of Kurtoglu (2002) and Okumus et al. (2004), the smoltiﬁcation process of the Black Sea trout has not been studied in detail. Hoar (1988) described sea trout as a poorer osmoregulator than Atlantic salmon. However, we found no difference in survival and growth between parr (7–12 cm) grown in fresh water and brackish water (15%) of the Black Sea (Okumus et al., 2004). Thus, smoltiﬁcation might not

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be necessary for migrating to these low salinities, and imposes no cost in terms of growth reduction. Similar results were reported by Landergren (2001) for Baltic Sea populations. Migration and marine life Very limited information is available on the marine life of the Black Sea trout in Turkish coastal waters, although Barach (1962) provided some information from the Georgian and Russian coasts. He reported that Abkhazian salmon (i.e. sea trout) migrated up the Crimean coast and into the Azov Sea, travelling up to 900 km from the point of release. In Turkish coastal waters, sea trout have been caught around 250–300 km west of the major smoltproducing rivers. It is believed that the Black Sea trout is conﬁned to coastal areas. Some observations from other localities (e.g. Norway and Scotland) indicate that post-smolt sea trout tend to shoal within 200 m of the shoreline. Shoals of post-smolts were often observed migrating back and forth along the littoral zone, without any directed migration (Lund & Hansen, 1992; Johnstone et al., 1995). Such behaviour in the Black Sea would make them very vulnerable to poaching and may be a factor in their decline. Temperature has been proposed as an inﬂuence on sea trout marine distribution (Lyse, 1998). In the Black Sea summer temperatures in the coastal zone rise to 28◦ C, well beyond the tolerance limits of salmonids, and this may lead to offshore movement to deeper, cooler waters. Duration of marine residency is not well-described. According to Barach (1962), most trout ﬁrst return to the rivers after 1 year in the sea. Some remain for 2 years, while a small proportion (mostly males) may come back to fresh water after only a few months. Individuals from different age groups gather in near-shore areas with low salinity, as part of the process of re-acclimatisation to fresh water before migrating into the rivers in spring. Catches of large sea trout and their appearance in ﬁsh markets and restaurants sharply increase in March, April and May. The sea age and size of ﬁsh entering rivers for spawning vary between 3+ and 5+ years and 40–80 cm (0.8–5.6 kg) for males, and 3+ and 8+ years and 45–99 cm (1.5–16.2 kg) for females. The male : female ratio of spawning migrants is around 1 : 2.5. The size range of sea trout returning to rivers in Georgia was reported as 0.5–16 kg (Solomon, 2000). According to Slastenenko (1956), ﬁsh weighing up to 26 kg have been recorded in the Black Sea. These ﬁgures indicate that the coastal waters of the Black Sea provide excellent conditions for growth, probably resulting from a combination of low salinity, suitable temperatures and abundant food. Feeding, age and growth The food of sea trout in the Turkish part of the Black Sea basin varies with life stage and location. Smolts in the river mouths and in the sea feed mainly on insects. The main prey categories of older sea trout in terms of frequency of occurrence are ﬁshes, mainly anchovy and sprat and crustaceans (Arthopoda) (Solomon, 2000; Tabak et al., 2001). In the streams and rivers, both the resident and anadromous forms depend heavily on aquatic insects and some animal detritus. The resident form feeds actively throughout the year, whereas the anadromous form does not feed during the coldest and the warmest months.

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At the end of the ﬁrst growth season (in December) juvenile trout in the rivers may reach a length of 9.5–16.5 cm and a weight of 13–35 g. They reach 16–36 cm at age 2 and 42.5–57.0 cm at age 3 (Barach, 1962; Tabak et al., 2001).

Fisheries and status of stocks Sea trout as a natural resource According to many researchers (Kessler, 1874; Arnold, 1896; Kavraisky, 1896 – all cited by Goradze, R. & Gadaeva, M. [2004]. The Black Sea Salmon in the past and present, development of the management strategy. Marine Ecology and Fisheries Research Institute of Georgia, Unpublished Report) there were abundant stocks and valuable commercial ﬁsheries in Georgia and Turkey at the beginning of the twentieth century. Edwards & Doroshov (1989) reported that the sea trout was still an important commercial species during 1980. As a conservation measure recreational and commercial ﬁsheries for sea trout stocks in the Black Sea and Azov basins have been closed for the past 20 years, although they are still exploited illegally. Many people believe that the species could still support important local commercial and recreational angling ﬁsheries, as well as having signiﬁcant potential for aquaculture. There is no available information on the numbers of sea trout anglers, or the monetary value of the angling ﬁshery and there is only one pilot project in Turkey that aims to develop separate broodstocks for commercial aquaculture and stock enhancement. Fisheries and current state of stocks The demand for wild trout, including sea trout, has made them a target for illegal ﬁshing and in spite of a complete ban on ﬁshing since the 1980s there has been increasing, but unquantiﬁed, ﬁshing pressure on the stocks. Illegal sea trout ﬁshing in Turkey is mostly conducted by small-scale ﬁshermen in coastal areas near the river entrances between the east of Rize and the Georgian border (Fig. 9.2). Sea trout are mostly caught by gill and trammel nets of 50–150 m length, particularly during March and June. Tabak et al. (2001) surveyed the area during the ﬁshing seasons from 1998 to 2000 and counted 15–72 nets per day and estimated monthly catches up to 9.2 tons or 2280 ﬁsh. In addition, sea trout are also caught as a by-catch during the mullet (Mugil so-iuy) ﬁshery, during April–June. Sea trout are also caught by amateur ﬁshermen and local people in rivers using cast nets. Illegal ﬁshing in the rivers continues nearly all year round, but catches increase between March and September, with monthly estimated quantities of 70–140 kg. Traps are also used for downstream migrating smolts and spent adults and upstream migrating mature ﬁsh. Other illegal ﬁshing methods include electroﬁshing, chemicals and explosives. Georgian authorities have monitored sea trout occurrence in city markets since 1991 and reported that catches have increased steadily, from 0.9 t in 1991 to 3.5 t in 1999 (Goradze, R. & Gadaeva, M. [2004]. The Black Sea Salmon in the past and present, development of the management strategy. Marine Ecology and Fisheries Research Institute of Georgia, Unpublished Report).

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Various ﬁsheries regulations, including the imposition of a closed season and size limits, and ﬁnally a complete ban on ﬁshing, have been imposed but have not been implemented effectively. Heavy exploitation, particularly in river entrances and in rivers, occurs during spawning migrations and on spawning grounds. Illegal ﬁshing still continues because of the high market value of sea trout and the lack of rigorous surveillance and enforcement. Current and potential threats for the Black Sea trout populations As elsewhere, the Black Sea trout populations are threatened by anthropogenic activities, including illegal over-ﬁshing (described above) and environmental degradation. The majority of the major rivers in Turkey are heavily modiﬁed for energy production. There are hydroelectric power plants on the Rivers Harsit, Iyidere and Coruh. In the near future, major changes can be expected at the mouth of the Coruh because of ﬂow control measures in Turkey. Failure to install passes for migrating trout at dams and power plants in many areas has cut-off access to spawning grounds, although the exact effects of this on stock status is unknown. However, decreases in sea trout populations of almost 50% have been found in other locations (e.g. Aarestrup & Jepsen, 1998). Considerable environmental problems have occurred in the Black Sea coastal environment (Mee, 1992). Land reclamation and road construction are major threats to the coastal environment in the region. Infrastructure for waste collection and disposal also is inadequate. Haphazard coastal construction, including the recent construction of the coastal highway, has further degraded physical and aesthetic aspects of the natural landscape in many areas. Problems also arise through pollution from industrial and domestic waste, over-fertilisation, increased sediment loads and the introduction of exotic species such as jellyﬁsh (Mnemiopsis leidy and Boroe ovata) and gastropods (Rapana thomasiana). There are no stock enhancement schemes in the Black Sea area currently, although an extensive hatchery-based enhancement project was carried out on the Chernaya River in Abkhazia from 1935 to 1957, with annual releases between 34 000 and 540 000 ﬁsh (Barach, 1962). With current threats to the Black Sea trout stocks, and various conservation/management efforts that are underway, enhancement programmes appear to be inevitable in the near future. Recently, trout farmers in the north-eastern Black Sea region of Turkey have imported brown trout eggs from Europe (France and/or Germany). However, there is no known incidence of introgression of alleles from Atlantic trout populations through enhancement programmes or escapements from ﬁsh farms.

Management and conservation As described earlier, the brown trout is an important resource in Turkey that used to support valuable commercial and sport ﬁsheries. Increasing demand for ﬁsh has increased ﬁshing pressure, while the cumulative effects of habitat destruction, water pollution, lack of management and illegal ﬁshing have reduced survival and recruitment. The number of rivers inhabited by sea trout has declined and the stocks in remaining rivers have been weakened drastically in the Black Sea basin. Thus, it is vital that current threats to the sea

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trout should be recognised, prioritised and that urgent measures are taken to halt and reverse the decline. During the 1980s major political, socio-economic and environmental changes occurred in many of the Black Sea countries. These changes caused environmental degradation and resulted in increasing ﬁshing pressure on valuable ﬁsh species such as trout. As a result of public anxiety, the sea trout was taken under protection in Turkey as a unique representative of its aquatic fauna in the early 1980s and was registered in the Red Book of the Russian Federation as endangered and protected (disappearing) in the Ukraine Red Book (Bushuev, S. [2000]. Information on Black Sea salmon population present in north-western part of sea. Unpublished notes.). However, the situation regarding numbers of ﬁsh was not really quite so critical during these years and, in spite of the regulations and complete ﬁshery bans, an extensive illegal ﬁshing has continued. The Ministry of Agriculture and Rural Affairs (MARA) is the competent authority responsible for ﬁsheries management in Turkey. However, protection and control activities in the ﬁeld are carried out by the Coast Guard and Corps of Genderme. The former body is responsible for marine ﬁsheries and the latter for inland areas. Unfortunately, the Coast Guard has not been effective in preventing illegal sea trout ﬁshing as the nets are set very close to shore. Also, there is a lack of coordination among MARA, the Coast Guard and the Corps of Genderme in enforcement in coastal and river zones. Other protection and control measures, such as prohibition of sale or purchase of sea trout, have not been considered yet. Furthermore, there is no ﬁshery specialisation in the Turkish judiciary system. The prosecution process is often very lengthy and ﬁnes are insufﬁcient to be a deterrent. Thus, the handling of cases of illegal ﬁshing and poaching is not very effective. An intensiﬁcation of the monitoring and surveillance activities both at sea and in the rivers, eradication of poaching and easing of migratory routes and access to spawning grounds will doubtless improve the state of stocks, or at least halt the decline. In the immediate future, the current regulations on ﬁshing should be implemented effectively and environmental degradations, particularly along the migratory routes and at the spawning grounds, should be repaired and natural spawning maintained. In the long term, however, a comprehensive regional management and conservation strategy is needed for Black Sea trout, based on regional and international cooperation, and addressing not only basic ﬁsheries regulation, but also environmental and local socioeconomic problems. It will need to address ways of improving marine environmental problems, the management of mixed-stock exploitation in coastal waters and the sharing of information, experiences and even some equipment and facilities (Solomon, 2000). The strategy for the management of the Black Sea trout should also include monitoring of water quality, rehabilitation of habitats, the construction of necessary upstream and downstream ﬁsh passages, the foundation of hatcheries for smolt production ranching and commercial aquaculture purposes, further research and monitoring of stocks. Development funding for research projects and conservation and management strategies is also an important issue that needs to be addressed. Some stocks may be opened for recreational ﬁsheries and the license fees will contribute to funding. The license holders should become stakeholders in the management plans (Goradze, R. & Gadaeva, M. [2004]. The Black Sea Salmon

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in the past and present, development of the management strategy. Marine Ecology and Fisheries Research Institute of Georgia, Unpublished Report). The management strategy for sea trout should be ﬁrmly based upon an understanding of the biology and status of the stocks of ﬁsh which, in different rivers, are likely to differ genetically, because of adaptation to local environmental conditions. Trout have such a distinct genetic population structure that it is particularly important for conservation measures and strategies to focus on the population level (Laikre, 1999), or on a catchment-by-catchment basis. Enhancement by restocking is another management option. A review of the status of aquaculture in six countries of the Black Sea coast was undertaken by an international mission, with the support of the Coordination Centre of the Black Sea Environmental Programme (BSEP). The review recommended the development of ranching and commercial sea trout culture in the region (GEF-BSEP, 1996). Some initiatives that followed in Georgia failed because of a lack of ﬁnance and adequate environmental quality. More recently, proposals for a hatchery production enhancement scheme have been made by Solomon (2000) that promoted three principles: obtaining brood ﬁsh or eggs from nearest stock(s) for restocking rivers that have lost their stocks; establishing broodstock from local depleted stocks and developing broodstock through selective breeding for commercial ﬁsh farming. However, it should be remembered that stocking or using brood ﬁsh that originated from non-native rather than native stocks present potential risks to the integrity of local, wild gene pools (Laikre, 1999; Okumus & Ciftci, 2004).

Conclusions and recommendations This review has shown that the Black Sea trout in Turkey and other countries such as Georgia, the Russian Federation and Ukraine is under serious threat from environmental degradation and illegal ﬁshing. As has been strongly recommended by TroutConcert (Laikre, 1999), threats to sea trout should be taken seriously. There is an urgent need for the development of an integrated management and conservation strategy for the genetically diverse stocks in the entire Black Sea basin. Unfortunately, the relatively complex life cycle of the sea trout, combined with inadequate data and many additional problems to do with infrastructure, funding and local circumstances make management particularly challenging. But some actions could be promoted as priority measures. Prevention of further decline through effective ﬁshery enforcement is essential to preserve the remaining stocks, but this will require integration between different agencies and countries and that requires shared aims and a common strategy. Provision of ﬁsh access to spawning grounds, by installation of ﬁsh passes and maintenance of suitable migration ﬂows is also vital to allow stocks to recover naturally. While natural recovery is generally regarded as desirable, to avoid the risks of intensive enhancement programmes, careful enhancement will be necessary to start or speed up recovery in some areas. This will have to be based on a precautionary approach to carefully control the genetic and ecological risks. In the long term, the sustainability of the Black Sea trout will require a mixture of better strategic planning and management supported by improved knowledge of stock genetic

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diversity and biology and comprehensive current stock assessment across the Black Sea stock groupings. The practical delivery of this management will require management action plans for each major river basin, based on their physical characteristics, hydrology, water quality, current ﬁsheries status and other constraints.

References Aarestrup, K. & Jepsen, N. (1998). Spawning migration of sea trout (Salmo trutta L.) in a Danish river. Journal of Fish Biology, 55, 767–83. Aras, S. (1974). Bioecological studies on trout inhabiting the Çoruh and Aras basins. PhD Thesis, Faculty of Agriculture, Ataturk University, Erzurum (In Turkish). Armstrong, J.D. & Herbert, N.A. (1997). Homing movements of displaced stream-dwelling brown trout. Journal of Fish Biology, 50, 445–49. Aydin, H. & Yandi, I. (2002). The general status of spawning areas of Black Sea trout in the East Black Sea regions (Salmo trutta labrax Pallas, 1811). E.Ü. Su Ürünleri Dergisi, 19, 501–06 (in Turkish). Baglinière, J.L. (1999). Introduction: the brown trout (Salmo trutta L.) – its origin, distribution and economic and scientiﬁc signiﬁcance. In: Biology and Ecology of the Brown and Sea Trout (Baglinière, J.L. & Maisse, G., Eds). Springer–Praxis, Chichester, pp. 2–12. Barach, G.P. (1962). The Black Sea Kumzha (trans. Terdzishvilli, N. & Solomon, D.). Black Sea Salmon Project, EU TACIS BSEP. Georgia Academy of Science of Georgia, 64 pp. Berg, L.S. (1948). Freshwater ﬁshes of the USSR and adjacent countries. Zoological Institute Akademy Nauk Moscow USSR 1(27), volume 1 (In Russian). English translation, 1962: Ofﬁce of Technical Services, Department of Commerce, Washington, DC. Bernatchez, L. (2001). The evolutionary history of brown trout Salmo trutta L. inferred from combined phylogeographic, nested clade and mismatch analyses of mitochondrial DNA variation. Evolution, 55, 351–79. Bernatchez, L. & Osinov, A.G. (1995). Genetic diversity of trout (genus Salmo) from its most eastern native range based on mitochondrial DNA and nuclear gene variation. Molecular Ecology, 4, 285–97. Bij De Vaate, A., Breukelaar, A.W., Vriese, T., De Laak, G. & Dijkers, C. (2003). Sea trout migration in the Rhine delta. Journal of Fish Biology, 63, 892–908. Ciftci, Y. & Okumus, I. (2004). Genetic structure of brown trout (Salmo trutta L.) populations in Turkey. Ongoing project supported by DG for Agriculture Research of Ministry of Agriculture and Rural Affairs. Dellefors, C. & Faremo, U. (1988). Early sexual maturation in males of wild sea trout, Salmo trutta L., inhibits smoltiﬁcation. Journal of Fish Biology, 33, 741–49. Edwards, D. & Doroshov, S. (1989). Appraisal of the sturgeon and sea trout ﬁsheries and proposals for a rehabilitation programme Project: FAO-FI–TCP/TUR/8853, Rome, Italy, 38 pp. Euzenat, G., Fournel, F. & Richard, A. (1999). Sea trout (Salmo trutta L.) in Normandy and Picardy. In: Biology and Ecology of the Brown and Sea (Baglinière, J.L. & Maisse, G., Eds). Springer–Praxis, Chichester, pp. 175–203. Ferguson, A. (1989). Genetic differences among brown trout (Salmo trutta) stocks and their importance for the conservation and management of the species. Freshwater Biology, 21, 35–46. GEF-BSEP (1996). Marine Aquaculture in the Black Sea Region: Current Status and Development Options. GEF Black Sea Environmental Programme, Black Sea Environmental Series, Vol. 2. New York: UN Publications, 239 pp. Geldiay, R. & Balık, S. (1996). Freshwater Fishes of Turkey. Aegean University, Faculty of Fisheries Publication No: 46 (In Turkish), Izmir, 532 pp. Hindar, K., Jonsson, B., Ryman, N. & Ståhl, G. (1991). Genetic relationship among landlocked, resident and anadromous brown trout, Salmo trutta L. Heredity, 66, 83–91. Hoar, W.S. (1988). The physiology of smolting salmonids. In: Fish Physiology (Hoar, W.S. & Randall, J.D., Eds), Vol. XIB, Academic Press, New York, pp. 275–343. Johnstone, A.D.F., Walker, A.F., Urquhart, G.G. & Thorne, A.E. (1995). The movement of sea trout smolts, Salmo trutta L., in a Scottish west coast sea loch determined by acoustic tracking. Scottish Fisheries Research Report, 56 pp.

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Klemetsen, A., Amundsen, P.-A., Dempson, J.B. et al. (2003). Atlantic salmon Salmo salar L., brown trout Salmo trutta L. and Arctic charr Salvelinus alpinus (L.): a review of aspects of their life histories. Ecology of Freshwater Fish, 12, 1–59. Kosswig, C. & Battalgil, F. (1943). Beitrage zur Türkischen Faunengeschichte I. Süsswasserﬂische. C.R. Soc. Turque Sci. Pys. Istanbul. Band, 8, 32–63. Kurtoglu, I.Z. (2002). Evaluation of brown trout (Salmo trutta) culture and its ranching potentials. PhD Thesis, Istanbul University, Graduate School of Natural and Applied Sciences, Istanbul, Turkey. ix+58 pp. (In Turkish). Kuru, M. (1971). Freshwater ﬁshes of Eastern Anatolia. Istanbul Uni. Fen Fak. Mec. Seri. B, 36, 137–47 (In Turkish). Kuru, M. (1975). Systematic and zoogeographical investigations on freshwater ﬁsh species (Pisces) living in Dicle (Tigris) – Fırat (Euphrates), Kura – Aras, Lake Van and the Black Sea basin. Thesis for Assoc. Professorship (In Turkish). Laikre, L. (Ed.) (1999). Conservation and genetic management of brown trout (Salmo trutta) in Europe. Report of the concerted action on identiﬁcation, management and exploitation of genetic resources in the brown trout (Salmo trutta) (‘TROUTCONCERT’; EU FAIR CT97-3882), 91 pp. Landergren, P. (2001). Survival and growth of sea trout parr in fresh and brackish water. Journal of Fish Biology, 58, 591–93. Lelek, A. (1988). Vorkommen, Taxonomie und Maβrahmen Zur Erhaltung der Forelle Salmo trutta labrax inder NO-Türkei. Cour. Forsch. – Inst. Senckenberg, 101, 1–44. Lund, R.A. & Hansen, L.P. (1992). Exploitation pattern and migration of the anadromous brown trout, Salmo trutta L., from the river Gjengedal, western Norway. Fauna Norvegica Serie A, 13, 29–34. Lyse, A.A., Stefansson, S.O. & Ferno, A. (1998). Behaviour and diet of sea trout post-smolts in a Norwegian fjord system. Journal of Fish Biology, 52, 923–36. Mee, L. (1992). The Black Sea in crisis: a need for concerted international action. Ambio, 21, 1278–86. Okumus, I. & Ciftci, Y. (2004). Fish population genetics and molecular markers: II. Molecular markers and their applications in ﬁsheries and aquaculture. Turkish Journal of Fisheries and Aquatic Sciences, 3, 51–79. Okumus, I., Atasaral, S. & Kocabas, M. (2006). A preliminary study on the growth of Black Sea trout parr in fresh and sea waters. Turkish Journal of Fisheries & Aquatic Sciences (in press). Osinov, A.G. & Bernatchez, L. (1996). ‘Atlantic’ and ‘Danubian’ phylogenetic groupings of brown trout Salmo trutta complex: genetic divergence, evolution, and conservation. Journal of Ichthyology, 36, 723–46. Pettersson, J.C.E., Hansen, M.A. & Bohlin, T. (2001). Does dispersal from landlocked trout explain the coexistence of resident and migratory trout females in a small stream? Journal of Fish Biology, 58, 487–95. Snoj, A., Marcjeta, B., Susjnik, S., Melkicj, E., Vladimir Meglicj, V. & Dovcj, V. (2002). The taxonomic status of the ‘sea trout’ from the north Adriatic Sea, as revealed by mitochondrial and nuclear DNA analysis. Journal of Biogeography, 29, 1179–85. Sahin, T., Okumus, I. & Celikkale, M.S. (1999). Evaluation of rainbow trout (Oncorhynchus mykiss) mariculture on the Turkish Black Sea Coast. Israeli Journal of Aquaculture – BAMIDGEH, 51(1), 17–25. Slastenenko, E. (1956). Karadeniz Havzası Balıkları (Turkish trans. H. Altan). Et ve Balık Kurumu Umum ˙ Müdürlügü Yayınlari, Istanbul – Turkey (In Turkish with English summary), 260 pp. Solomon, D.J. (1995). Sea trout stocks in England and Wales. National Rivers Authority R&D Report 25, Bristol, UK, 102 pp. Solomon, D. (2000). The biology and status of the Black Sea salmon Salmo trutta labrax. EU Tacis BSEP, Black Sea Salmon Project, Draft Report, 26 pp. ˙ Aksungur, M., Zengin, M. et al. (2001). Determination of bioecologic features and culture potential of Tabak, I., Black Sea trout (Salmo trutta labrax Pallas, 1811). Ministry of Agriculture and Rural Affairs, Central Fisheries Research Institute, Final Project Report, Trabzon (In Turkish), 193 pp. Togan, I., Fidan, A.Z., Yain, E., Ergüven & Emre, Y. (1995). Genetic structure of two Turkish brown trout populations. Journal of Fish Biology, 47 (Suppl. A), 164–9. Togan, I., Ergüven, A., Yalin, E. et al. (1996). Study on the genetic structure of populations: (i) Study on the genetic structure of human populations; (ii) Study on the genetic structure of brown trout (Salmo trutta L.) populations. A Technical report to the Scientiﬁc and Technical Research Council of Turkey, Ankara (In Turkish).

Chapter 10

The Status and Exploitation of Sea Trout on the Finnish Coast of the Gulf of Bothnia in the Baltic Sea E. Jutila1 , A. Saura1 , I. Kallio-Nyberg2 , A. Huhmarniemi3 and A. Romakkaniemi3 1

Finnish Game and Fisheries Research Institute, Viikinkaari 4, P.O. Box 2, FI-00791 Helsinki, Finland 2 Finnish Game and Fisheries Research Institute, Quark Fisheries Research Station, Korsholmanpuistikko 16, FI-65100 Vaasa, Finland 3 Finnish Game and Fisheries Research Institute, Oulu Game and Fisheries Research, Tutkijantie 2 A, FI-90570 Oulu, Finland

Abstract: The status of sea trout populations on the Finnish coast of the Gulf of Bothnia, the northernmost part of the Baltic Sea, was surveyed and assessed by electroﬁshing, recaptures of tagged ﬁsh and catch data. A great majority of the natural trout populations have died out and the remaining ones are endangered by the small size of the spawning populations. Supportive stocking has not strengthened natural reproduction. Trout have been ﬁshed during their feeding migration at a still younger age, most of them during the release year or in the following year, before they are sexually mature. The majority of small trout have been caught by gill nets with small mesh sizes used in coastal whiteﬁsh ﬁshing. In order to enhance the natural trout populations and to rationalise the ﬁshing, alternative procedures are presented for ﬁsheries management in the sea and rivers as well as recommendations for improving the management of the trout stocks and their environment. Keywords: Salmo trutta L., Sea trout, reproduction, stocking, ﬁshing regulation.

Introduction The Baltic Sea is the largest brackish water body in northern Europe. Its low salinity does not limit the survival of sea trout, which feed in the open sea (Landergren, 2001). The Baltic Sea with its northern part, the Gulf of Bothnia, is thus an especially large area for the feeding migration of sea trout (Christensen & Larsson, 1979). Sea trout populations differ in their migration patterns and both local and distant migrating populations exist in various parts of the sea (Svärdson & Fagerström, 1982; Kallio-Nyberg et al., 2002a). In the northern Baltic Sea, however, the migration of sea trout is mainly restricted to that area and 128

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129

the catch is based on releases of reared smolts that are subsequently caught in the coastal ﬁshery (ICES, 2002). In the Gulf of Bothnia, numerous brooks and small rivers and almost all salmon rivers previously had natural populations of sea trout. The reproduction of sea trout mostly takes place in the main stem and the lower parts and tributaries, while brown trout occupy the upper reaches and headwaters (Jutila et al., 1998; Haikonen et al., 2003). This distribution may, however, vary within the river system (Ryhänen, 1957) and sea trout have been found to have very ﬂexible modes of anadromy, also utilising the small spawning streams (Limburg et al., 2001). A large number of natural populations have died out because of a combination of overﬁshing and loss or degradation of the freshwater habitat (ICES, 2002). This chapter presents data on the status, exploitation and management of wild populations of sea trout on the Finnish coast of the Gulf of Bothnia. It also examines the reasons for the current situation and summarises the measures needed to reduce the most severe risks for the maintenance of the wild populations.

Data collection and assessment The data of this study were collected in a Finnish national survey on the sea trout populations in the Gulf of Bothnia (Kallio-Nyberg et al., 2002a). The occurrence of sea trout was studied by electroﬁshing in their natural and potential rivers in the 1980s and 1990s. The parr densities in natural sea trout rivers were monitored by annual electroﬁshing (400–600 V, 0.2 A, pulsed DC) at 8–14 permanent ﬁshing sites in each river according to the method described by Bohlin et al. (1989). In the Isojoki River, three successive removals were used at 3–5 sites to estimate the annual mean catchability (P-value) separately for parr 0+ and greater than 0+. These values were used to estimate the parr densities of the other sites where only one removal was used. The parr densities were estimated by the method of Junge & Libosvárský (1965). In the tributaries of the Tornionjoki River, correspondingly, the density estimation until 1998 was based on three successive removals and on 5-year moving averages of the P-value. Later, however, one removal with a standardised sampling duration of 10 min was applied in 1999–2000 concurrently with the earlier procedure, and since then solely at permanent ﬁshing sites. Only one removal was used in the Lestijoki River, where 0+ parr were rarely found. Supportive stocking has been carried out in all these rivers, but the released parr have not always been marked and thus the density of parr greater than 0+ is a combination of natural reproduction and stocking. Because only 0+ parr have been of natural origin, their density was considered as an index of the annual natural reproduction. The data on sea catches of trout were based on the ofﬁcial statistics of the professional marine ﬁshery (SVT, 2001, 2002, 2003). The data on the annual numbers of stocked sea trout were compiled by the Finnish Game and Fisheries Institute. The data on ﬁshing methods as well as on age and size distribution of the sea trout catches were obtained from Carlin tag returns of hatchery-reared sea trout, normally released as 2-year-old smolts. Annually 1–2% of the total number of smolts released has been tagged, and the tagging method has remained the same throughout the whole period.

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Results Status of the natural sea trout populations On the Finnish coast there are, altogether, over 40 rivers and brooks ﬂowing into the Gulf of Bothnia where sea trout originally occurred. Based on observations made of large (>1 kg) spawners in the coastal rivers, natural sea trout populations have been weak for almost 50 years (Kaukoranta et al., 2000), and they are now only found in three river systems: the Tornionjoki, the Lestijoki and the Isojoki (see Fig. 10.1). In addition, regular reproduction was generally found in two rivers and three brooks, but this was based on a mixed stock or dependent on supportive stocking. Electroﬁshing surveys in the other rivers revealed that they had lost their natural populations of sea trout. The disappearance of these populations has happened gradually since the nineteenth century by damming, dredging, pollution and the silting of rivers or by various combinations of these (Ikonen, 1984). The Tornionjoki River is over 500 km long and is a border river between Finland and Sweden. The water quality in the river has been assessed as good. Sea trout mainly spawn in several tributaries of the main stem. These streams have a total of over 250 ha of rapids that are suitable nursery areas for sea trout. Electroﬁshing carried out in the 1980s revealed the highest parr densities in the tributaries Äkäsjoki and Pakajoki. Regulation of ﬁshing, restoration of dredged rapids and stocking with hatchery-reared parr and smolts

Fig. 10.1 The Baltic Sea showing the existing and most important potential sea trout rivers on the Finnish coast of the northern (ICES subdivision 31) and southern (30) Gulf of Bothnia. Existing rivers: (1) Tornionjoki; (2) Kangosjoki; (3) Pakajoki; (4) Äkäsjoki; (5) Naamijoki; (6) Lestijoki and (7) Isojoki. Potential rivers: (1) Kiiminkijoki and (2) Merikarvianjoki.

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Table 10.1 Mean density of sea trout parr 0+ and greater than 0+ (individuals per 100 m−2 ) in the electroﬁshing sites in the Äkäsjoki (a tributary of the Tornionjoki River) and Lestijoki rivers (1990–2003) and in the Isojoki River (1993–2003). The density of parr greater than 0+ includes both wild and reared parr. Year

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Lestijokia

Äkäsjoki

Isojoki

0+

>0+

0+

>0+

0+

>0+

5.5 3.4 1.1 1.7 4.6 0 0 0.7 0.8 4.8 2.1b 6.8b 4.9b 15.4b

2.2 2.6 9.1 11.0 16.2 13.8 10.2 14.8 8.0 13.6 23.0b 11.9b 20.6b 17.0b

0.1 0.2 0.4 1.1 0.1 0 <0.1 0 0 <0.1 0.2 0.1 1.0 0.1

7.6 8.4 31.6 14.8 9.5 18.5 18.8 15.0 7.2 17.8 3.9 3.8 0.6 0.5

1.0 7.5 7.6 2.9 4.9 2.7 2.7 11.8 1.4

4.1 4.2 6.9 6.3 6.8 5.3 8.8 8.6 9.0

a Based on one removal. b Based on standardised duration of sampling.

have been employed to enhance the natural populations (Ikonen et al., 1986; Nylander & Romakkaniemi, 1995). The mean densities of 0+ parr gradually decreased in the early 1990s, but since the late 1990s they have risen again in many of the tributaries, such as the Äkäsjoki (Haikonen et al., 2003, see Table 10.1). The annual sea trout catches have been 2–3 tonnes in the river with the proportion of wild trout being 80–90% in the mid1990s (Haikonen et al., 2003). Despite the promising development of parr densities during the past few years in the Tornionjoki River, sea trout populations are still regarded as endangered. The Lestijoki River is 110 km long and ﬂows into the northern Gulf of Bothnia. There are about 17 ha of rapids downstream and 9 ha upstream, the lowest hydroelectric power plant being situated 32 km from the river mouth. The water quality has been assessed as satisfactory for sea trout reproduction, but low soil pH and increased sediment loading may occasionally reduce the survival of sea trout eggs and also hinder the smoltiﬁcation process (Soivio et al., 1998; Edén et al., 1999). Tagging experiments showed that most of the trout caught were undersized (TL < 40 cm) during their ﬁrst summer in the sea, and the heavy exploitation rate results in a shortage of spawners in the river. The sea trout population had already decreased in the 1960s, probably from dredging of the rapids, large-scale drainage of the catchment, and low pH caused by acidic compounds dissolved from the soil. Electroﬁshing carried out since the 1970s has only detected natural parr in some rapids and their densities have been low, on an average less than 1 parr 100 m−2 . The releases of

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reared parr have resulted in densities similar to other Gulf of Bothnia rivers (see Table 10.1), but have not achieved any major increase in the natural reproduction. Annual river catches of sea trout have been minimal. Some regulations control ﬁshing, for example gill net ﬁshing is prohibited in the river, but the basic problem of the intensive sea ﬁshing and overexploitation of sea trout remains unsolved. The natural population of sea trout is extremely endangered and on the verge of extinction. The Isojoki River is about 75 km long and situated on the southern Gulf of Bothnia. Two hydroelectric power plant dams serve as barriers to upstream migration, the lower one 11 km and the upper one 45 km from the river mouth. Although both of these have ﬁsh ladders, ﬁsh can only ascend the lower dam during high ﬂows and the functioning of the upper one is uncertain. In addition, there are also some minor dams that may hinder trout ascent. There are altogether 27 ha of rapids in the main stem below the upper power plant dam. Electroﬁshing carried out since 1995 has shown the mean density of wild 0+ parr to ﬂuctuate considerably (see Table 10.1). The density of parr greater than 0+ has been relatively less variable because of releases of reared parr. Rod ﬁshing is rather popular in the river, while net ﬁshing is prohibited. The river catch of sea trout has only been assessed in some years; varying between 200 and 600 kg (Jutila et al., 1998). According to tagging experiments, extensive sea ﬁshing is the most serious threat to the natural population, but the dam structures also prevent the ascent of spawners, especially in dry years. The natural population is endangered and dry years may increase this risk by limiting severely the extent of upstream migration. In addition, there are altogether over 30 partially or completely open rivers on the Finnish coast, with their total area of rapids rising to over 200 ha. Most of them have been dredged for log driving or for ﬂood protection and their water quality or hydrological characteristics may have deteriorated. They are, however, former sea trout streams and stocking experiments carried out and some of them indicate that they still may have the potential for sea trout production (Ikonen, 1984). Rearing and stocking At present, ﬁve different brood stocks of sea trout from rivers in the Gulf of Bothnia are maintained by hatchery rearing and used for egg production in Finland. The hatchery brood stocks have been founded by rearing parr or smolts caught from the river or eggs stripped from returning spawners. Depending on the river, one to three generations of brood stocks have been line bred in the hatcheries. As a result of the weak natural populations, brood stocks have been only partially completed by spawners or parr caught from the river in some years. The reared stocks are used for supportive stocking in their native rivers and for compensatory and sea ranching purposes near these rivers. In the 1990s, an average of 350 000 eyed eggs or newly hatched fry, 300 000 one-summer-old, 275 000 one-year-old or older parr and 61 000 smolts were released annually in the Gulf of Bothnia rivers to increase their smolt production. In addition, to reintroduce the hatchery stock of the Isojoki River to the wild, its offspring have since the 1990s been stocked in the Kiiminkijoki River, which lost its native stock in the 1970s.

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In the 1990s, an average of 487 000 sea trout smolts were released annually in the Gulf of Bothnia area as compensatory releases or for sea ranching. About 200 000 of these smolts have been released as compensation for environmental damage caused by the construction of hydroelectric power plants in the rivers. A considerable proportion of the annual releases are aimed at sea ranching and funded by the water owners and public associations. Almost all of these smolts have been released directly into the sea or near the river estuary without any possibility of the ﬁsh returning to spawn in the river.

Fishing The most accurate statistics on the sea trout catches from the Finnish side of the Gulf of Bothnia are available from the records of professional net ﬁshermen using gill nets and fyke nets, even though recreational ﬁshermen using mostly gill nets and not so commonly rod ﬁshing usually catch the majority of the sea trout. The annual catch of professional ﬁshermen in the 1990s ﬂuctuated from 50 to 160 tonnes (see Fig. 10.2), most of that being taken from the southern part of the Gulf. The catches of recreational ﬁshermen were only assessed in some years in the 1990s, when they ranged from about 60–70 tonnes. The total catch of the Finnish ﬁshermen from the Gulf of Bothnia thus varied roughly between 150 and 300 tonnes per year in the 1990s. The total annual catch of Finnish and Swedish ﬁshermen from the Gulf of Bothnia ranged from about 190–470 tonnes, comprising about 20% of the sea trout catch from the Baltic Sea. Nearly all these catches are based on reared smolts released into the rivers or directly into the sea. On the basis of tagging experiments, 75–80% of trout released in the northern or southern Gulf were recovered within 100–200 km of the point of release. This migration pattern is similar to the Swedish stocks (Larsson et al., 1979; Svärdson & Fagerström, 1982), but the migration distance is much greater than found in the Atlantic sea trout stocks, for example in northern Norway (Berg & Berg, 1987). About 80% of the recoveries of tagged sea trout came from gill net ﬁshing. In the northern Gulf of Bothnia, almost all of this catch was taken in coastal areas by bottom gill nets, mostly as a by-catch of whiteﬁsh ﬁshing. In the southern Gulf, about 20% of catch was taken with surface gill nets. In

Catch, tonnes

200 150 100 50

Fig. 10.2

2002

2000

1998

1996

1994

1992

1990

0

Annual sea trout catch of professional ﬁshermen in the Gulf of Bothnia (1990–2002).

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the northern Gulf, most recoveries came from ﬁsh caught with gill nets with mesh sizes (knot distance) of 38–50 mm; while in the southern Gulf the mesh size was a little larger, 40–55 mm. Fyke nets were the most common ﬁshing method for sea trout after gill nets, especially in the northern Gulf. Despite its increasing popularity, river ﬁshing practised mostly only by rod ﬁshing, only yielded less than 5% of the recoveries of tagged ﬁsh, while in the Atlantic rivers a considerable proportion of recaptures come from the river (Jonsson et al., 1994). Sea ﬁshing for migratory ﬁsh commonly predominates in the Baltic Sea (Svärdson & Fagerström, 1982), and because of the extensive mixed stock ﬁshing, some rivers in Finland, for instance, have lost their natural salmonid populations (Kaukoranta et al., 2000). Tagging studies have shown that the proportion of ﬁsh caught in the year of release has been continuously high and that group now comprises around 50% of all recoveries in the Gulf of Bothnia. The mean weight of trout caught during the ﬁrst sea year is less than 500 g. In the northern Gulf, over half of the recoveries were from ﬁsh released in the same year, while in the southern Gulf the largest proportion mostly comprised ﬁsh caught in the second year after release. In recent years the proportion of ﬁrst year ﬁsh has also increased there (see Fig. 10.3). The high proportion of ﬁsh caught during their ﬁrst sea year is also common in the Atlantic rivers (Jonsson et al., 1994), but in the Gulf of Bothnia this is especially disadvantageous because the sea trout here generally attain sexual maturity during their third sea year. During the past few decades, the annual output of sea trout smolts into the Gulf of Bothnia from Finland has comprised almost totally reared smolts, the proportion of natural smolts being less than 5%. In the 1980s, the stocking of sea trout yielded considerably higher recapture rates than current rates (Fig. 10.4). Normally the recapture rate has been higher in the southern than in the northern Gulf. At the beginning of the 1990s the recapture rate was exceptionally high in the Gulf of Bothnia, but has since declined. The recapture rate decreased to less than 5% by the mid-1990s. The results have improved a little in recent years in the southern Gulf (see Fig. 10.4). Higher recapture rates than these have earlier been reported in Sweden (Larsson et al., 1979; Svärdson & Fagerström, 1982). In the present circumstances, releases of sea trout for sea ranching purposes are not economically proﬁtable, as found, for example, in the Norwegian River Imsa with clearly higher recapture rates (Jonsson et al., 1994). Over half of the recoveries of Carlin-tagged trout come from undersized trout (TL < 40 cm) in the northern Gulf of Bothnia. The proportion has been lower in the southern Gulf, but in recent years it has also increased there. The mean weight of the captured sea trout has varied in the northern Gulf between 500 g and 1 kg, and in the southern Gulf between 1 and 2 kg. The mean weights have ﬂuctuated relatively more in the southern than in the northern Gulf, the highest values coinciding in both areas in the early 1990s.

Discussion The basic requirement for the maintenance and enhancement of sea trout populations is the available extent of suitable stream habitats for the natural reproduction. In the streams of

Status of Sea Trout in the Gulf of Bothnia

135

Northern Gulf of Bothnia 100

Percentage

80 60 40 20

1999

2000

1999

2000

1998

1997

1996

1995

1994

1993

1992

1991

1990

0

Year class Southern Gulf of Bothnia 100

Percentage

80 60 40 20

1998

1997

1996

1994

1993

1992

1991

1990

0

Year class 1. sea year

2. sea year

3. sea year

>3. sea year

Fig. 10.3 The annual age structure of the sea trout catch in the northern and southern Gulf of Bothnia based on Carlin-tagged smolts released (1990–2000).

the Gulf of Bothnia, environmental degradation has reduced the quality and quantity of the spawning and nursery habitat. Even though mostly tolerable, the conditions in streams are commonly suboptimal, and a reduction in sediment and nutrient loading from agriculture, forestry and settlements is necessary. Restoration of dredged rapids has been carried out since the 1980s in many rivers, but it has generally only been partial and should be supplemented. Furthermore, mill and power plant dams hinder or prevent the spawning migration in many rivers. Partial obstacles may also make ascent impossible, especially in

136

Sea Trout Northern Gulf of Bothnia

Southern Gulf of Bothnia

Recapture rate (%)

20 16 12 8 4

2000

1999

1998

1997

1996

1995

1994

1993

1992

1991

1990

0

Stocking year class

Fig. 10.4 The recapture rate (%) yielded from releases of Carlin-tagged sea trout smolts in the Gulf of Bothnia (1990–2000) in relation to the stocking year class.

dry seasons, and efﬁcient ﬁsh ladders should therefore be incorporated in the dams, at least in the Isojoki and Lestijoki rivers. During the past few decades, releases of reared parr and smolt have been used for supporting weak natural populations. Parr have also been stocked in river reaches upstream from impassable dams. At present, releases of reared parr increase or support natural parr production in empty or sparsely occupied nursery areas of the rivers. Smolt stocking may have a minor effect on the spawning stock if the undersized reared smolts are caught at the beginning of their sea migration. The numbers of ascending spawners have continuously been so low that stocking is obligatory for the maintenance of regular migration of trout into the natural spawning rivers until more effective ﬁshing regulation can be achieved. Releases of reared parr and smolts, are also necessary for reintroducing smolt production in more than ten potential sea trout rivers, which, on the basis of stocking experiments and existing nursery habitats, still have the potential for natural reproduction. Here, the effects of stocking will also depend on the regulation of ﬁshing, which does not properly protect sea trout during their sea migration at present. Intensiﬁed ﬁshing regulation had a beneﬁcial effect on the enhancement of the natural Atlantic salmon populations in the Gulf of Bothnia in the late 1990s (e.g. Romakkaniemi et al., 2003). The sea trout populations also have a basically similar potential for enhancement if ﬁshing is appropriately regulated. The need for intensive regulation is not necessarily permanent. Enhancement may proceed rapidly with effective regulation and the need may thus be temporary. At the moment the status of the Finnish sea trout populations in the Gulf of Bothnia is so critical that they should not be ﬁshed either at sea or in streams in order to allow the remaining natural populations to survive. The regulation of sea trout ﬁshing cannot be practised without also limiting the ﬁshing for other ﬁsh species. This includes the prohibition of all such ﬁshing where captured sea trout as a by-catch cannot be released unharmed back into the water. Gill net ﬁshing with small mesh sizes (<50 mm knot distance), which is a central ﬁshing practise in the Gulf of Bothnia, should be prohibited in particular.

Status of Sea Trout in the Gulf of Bothnia

137

Table 10.2 Summary of the possible options for ﬁshing regulation and an assessment of their effects on Finnish sea trout populations in the Gulf of Bothnia. Alternative

Minimum TL (cm)

Mesh size (knot distance) and number of gill nets/household

Fishing prohibited

Assessed effects on sea trout populations

I

65

45

III

40 55 (commercial)

≥50 mm max. 3 nets

IV

50

V (present)

40

≥45–50 mm ≤27–30 mm number of gill nets unrestricted Unrestricted

In natural sea trout rivers and the sea 1–30 km from the river mouth In natural sea trout rivers and the sea 5–15 km from the river mouth In natural sea trout rivers and the sea 10–30 km from the river mouth In natural sea trout rivers and the river mouth

Natural populations will recover

II

≥65 mm number of gill nets unrestricted ≥55 mm max. 6 nets

Only regulations included in the ﬁsheries legislation

Natural populations will recover slowly Natural populations may recover, but it is not certain Recovery of natural populations unlikely

Natural populations probably disappear

Recommendations In Table 10.2 we present ﬁve options for ﬁshing regulations that have different effects on the natural populations. In Option I the size of the sea trout is so large that they attain the minimum biological size for reproduction (TL > 65 cm) and can spawn at least once. Only this option can guarantee the enhancement of the natural populations. The basis of Options II and III is to reduce the disadvantages of sea trout ﬁshing considerably: even though they cannot entirely remove them. They are compromises between the measures needed for the enhancement of the sea trout populations and the continuation of the present ﬁshing of other ﬁsh species, especially whiteﬁsh. At best they could enhance natural sea trout populations, but their effect would probably be insufﬁcient. Option IV corresponds to the ﬁshing regulations proposed for the Gulf of Finland by several Finnish ﬁshing associations. If the present ﬁshing regulation continues (Option V), it would probably result in the disappearance of all natural sea trout populations in the near future, or only the strongest of them might survive. Furthermore, the maintenance of the reared brood ﬁsh populations might be uncertain in future.

References Berg, O.K. & Berg, M. (1987). Migrations of sea trout, Salmo trutta L., from the Vardenes River in northern Norway. Journal of Fish Biology, 31, 113–21. Bohlin, T., Hamrin, S., Heggberget, T., Rasmussen, G. & Saltveit, S.J. (1989). Electroﬁshing – theory and practice with special emphasis on salmonids. Hydrobiologia, 173, 9–43.

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Christensen, O. & Larsson, P.-O. (1979). Review of Baltic salmon research. ICES Cooperative Research Report, Copenhagen, Vol. 89, 124 pp. Edén, P., Weppling, K. & Jokela, S. (1999). Natural and land-use induced load of acidity, metals, humus and suspended matter in Lestijoki, a river in western Finland. Boreal Environment Research, 4, 31–43. Haikonen, A., Romakkaniemi, A., Keinänen, M., Mäntyniemi, S. & Vatanen, S. (2003). Monitoring of the salmon and trout stocks in the river Tornionjoki in 2002. Kala-ja riistaraportteja (In Finnish with English abstract). Finnish Game and Fisheries Research Institute, Oulu, Vol. 275, pp. 1–54. ICES (2002). Sea trout in the Baltic. ICES Cooperative Research Report, Copenhagen, Vol. 255, pp. 813–15. Ikonen, E. (1984). Migratory ﬁsh stocks and ﬁshery in regulated Finnish rivers ﬂowing into the Baltic Sea. In: Regulated Rivers (Lillehammer, A. & Saltveit, S.J., Eds). Universitetsforlaget, Oslo-Bergen-Stavanger-Tromsø, pp. 437–51. Ikonen, E., Jutila, E., Koljonen, M.-L., Pruuki, V. & Romakkaniemi, A. (1986). The status of the sea trout stocks, their genetic differences and needs for hatchery-rearing in the Tornionjoki river basin. Monistettuja julkaisuja (In Finnish). Finnish Game and Fisheries Research Institute, Helsinki, Vol. 57, pp. 1–103. Jonsson, N., Jonsson, B. & Hansen, L.-P. (1994). Sea ranching of brown trout, Salmo trutta L. Fisheries Management and Ecology, 1, 67–76. Junge, C.O. & Libosvárský, J. (1965). Effects of size selectivity on population estimates based on successive removals with electrical ﬁshing gear. Zoologicke Listy, 14, 171–78. Jutila, E., Ahvonen, A., Laamanen, M. & Koskiniemi, J. (1998). Adverse effects of forestry on ﬁsh and ﬁsheries in stream environments of the Isojoki basin, western Finland. Boreal Environment Research, 3, 395–404. Kallio-Nyberg, I., Jutila, E. & Saura, A. (Eds) (2002a). The status and ﬁshing of sea trout in the Gulf of Bothnia area. Kalatutkimuksia Fiskundersökningar (In Finnish with English abstract). Finnish Game and Fisheries Research Institute, Helsinki, Vol. 182, pp. 1–69. Kallio-Nyberg, I., Saura, A. & Ahlfors, P. (2002b). Sea migration pattern of two sea trout (Salmo trutta) stocks released in the Gulf of Finland. Annales Zoologici Fennici, 39, 221–35. Kaukoranta, M., Koljonen, M.-L., Koskiniemi, J., Pennanen, J. & Tammi, J. (2000). Atlas of Finnish Fishes (English summary). Lamprey, brook lamprey, Atlantic salmon, brown trout, Arctic charr, whiteﬁsh, vendace, grayling, asp, vimba, spined loach and bullhead – the distribution and status of stocks. Finnish Game and Fisheries Research Institute, Helsinki, 40 pp. Landergren, P. (2001). Survival and growth of sea trout parr in fresh and brackish water. Journal of Fish Biology, 58, 591–93. doi:10.1006/jﬁ.2000.1460. Larsson, P.-O., Steffner, N.G., Larsson, H.-O. & Eriksson, C. (1979). Review of Swedish sea trout (Salmo trutta L.) stocks based on results of tagging experiments. Salmon Research Institute Report (In Swedish with English summary). Swedish Salmon Research Institute Älvkarleby, Vol. 2, pp. –31. Limburg, K.E., Landergren, P., Westin, L., Elfman, M. & Kristiansson, P. (2001). Flexible modes of anadromy in Baltic sea trout: making the most of marginal spawning streams. Journal of Fish Biology, 59, 682–95. doi:10.1006/jfbi.2001.1681. Nylander, E. & Romakkaniemi, A. (1995). Sea trout and ﬁshing in the Tornionjoki River. Kalatutkimuksia Fiskundersökningar (In Finnish with English abstract). Finnish Game and Fisheries Research Institute, Helsinki, Vol. 89, pp. 1–63. Romakkaniemi, A., Perä, I., Karlsson, L., Jutila, E., Carlsson, U. & Pakarinen, T. (2003). Development of wild Atlantic salmon stocks in the rivers of the northern Baltic Sea in response to management measures. ICES Journal of Marine Science, 60, 329–42. Ryhänen, R. (1957). Summary of the observations on trout in the Isojoki (Finland). Rapports et Procès-verbaux des Réunions du Conseil International pour l’Exploration de la Mer, 148, 76–80. Soivio, A., Myllynen, K., Pakkala, J. & Jokela, S. (1998). Smolting of the brown trout (Salmo trutta L.) in Lestijoki water. Boreal Environment Research, 3, 387–93. Svärdson, G. & Fagerström, Å. (1982). Adaptive differences in the long-distance migration of some trout (Salmo trutta L.) stocks. Report of the Institute of Freshwater Research, Drottningholm, Vol. 60, pp. 51–80. SVT (2001). Finnish ﬁshery time series. Agriculture, Forestry and Fishery. Finnish Game and Fisheries Research Institute, Helsinki, Vol. 60, pp. 1–112. SVT (2002). Professional marine ﬁshery 2001. Agriculture, Forestry and Fishery. Finnish Game and Fisheries Research Institute, Helsinki, Vol. 57, pp. 1–55. SVT (2003). Commercial marine ﬁshery 2002. Agriculture, Forestry and Fishery. Finnish Game and Fisheries Research Institute, Helsinki, Vol. 55, pp. 1–55.

Chapter 11

Sea Trout (Salmo trutta L.) in European Salmon (Salmo salar L.) Rivers N.J. Milner1 , L. Karlsson2 , E. Degerman2 , A. Johlander2 , J.C. MacLean3 and L-P. Hansen4 1 Environment

Agency, C/O School of Biological Sciences, University of Bangor, Deiniol Rd, Bangor LL57 2UW, Wales, UK 2 National Board of Fisheries, Inst. of Freshwater Research, SE 178 93 Drottningholm, Sweden 3 Freshwater Fisheries Laboratory, Inchbraoch House, South Quay, Ferryden, DD10 9SL, Montrose, Scotland 4 NINA,P.O. Box 736, SentrumN-0105 Oslo, Norway

Abstract: Traditionally, ﬁsheries for sea trout (Salmo trutta L.) and Atlantic salmon (Salmo salar L.) have been assessed and managed independently. More recently the need to consider multi-species approaches to management has emerged. Sea trout and salmon have similar anadromous life cycles; adults spawn in same areas in rivers and as juveniles they frequently coexist in the same freshwater habitat. Sympatry presents potential practical problems, for example in setting biological reference points based on stock–recruitment relationships, if densities of one species inﬂuence the other. This chapter reviews the potential scale of this issue across the North-East Atlantic Commission (NEAC) area. The extent of mixed ﬁsheries and the relative importance of the two species were examined using rod catch data from 192 rivers in ﬁve countries on the Eastern Atlantic seaboard. Juvenile data from electro-ﬁshing surveys were used to investigate relative abundance in streams of different sizes. Estimates of egg deposition showed that by considering the contribution of only one species the total salmonid egg deposition could be underestimated by up to ﬁve times, an error that could be signiﬁcant when setting biological reference points for either. Sea trout were relatively more abundant than salmon in smaller catchments, which had proportionally more of their total wetted area contributed by smaller channels (<6 m) in which trout are shown to be relatively more abundant than salmon. However, in spite of extensive evidence of interspeciﬁc competitive interaction at micro to macrohabitat scale, examples of such effects at ﬁshery or catchment scale are scarce or inconsistent and some reasons for this are discussed. Keywords: Salmo trutta L., salmon, relative abundance, egg deposition, European distribution, interspeciﬁc interactions.

139

140

Sea Trout

Introduction This chapter reviews the relative abundance of sea trout (Salmo trutta L.) and salmon (Salmo salar L.) in salmon rivers of Europe. The study was part of an EU Concerted Action Programme (SALMODEL), developing the scientiﬁc basis of wild Atlantic salmon management (Crozier et al., 2003) across the North-East Atlantic Commission (NEAC) area. The aim was to assess the likely contributions of both species to total salmonid egg deposition in catchments and to assess the potential for interactions that might need to be considered when setting salmon conservation limits (CLs). A feature of S. trutta is the diversity of its life-history strategies ranging from complete freshwater residency to full anadromy, as typically seen in S. salar (Jonsson & L’Abée-Lund, 1993; Jonsson and Jonsson, 2006). By virtue of generally greater size and predominance of females in the sea migrants, that component is likely to be the dominant source of total trout egg depositions in most rivers having migratory trout populations. The management of salmonids has focused traditionally on individual species. This has arisen because of political and funding imperatives, directing science and management towards the most economically valuable species, which in Europe, has been the Atlantic salmon. More recently, the recognition that ecology of many river ﬁsh species are interdependent and are jointly linked to wider ecosystem function has led to recommendations for multi-species and ecosystem approaches to management (MAFF, 2000; Garcia et al., 2003). In most circumstances the assessment methods for salmon and sea trout are similar. Sport and commercial ﬁshing methods are usually the same, so regulations designed to protect one species can affect exploitation of the other. Thus, in developing the background for the future management of sea trout it is appropriate to review their relative abundance in the context of their principal cohabiting salmonid species. Sea trout, the migratory form of brown trout (S. trutta), are found throughout the range of the Atlantic salmon in continental Europe, the British Isles, Scandinavia and Iceland, extending as far south as latitude 42◦ on the Atlantic coast (Elliott, 1989; ICES, 1994). The returning adults of both anadromous species spawn in similar river habitats and in many cases spawning females of both species have overlapping or similar size distributions. Consequently, the egg deposition densities (numbers/area) may be similar. Both species spawn in stream gravels, often in the same areas. Salmon and trout juveniles are usually sympatric, with similar densities and closely overlapping habitat requirements (Bremset & Heggenes, 2001). These observations present a prima facie case that abundance of each species might be expected to inﬂuence egg-to-smolt survival of the other. Such effects on smolt production could lead to similar variation in adult abundance. There has been no systematic wide-scale assessment of relative abundance of the two species or its implications for management and this chapter provides a ﬁrst overview of this topic. Data sources were a problem, because much more information is available for salmon than trout in most cases; but for neither species are there extensive, long-term systematic data sets on stock abundance and dynamics. In this study rod catch data are used to explore across-river variation, and juvenile data from freshwater electro-ﬁshing surveys are used to explore abundance within rivers.

Sea Trout in European Salmon Rivers

141

Data sources and methods Rod catches River-speciﬁc rod catch data were obtained for the period 1994–99, from nine countries based upon various national recording methods. In several cases, catch statistics for both species were not available on a sufﬁciently consistent basis to contribute to across-country analyses. In particular, exploitation and catch reporting rates were unknown in many cases, but suspected to be different between the two species and probably across countries. In no rivers were catch data available for non-migratory trout, so all following comparisons were based on the migratory component. Simple comparison of size-based fecundity indicated that trout egg deposition was determined mainly by the migratory component, so this was not thought to be a serious limitation. To provide a simple index of the relative importance of the species the ratio value [S, salmon]/value[T, sea trout] was calculated for, variously, rod catch (N), biomass (W ) or egg deposition (E). Thus, a river having average rod catches of say 100 salmon and 50 sea trout has S/T(N) = 2.0; but, because on average sea trout are smaller than salmon, the ratio expressed as total eggs, S/T(E), will be somewhat greater. Annual egg deposition for sea trout and salmon was estimated for the four UK rivers (Tamar and Lune in England, Tywi and Dee in Wales), where data were available on exploitation rate and escapement composition for both salmon and sea trout (Milner et al., 2000; Environment Agency, 2003). For these four rivers a power function relationship between the S/T(E) (Y ) and S/T(N) (X) values was established as Y = 1.9067X 0.6121 (R2 = 0.958, n = 4), and this was used to estimate S/T(E) values for all rivers from S/T(N) values. Electro-ﬁshing surveys Rod catch data apply to whole catchments in most cases (Scotland has reach-speciﬁc rod catch data) and allow no discrimination across sub-catchments. In contrast juvenile abundance data from electro-ﬁshing surveys provide information on absolute and relative abundance at individual sites, from which inferences can be made about breeding and rearing distributions across the diversity of stream types within catchments. A major constraint however is that quantitative electroﬁshing has mostly been conﬁned to smaller streams (e.g. <10 m) and so populations in larger channels, such as main stem rivers, are less welldescribed. In most cases comprehensive national survey data for juvenile salmonids were difﬁcult to ﬁnd. Data were, however, available for Sweden (Swedish Electroﬁshing Register) and England and Wales, based on national monitoring programmes and from speciﬁc studies from Scotland (Gardiner, pers. comm.).

Results Patterns of variation Data suitable for analysis of coexisting sea trout and salmon ﬁsheries, were reported from 192 rivers (Table 11.1), of a total count of approximately 1500 NEAC salmon rivers. There

142

Sea Trout Table 11.1 Summary of catch data used for comparison of salmon and sea trout abundance across rivers. Column 3 includes only those rivers where data were considered to be suitable for comparative analyses. Country

Total salmon rivers

Salmon rivers with salmon and sea trout catch data

Overview of sea trout status in salmon rivers

Norway Scotland Ireland England and Wales Iceland France N. Ireland Sweden Finland

609 382 194 134 65 39 27 23 2

43 47 20 68 0 0 0 14 0

Locally important Ubiquitous, locally very important Ubiquitous, locally very important Ubiquitous, locally very important Few sea trout Few sea trout in salmon rivers Important in some rivers Locally very important Locally important within two large rivers

Table 11.2 Summary of river features and rod catch statistics for NEAC rivers having salmon and sea trout catch data. Country

Norway Scotland Ireland England and Wales Sweden Overall mean Median Min. Max.

Sample (n)

Catchment area (km2 )

ADF (m3 s−1 )

43 47 20 68

1 790 673 534 704

47.1 19.4 8.5 16.0

14

1 028

14.6

192

948 306 19.5 16 389

23 9 1.138 289

Salmon catch (N)

Sea trout catch (N)

Salmon avg. weight (kg)

Sea trout avg. weight (kg)

Mean S/T(N) ratio

1 612 1 198 646 252

696 447 1263 487

ND 3.3 2.6 3.3

ND 0.8 0.5 0.8

3.4 7.6 0.8 1.7

93

121

3.3

1.1

0.9

817

578

3.2

0.8

3.4

178 0.2 15 605

266 0.4 5357

3.2 1.0 5.5

0.7 0.3 1.8

2.4 0.002 74.6

Values are means for the river sample for each country. ADF = average daily ﬂow, N = annual rod catch (declared, uncorrected data 1994–98).

are many more rivers and streams where both species coexist, and even more where only sea trout occur, but where catch statistics are absent or have been inconsistently collected (ICES, 1994). The samples of rivers were highly selective in some countries; being inﬂuenced by the availability of contemporary sea trout catch data (Table 11.1). Consequently, the physical features (catchment area and average daily ﬂow [ADF]) and mean catches are unlikely to be unbiased samples of the salmonid rivers in those countries (Table 11.2). Across the whole sample of 192 rivers, mean annual salmon and sea trout rod catches (1994–98) were 817 and 578, respectively, but the sea trout catches were more statistically dispersed, the median catches being 178 and 266, respectively (Table 11.2). Mean (5 years)

Sea Trout in European Salmon Rivers

143

Salmon, R 2 = 0.363

5.0

Sea trout, R 2 = 0.201

Log10 (annual rod catch)

4.0

3.0

2.0

1.0

0.0

–1.0 0.0

1.0

2.0

3.0

4.0

5.0

Log10 (Catchment Area)

Fig. 11.1 The inﬂuence of catchment area (km2 ) on average salmon (solid line) and sea trout (dashed line) rod catches (1994–98) in 192 NEAC rivers.

catches in individual rivers of salmon and sea trout ranged up to 15 605 and 5357, respectively. In 50% of rivers salmon were at least 2.4 times greater than sea trout catches. Sea trout catches were relatively higher in smaller catchments (Fig. 11.1), so that the salmon/trout ratio (S/T(N)) increased with catchment size. The average weight of sea trout in all rivers was always less than that of salmon, although large (>5 kg) sea trout are characteristically common in certain rivers. Size distribution data (1994–98) for the total England and Wales rod catches demonstrated the smaller size of sea trout compared with salmon (Fig. 11.2), a pattern seen also in other countries. Consequently, the numerical dominance of sea trout (68% sea trout) in rod catches during that period in England and Wales was reversed in terms of total biomass (31% sea trout). Across the 192 rivers there was no signiﬁcant (P < 0.05) relationship between average sea trout weight and average salmon weight. However, average weights of both species were signiﬁcantly (P < 0.01) positively correlated with catchment size (Fig. 11.3), but less strongly so for sea trout (R2 = 0.15) than salmon (R2 = 0.24). The across-country variation in relative abundance (as catch or egg deposition) was described by ranking the S/T(N) and S/T (E) ratios respectively, tabulating cumulative salmon catch with increasing ratio and plotting cumulative proportion (%) against the ratios (Fig. 11.4a shows this for egg deposition). The variation across countries was marked (Table 11.3) and the percentage of the national salmon catch coming from rivers that were numerically dominated by sea trout (i.e. sea trout >50% of combined migratory catch) ranged between 6% (Scotland) and 50% (England and Wales). Overall, 13% of the NEAC

144

Sea Trout 75

Salmon (n = 83 335)

Percentage of frequency

Sea trout (n = 180 385) 50

25

0 0

2

4

6

8

10

12

14

16

18

20

22

24

Weight class (pounds)

Fig. 11.2 Size–frequency distribution of salmon and sea trout in rod catches, based on English and Welsh catch statistics, 1994–98, all data pooled.

Salmon

Log10 (mean weight) kg

1

Sea trout

R 2 = 0.239

0.5

0 R 2 = 0.148 –0.5

–1 1

2

3 Log10 (Area) ha

4

5

Fig. 11.3 The relationship between catchment area and average weight of salmon and sea trout caught in river rod ﬁsheries.

Sea Trout in European Salmon Rivers E&W Norway

(a)

Scotland Sweden

145

Eire

Cumulative percentage of catch

100

75

50

25

0 0.001

0.01

0.1

1

10

100

S/T(E ) ratio E&W

(b) Norway

Scotland Sweden

Eire

Cumulative percentage of rivers

100

75

50

25

0 0.001

0.01

0.1

1

10

100

S/T(E ) ratio

Fig. 11.4 (a) Cumulative percentage of national catches occurring at different ratios of salmon/trout egg deposition. (b) Cumulative percentage of national rivers having different ratios of salmon/trout egg deposition.

salmon rod catch came from rivers with more sea trout than salmon. An alternative way to express relative species abundance is to estimate the percentage of the national stock of rivers (as opposed to national catch) having more or less sea trout than salmon (Fig. 11.4b). By this method 58% of the 192 NEAC rivers had more sea trout than salmon in their catches (Table 11.3), and the range was 32% (Scotland) to 82% (England and Wales). To enable direct estimation of egg deposition rates of the two species, sufﬁcient data on exploitation rates, size distributions, sex ratios and fecundity were available only in four rivers. In those rivers (Tamar and Lune in England, Tywi and Dee in Wales) sea trout contributed between 24% and 65% of total catchment egg deposition (Table 11.4). However, inter-annual variation was high; thus in the river Tywi (Wales) the salmon and sea trout egg depositions were 8.8 and 32 million respectively in 1999, compared with 11.2 and 9.8 million

146

Sea Trout Table 11.3 Percentage of national salmon rod catch, coming from rivers with sea trout greater than 50%, expressed as ratios of catches (S/T[N]) and estimated ratio for egg deposition (S/T[E]). Country

From rivers with S/T(N) < 1.0

Norway Scotland Ireland England and Wales Sweden All rivers

From rivers with S/T(E) < 1.0

% National catch

% National rivers

% National catch

% National rivers

8 6 31 50 24 13

37 32 70 82 79 58

1 1 16 19 3 4

12 17 35 48 50 31

Table 11.4 Estimates of egg deposition (x million eggs) by salmon (S) and sea trout (ST) and salmon/sea trout ratios (S/ST). Tamar

Twyi

Dee

Lune

Year

Salmon

Sea trout

S/ST

Salmon

Sea trout

S/ST

Salmon

Sea trout

S/ST

Salmon

Sea trout

S/ST

1995 1996 1997 1998 1999

13.9 10.7 7.4 10.2 9

5.1 4.5 3.8 4.4 7

2.73 2.38 1.95 2.32 1.29

11.2 11.2 11.5 6.3 8.8

14.1 9.8 14.8 20.8 32.4

0.79 1.14 0.78 0.30 0.27

14.6 13 13.9 16.2 13.2

3.9 3.6 3.7 7.1 4.7

3.74 3.61 3.76 2.28 2.81

8.8 9.2 6 14.6 10.9

6.5 6.7 10.3 9.6 9.2

1.35 1.37 0.58 1.52 1.18

Mean SD

10.24 2.41

4.96 1.23

2.13 0.55

18.38 8.76

0.66 0.37

14.18 1.29

4.6 1.46

3.24 0.66

9.9 3.16

9.8 2.24

8.46 1.74

1.20 0.37

in 1996. The analysis of these four rivers also showed that the S/T(N) ratio underestimated the S/T(E) ratio based on egg deposition, by on average 2.4. An adjustment factor derived from the power function relationship between the two variables in the four rivers was used to estimate S/T(E) for all other rivers. The adjusted S/T ratios, S/T(E), based on eggs rather than rod catch, showed that overall only 4% of the total NEAC salmon rod catch came from rivers where catches were dominated by sea trout. However, 31% of the NEAC rivers had more sea trout than salmon egg deposition (Table 11.3). The difference is because many rivers produce comparatively small catches of salmon. Sweden, England and Wales and Ireland had the highest incidences of sea trout egg deposition (50%, 48% and 35% of rivers respectively had egg depositions dominated by sea trout), compared with Norway and Scotland (12% and 17% respectively). The proportions of national salmon catch coming from rivers where sea trout dominated egg deposition was much less, ranging between 1% (Norway and Scotland) and 19% (England and Wales). A signiﬁcant, but undetermined, source of error in this analysis is that the correction factor was based on a small sample of rivers with comparatively large sea trout

Sea Trout in European Salmon Rivers 100

147

England and Wales, n = 2164 Sweden, n = 1658

Percentage salmon .0+

Scotland, n = 227 75

50

25

0 0–2 2–4 4–6 6–8 8–10 10– 12– 14– 16– 18– 20– >35 12 14 16 18 20 35 Width (m) Fig. 11.5 The occurrence of salmon as a percentage of the total salmonid (salmon + trout) fry (summer 0+) in streams of three NEAC countries.

and catchment areas ranging between 930 and 1793 km2 . The correction factor would be smaller for smaller rivers, which typically have smaller total catches of both species and smaller sea trout, leading to lower sea trout egg deposition. Relative abundance of juveniles A limitation of using rod catch data is that, because catch returns are normally based on whole rivers, it gives no information on the distribution or relative abundance of the two species within catchments. Notwithstanding uncertainties about the true relationship between catch and escapement or egg deposition, the simple assumption of uniform distribution around catchments, in proportion to catches, is certainly ﬂawed in most cases. No systematic surveys of spawning distribution of the two species were found for any catchment. Examination of within-river spatial patterns was therefore based on the abundance of juveniles, derived from electro-ﬁshing surveys, usually in late summer (July–September). Three data sets, for Sweden, England and Wales together and Scotland demonstrated the same result that the proportions of the two species varied systematically with stream width (Fig. 11.5). Trout were dominant (>50% of total juvenile salmonids) in streams narrower than 6 m in all data sets. In order to translate this result in terms of total catchment salmonid production, it was necessary to know what proportion of nursery areas lie within different channel width categories. The starting hypothesis was that stream widths vary predictably around catchments (Ferguson, 1981), such that narrow, low-order streams contribute most to total catchment stream length whereas most wetted area is contributed by higher order, wider channels. It follows that in smaller streams having higher proportions of low-order channels

148

Sea Trout 100

Percentage of area at width <5 m

90 80 70 60 50 40 30 20 10 0 0

250

500

750

1000

1250

1500

Wetted area (Ha)

Fig. 11.6 The relationship between wetted stream area (as % of total stream wetted area) lying within channels of less than 5 m width and total wetted area (Ha).

a higher proportion of total wetted area will be contributed by smaller, narrower channels. However, the data to test this hypothesis over a wide range of river types (e.g. glacial/surface water or ground water sources, etc.) were hard to ﬁnd, but an example from England and Wales illustrates the point. The wetted area of small stream (<5 m wide) was greatest in smallest catchments (Fig. 11.6) and overall about 20% of the wetted area of the 39 rivers in that survey was contributed by channels narrower than 6 m (Fig. 11.7).

Discussion Rod catch data demonstrate the extent to which salmon and sea trout coexist in streams and rivers across the NEAC area, but there was systematic variation across countries. For example, Scotland and Norway tended to have relatively smaller sea trout catches for a given catchment size than Sweden or England and Wales. The reasons for this are not clear, but may be partly related to variation in river structure reﬂecting geomorphological characteristics differing across large-scale geographical regions. In addition, a variety of biological mechanisms have been proposed to account for regional variation in sea trout abundance through their inﬂuence on the incidence of anadromy in trout (see Jonsson & Jonsson, 2006). The numerical dominance of sea trout, particularly in smaller catchments, was much reduced when their relative abundance was expressed in terms of egg deposition, because of their smaller size and fecundity. Nevertheless, sea trout dominated salmonid egg deposition in between 12% and 50% of rivers. The relative importance of sea trout egg deposition compared with salmon varied, with a rank order of Sweden > England and Wales > Ireland > Scotland > Norway. It should be noted that sea trout also occur in

Sea Trout in European Salmon Rivers

149

100

Cumulative % area

80

60

40

20

0 0

5

10

15

20

25

30

35

40

Width (m) Fig. 11.7 The relationship between cumulative proportion of total stream area and stream width, in 39 English and Welsh streams (total area = 9269 ha).

other countries and regions, where salmon are less abundant (see ICES, 1994). In some of these, sea trout are found at very high abundance. For example, large sea trout (mean weight 2.25 kg) are abundant in the Normandy rivers of France, giving egg depositions up to 1300 per 100 m2 (Euzenat et al., 2006). Iceland, Finland, Poland, Denmark and northern Spain also have signiﬁcant sea trout stocks (ICES, 1994), but were not included in the SALMODEL analysis because of the limited information on or the absence of coexisting signiﬁcant salmon stocks. Relative abundance of both species, based on rod catches, increased with catchment size, but sea trout were increasingly numerically dominant in smaller catchments and this pattern of adult abundance ﬁtted qualitatively with observed patterns of juvenile abundance. The latter were characterised by increasing dominance of trout as channel size (width) decreased below 6 m. It was shown that such smaller channels (e.g. width <5 m) contributed to an increasing proportion of total wetted area as catchment size decreased. If adult and juvenile abundance are positively related then adult sea trout catch would also be expected to increase relative to salmon, on average, as catchment size decreases. For both species the average size of rod caught ﬁsh also reduced with decreasing catchment size. Jonsson et al. (2001) have discussed the life-history traits that may be associated with this effect in trout. Egg deposition estimates, from the few rivers where this was directly possible, showed that ignoring egg numbers of the other species could lead to underestimates of total salmonid egg deposition by ×2 to ×5. Depending upon the starting point on the stock–recruitment curve, a ﬁve-fold variation in egg deposition for either species might be expected to cause substantial variation in early fry mortality and later smolt production. Given this evidence

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of sympatry and relative abundance the potential for interspeciﬁc interaction is clear, but the realisation of such potential impacts is crucially dependent upon the effect of scale. Salmon have long been known to dominate the larger channels of river systems (Huntsman, 1936), and spawning ground selection, through for example depth and substrate size, is a major reason for this. This includes preferential usage of smaller channels by sea trout which, being generally smaller, are able to spawn in smaller gravels in shallower water with slower ﬂows (Heggberget et al., 1988; Crisp & Carling, 1989). However, where spawning does overlap, the timing of spawning may be important, because the salmon, which spawn later and are larger, could displace the eggs of sea trout. The prevalence of trout in smaller channels of big catchments and in smaller whole catchments (by virtue of relatively more small channel wetted area) is initially determined by spawning. But this segregation is only partial and reﬂected in the distribution of juveniles that commonly live in sympatry, thereby still maintaining potential for interspeciﬁc interaction. Competition among juveniles of the two species is well established (Kalleberg, 1958) and there is an extensive literature on their habitat requirements in fresh water and of their competitive interactions (e.g. Gibson, 1993; Bremset & Heggenes, 2001). In summary, these show that trout tend to dominate salmon through interference competition, such that salmon will extend their range of utilised habitat when trout are removed from stream sections (e.g. Kennedy & Strange, 1986). But such competition is moderated by partial niche separation reinforced by adaptations to microhabitats (Jones, 1975). Salmon juveniles tend to occur at higher densities in deeper, faster water over larger substrate while trout are more abundant in shallower, slower water (Gibson, 1993; Bremset & Heggenes, 2001). Relatively higher proportions of bankside cover, resulting from edge effects in smaller channels, will also favour juvenile trout, in contrast to salmon which are better able to make use of rearing area further away from banks. (Lindroth, 1955; Karlström, 1977). The greater total juvenile salmonid production reported when both species are present, compared with that when only one is found (e.g. Kennedy & Strange, 1986) further demonstrates that niche separation occurs. Thus, the potential for direct competition among juveniles is reduced to some extent by the combined expression of competitive and selective segregation (Bremset & Heggenes, 2001). The separation, initially of spawners and then of sympatric juveniles, is incomplete however and there remains signiﬁcant overlap in both habitat use and feeding. So the potential for interspeciﬁc interaction, by which the abundance of one species inﬂuences the survival of the other, cannot be discounted. A number of studies have reported that active removal or naturally lower abundance of one species, in microhabitats (e.g. <101 m such as rifﬂes or pools in small streams) or macrohabitats (e.g. 101 –102 m, typical electro-ﬁshing survey reaches), leads to an increase in the other species (Jones, 1975; Karlström, 1977; Baglinière et al., 1979; Egglishaw & Shackley, 1982; Kennedy & Strange, 1986). In contrast, a statistical comparison of juvenile (0+ to 2+) abundance across several sites did not demonstrate signiﬁcant associations between the two species (Wyatt & Barnard, 1997), but this spatial analysis may have reﬂected more the variation in habitat features across the sites. Similarly, analysis of many Swedish sites showed no association between salmon and trout juveniles (Crozier et al., 2003).

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Demonstration of species interaction at larger scale (>103 m, sub-catchment to whole catchments) is more equivocal. Milner et al. (1993) reported synchrony in abundance between the species in some tributaries of the River Conwy, North Wales, but not in others. Euzenat et al. (2006) suggested that salmon decline in the River Bresle may have contributed to an increase in sea trout spawning escapement. The decline in sea trout in western Ireland because of marine salmon farming, may have contributed to an increase in salmon catches, possibly attributable to increased availability of freshwater rearing capacity (Ó Maoiléidigh, pers. comm.). In contrast, some catch series show that sea trout and salmon can vary synchronously in both net and rod ﬁsheries (Crozier et al., 2003) or vary independently (Evans & Greest, 2006), an inconsistency that may reﬂect in part the duration of different time series. Long-term recovery of salmon and sea trout catches in formerly depleted rivers has been shown to follow similar patterns in both species (Environment Agency, 2004), suggesting a common response to improving environmental quality. In general, catch–time series data are not very informative with respect to ecological interactions because, although catches are in general related to stock size, the relationship is complicated and often confounded by environmental factors affecting stock size, ﬁshing levels or efﬁciency simultaneously (Shelton, 2002). The apparent conﬂict between the potential for competitive interaction and the inconsistent evidence of effects at population or ﬁshery scales may be explained by some combination of the following factors: •

•

•

The interaction effect is controlled by juvenile niche separation to levels below that detectable at sub-catchment or catchment level. The extent of this separation is likely to vary between rivers and habitat types. As a result of partial spawning segregation and low trout densities in large channels, where much salmon production takes places, any signiﬁcant interaction is conﬁned to smaller streams. However, smaller channels constitute a comparatively small proportion of whole-river rearing area and so, at that scale, the competition effect is diluted beyond detection. Even in the face of interaction within small tributaries, a low level of synchronous variation across tributaries (Milner et al., 1993) may limit any whole catchment response. Most studies on juveniles have been on electro-ﬁshing estimates of abundance made usually in mid- to late summer and therefore after the acute density-dependent regulatory phase. It is possible therefore that such data will never reveal strong interspeciﬁc densitydependent effects. Crozier et al. (2003) found no reported studies where egg or early fry densities of both species were reported or analysed together, but this is the stage when density-dependent competition is highest (Elliott, 1994; Elliott & Elliott, 2006).

The tentative conclusion is that, in spite of unambiguous evidence of interspeciﬁc competition, there is as yet no clear demonstration of negative interactions between salmon and sea trout populations at ﬁshery or catchment management scales. However, the data on this topic are few and inconsistent, so the possibility of such effects cannot be eliminated, especially in smaller streams and rivers, because the required studies into combined species

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stock–recruitment relationships have never been carried out and this represents a continuing research need.

References Baglinière, J.-L., Champigneule, A. & Nihouarn, A. (1979). La fraie du saumon Atlantique (Salmo salar L.) et de la truite (Salmo trutta L.) sur le bassin du Scorff. Cybium. 3ème série 7, 75–96. Bremset, G. & Heggenes, J. (2001). Competitive interactions in young Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) in lotic environments. Nordic Journal of Freshwater Research, 75, 127–42. Crisp, D.T. & Carling, P.A. (1989). Observations on siting, dimensions and structure of salmonid redds. Journal of Fish Biology, 34, 119–34. Crozier, W.W., Potter, E.C.E., Prévost, E., Schön, P.-J. & Ó Maoiléidigh, N. (Eds) (2003). A Coordinated Approach Towards the Development of a Scientiﬁc Basis for Management of Wild Atlantic Salmon in the North-East Atlantic (SALMODEL), Queen’s University of Belfast, Belfast, 431 pp. Egglishaw, H.J. & Shackley, P.E. (1982). Inﬂuence of water depth on dispersion of juvenile salmonids (Salmo salar L. and Salmo trutta L.) in a Scottish stream. Journal of Fish Biology, 21, 141–55. Elliott, J.M. (1989). Wild brown trout Salmo trutta: an important national and international resource. Freshwater Biology, 21, 1–5. Elliott, J.M. (1994). Quantitative Ecology and the Brown Trout. Oxford University Press, Oxford. Elliott, J.M. & Elliott, J.A. (2006). A 35-year study of stock–recruitment relationships in a small population of sea trout: assumptions, implications and limitations for predicting targets. In: Sea Trout: Biology, Conservation and Management (Harris, G. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 257–78. Environment Agency (2003). Guidelines on the Preparation of Salmon Action Plans, Version IV, Environment Agency, Bristol. Environment Agency (2004). Our Nations’ Fisheries: The Migratory and Freshwater Fisheries of England and Wales – A Snapshot. Environment Agency, Bristol, 97 pp. Euzenat, G., Fournel, F. & Fagard, J-L. (2006). Population dynamics of sea trout in the River Bresle, a coastal chalk stream in Normandy, France. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the 1st International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 307–23. Evans, R. & Greest, V. (2006). The rod and net sea trout ﬁsheries of England and Wales. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 107–14. Ferguson, R.I. (1981). Channel form and channel changes. In: British Rivers (Lewin, J., Ed.). George Allen and Unwin, London, pp. 90–125. Garcia, S.M., Zerbi, A., Aliaume, C., Do Chi, T. & Lasserre, G. (2003). The ecosystem approach to ﬁsheries. Issues, terminology, principles, institutional foundations, implementation and outlook. FA Technical Paper. No. 443, Rome, FAO2, 2003, 71 pp. Gibson, R.J. (1993). The Atlantic salmon in fresh water: spawning, rearing and production. Reviews in Fisheries Biology, 3(1), 39–73. Heggberget, T.G., Haukebö, T., Mork, J. & Ståhl, G. (1988). Temporal and spatial segregation of spawning in sympatric populations of Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L. Journal of Fish Biology, 33, 347–56. Huntsman, A.G. (1936). The sucker (Catastomus commersonii) in relation to salmon and trout. Transactions of the American Fisheries Society, 65, 152–56. ICES (1994). Review of the study group on anadromous trout. Trondheim, Norway, 29–31 August 1994. ICES C.M. 1994/M:4, 80 pp. Jonsson, B. & L’Abée-Lund, J.H. (1993). Latitudinal clines in life-history variables of anadromous brown trout in Europe. Journal of Fish Biology, 43 (Suppl. A), 1–16. Jonsson, B., Jonsson, N., Brodtkorb, E. & Ingebrigsten, P.-J. (2001). Life history traits of brown trout vary with the size of small streams. Functional Ecology, 15, 310–17.

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Jonsson, B. & Jonsson, N. (2006). Life history of the anadromous trout Salmo trutta. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the 1st International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 196–223. Kalleberg, H. (1958). Observations in a stream tank of territoriality and competition in juvenile salmon and trout (Salmo salar L. and S. trutta L.). Institute of Freshwater Research Drottingholm. Report No. 39, pp. 55–98. Karlström, Ö. (1977). Habitat selection and population densities of salmon (Salmo salar L.) and trout (Salmo trutta L.) parr in Swedish rivers with some references to human activities. Acta University of Upsala, 404, 3–12. Kennedy, G.J.A. & Strange, C.D. (1986). The effects of intra- and inter-speciﬁc competition on the distribution of stocked juvenile Atlantic salmon, Salmo salar L., in relation to depth and gradient in an upland trout, Salmo trutta L., stream. Journal of Fish Biology, 29, 199–214. Lindroth, A. (1955). Distribution, territorial behaviour and movements of sea trout fry in the river Indalsälven. Institute of Freshwater Research Drottingholm. Report No. 36, pp. 104–19. MAFF (2000). Salmon and Freshwater Fisheries Review. Ministry of Agriculture and Fisheries, London, 199 pp. Milner, N.J., Wyatt, R.J. & Scott, M.D. (1993). Variability in the distribution and abundance of stream salmonids and the associated use of habitat models. Journal of Fish Biology, 43 (Suppl. A), 103–19. Milner, N.J., Davidson, I.C., Wyatt, R.J. & Aprahamian, M.A. (2000). The use of spawning targets for salmon ﬁshery management in England and Wales. In: Management and Ecology of River Fisheries (Cowx, I.G., Ed.). Fishing News Books, Oxford, pp. 361–72. Shelton, R. (Ed.) (2002). The interpretation of rod and net catch data. Proceedings of the Atlantic Salmon Trust Workshop, Lowestoft, November 2001. Atlantic Salmon Trust, Moulin, 107 pp. Wyatt, R.J. & Barnard, S. (1997). The transportation of the maximum gain spawning target from the river Bush (NI) to England and Wales. Environment Agency, R&D Technical Report W65. Environment Agency, Bristol, 80 pp.

Section 2

GENETICS AND LIFE HISTORY

Chapter 12

Genetics of Sea Trout, with Particular Reference to Britain and Ireland A. Ferguson School of Biology & Biochemistry, Queen’s University, Belfast BT7 1NN, Northern Ireland

Abstract: In common with much of north-western Europe, Britain and Ireland was ice covered during the last glaciation and thus current populations of brown trout have been derived postglacially (<14 000 years) from several genetically distinct lineages of anadromous brown trout (sea trout). Evolution from sea trout to freshwater (river and lake) brown trout in Britain and Ireland has occurred independently in each water system and it is thus not possible to generalise on the relationship between sea and freshwater trout. The primary difference between sea trout and most freshwater trout is that their migration takes them to the sea rather than to a lower region of a river or to a lake. Sea trout life history involves physiological and other changes necessary to adjust to the different requirements of freshwater and marine environments as well as accurate migration to reach appropriate feeding and spawning habitats in these environments. Many lines of evidence indicate genetic bases to each of these components and anadromy in brown trout is thus best explained as a threshold quantitative trait; it is a trait that is controlled both by multiple genes and by environmental inﬂuences and is expressed (i.e. anadromy occurs) when this combination of factors exceeds a threshold level. Sea trout populations generally show substantial genetic heterogeneity among populations within and among water systems as a result of colonisation by multiple lineages as well as selection and drift since colonisation. In some areas, but not others, studies with molecular markers indicate a pattern of increasing genetic differentiation with increasing geographical distance, that is, reproductive isolation by distance. Sea trout populations within and among river systems also differ in many biological characteristics, which are quantitative genetic traits, and this variability results in local adaptation and diverse angling challenges. A key implication of its genetic basis is that anadromy will evolve in response to differential costs and beneﬁts and thus as a result of accidental or deliberate selection. Stocking with domesticated and non-native trout is likely to have a greater detrimental impact on sea trout than freshwater populations, although in both cases a reduction in population ﬁtness is likely to ensue. Where stocking is shown to be necessary it should be carried out using only the offspring of native broodstock (supportive breeding) although this should be undertaken with due regard to possible detrimental genetic effects.

What is a sea trout? The simple answer to this question is that a sea trout is a brown trout (Salmo trutta L. species complex) that has spent a period of time feeding in the sea before returning to fresh water to reproduce, a life history referred to as anadromous. Why should some brown 157

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trout go to sea while other individuals remain in fresh water throughout their lives? To understand this it is necessary to consider the origins of current brown trout populations. During the last Ice Age, which started about 75 000 years ago and had its maximum extent about 18 000 years ago, most of Britain and Ireland, in common with much of north-western Europe, was ice covered (Denton & Hughes, 1982). It is unlikely that brown trout or other freshwater ﬁsh could have survived in most of this region during maximum glaciation. By 14 000 years ago the ice was well in retreat and recolonisation could commence. However, ice cover was not continuous throughout north-western Europe and there were several refuge areas where brown trout could have survived, in addition to survival in the nonglaciated region to the south of Britain and Ireland. Current genetic evidence (Ferguson & Fleming, 1983; Hamilton et al., 1989; Hynes et al., 1996; Bernatchez, 2001; Duguid, 2002; McKeown, 2005) suggests that, as well as this latter southern refuge, there were at least ﬁve other refuges in more northern parts. As a result of isolation in these separate refuges several genetically distinct ancestral groups (lineages) evolved, which then independently colonised Britain and Ireland in the postglacial period. Some of these lineages were further split as a result of shorter periods of isolation, and in some places interbreeding and introgression across lineages has taken place since postglacial colonisation possibly as a result of human environmental perturbations, which has resulted in the breakdown of reproductive isolation. As the same diversity of lineages occurs in Ireland, which did not have any postglacial freshwater connection with the rest of Europe, as in Britain (McKeown, 2005), it is clear that all colonisation of Britain and Ireland by brown trout was as sea trout. Thus anadromy is the ancestral state (at least in respect of recent evolutionary history) and the change from sea trout to freshwater (river and lake) brown trout in Britain and Ireland has occurred independently in each river system within the past 14 000 years. As well as this, freshwater trout have arisen on several occasions in some river systems from lineages that colonised at different times postglacially giving rise to sympatric reproductively isolated populations (Ferguson, 2004). This anadromous origin has important implications for study of sea trout–freshwater trout interrelationships. In particular it means that it is not possible to generalise on the relationship among these forms as freshwater trout have evolved independently in each catchment and what may apply in one catchment may not be the case in others. In addition we need to focus on why some brown trout become purely freshwater rather than continue with the ancestral trait of anadromy. In others words the question is, why do some brown trout not go to sea rather than why do some brown trout become sea trout, as more frequently posed. While this may seem somewhat semantic, an understanding of this difference is important for a correct focus on investigations. The nineteenth century placement of sea trout as a separate species is clearly inappropriate as was also the recognition of lacustrine and riverine ecotypes as species. Rather the ancestral lineages, and sub-lineages of these, are the basic evolutionary and thus taxonomic and conservation units (Ferguson, 2004). Brown trout show a wide range of life-history strategies. Throughout this review the term ‘brown trout’ is used in the general taxonomic sense with freshwater trout referring to brown trout that remain in fresh water throughout their lives (i.e. non-anadromous) and ‘sea trout’ referring to anadromous brown trout. Brown trout that do not go to sea have been

Genetics of Sea Trout: Britain and Ireland Table 12.1

159

Main life-history types of brown trout.

Type

Life history

Resident (river trout) Migration to lower region of the river (river trout) Migration to lake (lake trout) Migration within lake (lake trout) Migration to estuary (slob trout) Migration to sea (sea trout)

All life stages spent in same region of the river Reproduction upstream, adult feeding downstream Reproduction in afferent or efferent rivers, adult feeding in lake Spawning and main feeding areas in different regions of lake Reproduction upstream, adult feeding in estuary Reproduction in river, adult feeding in sea

Note that these types are not always mutually exclusive as the same individual ﬁsh may make different migrations at different ages.

referred to by some authors as freshwater resident. However, this is a misnomer as, while some brown trout may remain resident within a restricted stretch of a river, most freshwater brown trout undergo some form of migration (Table 12.1). Within a river, migration may occur between upstream spawning areas and downstream adult feeding grounds with, in some cases, migration occurring as far as the river estuary. In Ireland, as well as other parts of Europe, the most abundant life history for brown trout involves a migration between spawning areas in rivers and feeding grounds in lakes. Occasionally the entire life cycle may be spent in a lake if suitable spawning habitat and water ﬂow are present, although again a migration within the lake between spawning and adult feeding areas is usually involved. Thus the primary difference between sea trout and most freshwater trout is that their migration takes them to the sea rather than to a lower region of a river or to a lake. That is, this anadromous life history is just the extreme of this continuum of migratory patterns. Therefore, in studies of sea trout biology, it is artiﬁcial to separate sea trout from freshwater trout as sea trout have much in common with lake migratory trout. The distinction breaks down further in low salinity areas such as the Baltic Sea where fry and 0+ parr migrate to the sea (Landergren, 2001) and where spawning can occur in brackish water (Landergren & Vallin, 1998).

Components of anadromy The sea trout life history involves two major components: (1) physiological, morphological and behavioural changes necessary to adjust to the different requirements of freshwater and marine environments and (2) accurate migration to reach appropriate feeding and spawning habitats in these environments. In this review evidence for genetic involvement in these components is considered. Given the limited information on some aspects of sea trout genetics, it is necessary to draw on information from other salmonids that show similar life-history variation. Of particular relevance are studies on rainbow trout (Oncorhynchus mykiss), which, in its native western North America, shows both freshwater and anadromous forms, the latter being commonly referred to as steelhead. Other salmonids with anadromous and non-anadromous forms in some part of their range include Atlantic

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salmon (Salmo salar), Arctic charr (Salvelinus alpinus), brook charr (Salvelinus fontinalis) and sockeye salmon (Oncorhynchus nerka).

Physiological and other changes in marine and freshwater environments When a trout is in fresh water its body has a higher ionic strength than the surrounding water and thus it is faced with the loss of ions by diffusion and the gain of water by osmosis. The converse is the case in sea water with the gain of ions and loss of water occurring. Movement from one medium to another thus requires physiological adjustment for both ionic and water regulation. In teleost ﬁshes most of the ionic regulation is carried out by the gills. Thus sea trout and other salmonid smolts typically show an increase in gill Na+ , K+ -ATPase and H+ -ATPase in the period immediately prior to migration to sea, as well as changes in several other enzymes and proteins (Hoar, 1988; McCormick et al., 1998; Stefansson et al., 2003). Measurement of increasing gill Na+ , K+ -ATPase can be used to predict smolting in individual brown trout before morphological changes are obvious (Nielsen et al., 2004). As well as changes in total activity, differential gene expression of Na+ , K+ -ATPase isoenzymes also occurs on movement from fresh water to the sea (Richards et al., 2003). Developmental and physiological changes occur in the retina of salmonid ﬁshes during smolting including loss of ultraviolet-sensitive cone receptors and switching of visual pigments (Dann et al., 2003). Physiological changes on movement to seawater are mediated by the pituitary, thyroid, growth and cortisol hormones (McCormick, 1995; Finstad & Ugedal, 1998; Martin et al., 1999; Agustsson et al., 2001). Not surprisingly, given the changes in protein expression noted above, recent studies looking directly at gene expression during smolting have shown increases in the expression of various genes (Hardiman & Gannon, 1996; Seidelin et al., 2001; Dann et al., 2003; Giger et al., 2004) although, in most cases the genes involved cannot as yet be identiﬁed. It is possible that in the future, these molecular studies may reveal new insight into the changed pattern of gene expression involved in smolting, through the identiﬁcation of genes not previously known to be involved in the smolting process. It should be emphasised, however, that just because there is changed expression of various genes this does not mean that the process of smolting is under genetic control. Thus it is not possible to determine whether this involves environmental control of gene expression, operating through the neuroendocrine system, or endogenous gene control by other genes, or indeed, as is most likely the case, a combination of the two. However, indirect evidence suggests that the ability to adjust to marine conditions is at least partly genetically controlled. Thus different stocks of rainbow trout and Atlantic salmon have been shown to differ in their ability to survive in sea water (Burton & Idler, 1984). Boula et al. (2002) found that in domestic anadromous brook charr, rearing conditions had no negative impact on osmoregulatory activity or gill Na+ , K+ ATPase suggesting an important hereditary component of gill Na+ , K+ -ATPase activity. Brown trout and other salmonid populations that have been in fresh water for many thousands of generations have retained the ability to bring about the physiological changes necessary to adjust to marine conditions (Krieg et al., 1992; Staurnes et al., 1992; Foote et al., 1994).

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As well as physiological changes associated with ionic and water regulation, morphological changes also occur in respect of camouﬂage. In rivers, trout are primarily bottom dwellers and their colouration allows them to ‘blend’ with the background when viewed from above. To be camouﬂaged in mid-water in the sea requires that the individual ﬁsh does not stand out against the background against which it is viewed irrespective of direction. This is achieved by having the skin on the sides of the body act as a mirror and reﬂecting light falling upon it. Sea trout smolts, in common with other salmonid smolts, become silvery in appearance before migration to sea. This silvering is achieved through the deposition of guanine and hypoxanthine in the skin. Silvering is also seen in some degree in many lake brown trout (Olsson & Greenberg, 2004). For example, the so-called salmon trout of Lough Neagh (Northern Ireland) superﬁcially resembles a sea trout and has led to reports of sea trout being caught in this freshwater lake (Crozier & Ferguson, 1986, 1993).

Evolution of migratory strategies Migration is a key aspect of the life history of the sea trout and most freshwater trout, as well as salmonids in general. Many lines of evidence (summarised in Table 12.2) suggest that migratory behaviour, deﬁned as purposeful movement, is under genetic control and has a high heritability. A genetically determined trait that results in an individual surviving and reproducing better than other individuals, and thus leaving relatively more offspring (i.e. having a higher ﬁtness) will increase in frequency over the generations. In evolutionary Table 12.2

Evidence for genetic control of migratory behaviour in brown trout and other salmonids.

Observation

References

Loss of anadromy in brown trout populations above impassable waterfalls Different sea trout stocks showed different migration pathways when released in the Baltic Sea Genetic control of migration direction in outlet and inlet spawning brown trout, rainbow trout and sockeye salmon In experimental studies with brown trout more anadromous offspring were produced from anadromous parents than from freshwater ones Different male and female migratory behaviour in the same population of brown trout and other salmonids Variation among salmonid species in extent of anadromy when in sympatry Occurrence of related individuals of steelhead trout spawning at the same time implies a heritable basis for upstream migration date and maturation date Salmonid populations that differ in migratory behaviour show genetically based differences in swimming ability Timing of downstream and upstream migration is genetically controlled in Atlantic salmon

Jonsson, 1982; Svärdson & Fagerström, 1982; Fleming, 1983 Svärdson & Fagerström, 1982 Raleigh, 1971; Kelso et al., 1981; Ferguson & Taggart, 1991; Jonsson et al., 1994 Skrochowska, 1969

Campbell, 1977; Jonsson, 1985; Fleming, 1983 Stearns & Hendry, 2004 Bentzen et al., 2001

Hendry et al., 2004 Nielsen et al., 2001; Stewart et al., 2002

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terms, the trait is said to be subject to positive natural selection. Conversely, traits that lower ﬁtness will be selected against. Migration to sea has several advantages including better feeding opportunities and enhanced food utilisation. Larger size results in increased fecundity especially for females with greater energy stores. In addition, larger size possibly conveys advantages in terms of being able to cope with physical conditions at spawning sites and competition for mates, all of which potentially increase ﬁtness (Holtby & Healey, 1990). However, numerous lines of evidence demonstrate that anadromy has costs as well as beneﬁts (reviewed by Hendry et al., 2004). For example, there are increased chances of predation during migration and when in the sea, as well as increases in parasite burdens in the marine environment. Energy requirements are also increased as a result of physiological adjustments and the actual migration. Thus, Bohlin et al. (2001) found a negative correlation between the extent of anadromy and altitude at which brown trout populations were situated in Sweden, suggesting that anadromy is less common in populations where migration has a greater energy requirement. Compensatory adaptations to the costs of both anadromy and non-anadromy have been indicated by a number of studies (reviewed by Hendry et al., 2004). The feeding advantage of migration to the sea can also be obtained to some degree in lakes without increasing costs to the same extent. Not surprisingly, in areas where suitable lakes occur, lake feeding has often evolved to be the main strategy rather than the ancestral trait of feeding in the sea. However, in individual water systems the balance of beneﬁts and costs results in different life-history strategies evolving with, in some cases, several strategies coexisting in the same water system possibly as a result of frequencydependent selection, that is, a particular strategy is only advantageous when a small proportion of the population participate and advantage decreases as the frequency of the trait increases. Anadromy can also be inversely density-dependent but this may be purely environmentally induced, although potentially there could be a genetic component to density where, for example, higher survival is genetically based. The differential occurrence of freshwater and anadromous forms within large water system as well as among adjacent water systems suggests that costs and beneﬁts are in ﬁne balance. This ﬁne balance means that relatively minor changes in environmental conditions and/or genes can result in a change in life-history strategy. For female trout, as discussed above, it can be advantageous, in the ﬁtness sense, to grow larger. However, for males in terms of sperm production there probably is less advantage in growing large although large size can be important in competition for mates. For many sea trout populations, differential selection on males and females has resulted in sea trout being predominantly females with males remaining in fresh water throughout their lives (Jonsson, 1985; Campbell, 1997). For example, in the Glynn River (Northern Ireland) Fleming (1983) found that above impassable waterfalls the sex ratio was not signiﬁcantly different from equality. However, in the below-falls section, of 248 mature freshwater trout only two were females (0.8%) but in 111 smolts and spawning sea trout that were examined 104 (94%) were female. If the tendency to migrate is genetically controlled then it would be expected that there would be strong selection against the migratory phenotype in populations living above

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waterfalls and other impassable barriers as migratory individuals could not return and would have zero ﬁtness with respect to that population. Thus there would be loss of the genotypes for migration from the population unless maintained by some form of balancing selection. Several studies have shown that downstream gene ﬂow from populations isolated above waterfalls to those below is extremely limited or non-existent. For example, alleles have been found in above-falls samples that are absent in samples taken below, suggesting that above-falls populations are subject to strong selection for stream residency (Fleming, 1983; Thompson, 1995). Again in the Glynn River, during 3 years of extensive electroﬁshing in the section above the waterfalls no trout showing external signs of smolting were found even though there were many smolting ﬁsh just below the falls at the same time (Fleming, 1983). Of some 400 (age 2+) trout that were adipose ﬁn-clipped in a 300 m stretch above the waterfalls in April 1981, none were subsequently detected below the falls although they were still recovered in the original selection on several subsequent occasions. Svärdson and Fagerström (1982) found that an above-waterfalls stock of brown trout, which was transplanted to below the falls, showed much less movement than the below-falls stock from the same river. Genetic basis of anadromy As indicated above there are clearly genetic bases to both the physiological adjustments required for anadromy and especially to the process of migration. Genetics can also indirectly inﬂuence anadromy through other traits such as maturation, energy efﬁciency, etc. Thus migration and sexual maturation appear to be conﬂicting processes in brown trout (Thorpe, 1987) such that parr that mature do not usually become smolts in the same year. Timing of maturation in brown trout and other salmonids is a quantitative trait with both genetic and environmental components (Thorpe et al., 1983; Palm & Ryman, 1999). Energy efﬁciency and growth have also been shown to be involved in anadromy and, based on studies in other salmonids, it is likely that these traits also have a genetic basis. In brown trout (Forseth et al., 1999) and brook charr (Salvelinus fontinalis) (Morinville & Rasmussen, 2003) it has been shown that anadromous individuals have higher metabolic costs, that is, they are less efﬁcient at converting food to growth. In addition, there are clearly environmental factors responsible for anadromy as studies have shown that by changing food availability the proportion of ﬁsh emigrating to sea can be altered (Morinville & Rasmussen, 2003). Given the clear involvement of multiple genetically based traits together with environmental inﬂuences, anadromy in brown trout is thus best explained as a threshold quantitative trait, that is, a trait that is controlled by multiple genes and by environmental inﬂuences. Anadromy thus occurs when the combination of these genetic and environmental inﬂuences exceeds a threshold level, while below this level trout adopt one of the freshwater life histories. This explanation has been hypothesised for all salmonids with anadromous/non-anadromous life histories (Hallerman, 2003). It is to be expected that various combinations of genetic and environmental inﬂuences can result in the threshold being exceeded with a low genetic propensity being offset by a high environmental inﬂuence

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and vice versa. Thus anadromy can be dormant in a population and only occur when environmental conditions change. Gargan et al. (2006, this volume) found that substantial numbers of sea trout smolts were produced in two rivers in the west of Ireland with extremely small spawning escapements of sea trout implying that the freshwater trout had contributed signiﬁcantly to the smolt runs. However, this contribution decreased with time. As yet there are no estimates of the relative contributions of genetic and environmental inﬂuences to anadromy (heritability in the broad sense) for brown trout. Thrower et al. (2004) estimated heritabilities for smolting, precocious maturation and growth in 75 families of within line and reciprocal between line crosses of rainbow trout (Oncorhynchus mykiss) involving steelhead (anadromous) and lake-resident ﬁsh. The lake resident stock was derived from the same anadromous stock 70 years earlier by artiﬁcial transplantation above two 15 m waterfalls. As noted above, it would be expected that genes related to smolting and migration would be strongly selected against in such a situation. Differences in smolting, maturation and growth were found among lines, and among families within lines, reﬂecting additive genetic variation for these traits, with a heritability greater than 0.5 being indicated. The lake population still retained a large amount of genetic variability associated with smolting. Smolting and precocious maturation (males at age 2 years) were found to be negatively genetically correlated. Thrower et al. (2004) suggest that the high heritability of smolting, together with migrants being unable to return to the lake (i.e. complete selection against downstream migration), indicates that the genetic potential for smolting can lie dormant for decades or be maintained through a dynamic interaction between smolting and early maturation.

Genetics of sea trout populations Molecular marker studies of freshwater–sea trout relationships Various studies have examined samples of sea trout and freshwater trout from the same river using allozyme, mitochondrial DNA (mtDNA), minisatellite and microsatellite genetic markers. As, in many cases, the aim is to use speciﬁc variants as indicators or markers to characterise the overall genetic make-up of an individual or population, genes that can be studied by molecular techniques are often referred to as ‘molecular markers’. Most such genes (alleles) are selectively neutral or effectively so and not subject to direct natural selection. Two major problems arise in studies of genetic differences between anadromous and freshwater trout. First, the two types have to be truly sympatric. Some of these studies have taken the respective samples from different parts of the river, with freshwater trout in some cases from above barriers to upstream migration and not surprisingly differentiation was found (e.g. Skaala & Naevdal, 1989). The second problem concerns the identiﬁcation of the life-history type to which an individual belongs, and linked to this is the problem of ensuring that the same cohort is examined. Thus freshwater trout can generally only be determined as individuals that show no evidence of marine growth on their scales and are older than ﬁsh which would have gone to sea in that river. However, this means that

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they are often compared with sea trout from a younger cohort. Where effective population size is small, genetic differences among cohorts can occur (Laikre et al., 2002). In most, but not all, situations where authenticated and truly sympatric samples of the two types have been studied, no differentiation has been found (e.g. Fleming, 1983; Hindar et al., 1991; Cross et al., 1992; Pettersson et al., 2001). This suggests that interbreeding occurs between the two types at a higher level than say between adjacent river populations of sea trout, which generally shows signiﬁcant differentiation using similar techniques (see below). However, it should be remembered that interbreeding only needs to exceed quite a low level (more than four or ﬁve individuals per generation) for the two populations to behave effectively as a randomly breeding unit resulting in neutral alleles not differing signiﬁcantly in frequency (Hartl & Clark, 1989; Templeton, 1998). Extensive interbreeding is also implicated in some rivers where the freshwater trout are almost exclusively males. Occasionally genetic differentiation has been found been freshwater and sea trout. In the Glenariff River (Northern Ireland) signiﬁcant allozyme and mtDNA differentiation was found between freshwater trout in the section immediately below the waterfalls and an adjacent (∼1 km – distance) sea trout population (Fleming, 1983; Hamilton, Hynes & Ferguson, unpublished studies). The freshwater trout sample was genetically more similar to brown trout above the waterfalls than to the sea trout sample, suggesting that this freshwater population had been established by individuals from above the falls. Although, as argued above, strong selection against migration is expected in above-falls populations, occasional displacement downstream may occur during unusual ﬂood or other conditions. In a Swedish stream, Pettersson et al. (2001) found no genetic differentiation between sympatric sea and freshwater trout and no evidence, from individual assignment analyses, that the freshwater trout were immigrants from the above-falls population. Molecular marker studies of sympatric anadromous and freshwater forms of other salmonids, especially rainbow trout, have found similar results to the above with lack of genetic differentiation being more commonly found than differentiation (Taylor et al., 1996; Jones et al., 1997; Boula et al., 2002; Docker & Heath, 2003). For example, Docker & Heath (2003) found genetic divergence between sympatric steelhead and freshwater rainbow trout in one out of ﬁve river systems examined in British Columbia, Canada. Narum et al. (2004) found genetic divergence between anadromous and freshwater rainbow trout in one tributary of a river system but not in another. Reproductive isolation, even when sea trout and freshwater trout spawn in sympatry may be produced by size assortative mating limiting interbreeding. However, mature male parr of freshwater trout may fertilise the eggs of sea trout females as multiple males have been shown to fertilise the eggs of a single female in a redd (Hindar, 1997). Differences in spawning time and substrate could also act as isolating mechanisms. For example, in the Deschutes River (Oregon, USA) Zimmerman & Reeves (2000) found that the mean spawning date for steelhead was 9–10 weeks earlier than for freshwater rainbow trout and that steelhead tended to spawn in deeper water with larger substrate. Selection against hybrids between sea and freshwater trout could also reduce or eliminate gene ﬂow. The fact that often no genetic differentiation has been found, using molecular marker techniques, between sympatric sea and freshwater trout indicates that there is a signiﬁcant

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interbreeding between the two types, that is, they do not usually form reproductively isolated populations. It does not mean that there is no genetic basis to anadromy, a point that has been misinterpreted by some authors. Quantitative genetic traits can segregate among families within a single interbreeding population as typically seen, for example, in a number of human disease conditions. As indicated above, anadromy is best explained as a threshold quantitative trait, which is not in any way in contradiction to the molecular marker ﬁndings. Molecular marker studies of intra- and inter-population variation Brown trout populations generally show substantial genetic heterogeneity both within and among water systems (Ferguson, 1989). Much of this heterogeneity is the result of independent colonisation by several genetically distinct lineages. Brown trout probably ﬁrst colonised the Atlantic region around 750 000–1 000 000 years ago (Bernatchez, 2001) and differentiation has occurred during this time as the result of multiple glaciations, which have resulted in periods of isolation in separate regions followed by secondary contact and introgression. In addition to the genetic differentiation of different lineages, postcolonisation genetic changes have occurred as a result of independent natural selection resulting in adaptation to local conditions. Random genetic changes have also occurred because of genetic drift. Postglacial differentiation as a result of selection and drift is counteracted by gene ﬂow among populations as a result of straying; a force potentially greater in sea trout compared with freshwater populations where the sea acts as a barrier to gene ﬂow. Most studies of genetic variation among brown trout populations in Britain and Ireland have been concerned with freshwater trout, with attention often having focused on morphologically distinct populations especially where these occur sympatrically (Ferguson, 2004). In many cases sea trout rivers have been included in overall brown trout geographical surveys but have not involved speciﬁc sampling for sea trout smolts or adults, although parr have been collected from sections of rivers where there is no barrier to anadromy and where sea trout are known to occur. Given the lack of differentiation between sympatric sea trout and freshwater trout in most cases (see above) this is unlikely to result in serious bias. Studies of sea trout population genetics have also been undertaken in various countries in western Europe including Denmark, Norway, Sweden and Spain. The general conclusion from all studies is that, in spite of their often more uniform external appearance as a result of silvering, sea trout in different catchments, in common with freshwater trout, are genetically different, and genetically distinct populations can occur in different tributaries within a river system. This implies that effective straying (i.e. straying that results in successful breeding and contribution to succeeding generations – gene ﬂow), even at the level of tributaries within rivers, is very limited, otherwise these differences among adjacent rivers would not exist. Based on scale samples from the 1910s and 1950s together with tissue samples from the 1990s, Hansen et al. (2002) found that differentiation at eight microsatellite loci among ﬁve anadromous populations in Denmark was temporally stable with much less variation among temporal samples from the same population than among samples from different populations.

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In some cases, but not all, weaker genetic differentiation has been found among sea trout populations than freshwater populations (e.g. Paaver, 1989) suggesting relatively greater gene ﬂow in sea trout, although still at a very low level as effective straying of more than four or ﬁve individual ﬁsh per generation would prevent detectable differentiation in neutral alleles (Hartl & Clark, 1989; Templeton, 1998). Again, in contrast to freshwater populations (Ferguson, 1989) a pattern of increasing genetic differentiation with increasing geographical distance has been found for sea trout populations in some areas but not in others. In other words, there is increasing reproductive isolation with increasing distance among populations and gene ﬂow follows a ‘stepping-stone’ model as compared with an ‘island’ model for freshwater populations. In contrast to studies indicating a higher level of gene ﬂow among sea trout populations relative to freshwater ones, in Spence’s River (Co. Down, Northern Ireland) a GPI-A2*QO allele was found at a frequency of 0.4 but this allele was absent in brown trout sampled from an adjacent river 2 km along the coast and in 11 other rivers at a distance of 3–30 km (Taggart et al., 1981; Ferguson & Fleming, 1983; Ferguson, 1989, unpublished data). All these rivers are known to contain sea trout. The frequency of the allele was found not to be signiﬁcantly different in 1989 compared with 1981 and was still absent in all 12 other rivers in the area at this later date (unpublished data). It is unlikely that differential selection could account for the presence in Spence’s and absence in adjacent rivers as GPI-A2*QO is a null allele, although directional selection could have resulted in a rapid increase in frequency in Spence’s. The unique occurrence in Spence’s would indicate a very low level of gene ﬂow from Spence’s to the other populations in the area. Fleming (1983) examined variation at 11 polymorphic enzyme coding loci in brown trout populations from 19 river systems in north-eastern Ireland and seven river systems in north-western England all of which were known to have sea trout runs. Signiﬁcant genetic heterogeneity was found among the river systems within and between the two regions. Greater genetic differentiation in the two regions combined, together with several unique alleles in north-western England indicated a lack of gene ﬂow across the Irish Sea. Thompson (1995) surveyed genetic variation in 835 brown trout individuals from potentially anadromous populations in Scotland. Six rivers from the east coast and nine rivers from the west coast were represented. All samples were screened at ﬁve minisatellite loci with 7–19 alleles being found to segregate per locus. Samples from 11 rivers were also screened at 30 enzyme coding loci, 19 of which were polymorphic. Results from minisatellite and allozyme screening generally concurred. A signiﬁcantly greater number of minisatellite alleles were detected in the east coast group of populations compared with the west coast group. Minisatellite data showed a signiﬁcantly lower level of genetic differentiation in the east coast group (FST = 0.027) than in the west coast group (FST = 0.089) implying greater levels of gene ﬂow among the east coast populations. For the east coast group there was a signiﬁcant positive correlation between genetic differentiation and geographical distance. However, no correlation of genetic and geographical distances was found for the west coast populations, as was also the case for the Irish Sea populations examined by Fleming (1983). The difference in the two areas may be related to different migratory behaviour of these sea trout stocks in the sea. Scottish east coast sea trout are

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known to make long distance migrations across the North Sea (Pratten & Shearer, 1983), and may thus be more likely to stray, while Irish Sea and Scottish west coast populations may remain within the coastal zone. Presence and absence of isolation by distance has been seen elsewhere in north-western Europe. Skaala (1992) found no relationship between genetic differentiation and distance in western Norwegian sea trout populations. However, on the south-east coast of Norway, Knutsen et al. (2001) found evidence of isolation by distance. Studies of Danish sea trout populations using mtDNA (Hansen & Mensberg, 1998) also indicated increased genetic differentiation with increased geographical distance. A much stronger pattern of isolation by distance was observed when samples from different tributaries within a river were pooled rather than treating all samples separately. Ruzzante et al. (2001) studied microsatellite variation in brown trout from 32 rivers in Denmark and found isolation by distance at the larger geographical scale when these were pooled into nine geographical regions but not when populations were considered individually. In Spanish brown trout populations at the southern limit of anadromy, Bouza et al. (1999) found a signiﬁcant pattern of isolation by distance and that intra-population genetic variability decreased and genetic differentiation among populations increased as the extent of anadromy within populations decreased. In northern Spanish sea trout populations, Morán et al. (1995) also found a pattern of isolation by distance. Studies of differentiation of sea trout populations within river systems, other than for inter-tributary variation, have been more limited than for lake trout populations. However, Hall (1995) found that early and late runs of adult sea trout in the River Dee (Wales) were genetically differentiated based on mtDNA and microsatellite analyses, with mtDNA indicating higher differentiation. Similarly, early and late run groups of steelhead in British Columbia were found to have highly signiﬁcant genetic differences over 10 microsatellite loci (Hendry et al., 2002). Quantitative trait variability among populations Sea trout populations within and among river systems differ in many biological characteristics including, age and size at smolting, age at ﬁrst return from sea, timing of smolt migration and return from sea, spawning time, number of repeat spawnings, maximum age and size, morphology and distance migrated from home river (see other chapters in this book). Based on studies in salmonids in general, it is likely that all these characteristics are quantitative traits and thus controlled by multiple genes and by environmental inﬂuences with varying levels of heritability among traits and among populations. Behavioural and morphological differences exist among sea trout strains even when reared under the same conditions (Petersson & Järvi, 1997). Indirect evidence from other salmonids (see review by Quinn, 2005 and references therein) suggests that much, if not all, of this inter-population variability results in adaptation to the local conditions in speciﬁc rivers and tributaries. For example, in salmonids, precise timing (Stewart et al., 2002) and navigational aspects of migration are important in ensuring ﬁtness under the conditions speciﬁc to individual waters, and thus have an important contribution to

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productivity. Variation in these characteristics also has a direct bearing on the perceived angling quality of different waters and maintenance of this diversity is a key challenge for ﬁsheries management. As discussed above, as well as genetic differentiation acquired since postglacial colonisation as a result of selection and drift, part of the genetic variation among populations reﬂects differences among separate evolutionary lineages as for freshwater populations (Ferguson, 2004; Duguid et al., 2005). Some rivers currently show a single mtDNA lineage while others show multiple lineages (McKeown, 2005) that have most probably introgressed. However, the existence of reproductively isolated sympatric populations of sea trout representing different lineages has not yet been examined in the way that it has been done for lake-dwelling brown trout (Ferguson, 2004). Different mtDNA lineages are geographically restricted within Britain and Ireland with, for example, one lineage being present in western Ireland, western Scotland, eastern Scotland and eastern England at substantial frequency (including sea trout populations) but absent from the other regions including the Irish and Celtic seas and southern coasts (McKeown, 2005). Regional and local genetic differences may in part account for the variability seen in sea trout characteristics. For example, the Currane system (south-western Ireland) is renowned for its long-lived sea trout (Fahy, 1985) and the system has contributed some 50% of the angler-caught specimen sea trout in Ireland. Interestingly trout from the Currane system have a high frequency of the LDH-C1*100 allele, an allele which is rare in sea trout populations and which has been shown to be associated with long-lived ferox trout in some lakes (Hamilton et al., 1989; Duguid et al., 2006). Differentiation among populations in molecular markers is generally summarised in the statistic FST , while quantitative gene differentiation can likewise be summarised as QST . Studies on numerous organisms have shown that FST and QST are highly correlated although QST is generally larger than FST (Merilä & Crnokrak, 2001; Moran, 2002). This is what would be theoretically expected since natural selection will result in more rapid genetic change among populations compared with genetic drift of neutral alleles, except where populations have a very low effective population number (<100) or selection is very weak (<1%). Thus divergence based on neutral genetic markers is likely to considerably underestimate divergence in adaptive quantitative genetic variation. It should not be assumed that because there is no signiﬁcant molecular divergence (i.e. FST not signiﬁcantly different from 0) or because the level of divergence is low, that important adaptive differences do not exist among populations. On the other hand, if molecular genetic variation exists across populations then almost certainly there are adaptive genetic differences as well. Moreover local adaptation can be based on large changes at a relatively small number of gene loci.

Hybridisation between sea trout and Atlantic salmon As brown trout often spawn in the same locations as Atlantic salmon, hybridisation can occur and it has been suggested that this is more frequent when large sea trout are present (Jordan & Verspoor, 1993). Morphological identiﬁcation of hybrids is not reliable (Wilkins et al., 1994) but these can be identiﬁed by various molecular markers that show ﬁxed differences

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between the two species (e.g. Crozier, 1984; Matthews et al., 2000). The female parent of the hybrid can also be determined using maternally inherited mtDNA differences between brown trout and Atlantic salmon (McGowan & Davidson, 1992). In a survey of 4135 mixed brown trout and Atlantic salmon fry and parr from 13 rivers in Ireland, Matthews et al. (2000) found an overall F1 hybrid frequency of 1.2% with a maximum of 3.1% per river and 17.4% in a small tributary of one river. MtDNA analysis indicated that all hybrids arose from Atlantic salmon female × brown trout male crosses. In the River Tweed (Scotland), Jordan & Verspoor (1993) found 3.4% hybrids in a sample of supposed salmon fry and suggested that this high rate of hybridisation was because of the presence of large sea trout in the river possible resulting in increased errors in mating. A hybridisation rate of 18.2% was recorded in a sample of supposed juvenile salmon from the River Leven in northwestern England with one individual being the result of a female trout × male salmon cross (Hartley, 1996). The latter cross was also found in one individual of supposed trout parr from the River Tweed (McMeel, 1996). From studies in Scotland (Youngson et al., 1993) and in Norway (Hindar & Balstad, 1994; Hindar, 1997) it has been concluded that escaped-farm salmon are more likely to hybridise with brown trout than wild salmon. As backcrossing of hybrids to either salmon or trout is likely to be very rare, given the low ﬁtness of F1 hybrids and poor survival of backcrosses (Hindar, 1997), hybridisation is probably of little genetic importance to natural populations of sea trout, although where high rates of hybridisation occur it will reduce ﬁtness. However, the occurrence of a moderate frequency of F1 hybrids in some rivers means that hybrids are likely to be included in sea trout sampling and thus may bias biological studies. Hybrids may also be the reason why debate ensues among anglers on some occasions as to whether an individual ﬁsh is a sea trout or an Atlantic salmon.

Management and other implications Implications of genetic basis of anadromy The clear evidence of genetic bases to the various components of anadromy, indicating that anadromy is a quantitative threshold trait, has implications for both biological studies and management of sea trout. A key implication is that anadromy will evolve in response to differential costs and beneﬁts and thus as a result of accidental or deliberate selection. The likelihood of moderate heritability, together with the high fecundity of sea trout, means that substantial changes can occur in a few generations. The fact that sea trout can be present in one river and rare or absent in an adjacent one, together with the co-occurrence of sea and freshwater trout in many systems, suggests that the ﬁtness advantages of sea and freshwater life-history strategies are similar, that is there is ‘knife-edge’ balance. Thus relatively small changes can change the balance in favour of an alternative life history. For example, a small decrease in marine survival or greater exploitation of sea trout could potentially result in increased selection against anadromy. In western Scotland and Ireland infestations with sea lice and other problems associated with Atlantic salmon farming have led to reduced marine survival (e.g. Gargan et al., 2006). While such poor survival has a direct impact on sea trout

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numbers it also acts as a source of selection against anadromy and in favour of a freshwater life history. Greater exploitation of sea trout relative to freshwater trout in a river system can also select against anadromy. The selection effect is magniﬁed by the fact that female sea trout are typically larger than freshwater trout and thus produce proportionately more eggs. It could be speculated that the apparent decline in some areas prior to the onset of salmon farming could in part be the result of selection against anadromy as a result of differential exploitation although it is impossible to separate this from the direct impact of heavy exploitation on recruitment. Stocking with domesticated brown trout could result in a decrease in the genetic potential for anadromy in a wild population (Table 12.3). Many farm strains of brown trout in Britain and Ireland are derived from strains established in the late nineteenth century. These strains were established using broodstock from Loch Leven (Scotland) together with those from other populations including sea trout ones (Maitland, 1887; Armistead, 1895). Thus many of these stocks have been under domestication for at least 25 generations during which time they have been subjected to deliberate and accidental selection and to random genetic changes as a result of genetic drift. Artiﬁcial selection of larger ﬁsh was undertaken even in the early days of trout farming as clearly documented by Maitland (1887) and Armistead (1895). In addition to faster growth, selection can result in increased aggression and other behavioural changes (Fleming et al., 2002). Domestication also results in lowered awareness of predators (Fernö & Järvi, 1998; Johnsson et al., 2001; Álvarez & Nicieza, 2003). Inadvertent selection can also occur as a result of hatchery procedures, for example because of different feeding levels (Glover et al., 2004). Ruzzante et al. (2004) found that although domesticated trout, which had been stocked into rivers, produced smolts, they experienced high mortality at sea and were therefore largely absent in returning spawners. This suggests that genetic changes during domestication have reduced the ability to survive in the sea and the authors conclude that sea trout of domesticated origin are unlikely to reproduce to any signiﬁcant extent. However, stocked domesticated trout that remain in fresh water have been shown to breed successfully (Hansen et al., 2000a) and thus may increase the proportion of freshwater trout relative to sea trout. Thrower et al. (2004) note that an experimental line derived from a lake population of rainbow trout, which had originally been established above a waterfall from steelhead parents (see above), produced smolts but these had poor survival relative to smolts derived from anadromous parents. The authors speculated that this reduced survival could be attributed to reduced genetic variability in the lake population or because of lack of natural selection for marine survival. That is, the lake population had not been exposed to marine conditions for 70 years and thus, while it retained smolting ability, it had not been subject to ongoing selection in relation to changing marine conditions. A further implication of the genetic basis of anadromy is that genetic background needs to be taken into account when studying the inﬂuence of environmental factors on sea trout production. As noted above, there is considerable genetic diversity within and among sea trout populations and failure to account for this variability in ecological and

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Table 12.3 Some potential impacts of stocking domesticated brown trout into sea trout populations, based on studies of brown trout and other salmonids. Effect

Cause

Impact

References

Competitive displacement of native trout

Domesticated trout larger because of selective breeding, earlier hatching and greater food availability; domesticated trout more aggressive Although they have lower survival, some domesticated trout survive to reproduce as parr or adults

Reduced survival of wild parr, reduced smolt production, and reduction in sea trout return

Fernö & Järvi, 1998; Einum & Fleming, 2001; Johnsson et al., 2001; Fleming et al., 2002; Álvarez & Nicieza, 2003; Glover et al., 2004 Hansen et al., 1997, 2001b; Fleming & Petersson, 2001; Hansen, 2002; Chilcote, 2003; McGinnity et al., 2003; Ferguson et al., 2007 Hansen et al., 2000a, 2001b; Ruzzante et al., 2001, 2004

Hybridisation of wild and domesticated trout

Reduced anadromous component in population

Propensity of domesticated trout to remain in fresh water; poor survival and return of domesticated trout that migrate to sea

Change in characteristics of population such as timing of adult return

Genetic differences between domesticated strain and native population

Loss of inter-population genetic heterogeneity

Most domesticated strains are from a few wild populations and only contain a small part of natural genetic variability

Loss of intra-population genetic variability

Some farm strains have low genetic variability as a result of founder effects and inbreeding

Lowered recruitment

All of above which are cumulative over years as stocking continues

Reduced population ﬁtness (i.e. lower production of offspring in subsequent generations) compared with pure wild population Change in genetic composition of population in favour of freshwater genotypes with lowered sea trout yield Change in angling quality of population in respect of run timing, age, behaviour and other stock characteristics Loss of local adaptations and loss of unique stock characteristics; reduction in ability of species to respond to changing environmental conditions Inbreeding depression resulting in lowered ﬁtness and greater uniformity in population characteristics; reduction in ability of population to respond to changing environmental conditions Extinction vortex; population ceases to be self-sustaining and declines exponentially, eventually becoming extinct

Palm & Ryman, 1999; Stewart et al., 2002; McGinnity et al., 2003

Maitland, 1887; Armistead, 1895; Wang et al., 2002a, b; Utter, 2003

Hansen et al., 1997, 2001a; Hansen, 2002; Utter, 2003; Skaala et al., 2004

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other experiments renders these of questionable value. Thus, just as it is important when looking at the inﬂuence of genetic factors to ensure a uniform environment through, for example, common garden experiments, it is similarly important to ensure uniform genotype in experiments with environmental variables. Implications of genetic differentiation among populations As noted above, most, if not all, brown trout traits of interest to the ﬁshery manager and angler have a genetic component, as well as an environmental one. Loss of genetic diversity leads to lowered abundance, lowered recruitment and greater uniformity in life-history characteristics. Genetic differences in these characteristics are also important in adaptation to local environmental conditions. It is thus vital to consider both genetic and environmental aspects in the management of sea trout populations. Effective management and conservation must take account of the genetic differences within and among catchments. From the angler’s point of view, genetic diversity is also directly important because it results in angling diversity in terms of size, run timing and behaviour. Some commentators in angling magazines have expressed the opinion that, as a result of previous stocking with domesticated trout, the natural genetic diversity of brown trout in Britain and Ireland has been wiped out. However, no evidence for this contention is given! Detailed studies, which have examined genetic diversity in both natural and farm stocks of brown trout from throughout Britain and Ireland, and placing this diversity in the context of that found elsewhere in north-western Europe, show that this contention is untrue (Ferguson, 1989, 2004; Stephen & McAndrew, 1990; Hynes et al., 1996; Duguid, 2002; McKeown, 2005). Although some waters contain trout of farm origin or show evidence of substantial genetic inﬂuence from farm strains, the vast majority of natural populations show no evidence of farm genetic inﬂuence. This is especially so in the more western and northern parts of both Britain and Ireland. There are several reasons why supplemental stocking has had a lesser impact than might be anticipated. First, stocking often took place in waters with plentiful trout already, perhaps in a misguided attempt to increase growth rate or in pursuit of the Victorian ‘new blood’ philosophy. In competition with native trout, introduced domesticated trout, as with other domesticated salmonids (McGinnity et al., 2003) generally have low survival and so contribute little to future generations. In addition, the number of stocked trout is often a very small proportion of the natural population. As female trout produce some 2000 eggs per kg, in larger water systems at least, natural recruitment is of the order of many millions. Even today stocking often consists of a few thousand fry or parr. Studies in Denmark, France, Ireland and Norway have shown that after 10 to 40+ years of stocking, involving millions of farm trout, only a small percentage of the naturally reproduced trout show farm genotypes (Taggart & Ferguson, 1986; Skaala et al., 1996; Borgstrøm et al., 2002; Heggenes et al., 2002; Champigneulle & Cachera, 2003). Hansen (2002) in Denmark has determined the genetic composition of trout populations prior to stocking, using DNA taken from historical scale collections. In the majority of, but not in all populations, the genetic contribution by stocked domesticated trout was surprisingly low. However, in some populations very

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substantial genetic inﬂuence of stocked trout was found, as has been noted elsewhere (e.g. Lagiardèr & Scholl, 1996). Thus although the impact of past stocking has been much lower than anticipated in many situations, stocking with domesticated trout continues to be a threat to native genetic diversity as the more it is undertaken the more likely it is that detrimental effects on wild populations will occur. It has been argued that stocking is no different from natural straying. There are few estimates of straying rates for sea trout but those that do exist suggest that it may be of the order of 1–3%. However, strays do not necessarily breed in the non-home river. If they do breed, the success of their offspring, generally hybrids with native ﬁsh, is likely to be lower than native ﬁsh (McGinnity et al., 2004). Thus effective straying rates, in terms of gene ﬂow, are probably less than 1%. Indeed if they were higher than a few individuals per generation, the genetic differences among sea trout populations in adjacent tributaries and rivers, as seen in molecular marker studies, would not exist. At this low level, straying does not disrupt adaptations to local conditions as these are maintained by natural selection. Several studies have shown higher genetic variability in some sea trout populations compared with isolated freshwater ones (Ferguson, 1989; Bouza et al., 1999). The stronger natural selection is on a trait the higher gene ﬂow would need to be before differences among adjacent rivers would be eliminated. Thus molecular studies (primarily based on neutral markers) overestimate the impact of straying as adaptive differences can be maintained at higher levels of gene ﬂow than neutral allele differentiation. A little gene exchange, through straying, is advantageous as it helps maintain genetic variability in small populations, but too much can be damaging. Supplemental stocking with non-native and domesticated trout can reduce the genetic diversity within and among sea trout populations and can lower ﬁtness through hybridisation with the wild population (Table 12.3). There is also little, if any, rigorous scientiﬁc evidence that supplemental stocking is of value in increasing stocks. Stocked domesticated ﬁsh have two main impacts on wild populations: (1) competition and (2) interbreeding (McGinnity et al., 1997, 2003; Fleming et al., 2000; Einum & Fleming, 2001; Fleming & Petersson, 2001). As a result of selection for faster growth and earlier hatching, together with possibly favourable conditions for growth in the hatchery, stocked trout are often larger than the equivalent wild cohort. As a result of this larger size, together with the more aggressive behaviour typical of domesticated ﬁsh, stocked ﬁsh can competitively displace wild ﬁsh. Poorer later survival of the farm ﬁsh means that they do not compensate for the displaced wild ﬁsh and so the overall number of adults and thus subsequent juvenile recruitment is reduced. As some stocked ﬁsh do survive to maturity, in the next generation part of the wild production is converted to hybrids with these domesticated trout. In terms of survival, growth and other aspects of performance, hybrids have been found to be intermediate between the wild and farm ﬁsh. In the many experiments where wild farm salmonid hybrids have been produced there is no evidence for hybrid vigour, that is, where the hybrids do better than either of the parents (McGinnity et al., 2003). Thus suggestions that adding ‘new genes’ will improve the wild stock are unfounded and are based on a false theoretical premise derived from highly inbred pure lines of agricultural plants and animals. Although smaller than farm ﬁsh, hybrids are often larger than wild trout and also have a detrimental competitive

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impact. As hybrids have reduced survival compared with wild ﬁsh, the conversion of part of the potential wild production to hybrids results in an overall reduction in survival in the population giving fewer ﬁsh and lower recruitment of juveniles in the next generation. As stocking is often carried out on an annual basis, there is a cumulative reduction in numbers over generations, with eventually the population ceasing to be self-sustaining and is only maintained by increasing levels of annual stocking. Chilcote (2003) demonstrated that a spawning population comprised equal numbers of hatchery and wild steelhead rainbow trout would produce 63% fewer recruits per spawner than one that comprised entirely wild ﬁsh and he concluded: ‘For natural populations, removal rather than addition of hatchery ﬁsh may be the most effective strategy to improve productivity and resilience’. Given this lowered ﬁtness as a result of stocking with domesticated trout, together with the potential reduction in the anadromy as discussed above, such stocking is likely to be even more damaging in the case of sea trout stocks than in freshwater ones. In addition to stocking with domesticated trout, supplementation is also carried out using hatchery-reared ﬁrst generation offspring of broodstock taken from other rivers. In an experiment undertaken under communal conditions, McGinnity et al. (2004) found that the offspring of non-native Atlantic salmon had only 35% of the lifetime success of the native group. In this case study, the two rivers arise on the same mountain, about 1 km apart, and ﬂow into the sea about 50 km apart. Again, as with domesticated ﬁsh, the problem occurs when non-native individuals that do survive hybridise with the native ﬁsh resulting in a lowering of ﬁtness in that population. Thus while stocking with the offspring of non-native trout may not be as damaging as stocking with domesticated ﬁsh, such stocking can lead to a loss of ﬁtness in the recipient population. Stocking with non-native trout may also be ineffective as Gargan et al. (2006) found no increase in sea trout smolt runs following substantial stocking in two western Irish rivers. Before stocking is undertaken, it needs to be established why stock levels are too low in the ﬁrst place. Often this will be because of deterioration in habitat and pollution. The ﬁrst line of action should be to reverse these detrimental effects. Given the will and the ﬁnance, habitat restoration can be effectively undertaken. This should be the way forward. In some cases, this may not be possible in the short term and artiﬁcial stocking may be necessary for the time being to overcome a clearly identiﬁed ‘bottleneck’ in production (Aprahamian et al., 2003). Where stocking is undertaken it should be carried out using only the offspring of native broodstock, a procedure referred to as supportive breeding (Hansen et al., 2000b). The period in the hatchery should be as short as possible commensurate with overcoming the natural bottleneck to production. It is also important that it is carried out in such a manner so as not to result in inbreeding and loss of genetic variability (Ryman & Laikre, 1991; Ryman et al., 1995) or inadvertent selection (Glover et al., 2004).

Future genetic research requirements (1)

The heritability of anadromy needs to be determined for brown trout. This would require full smolt and adult trapping facilities on a sea trout river and the use of DNA proﬁling (e.g. Ferguson et al., 1995a, b) to determine parentage of sea trout.

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Heritability estimates also need to be made in several different brown trout–sea trout sympatric populations of different genetic background (lineages). (2) The relative ﬁtness, and thus the extent of local adaptation, of sea trout populations with different biological characteristics needs to be investigated (e.g. age and size at smolting, age at ﬁrst return from sea, timing of smolt migration and return from sea, spawning time, number of repeat spawnings, maximum age and size, morphology and distance migrated from home river). These objectives could be accomplished by using DNA proﬁling to assign parentage and grand-parentage in natural populations, through common garden experiments (e.g. McGinnity et al., 2003, 2004) and by genome scans with QTL mapping (e.g. see reviews by Rogers and Bernatchez [2005], Slate [2005] and Storz [2005]). (3) The diversity of lineages within and among sea trout populations should be investigated further using complete mtDNA genome analysis and the possible relationships of this ancestry to current biological characteristics need to be determined. Appropriate populations should be prioritised for conservation. (4) The impact of stocking domesticated and non-native brown trout on the extent of anadromy and ﬁtness of sea trout populations should be investigated using common garden experiments (e.g. McGinnity et al., 2003, 2004) and comparative genomics.

Acknowledgements I am grateful to Michael M. Hansen, John B. Taggart and Paulo A. Prodöhl for their valuable and constructive comments on a draft of this chapter.

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McCormick, S.D. (1995). Hormonal control of gill Na+ ,K+ -ATPase and chloride cell function. In: Cellular and Molecular Approaches to Fish Ionic Regulation (Wood, C.M. & Shuttleworth, T.J., Eds). Academic Press, San Diego, CA, pp. 285–315. McCormick, S.D., Hansen, L.P., Quinn, T.P. & Saunders, R.L. (1998). Movement, migration, and smolting of Atlantic salmon (Salmo salar). Canadian Journal of Fisheries and Aquatic Sciences, 55 (Suppl. 1), 77–92. McGinnity, P., Stone, C., Taggart, J.B. et al. (1997). Genetic impact of escaped farmed Atlantic salmon (Salmo salar L.) on native populations: use of DNA proﬁling to assess freshwater performance of wild, farmed and hybrid progeny in a natural river environment. ICES Journal of Marine Science, 54, 998–1008. McGinnity, P., Prodöhl, P., Ferguson, A. et al. (2003). Fitness reduction and potential extinction of wild populations of Atlantic salmon Salmo salar as a result of interactions with escaped farm salmon. Proceedings of the Royal Society of London B, 270, 2443–50. McGinnity, P., Prodöhl, P., Ó Maoiléidigh, N. et al. (2004). Differential lifetime success and performance of native and non-native Atlantic salmon examined under communal natural conditions. Journal of Fish Biology, 65 (Suppl. A), 173–87. McGowan, C. & Davidson, W.S. (1992). Unidirectional natural hybridization between brown trout (Salmo trutta) and Atlantic salmon (S. salar) in Newfoundland. Canadian Journal of Fisheries and Aquatic Sciences, 49, 1953–8. McKeown, N. (2005). Phylogeography and population genetics of brown trout in Britain and Ireland. PhD Thesis, Queen’s University of Belfast. McMeel, O.M. (1996). Molecular phylogenetics of brown trout (Salmo trutta L.) populations. PhD Thesis, The Queen’s University of Belfast, 214 pp. Maitland, J.R.G. (1887). The History of Howietoun. Howietoun Fishery, Stirling. Martin, S.A.M., Youngson, A.F. & Ferguson, A. (1999). Atlantic salmon (Salmo salar) prolactin cDNA sequence and its mRNA expression after transfer of ﬁsh between salinities. Fish Physiology and Biochemistry, 20, 351–9. Matthews, M.A., Poole, W.R., Thompson, C.E. et al. (2000). Incidence of hybridization between Atlantic salmon, Salmo salar L. and brown trout, Salmo trutta L., in Ireland. Fisheries Management and Ecology, 7, 337–47. Merilä, J. & Crnokrak, P. (2001). Comparison of genetic differentiation at marker loci and quantitative traits. Journal of Evolutionary Biology, 14, 892–903. Moran, P. (2002). Current conservation genetics: building an ecological approach to the synthesis of molecular and quantitative genetic methods. Ecology of Freshwater Fish, 11, 30–55. Morán, P., Pendás, A.M., Gárcia-Vázquez, E., Izquierdo, J.L. & Lóbon-Cerviá, J. (1995). Estimates of gene ﬂow among neighbouring populations of brown trout. Journal of Fish Biology, 46, 593–602. Morinville, G. & Rasmussen, J. (2003). Early juvenile bioenergetic differences between anadromous and resident brook trout (Salvelinus fontinalis). Canadian Journal of Fisheries and Aquatic Sciences, 60, 401–10. Narum, S.R., Contor, C., Talbot, A. & Powell, M.S. (2004). Genetic divergence of sympatric resident and anadromous forms of Oncorhynchus mykiss in the Walla River, USA. Journal of Fish Biology, 65, 471–88. Nielsen, C., Holdensgaard, G., Petersen, H.C., Björnsson, B.Th. & Madsen, S.S. (2001). Genetic differences in physiology, growth hormone levels and migratory behaviour of Atlantic salmon smolts. Journal of Fish Biology, 59, 28–44. Nielsen, C., Aarestrup, K., Norum, U. & Madesen, S.S. (2004). Future migratory behaviour predicted from premigratory levels of gill Na+ /K+ -ATPase activity in individual wild brown trout (Salmo trutta). Journal of Experimental Biology, 207, 527–33. Olsson, I.C. & Greenberg, L.A. (2004). Partial migration in a landlocked brown trout population. Journal of Fish Biology, 65, 106–21. Paaver, T.K. (1989). Genetic differentiation of sea trout, Salmo trutta, populations of Estonian rivers. Journal of Ichthyology, 29, 37–43. Palm, S. & Ryman, N. (1999). Genetic basis of phenotypic differences between transplanted stocks of brown trout. Ecology of Freshwater Fish, 8, 169–80. Petersson, E. & Järvi, T. (1997). Reproductive behaviour of sea trout (Salmo trutta) – the consequences of searanching. Behaviour, 134, 1–22. Pettersson, J.C.E., Hansen, M.M. & Bohlin, T. (2001). Does dispersal from landlocked trout explain the coexistence of resident and migratory trout females in a small stream? Journal of Fish Biology, 58, 487–95. Pratten, D.J. & Shearer, W.M. (1983). The migrations of north Esk sea trout. Fisheries Management, 14, 99–113.

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Chapter 13

The Genetic Basis of Smoltiﬁcation: Functional Genomics Tools Facilitate the Search for the Needle in the Haystack T. Giger1 , U. Amstutz1 , L. Excofﬁer1 , A. Champigneulle3 , P.J.R. Day2 , R. Powell4 and C.R. Largiadèr1 1 CMPG

(Computational and Molecular Population Genetics Lab), Zoological Institute, University of Bern, Baltzerstrasse 6, CH-3012 Bern, Switzerland 2 Centre for Integrated Genomic Medical Research, University of Manchester, Stopford Building, Oxford Road, Manchester, M13 9PT, UK 3 Station d’Hydrobiologie Lacustre de Thonon, INRA (Institute National de la Recherche Agronomique), 75 avenue de Corzent, F-74203 Thonon-les-Bains, France 4 Department of Microbiology, National University of Ireland – Galway, Galway, Ireland

Abstract: This chapter describes the potential for the application of cDNA microarray technology for the investigation of the genetic basis underlying parr-smolt transformation, that is, the smoltiﬁcation process, in salmonids. In the Section Materials and Methods, an experimental approach is presented, which aims at the identiﬁcation of a tissue that is particularly suitable for a ﬁrst cDNA microarray screen of global gene expression patterns in smoltifying and non-smoltifying individuals. To this aim, the global gene expression diversity in four tissues, which have been identiﬁed in previous studies as being involved in the regulation of the smoltiﬁcation process were analysed with a cDNA microarray containing 1076 non-redundant salmonid gene probes. These experiments included liver, brain, kidney and gill tissue samples from three brown trout (Salmo trutta L.) individuals. The liver tissue was identiﬁed as being the most informative tissue among these four, as it showed the highest level of measurable inter-individual gene expression diversity. In the second part of this article follow-up experiments for a candidate gene are described as an example of how the role in smoltiﬁcation of new genes that have been identiﬁed in cDNA microarray experiments can be studied in further detail. In particular, a northern blot analysis of the Transaldolase 1 gene for sedentary and migratory brown trout at the smolt and a pre-smolt stage was conducted. This gene was identiﬁed as being differently regulated in the liver between migratory and sedentary populations of brown trout in a previous cDNA microarray study. This analysis revealed, that in both phenotypes and age stages only one type of Transaldolase 1 transcript is synthesised, which consists always of the same exon composition. Additionally, a real-time quantitative PCR approach was used to assess the temporal dynamics of Transaldolase 1 expression across phenotypes at the smolt and a pre-smolt age stage. In the investigated pre-smolt stage, no signiﬁcant difference in Transaldolase 1

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expression levels between migratory and sedentary individuals was found, whereas in individuals at the smolt stage a signiﬁcant difference was detected. Keywords: Smoltiﬁcation, gene expression, cDNA microarray, Salmo trutta life-history, Transaldolase 1.

Introduction Partial migration describes the phenomenon of populations divided into migratory and sedentary individuals and occurs among animal taxa from insects to higher vertebrates (Jonsson & Jonsson, 1993). In many brown trout (Salmo trutta) populations, the two lifehistory forms (sedentary and migratory) spawn together and may be offspring of the same parental ﬁsh. This life-history variation appears to be inﬂuenced by a complex interaction of genetic and environmental factors (reviewed in Jonsson & Jonsson, 1993). Migratory individuals have to undergo a process termed smoltiﬁcation, which is the transition from a sedentary juvenile (parr) to a migratory (smolt) individual. This process involves dramatic morphological, physiological and behavioural changes, as migratory individuals have to adapt to long distance migration and to life in the open water column. Anadromous individuals additionally have to adapt to changing osmotic conditions (Jonsson & Jonsson, 1993). The phenomenon of partial migration is not well understood. For example, there are various theoretical explanations on how genetic variation for life-history traits is maintained in natural populations (Stearns, 1992), but there has been little success to date in ﬁnding empirical data supporting one of these models. Most likely, the tendency to migrate corresponds to a threshold character (Falconer, 1989) with only two phenotypic classes (i.e. sedentary and migratory trout). A similar model of inheritance may be assumed, for example for a quantitative trait, where many genes affect a character also subject to a large environmental component, thus producing continuous variation. In order to obtain discrete phenotypes under such conditions the underlying continuity of genetic and environmental effects needs a threshold, above which one phenotype is expressed and below which the alternative phenotype is expressed (Falconer, 1989). The study of the phenomenon of migratory behaviour and in particular of the associated smoltiﬁcation process in salmonids has focused on the anadromous forms because of their tremendous economic importance in aquaculture. It is evident that smolts are the desired life-history type for aquaculture, partly because their schooling behaviour can result in higher densities than for sedentary individuals. A major problem in ﬁsh farming concerns ﬁsh that turn from smolts into adults before they reach the optimal size to be sold on the market. Maturation arrests growth and signiﬁcantly reduces the quality of the meat, and thus, its commercial value. It is therefore not surprising that there is a vast literature concerning physiological changes in smoltifying ﬁsh and the environmental factors that have an inﬂuence on this process. For example, it has been shown that smolting involves a number of interacting endocrine systems. Growth hormone (GH), insulin-like growth factor-1 (IGF-1), cortisol and thyroid hormones (THs) increase during spring in response to seasonal changes in

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photoperiod and stimulate smolting (Hoar, 1988). Previous studies postulate the existence of a ‘light-pituitary’ regulatory axis (reviewed in Björnsson, 1997). These studies imply that increased photoperiod triggers an elevation in plasma levels of GH and thereby the onset of the parr-smolt transformation. A recent study provides evidence, that this ‘light-pituitary’ axis could be mediated by the neuroendocrine hormone melatonin, which is released into the blood from the pineal gland. It has been shown that melatonin contributes to controlling GH secretion in vitro (Falcon et al., 2003). In all vertebrate species examined thus far, pineal and blood melatonin levels are high during the night and low during the day, and this daily pattern changes on an annual basis as a consequence of the seasonal variations in day length (Falcon et al., 2003). The functional relationship of the GH/IGF axis is conserved in ﬁsh and is the primary endocrine system that regulates body growth in vertebrates. The GH stimulated production of IGF-1 in the liver is a major point of control in this axis (Pierce et al., 2005). Additionally, thyroid hormones are necessary for normal growth in salmonids, but rather than acting directly, it has been suggested that these hormones may play a permissive role in growth acting at different levels of the GH/IGF-1 axis (Nicoll et al., 1999; reviewed in Björnsson et al., 2002). Another characteristic of smoltiﬁcation is that anadromous individuals also have to adapt to a saline environment. The main osmoregulatory organs that enhance seawater adaptability are the gills, the kidneys and the intestines (Hoar, 1988) as teleosts are hypo-osmotic in seawater and balance the inﬂux of ions and the efﬂux of water by excreting excess monovalent ions in the gills, absorbing ﬂuid from ingested saltwater in the intestine, and excreting excess divalent ions via the kidney. The key enzyme to transport processes in the gill and the intestine is the membrane-spanning protein Na+ , K+ -ATPase. Therefore the regulation of Na+ , K+ -ATPase expression in these organs is of major importance to ﬁsh during saltwater acclimation (Seidelin & Madsen, 1999). GH has both growth-promoting and seawater-adapting actions in salmonids and together with the interrenal gland secreted cortisol, it is considered to be the major seawater-adapting hormone in salmonids. Both hormone levels increase during spring to stimulate smolting. GH and cortisol may act both individually as well as in synergy to increase saltwater tolerance, chloride cell number and size and gill Na+ , K+ -ATPase activity in salmonids (Björnsson et al., 2002). Whereas many studies have investigated physiological and endocrinological changes during the parr–smolt transformation, relatively few studies have addressed the genetic mechanisms underlying the life-history pattern of migratory behaviour in salmonids (e.g. Birt et al., 1991; Clarke et al., 1992; Nielsen et al., 2001). These studies have been able to show that this trait has a heritable component, but that it differs across genetically differentiated populations. Yet, there is no clue to which genes may be responsible for initiating and regulating smoltiﬁcation. Novel molecular technologies, such as cDNA microarrays (Schena et al., 1995) – also known as DNA chips – and constantly increasing genetics and bioinformatics resources from salmonids (Davey et al., 2001; Martin et al., 2002; Rise et al., 2004) open the possibility to screen genes with known or unknown function whether they are involved in smoltiﬁcation.

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DNA chips are glass surfaces that represent thousands of DNA fragments arrayed at discrete sites. Hybridisation of mRNA derived samples to DNA chips allow monitoring of the expression of thousands of different genes simultaneously. cDNA microarray technology should not be viewed as an alternative or a replacement for ‘classical’ hypothesis-driven approaches, where the role of known candidate gene or gene products in a particular process are investigated based on their previously known biological properties. The application of cDNA microarrays rather represents an important complement to existing techniques by generating new hypotheses, that is the identiﬁcation of new candidate genes independent on any a priori knowledge on their identity and function, and thus allow the identiﬁcation of new pathways that are involved in the expression of a trait of interest, such as smoltiﬁcation (Giger et al., 2006a, b). As the development and application of cDNA microarrays is currently a time-consuming process and requires considerable ﬁnancial resources, there is a great interest in limiting the number of single microarray experiments needed in a particular study. In this context, the questions of which tissue and which age stage of brown trout should be considered for an optimal cDNA microarray screen in order to identify new candidate genes involved in the smoltiﬁcation process are of central importance. Here, we focused on the ﬁrst question. Based on the literature reviewed earlier, the gene expression of 1076 genes in the four presumably most relevant tissues (brain, liver, kidneys, gills) for the regulation of smoltiﬁcation were screened in a few brown trout individuals using cDNA microarrays. The aim of this experiment was to identify the ‘most informative’ tissue among these four for a more extensive cDNA microarray study on the smoltiﬁcation process in salmonids. Because the cDNA microarray used here contained only a rather small fraction of the total salmonid transcriptome, the amount of detectable gene expression differences among individual ﬁsh was chosen as a selection criterion for the most ‘most informative’ tissue. In order to demonstrate how gene expression differences could be studied in further detail after the successful cDNA microarray-based identiﬁcation of a gene as a potential candidate being involved in the smoltiﬁcation process, various follow-up experiments for the Transaldolase 1 gene (Taldo 1) are described in the second part of this article. As a previous microarray study has shown, this gene is differently regulated in the liver between migratory and sedentary populations of brown trout and Atlantic salmon (Salmo salar) (Giger et al., 2006b). Once particular candidate genes have been detected in a cDNA microarray screen, such as Taldo 1, it is not possible to say whether these genes are involved in the onset of the smoltiﬁcation process or are a by-product of this phenotypic change. Taldo 1 is involved in the non-oxidative element of the pentose phosphate pathway, which maintains the monosaccharide pool in the cytosol, especially ribose-5-phosphate needed for nucleotide and nucleic acid synthesis (Novello & Mclean, 1968). This function does not provide any obvious hypothesis for its potential role in the smoltiﬁcation process. Thus, the ﬁrst followup experiment consisted of a test for the presence of alternative splice variants of this gene, which could assume different and unknown functions. Here, a northern blot analysis (Alwine et al., 1977) of the two different phenotypes in presmolt and smolt age stages was conducted. This analysis allows the potential identiﬁcation of alternatively spliced variants (Stamm et al., 2005) of Taldo 1 transcripts. Alternative

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splicing is a modiﬁcation mechanism by which splicing out introns in eukaryotic premRNAs occurs in various ways, resulting in one gene producing several different mRNAs and protein products. Alternative splice variants of Taldo 1 transcripts would therefore have different RNA sequences, which would affect the hybridisation afﬁnity onto the Taldo 1 gene probe. This would imply that the measured expression differences would be because of phenotypic speciﬁc expression of Taldo 1 protein isoforms. Even if the expression differences observed for a candidate gene are a by-product of smoltiﬁcation, the information at what age stage this expression difference is ﬁrst visible would be important in order to more precisely determine the age stage when the onset of smoltiﬁcation occurs, for future cDNA microarray experiments. Here, in a second follow-up experiment, the temporal dynamics of Taldo 1 expression in pre-smolt and smolt age stages of migratory and sedentary brown trout was assessed by using a real-time quantitative PCR (reviewed in Bustin, 2000) approach.

Materials and methods Sampling All trout were caught by electroﬁshing and immediately killed with an overdose of the anaesthetic 2-phenoxyethanol (Sigma). The tissue samples were immediately extracted, transferred into an RNA stabilisation reagent (RNA Later, Ambion) and stored at −30◦ C. For the microarray analysis of gene expression diversity across different tissues, two migratory individuals from a migratory population at the smolt stage from the Redon River (46◦ 21 03 N, 6◦ 24 06 E) and one sedentary individual from the same river (46◦ 20 06 N, 6◦ 25 21 E) were sampled in May 2002. From these ﬁsh brain, gill, kidney and liver tissue samples were taken. For the northern blot analysis liver tissue from migratory and sedentary brown trout individuals at the age of 0+ and 1+ years from the two Redon populations (three individuals per phenotype and age class) were investigated. The 0+ individuals were sampled in November 2004. For the subsequent assessment of the dynamics of Taldo 1 expression in liver, nine migratory 1+, eight sedentary 1+ (both sampled in April 2003), and ten individuals each from both phenotypes of the 0+ age class were analysed. Microarray construction, probe labelling, hybridisation and image analysis In order to minimise redundancy, the 1076 cDNA probes were selected from an expressed sequence tag (EST) library of Salmo salar (Davey et al., 2001; Martin et al., 2002) containing 3133 salmon cDNA clones after performing a clustering analysis using the program STACKPACK version 2.1 (Electric Genetics). Of the 1076 spotted cDNAs, 505 were singleton ESTs and the remaining 571 were arbitrarily selected representatives of 533 different cDNA clusters. The spotted constructs consisted of puriﬁed plasmids containing a cDNA strand as inserts, with sizes that range between 124 and 1179 bp, at a concentration of 1 μg/μl in 3× SSC. The plasmids were isolated with the HiSpeed plasmid puriﬁcation kit (Qiagen) from Escherichia coli cultures grown overnight.

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The cDNA microarrays were manufactured by the company Tecan (Maennedorf, Switzerland, http://www.tecan.ch/index.html) by spotting three times 1152 gene probes using a Genesis NPS 100 nanopipetting liquid handling system with non-contact piezo printing technology onto epoxy-coated glass slides (NoAb BioDiscoveries). The 1152 spotted features included the selected 1076 cDNA probes and 76 negative controls, consisting of 3× SSC. Total RNA was extracted using the Absolutely RNA RT-PCR Miniprep kit (Stratagene). For the labelling reaction, 50 μg of the extracted total RNA stored in 14.4 μl DEPC treated water were used. cDNA was synthesised by incorporating cyanine 3 ﬂuorochromes (CYTM 3, Amersham) at a concentration of 0.1 mM into the newly synthesised cDNA strand. The strands were primed with 4 μg of an anchored oligo dT (5 (TTT)6TTVN3 ) at a concentration of 1 μg/μl, and with 1.6 μg of a random hexamer (Invitrogen) at a concentration of 3 μg/μl, and catalysed with 400 units of Superscript II reverse transcriptase enzyme (Invitrogen) at 42◦ C for 3 h. After 2 h of incubation another 200 units of the reverse transcriptase was added. The RNA was hydrolysed with 5 μl 0.5 M EDTA and 10 μl 1 M NaOH during 20 min at 65◦ C. A neutralisation reaction was performed with the addition of 6 μl 1 M HCl and 2 μl 1 M Tris (pH 7.5). After this step, the synthesised products were cleaned using the Qiaquick PCR puriﬁcation kit (Qiagen). The products were then concentrated to a volume of 30 μl and stored at −30◦ C until they were hybridised onto the microarray slides. Prior to hybridisation, the slides were blocked with incubation in blocking buffer (NoAb BioDiscoveries) for 2 h at room temperature. Afterwards, the slides were denatured in boiling water for 2 min. Then the slides were put into 50% formamide in 3× SSC buffer for 2 min at room temperature. In a next step, the slides were washed at room temperature with incubation in double-distilled water for 2 min, in 70% ethanol for 2 min, and in absolute ethanol for 2 min. After this step, the slides were dried in a centrifuge. The hybridisation mix comprised 220 μl Slidehyb buffer 1 (Ambion) and 30 μl of labelled cDNA. The hybridisation of the labelled target molecules onto the microarray was performed during 16 h in an Automated Slide Processor (ASP, Amersham) at 42◦ C. The slides were subsequently washed at 45◦ C with 0.1× SSC, 1× SSC+0.2% SDS, 0.1× SSC+0.2% SDS and dried with isopropanol. After hybridisation, the slides were scanned with a ScanArray 4000 Microarray Analysis System (Packard BioScience). The photomultiplier tube (PMT) setting was kept constant at 90%. Median signal intensities of each spot and its background were extracted from the TIFF images of the scanned slides using the QUANTARRAY software version 3.0 (Packard BioScience). In order to virtually increase the dynamic range of the microarray scanner with the common linear scaling technique of multiple scans (Dudley et al., 2002) the slides were scanned one to two times at laser powers ranging between 70% and 100%. This technique consists in extrapolating saturated signals of the most sensitive scan based on a linear regression of the signal intensities for the high versus low intensity scans for spots in the linear range of both scans (>2000 and <50 000 intensity units, respectively). After this step the background signal was subtracted from the spot reading.

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Table 13.1 Average coefﬁcient of variation (CV) of gene expression levels of 1076 genes and its standard error (SE) from kidney, liver, brain and gill tissue. Tissue

Average CV (%)

SE

Kidney Liver Brain Gills

3.40 5.11 3.29 3.20

2.94 3.51 2.87 2.45

In order to make the data comparable across chips, the expression values were log2 transformed and normalised to have equal average gene expression intensities for all chips using a trimmed mean globalisation method (Kroll & Wölﬂ, 2002) cutting off 1% of the highest and 1% of the lowest values. This normalisation was carried out for each tissue separately. Reporters were excluded from the analysis if they were negative controls or if they showed aberrant spot morphologies. Additionally, an intensity-based ﬁltering of array elements (Quackenbush, 2002) with an arbitrarily chosen threshold of 512 signal intensity units was performed in order to cope with the limited precision of the microarray technology at very low expression levels. After this step, the mean from the signals of the three spot replicates for each gene was computed. To measure the variability among the tissues the average coefﬁcient of variation (CV) for all genes within each tissue and its standard error (SE) was computed and are reported in Table 13.1. As a measure for the differences in global gene expression levels (dx,y ) the squared gene expression differences between two experiments averaged over all genes was calculated according to the following formula: 1 (log x¯ i − log y¯ i )2 n n

dx,y =

(Khaitovich et al., 2004)

i=1

where x¯ i and y¯ i represent the average signal from the spot replicates from gene i from the two slides x and y. This distance is calculated over all detectable genes (n). A distance tree was constructed by UPGMA clustering (Fig. 13.1) of the pairwise distances with the program NEIGHBOR contained in the PHYLIP package version 3.6 (Felsenstein, 1989). In order to assess the conﬁdence of individual tree nodes bootstrapping (1000 pseudo replicates) was performed by drawing genes with replacement.

Real-time PCR and northern blotting Total RNA was extracted using the Absolutely RNA RT-PCR Miniprep kit (Stratagene). cDNA was synthesised by incubating 1 μg RNA with 10 mM of each dNTP and 50 ng/μl

190

Sea Trout K24

91 91 100

K28 K50 G24

91 100

100

G28 G50 B24

100 100

B28 B50 L24

100 100 0.1

L28 L50

Fig. 13.1 UPGMA tree based on pairwise distance measures of global gene expression in kidney (K), gill (G), brain (B) and liver (L) tissues of two migratory (24 and 28) and one sedentary (50) brown trout individuals at the age of 1+ years. The distance measures are based on expression data of 1076 genes. Bootstrap values (percentage out of 1000 replicates) are given at corresponding branch nodes.

random hexamer (Invitrogen) at 65◦ C for 5 min. After 2-min incubation on ice DTT (2 μM, Invitrogen), 5× Superscript ﬁrst strand buffer (Invitrogen) and 40 units RnasIn (Promega) or RNaseOUT (Invitrogen) were added. The reaction was catalysed with 100 U Superscript II reverse transcriptase (Invitrogen) with 10 min incubation at room temperature, 5 min at 37◦ C and 50 min at 42◦ C. The reaction was stopped by inactivating the enzyme with 15 min incubation at 70◦ C. Afterwards the samples were stored at −30◦ C. The real-time quantitative PCR for Taldo 1 was carried out on an ABI Prism 7000 Sequence Detection System (Applied Biosystems). cDNA synthesised from 12.5 ng RNA was used in a 25 μl reaction volume containing 12.5 μl TaqMan Universal PCR Master Mix, No AmpErase UNG (Applied Biosystems) and 1.25 μl 20× Assay Mix (TaqMan Assay-by-Design, Applied Biosystems). The thermal proﬁle was used according to the manufacturer’s instructions. The starting template quantity was determined as described in Peirson et al. (2003, Equation 2). The efﬁciency (E) of each assay was established from the slope of a standard curve of serial dilutions according to the manufacturer’s instructions with the formula E = 10−1/slope . Such a dilution series was performed for two samples and the mean of the two computed efﬁciencies was used for all subsequent calculations. In order to minimise the impact of technical variation on these gene expression measurements, each sample was run in quadruplicate and the quantities were normalised to the geometric mean of three endogenous controls (Vandesompele et al., 2002). These controls comprised the ribosomal protein L10, coagulation factor VII and prosaposin which have been shown to be expressed at relatively constant quantities in trout and Atlantic salmon liver (Amstutz et al., 2005). For the construction of the northern blot, total RNA (12.5 μg) was separated on a 1.2% agarose gel containing 1× MOPS and 1% formaldehyde. RNA was transferred to

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positively charged nylon membranes (Roche) in 20× SSC by a standard capillary blotting method. Following UV crosslinking of the RNA to the nylon ﬁlter, pre-hybridisation and hybridisation were carried out in 6× SSC, 5× Denhardt’s reagent and 0.5% SDS at 60◦ C. Pre-hybridisation was with 50 mg/ml denatured herringsperm DNA and 100 mg/ml denatured calf thymus DNA. For hybridisation, 100 ng of amplicons from the clone of the cDNA library (Davey et al., 2001; Martin et al., 2002), that contained Taldo 1 cDNA in the plasmid insert, was labelled with [a-32P]dCTP using the Ready-To-GoTM DNA labelling kit (Amersham). After overnight hybridisation, membranes were washed twice with 2× SSC/0.2% SDS and twice with 0.2× SSC/0.1% SDS at 60◦ C before exposure to a PhosphorImager screen.

Results and discussion Gene expression diversity among tissues The average CV of gene expression was considerably higher in liver than in kidney, brain and gills (Table 13.1), indicating that gene expression variability among individuals is highest in the liver. This could reﬂect the functional pleiotropy of the liver in comparison with the other three tissues. This difference in variability directly translates into larger global gene expression differences among individual ﬁsh in liver tissue than in the other screened tissues, which is summarised in an UPGMA tree based on global gene expression diversity indices (Fig. 13.1). The four tissues of the three analysed individual ﬁsh form distinct clusters separated with strong bootstrap support, and within tissues, the longest terminal branch lengths (i.e. the highest level of inter-individual diversity) are observed in the liver tissue. Although the sedentary individual (no. 50) clusters between the two migratory individuals (nos. 28 and 24), the distance between the sedentary individual and the closer migratory individual is higher in liver than all other distances across individuals in other tissues. This suggests that the highest level of gene expression differences between phenotypes can also be found in liver tissue, and thus, that the liver is probably the most informative tissue for more extensive microarray studies that aim at detecting new candidate genes involved in smoltiﬁcation using this particular chip. Finally, it is worth mentioning, that all individual experiments cluster with high statistical support as indicated by high bootstrap values greater than 90% for all nodes (Fig. 13.1), which suggests that for each tissue, each individual ﬁsh shows a very distinct global gene expression proﬁle.

Splice variant analysis and expression dynamics of Taldo 1 across different phenotypes and age stages The northern blot analysis (Fig. 13.2) did not reveal any indication for the presence of alternative splice variants. All the phenotypes and age stages investigated showed one distinct band of equal and expected size. Thus, the expression differences of Taldo 1

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Sea Trout

1

2

3

4

5

6

7

8

9

10

11

12

Fig. 13.2 Northern blot of the Transaldolase 1 gene from migratory (1–3) and sedentary (4–6) brown trout individuals at the smolt stage, migratory (7–9) and sedentary (10–12) individuals at the pre-smolt stage.

Relative expression level (arbitrary units)

1.2

1.0

0.8

0.6

0.4

0.2 Rmig 0+

Rsed 0+

Rmig 1+

Rsed 1+

Fig. 13.3 Box plot from expression data obtained by real-time quantitative PCR from migratory (Mig) and sedentary (Sed) brown trout individuals at the smolt stage (1+) and pre-smolt stage (0+). The whiskers indicate the 1.5 interquartile ranges.

measured between the two phenotypes are attributable to different quantities of one particular type of transcripts, always consisting of the same exon composition. As the measured expression differences of Taldo 1 are the consequence of different transcript quantities of the same type, it is now worth asking the question, whether the same pattern of expression differences can already be observed in pre-smolt age stages. Two-sided Mann–Whitney U tests of expression data obtained by quantitative real-time PCR revealed that at the age of 1+, the difference in Taldo 1 expression between the two phenotypes is statistically signiﬁcant with a P-value of 0.0061 with lower expression levels in migratory individuals (Fig. 13.3). This difference is not signiﬁcant between migratory and sedentary

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individuals of the 0+ age class (P-value 0.2413; Fig. 13.3). Overall, there is a signiﬁcant decrease with time of Taldo 1 expression levels within both phenotypes with a P-value of 0.0267 among sedentary and a P-value of 0.0007 among migratory individuals (Fig. 13.3). The investigation of age stages that are intermediate to 1+ individuals sampled in May and 0+ individuals sampled in November are a subject of ongoing experiments. This type of time series experiments allows one to trace the onset of smoltiﬁcation. Once this is achieved, the experimental setup can be updated and additional cDNA microarray experiments can be performed at age stages where differences in gene expression can be contrasted best possibly between the two phenotypes. Additionally, the assessment of genetic differences in the coding and regulatory regions of the Taldo 1 gene between the different phenotypes is also the subject of ongoing analyses. So far, after having sequenced exons 2–4 in both phenotypes, no variation at the DNA level leading to structural changes in the gene has been found (Amstutz et al., unpublished).

Conclusions Novel molecular biological technologies, such as cDNA microarrays, which allow one to monitor the expression of thousands of different genes simultaneously, and the constantly increasing genetic and bioinformatic resources for salmonid species open the possibility of screening a large number of genes with known or unknown function for their involvement in smoltiﬁcation. The cDNA microarray technology should not be viewed as an alternative or a replacement for ‘classical’ hypothesis-driven approaches, where the role of known candidate genes or gene products in a particular process is investigated based on their previously known biological properties. Rather, the application of cDNA microarrays represent an important complement to existing techniques by generating new hypotheses, that is the identiﬁcation of new candidate genes independent on any a priori knowledge on their identity and function, and thus allow the identiﬁcation of new pathways that are involved in the expression of a trait of interest, such as the smoltiﬁcation in salmonids. As the development and application of cDNA microarrays is currently very timeconsuming and expensive, there is a great interest in limiting the total number of microarray experiments in a study. In this context, the questions of which tissue and which age stage of brown trout should be best considered for a cDNA microarray study of the genetic basis underlying the smoltiﬁcation process remain of central importance.

Acknowledgements We thank G.C. Davey, N.C. Caplice and S.A. Martin for their work on the cDNA clone libraries. We thank R. Häner and his group, and also the Functional Genomics Unit, Universities of Bern and Zurich, for permitting access to their lab infrastructure. We are grateful to O. Greiner for technical advice and valuable discussion, R. Stoerrlein for the help in microarray construction and G. Rigoli for technical laboratory assistance. This work was supported by the Swiss National Science Foundation grant 3100-067136.01 to C.R.L. and P.J.R.D.

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References Alwine, J.C., Kemp, D.J. & Stark, G.R. (1977). Method for detection of speciﬁc RNAs in agarose gels by transfer to diazobenzyloxymethyl-paper and hybridization with DNA probes. Proceedings of the National Academy of Sciences of the United States of America, 74, 5350–54. Amstutz, U., Giger, T., Champigneulle, A., Hansen, M.M., Powell, R. & Largiader, C.R. (2006). Distinct temporal patterns of Transaldoase I gene expression in future migratory and sedentary brown trout (Salmo trutta). In press. Birt, T.P., Green, J.M. & Davidson, W.S. (1991). Contrasts in the development and smelting of genetically distinct anadromous and nonanadromous Atlantic salmon, Salmo salar. Canadian Journal of Zoology, 69, 2075–84. Björnsson, B.Th. (1997). The biology of salmon growth hormone: from daylight to dominance. Fish Physiology and Biochemistry, 17, 9–24. Björnsson, T.B., Johansson, V., Benedet, S. et al. (2002). Growth hormone endocrinology of salmonids: regulatory mechanisms and mode of action. Fish Physiology and Biochemistry, 27, 227–42. Bustin, S.A. (2000). Absolute quantiﬁcation of mRNA using real-time reverse transcription polymerase chain reaction assay. Journal of Molecular Endocrinology, 25, 169–93. Clarke, W.C., Whitler, R.E. & Shelbourn, J.E. (1992). Genetic control of juvenile life history pattern in chinook salmon (Oncorhynchus tshawytscha). Canadian Journal of Fisheries and Aquatic Sciences, 49, 2300–06. Davey, G.C., Caplice, N.C., Martin, S.A. & Powell, R. (2001). A survey of genes in Atlantic salmon (Salmo salar) as identiﬁed by expressed sequence tags. Gene, 263, 121–30. Dudley, A.M., Aach, J., Steffen, M.A. & Church, G.M. (2002). Measuring absolute expression with microarrays with a calibrated reference sample and an extended signal intensity range. Proceedings of the Academy of Sciences of the United States of America, 99, 7554–59. Falcon, J., Besseau, L., Fazzari, D. et al. (2003). Melatonin modulates secretion of growth hormone and prolactin by trout pituitary glands and cells in culture. Endocrinology, 144, 4648–58. Falconer, D.S. (1989). Introduction to Quantitative Genetics, 3rd edn. Longman Scientiﬁc & Technical, Burnt Mill, England. Felsenstein, J. (1989). PHYLIP – Phylogeny Inference Package (Version 3.2). Cladistics, 5, 164–6. Giger, T., Excofﬁer, L., Day, P.J.R., Champigneulle, A., Hansen, M.M., Powell, R. & Largadèr, C.R. (2006). Life history shapes gene expression in salmonids. Current Biology, 16(8), R281–2. Hoar, W.S. (1988). The physiology of smolting salmonids. In: Fish Physiology, Vol. XIB (Hoar, W.S. & Randall, D.J., Eds). Academic Press, New York, pp. 275–343. Jonsson, B. & Jonsson, N. (1993). Partial migration: niche shift versus sexual maturation in ﬁshes. Reviews in Fish Biology and Fisheries, 3, 348–65. Khaitovich, P., Weiss, G., Lachmann, M. et al. (2004). A neutral model of transcriptome evolution. PLOS Biology, 5, 682–89. Kroll, T.T. & Wölﬂ, S. (2002). Ranking: a closer look on globalisation methods for normalisation of gene expression arrays. Nucleic Acids Research, 30, e50. Martin, S.A., Caplice, N.C., Davey, G.C. & Powell, R. (2002). EST-based identiﬁcation of genes expressed in the liver of adult Atlantic salmon (Salmo salar). Biochemical and Biophysical Research Communications, 293, 578–85. Nicoll, C.S., Rodgers, B.D. & Kelley, K.M. (1999). Hormonal regulation of growth and development of nonmammalian vertebrates. In: Handbook of Physiology (Kostyo, J., Ed.). Oxford University Press, New York, pp. 73–98. Nielsen, C., Holdensgaard, G., Petersen, H.C., Björnsson, B.Th. & Madsen, S. (2001). Genetic differences in smolt physiology, growth hormone levels and migratory behaviour of Atlantic salmon smolts. Journal of Fish Biology, 59, 28–44. Novello, F. & Mclean, P. (1968). Pentose phosphate pathway of glucose metabolism – measurement of nonoxidative reactions of cycle. Biochemical Journal, 107, 775–791. Peirson, S.N., Butler, J.N. & Foster, R.G. (2003). Experimental validation of novel and conventional approaches to quantitative real-time PCR data analysis. Nucleic Acids Research, 31, e73. Pierce, A.L., Fukada, H. & Dickhoff, W.W. (2005). Metabolic hormones modulate the effect of growth hormone (GH) on insulin-like growth factor-1 (IGF-1) mRNA level in primary culture of salmon hepatocytes. Journal of Endocrinology, 184, 341–49. Quackenbush, J. (2002). Microarray data normalization and transformation. Nature Genetics, 32, 496–501.

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Rise, M.L., von Schalburg, K.R., Brown, G.D. et al. (2004). Development and application of a salmonid EST database and cDNA microarray: data mining and interspeciﬁc hybridization characteristics. Genome Research, 14, 478–90. Schena, M., Shalon, D., Davis, R.W. & Brown, P.O. (1995). Quantitative monitoring of gene expression patterns with a complementary DNA microarray. Science, 270, 467–70. Seidelin, M. & Madsen, S.S. (1999). Endocrine control of Na+ , K+ -ATPase and chloride cell development in brown trout (Salmo trutta): interaction of insulin-like growth factor-I with prolactin and growth hormone. Journal of Endocrinology, 162, 127–35. Stamm, S., Ben-Ari, S., Rafalska, I. et al. (2005). Function of alternative splicing. Gene, 344, 1–20. Stearns, S.C. (1992). The Evolution of Life Histories. Oxford University Press, Oxford. Vandesompele, J., De Preter, K., Pattyn, F. et al. (2002). Accurate normalization of real-time quantitative RT-PCR data by geometric averaging of multiple internal control genes. Genome Biology, 3, 0034.1–0034.11.

Chapter 14

Life History of the Anadromous Trout Salmo trutta B. Jonsson and N. Jonsson Norwegian Institute for Nature Research, Dronningensgt 13, PO Box 736 Sentrum, N-0105 Oslo, Norway

Abstract: Coastal populations of brown trout Salmo trutta L. vary from freshwater resident trout via partially migratory stocks consisting of anadromous (sea trout) and freshwater resident ﬁsh to chieﬂy anadromous populations. Within rivers, anadromous and freshwater resident ﬁsh may spawn separately or together on the same spawning grounds. When they are together, resident females spawn later during the spawning period. This results in reduced nest superimposition from larger females which dig deeper nests. Sea trout grow faster than resident trout and their somatic energy density and gonadal energy allocation are higher. But resident females have large eggs with higher energy density than anadromous conspeciﬁcs of similar size. Within populations, there is little genetic difference between migratory and resident ﬁsh, and lipid content, growth and metabolic rates during the parr stage appear important for whether a ﬁsh migrates or stays as resident. Among populations, there is genetic diversity even within a morph, and the genetic differentiation increases with distance. The proportion of anadromous trout appears to increase with decreasing growth opportunities in fresh water. Within populations, smolt age and size tend to increase with decreasing growth rate, and among populations, smolt size and age are inclined to decrease with decreasing growth rate. Smolt size and adult size of sea trout decrease with stream size for ﬁsh spawning in very small brooks. Adult size of anadromous trout is usually larger than that of resident trout from the same population, and females are on average larger than males because more of them migrate for feeding at sea, and they attain maturity at an older age. In spite of the large amount of information already available, there are still some questions that remain unanswered and some suggestions for future work are given. Keywords: Sea trout, life history, polymorphism, variation, dimorphism, research needs.

Introduction Few ﬁsh species exploit a broader ecological niche than brown trout Salmo trutta L. It feeds in fresh and salt waters and spawn in small streams as well as large rivers. Natural distribution is from the Atlas mountains in North Africa to the Barents Sea, and from the western limits of the European coastline to the Ural mountains and Caspian Sea. Anadromous populations (sea trout) occur in western Europe from north Portugal to the White Sea and Cheshkaya Gulf, including Iceland and the Baltic Sea (Elliott, 1994). To succeed in this wide area and range of habitats, populations exhibit polymorphism, sexual size dimorphism and polyphenism. The populations are often partially migratory, split between freshwater resident and sea-run migratory morphs (deﬁned as any of the 196

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sympatric forms [individual variants] that account for polymorphism [Jonsson & Jonsson, 1993]), which meet at the spawning ground, and the young compete for shared resources in the nursery area for parts of their life (Jonsson, 1985). Males are more often resident and usually smaller than females (Jonsson et al., 2001). The offspring are developmentally plastic and can adjust smolt age and maturity age in response to environmental variation. The variability of brown trout has created a systematic problem and altogether, about 50 species have been described for varieties of brown trout (Behnke, 1986). These varieties are presently grouped together as one species, often with more than one variant within the same locality with a varying degree of genetic differentiation (Hindar et al., 1991; McVeigh et al., 1995; Hynes et al., 1996). Co-occurring resident and anadromous forms can spawn together, and resident ﬁsh can develop from anadromous parents and vice versa (Skrochowska, 1969; Jonsson, 1989). Sea trout colonised the catchments in northern Europe postglacially. Resident trout have evolved from these, independently in the various catchments. Although there are several genetically differentiated lineages which have invaded European water courses, they should be considered as one species, similarly to some other species such as Arctic charr, Salvelinus alpinus (L.) (Jonsson & Jonsson, 2001). This chapter describes (1) polymorphism in brown trout, distinguishing between the anadromous and resident forms; (2) variation in life-history characters of sea trout and (3) sexual size dimorphism in brown trout. Finally, we present views on further research on the life history of trout.

Polymorphism in brown trout: anadromous and resident forms Morphs Brown trout are adapted to a wide range of aquatic habitats, with freshwater resident and anadromous forms coexisting in many rivers (Elliott, 1994). Anadromy involves the migration of juveniles from their freshwater nursery to marine feeding habitats and the return to fresh water for spawning, whereas freshwater residents carry out the entire life cycle within the watercourse. The two morphs can be geographically isolated by physical barriers impassable for trout, or partially so by the distance in upstream and downstream areas of the same river. But the two can also co-occur in systems without barriers such as coastal rivers with free access from the sea. The latter populations are composed of individuals adopting anadromy or residency as alternative life-history strategies (partial migration; Jonsson & Jonsson, 1993). Table 14.1 summarises the differences in life-history traits between resident and anadromous brown trout and these differences are discussed here. The morphs differ in habitat use as well as various physiological and morphological characters, for example spawning dress and camouﬂage colours, secondary sexual characters, growth rate, size at maturity, egg number and size and sex ratio; among these, the variation in colours and size may be the most conspicuous ones. At sea, brown trout have pelagic camouﬂage dress, with silvery or silvery-grey sides with dark spots, white belly and dark back, whereas residents in streams have darker sides with light brown or tawny overall colouration, brown to dark brown back, with pronounced black and often red spots along

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Table 14.1 Life-history traits (population means) of resident and anadromous brown trout in partially migratory populations. Variable

Resident

Anadromous

References

Main food items

Zoobenthos, zooplankton

Fish, zoobenthos

Adult mass (g)

50–150

300–4000

Length at maturity (cm)

15.2–23.5

19.9–75.8

Age at maturity (mean) Sex ratio (% males)

1.5–5.2

2.3–8.2

67–96

27–49

% Morphs (resident/sea trout) Somatic energy density (kJ/g) Fecundity Resident 300 g Sea trout 500 g

0.2–15

0.2–15

Pemberton, 1976a; Fahy, 1983; Jonsson & Gravem, 1985; Lyse et al., 1998; Knutsen et al., 2001b, 2004 Jonsson ,1985; L’Abée-Lund et al., 1989; L’Abée-Lund & Jonsson, 1993 Jonsson, 1981; L’Abée-Lund et al., 1989; L’Abée-Lund, 1991; Jonsson et al., 2001 Campbell, 1977; Jonsson, 1989; L’Abée-Lund et al., 1989, 1990 Campbell, 1977; Jonsson, 1981; Jonsson et al., 2001 Jonsson, 1981; Jonsson et al., 2001

4.5

5.0

Jonsson & Jonsson, 1997

GSI females (energy, kJ %) (mass %)

28.6 16–17a 10–15.8

Egg weight (mg) Resident 300 g Sea trout 500 g Depth of nest (cm) Smolt age (population mean, years) Smolt size (population mean, cm)

667 909–1537 34 30–37a 17.6–26

69–100 4a

58–78 17a 1.2–5.6 6.7–25.2

Jonsson, 1981; Elliott, 1995; L’Abée-Lund & Hindar, 1990; Jonsson & Jonsson, 1999; Maisse & Baglinière, 1999 Jonsson & Jonsson, 1997 Elliott, 1988 Elliott, 1984; L’Abée-Lund & Hindar, 1990; Jonsson & Jonsson, 1997 Jonsson & Jonsson, 1999; Olofsson & Mosegaard, 1999 Elliott, 1984 L’Abée-Lund et al., 1989; Jonsson et al., 2001 L’Abée-Lund et al., 1989; Jonsson et al., 2001

a Figures refer to allopatric populations because information from partially migratory trout is missing. When information on mass (W , g) is lacking in the original reference, it was estimated from length (L , cm) using the relationship 100 = L 3 /W .

the sides of body. The spawning colours of the two are more similar, but residents usually have a darker back and sides than their anadromous conspeciﬁcs. Before migration, sea trout juveniles undergo smolting (Tanguy et al., 1994; Finstad et al., 1998). This process is pre-adaptation for the oceanic life as an increased ability to hypoosmoregulate in sea water is developed (McCormick & Saunders, 1987; Hoar, 1988). One characteristic change is increased Na+ , K+ -ATPase enzyme abundance in gill chloride cells during spring (McCormick & Saunders, 1987). A study of wild brown trout in tributaries of the River Lille Aa, Denmark, showed that two months prior to migration, a bimodal morphological (silvering) and physiological (Na+ , K+ -ATPase) development concurred.

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This was related to subsequent differentiation into resident and migratory fractions of each population (Nielsen et al., 2003). In wild brown trout, gill Na+ , K+ -ATPase activity did not increase further once migration was initiated. In brown trout, differentiation of populations into groups of migratory and resident individuals seemed to be inﬂuenced by sex (Rasmussen, 1986) and genetics (LeCren, 1985; Zaugg et al., 1985; Muir et al., 1994). In brook charr Salvelinus fontinalis Mitchill gill Na+ , K+ -ATPase activity appears to have an important hereditary component (Boula et al., 2002), and the same holds probably for brown trout. Smolting seems not obligatory for survival at sea. Also ﬁsh from inland populations survive and grow well in sea water. Fjord releases in Norway have revealed that ﬁsh from the sub-Alpine Lake Tunhovdfjord were very well suited for sea ranching and gave higher return than most sea trout populations tested (Jonsson et al., 1994b, 1995). However, the survival of freshwater trout at sea can be improved by sea-water acclimatising the ﬁsh for 4–8 weeks before release into salt water (Jonsson et al., 1994a). The experiment exhibited that the acclimatisation was less important for offspring of anadromous trout, and that increased ability for ionic regulation can be achieved by exposing inland trout to sea water before release. In the ranching experiments, anadromous stocks stay at sea for a longer time than the resident stocks, the mean number of days were 148 and 77, respectively, suggesting genetic variation in migratory tendencies among populations. Genetic variation between sea trout and freshwater resident trout was also indicated by differences in susceptibility to sea lice Lepeophtheirus salmonis (Krøyer) (Glover et al., 2001). Sea lice abundance was approximately twice as high on resident trout as on sea trout when tested in common garden experiments. How are sea trout distinguished from sympatric resident trout when caught in streams? The growth rate increases abruptly when the trout enter sea water, and sea trout are often identiﬁed by inspection of the scales (Jonsson, 1985; Elliott & Chambers, 1996). Chemically, they are distinguished by the strontium density in the scales which is the highest in sea trout (Bagenal et al., 1973; Coutant & Chen, 1993; Eek & Bohlin, 1997). In the River Jörlandaån, Sweden, the mean strontium content for sea trout was 266 μg/g and for resident trout 93 μg/g. Differences in carotenoid pigment proﬁles are detected in sea and resident trout (Youngson et al., 1997). Among sea trout, only two carotenoids were detected, astaxanthin and lutein, whereas among resident trout six carotenoids, zeaxanthin, lutein, astaxanthin, canthaxanthin and two unidentiﬁed pigments, were detected. Carotenoid pigment proﬁles of the yolk may also be used to separate alevins of sympatric freshwater residents and anadromous trout, young ﬁsh which have never been to sea. Sea trout usually grow much larger than resident trout (Table 14.1). The size difference is mainly because of higher growth rate at sea than in fresh water. Residents can mature as small as approximately 10 cm in length, whereas the smallest adult sea trout are approximately 20 cm (Olofsson & Mosegaard, 1999; Jonsson et al., 2001). The mass of 4-year-old resident trout living in small streams may be approximately 20 g, whereas similar aged sea trout can be between 0.5 and 1 kg (Jonsson & Sandlund, 1979; Swales, 1986; Champigneulle et al., 1999; Landergren, 2001). Variation in food consumption rate is probably the main reason for the vast size difference (Alm, 1959; Elliott et al., 1995), but also food particle size may

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be important (Wootton, 1998). There is little knowledge about the meaning of differences in ionic or hormonal differences (Nielsen et al., 1999; Seidelin & Madsen, 1999) for observed growth or size differences between sea trout and conspeciﬁc freshwater residents. With increasing size, ﬁsh need and can exploit gradually larger food items. As they grow, they can feed on previously inaccessible items, whereas previously used resources may gradually become unavailable (Rincón & Lobón-Cerviá, 2002). Furthermore, sea trout grow faster because they start feeding on ﬁsh earlier than residents (Keeley & Grant, 2001), and ﬁsh is their main food item (Pemberton, 1976a; Fahy, 1983; Elliott, 1997; Knutsen et al., 2001b, 2004). Freshwater residents feed chieﬂy on small food items such as zoobenthos and zooplankton (Jonsson & Gravem, 1985). Earlier ﬁsh feeding in sea trout than resident trout may be associated with their greater aggressiveness, as found when testing sea-run, lake-run and stream-resident brown trout populations in Finland (Lahti et al., 2001). The sea-run form was more aggressive than the lake-run and stream residents. Among 0+ ﬁsh, migratory offspring are usually larger than resident offspring as a result of their larger egg size. During exogenous feeding, their growth rates are similar, but the smaller resident trout simply never catch up with the larger migratory juveniles (Elliott, 1994). The higher population density in rivers than at sea can have a negative effect on growth affecting residents more than sea trout (Jonsson, 1985; L’Abée-Lund et al., 1989; Jenkins et al., 1999). At high densities, feeding reduces food abundance, but there may be also a negative effect of intensiﬁed competition and increased activity resulting from aggressive encounters between conspeciﬁcs. In addition to higher growth rate and ﬁsh size, the rich feeding environment at sea may result in higher somatic energy density in sea trout than in freshwater resident trout. On the spawning grounds, somatic energy density was approximately 5.0 kJ/g wet mass in sea trout and 4.5 kJ/g or 10% less in residents from a population in southern Norway (Jonsson & Jonsson, 1997). Elliott (1988) gave similar ﬁgures for sea trout females from Black Brows Beck and residents from the neighbouring Wilﬁn Beck. This difference was chieﬂy caused by a higher somatic lipid content in migratory than in resident ﬁsh. Furthermore, the size-speciﬁc somatic energy content increased with ﬁsh size in resident ﬁsh (Elliott, 1976; Jonsson & Jonsson, 1997) and was chieﬂy attributable to a higher density of somatic lipids in larger than smaller individuals. In sea trout, the energy density was higher, but independent of body size meaning that the energy densities of small and large sea trout spawners were similar. One reason for the difference may be that the production of somatic proteins relative to lipid reserves has higher priority in resident than migratory ﬁsh. Body growth can be viewed as a defence against predators, and this defence may be more important for the smaller than the larger morph as the number of potential predators decreases with trout size (cf. Wootton, 1998). The investment in lipid energy reserves may be more suitable for use during long migrations and spawning competition in the river. Activity related to migration and spawning is probably lower in resident than in sea trout, so their need for somatic energy reserves may be smaller. Furthermore, large adults may also need larger energy reserves to survive spawning than smaller ones. In both Atlantic salmon Salmo salar

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201

L. and Arctic charr mortality resulting from spawning is higher in larger than smaller spawners (Dutil, 1986; Jonsson et al., 1997). Furthermore, small salmon appear to regain their strength after spawning more rapidly than large salmon. Iteroparous one-sea winter (SW) ﬁsh spawn annually whereas repeatedly breeding multi-sea winter salmon spawn biennially (Jonsson et al., 1991). Thus, there is reason to believe that large trout spawners need a higher lipid density than smaller ones. Together, this may explain why sea trout exhibit higher energy densities than resident trout, and why the energy density increases with the size of resident ﬁsh. Age at sexual maturity is usually lower in resident than anadromous trout. Maturity is often attained when growth rate starts to level off, and that occurs earlier in resident than anadromous ﬁsh (Jonsson & Jonsson, 1993). As residents and anadromous ﬁsh belong to the same populations, female surplus among sea trout leads to male surplus among resident ﬁsh. Resident trout can be almost only males, but in some populations, resident females are also numerous although less so than in the migratory part of the populations. Variations in early energy allocation may be associated with the adoption of migration versus resident life histories. Studies in salmonids show that fast-growing individuals shift their niche earlier and at a smaller body size than more slow-growing individuals feeding in the same habitat (Jonsson, 1985; Jonsson & Gravem, 1985; Bohlin et al., 1993, 1996; Økland et al., 1993; Forseth et al., 1994, 1999). Forseth et al. (1999) found that the food consumption and energy use were much higher for lake migratory brown trout than stream residents of the same population. The absolute daily ration for 2+ migrants was more than four times higher and the energy budget 4.5 times higher than for similar-aged residents. Despite this large difference in food consumption, the speciﬁc growth rate did not differ between resident and migratory individuals at the time of migration, and the proportion of the energy used for growth was much smaller than that of the residents. The reason is that their energy use is much higher, not the least because of their larger body mass. Similar results were found in brook charr (Morinville & Rasmussen, 2003). This may be linked to higher aggressiveness and activity mentioned above (Lahti et al., 2001), and therefore higher energy needs. Migratory brook trout had noticeably different energy budgets than residents from the same system. No difference in speciﬁc growth rate prior to migration was found between migrants and residents of the same age class. Although it appeared that migrants obtain more food, the fact that they migrate suggests that they do not receive enough energy to satisfy their higher metabolic demands. They most likely enter growth bottlenecks sooner than residents. These ﬁndings suggest that migrants adopt migration as a consequence of energetic limitation in fresh water (Jonsson & Jonsson, 1993). However, factors initiating and regulating smolting of brown trout are not well understood (McCormick & Saunders, 1987; Finstad & Ugedal, 1998; Claireaux & Audet, 2000; Boula et al., 2002). Spawning Resident and anadromous brown trout can spawn together, or separately. In tributaries to the Vangsvatnet lake, Norway, the two morphs breed together (Jonsson, 1985). On some spawning grounds, resident males occur together with the anadromous ﬁsh. There, resident

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females enter the spawning grounds relatively late in the season, towards the end of the spawning period (Jonsson, 1981). The proportion of resident spawners increased with decreasing size of the spawning stream, and in the smallest streams sea trout were seldom observed. Also in the Kirk Burn, Scotland, Campbell (1977) noted an overlap between the spawning zones of resident and sea trout. And in a branch of the Scorff River in Brittany, France, freshwater resident trout spawn in the upper reaches of the system whereas the migratory population spawn mainly in lower reaches with some overlap between populations occasionally in the middle reaches (Bagliniére et al., 1989). In that system, there are suitable spawning grounds in the lower and upper reaches of the brook, but anadromous ﬁsh do not move to the upper spawning ground. Similar observations have been given by, for example, Skaala & Nævdal (1989) and Bohlin et al. (2001). Trout spawners may also be segregated by size (and thereby morph) according to current velocity and the coarseness of the bottom substrate. The largest females spawn on coarser gravel and in deeper zones with higher current speeds (Ottaway et al., 1981), and they bury their eggs deeper in the gravel substrate than small females similar to observations from other salmonid species (Fleming, 1996). Elliott (1984) showed that the mean burial depth of eggs is 17 cm for sea trout, with body lengths between 25 and 45 cm, and 4 cm for resident females, with body lengths between 17 and 30 cm. The risk of nest superimposition by an anadromous spawner is decreased by the late spawning of smaller, resident trout and smaller anadromous trout (Elliott & Hurley, 1998) in sea trout rivers. Morph differentiation: genetics and environment Trout populations often include both migratory and resident individuals. The species was one of the ﬁrst to invade rivers from the ocean, as the ice-cap retreated from northern Europe approximately 10 000 years ago. These early immigrants were the founders of both the resident and anadromous trout in these water courses (Huitfeldt-Kaas, 1918). A similar population splitting has been observed in coastal rivers where brown trout have been released, whether the parental ﬁsh were anadromous or freshwater resident (Allen, 1951; Rounsefell, 1958; Frost & Brown, 1967), showing that both behavioural tendencies occur within single gene pools. Morph differentiation is partly inherited and partly inﬂuenced by environment. The environmental effect is developmental, meaning that incidences during early development inﬂuence the development of parr into resident or migratory forms similar to that described for other types of organisms such as amphibians and insects (Laurila et al., 2002; Moczek & Nijhout, 2003). Phenotypic plasticity is a bet-hedging strategy which provides means for adapting to environmental unpredictability. Phenotypic plasticity contrasts populationlevel adaptation where phenotypic diversity is caused by genetic diversity, chieﬂy inherited because of the evolutionary history of the organisms and natural selection in spatially or temporally heterogeneous environments (Meyers & Bull, 2002). A combination of these two mechanisms appears important in brown trout life history. Apparently, the seaward migration and sexual maturation are competing processes at the parr stage (Thorpe, 1986; Hansen et al., 1989; Jonsson & Jonsson, 1993). Usually, trout that mature sexually at the

Life History of Anadromous Trout

203

parr stage never smolt and migrate to sea (Jonsson, 1985; Dellefors & Faremo, 1988). What is the trade-off between these two processes? Growth is often limited by present feeding opportunities, and ﬁsh are often attaining maturity near the asymptotic body size determined by the food ration consumed (Jonsson & Jonsson, 1993; Forseth et al., 1994; He & Steward, 2002). However, if ration size can be increased by moving to a new area where feeding opportunities are better, migration is an alternative to maturation. By this shift, the trout postpones growth stagnation and thereby increases future reproductive output if it survives. Thus, the ‘decision’ of an organism to migrate or not is likely to be dependent on a trade-off between beneﬁts and costs of migration compared with residency. The beneﬁts and costs are balanced through their effect on ﬁtness. For individual ﬁsh, their ﬁtness should be maximised when minimising the ratio μ/g; μ is size-speciﬁc mortality rate and g is size-speciﬁc growth rate (Werner & Gilliam, 1984). Thus, growth rate may be assumed as an index of reproductive success, and should inﬂuence whether a ﬁsh becomes resident or migrates to sea. The variation in the proportion of resident relative to migratory ﬁsh in habitats offering different growth opportunities (L’Abée-Lund et al., 1990) and migratory costs (Bohlin et al., 2001), lends support to this theory. The higher the growth rate in fresh water and the higher the migratory cost, the higher the proportion of resident ﬁsh. Furthermore, offspring of sea trout reared to smolt size in the benign conditions of hatcheries hardly migrate when released in rivers (Jonsson et al., 1995). They often attain maturity instead and thereby become freshwater residents, probably, because the feeding rate in fresh water is too high. Anadromous brown trout are therefore often ranched from eggs or fry, not smolts (Jonsson et al., 1994b; Dannewitz et al., 2003). Maturation as parr appears to be inﬂuenced by food ration size, either through the level of lipid storage (Thorpe & Metcalfe, 1998), metabolic rate (Morinville & Rasmussen, 2003), or a relationship between metabolism and the amount of surplus energy which can be allocated for growth (proteins and lipids) (Forseth et al., 1999). Trout appear to be able to ‘decide’ whether to migrate relative to their present energetic state and expected future ﬁtness return, although the only variables they physiologically ‘know’ are present size, growth and metabolic rates and energetic state. The reaction norms are probably genetically programmed through selection. Possibly, rich feeding opportunities in fresh water relative to that at sea select for a resident life history, and poor feeding opportunities select for a migratory way of life (Nikolsky, 1963; Northcote, 1978; Gross et al., 1988). The response to the feeding opportunities offered appears to be partly phenotypically plastic (Forseth et al., 1999; Gowan & Fausch, 2002). The energetic threshold level coding for migration differs probably among populations. Thus an energetic state triggering migration in one population, gives residency in others (Svärdson & Fagerström, 1982; Jonsson et al., 1994b). This level is probably adapted through natural selection. Brown trout are divided into a number of discrete demes, spawning in geographically separated localities (Ferguson & Fleming, 1983; Skaala & Nævdal, 1989; Cross et al., 1992). The farther they are apart, the greater the genetic differentiation between populations (Hindar et al., 1991; Knutsen et al., 2001a). There may be one population in small streams, whereas larger rivers may support several local populations. For instance, two brown trout populations in neighbouring streams, exhibit genetic differences in migratory

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behaviour (Elliott, 1989). Similarly, genetic differences occur between trout populations above and below barriers, such as impassable waterfalls (Hindar et al., 1991). Furthermore, a ﬁeld experiment showed that this genetic difference was associated with reduced migratory tendency above the falls, probably resulting from strong selection pressure for maintenance of position in the river, whereas those below the falls migrated freely between fresh and salt waters (Jonsson, 1982). In partially migratory trout populations, there is little genetic variation between resident and migratory ﬁsh. Hindar et al. (1991) were unable to detect genetic differentiation between sympatric resident and anadromous trout. But there were signiﬁcant differences between anadromous ﬁsh spawning at different localities in the same watercourse. More recently, Pettersson et al. (2001) found that brown trout from a common gene pool could adopt alternative migratory strategies. They tested the genetic differentiation in stream-resident females living in sympatry with sea-migrant females, and females living in an upstream allopatric landlocked population. Genetic differentiation was not detected between the sympatric morphs whereas both diverged signiﬁcantly from the landlocked population. The study could not preclude that a modest degree of gene ﬂow takes place from the landlocked population and that this may play a role in maintaining the two coexisting life-history morphs among females in the downstream population. However, the resident ﬁsh in the migratory populations were not residents originating from non-anadromous ﬁsh invading from upstream areas. Similarly, Jonsson et al. (2001) observed both morphs in streams with no resident ﬁsh above the stretches occupied by sea trout, indicating that brown trout can give both resident and migratory offspring. The fact that partial migration in brown trout is inﬂuenced both by the genetics and the environment was also revealed by rearing experiments. Skrochowska (1969) showed that freshwater residents can develop from anadromous parents, and anadromous offspring from resident ﬁsh, but resident parents tend to produce a lower proportion of migratory and more resident offspring than do anadromous parents. In another experiment, offspring of resident and sea-run migrant brown trout from the Norwegian River Imsa were reared for 3 years in a hatchery (Jonsson, 1989). Each of the two groups was split into two groups reared in separate tanks with different densities. Within each morph, those reared at the highest density grew most slowly. Both within resident trout and sea-run trout, more ﬁsh matured as 1-year-olds in the fast-growing than in the slow-growing group. But offspring of resident trout matured signiﬁcantly earlier than offspring of migrants, even though the residents were more slow-growing than the sea-run migratory ﬁsh. This indicated a genetic difference in growth rate and maturity age between these two brown trout morphs. Elliott (1988, 1989), studying trout in two small Lake District streams, found that offspring of resident ﬁsh in all age groups were smaller than those of migrants. This was probably not because of genotypic differences in growth rates, but a result of the differences in egg and fry size. Offspring from large eggs grew the largest. The size advantage was retained during their entire life span. Hence, there is inherited population variation in migratory behaviour among populations, whereas within populations, observed variations are chieﬂy environmental. The migrants probably have a very low level of energy consumption relative to an inherited threshold.

Life History of Anadromous Trout

205

The frequency of anadromous ﬁsh should be low if feeding opportunities are favourable in fresh water, and high if food conditions there are poor (Gross, 1987, 1996; Gross et al., 1988; Jonsson & Jonsson, 1993; Gross & Repka, 1998). The switch point between the two behaviours is probably adapted so that individual ﬁtness is maximised, but constrained by the energetic state of the ﬁsh when the decision is made. This view is supported by ﬁndings also from other salmonids such as white-spotted charr Salvelinus leucomaenis (Pallas) (Morita et al., 2000) and sockeye salmon Oncorhynchus nerka (Walbaum) (Wood & Foote, 1996; Altukhov et al., 2000). Beneﬁts and costs of migration The proﬁt of migration is increased food consumption with higher growth rate, energy density and reproductive potential than those of resident ﬁsh. Increased reproductive potential results from larger body size, higher gamete production and improved competitive ability during spawning. On the other hand, migration may be costly with high mortality at sea (Berg & Jonsson, 1990). Estimates from the River Vosso sea trout indicate that approximately 40% of a cohort of immature ﬁsh died during the summer at sea (Jonsson, 1981) and adult mortality was higher. Only about 30% of the spawners returned to spawn a second time, indicating an overall mortality of approximately 50%. Monitoring of sea trout from the River Vardneselva, northern Norway, showed a mean loss during the summer at sea of 56% when averaged over all age groups, with a tendency of higher sea survival among young than old migrants. The mortality of resident trout appears lower. Estimates from several lakes and years were summarised by Jensen (1977) giving annual mortality rates for freshwater resident trout 2 years and older of between 0.16 and 0.47, with most ﬁgures below 0.4. We have not been able to ﬁnd any estimates from resident trout living in sympatry with sea trout. The movement between the spawning and nursery grounds in fresh water and the sea may also have a cost. Bohlin et al. (2001) provided a direct test of how this migratory cost inﬂuences the distribution of sea trout and resident populations in southern Sweden. They used elevation of the spawning area as an index of migratory difﬁculty. At low elevations, juvenile density (as a measure of population productivity) was higher for migratory than for nearby non-migratory populations, showing that migration was beneﬁcial because it increased the production of juveniles. Juvenile density then decreased with increasing elevation in sea trout but not resident trout. At an elevation of about 150 m, juvenile density was similar for the two, suggesting that the cost of migration had reached the point where it offset the beneﬁts. As expected, the presence of sea trout also disappeared at higher elevations. This evidence suggests that the cost of migration (elevation and distance from the sea) in addition to impassable barriers such as waterfalls, limits the distribution of sea trout inland. Reproductive characters Gonadal energy investment by unit of somatic energy (GSI kJ) was 1.2 times higher for sea trout (34%) than resident females from the same population (28.6%), but not in males

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(3%) (Jonsson & Jonsson, 1997). Higher somatic energy density in migratory relative to resident females enables the migrants to invest more energy in gonadal development. But the gonadal mass relative to the somatic mass increase in the year of sexual maturation, was similar for sea trout and resident ﬁsh of the same sex. Thus, the ﬁsh appeared to allocate a certain percentage of their available energy into gonadal production. As a consequence of their higher energy content, the gonadal investment was higher in anadromous than resident trout. A similar GSI, as reported from Norway, was found in studies of sea trout from a stream in the Lake District, England (Elliott, 1988). The female trout from Black Brows Beck invested 30–37% of their total energy in egg production. Resident females from the nearby Wilﬁn Beck, however, invested only 16–17%, which is considerably lower than in the Norwegian population of sympatric anadromous and non-anadromous trout, described above (Jonsson & Jonsson, 1997). An intermediate estimate for the energy loss in non-anadromous trout was given by Berg et al. (1998). They found that ﬁrst time spawning females lost about 20% of their total energy during spawning. Resident females seem to spawn larger but fewer eggs compared with similarly sized sea trout (Jonsson & Jonsson, 1999; Olofsson & Mosegaard, 1999). This appears to be in contrast to the hypothesis that egg mass increases with the fat content of the ﬁsh, because the fat content in resident trout is lower than that of sea trout (Jonsson & Jonsson, 1997). Mean egg mass in sea trout females weighing 300 and 500 g were found to be 0.047 and 0.058 g, respectively. Offspring of freshwater residents reared under similar conditions were larger: 0.069 and 0.074 g (Jonsson & Jonsson, 1999). However, because egg mass increases with the body size of the trout, sea trout often exhibit similar sized or larger eggs than residents. Furthermore, mean fecundity in wild sea trout from southern Norway with body mass of 100 and 500 g were approximately 300 and 1500 eggs, respectively. The corresponding fecundities of resident trout were 270 and 1100 eggs (Jonsson & Jonsson, 1999). Resident females from the Wilﬁn Beck and sea trout from the Black Brows Beck, England, both with body lengths at 25 cm, laid 239 and 512 eggs, respectively (Elliott, 1988). Thus, large eggs of resident trout may be a speciﬁc adaptation of this morph, and at the expense of egg numbers. Selection for this may be the competitive ability of the young towards sea trout offspring. After swim-up, the offspring of the two morphs compete for the same food resources. Because of the larger body size of their mother, most sea trout offspring have large yolk sacs. To compensate, the smaller resident females may produce relatively large eggs for their body size. If this is correct, resident trout co-occurring with sea trout, should have larger eggs than corresponding resident trout competing only among themselves. This accords with observations of sympatric resident and anadromous brown trout in Jördalsån stream, south-western Sweden (Olofsson & Mosegaard, 1999). In this stream, the residents had equally large or larger eggs than sea trout, 65.9–108.5 mg versus 76.8–84.2 mg. The eggs of allopatric resident trout were 23.7–80.1 mg, and migratory populations 44.5–121.9 mg. Furthermore, the sympatric resident population had a lower absolute fecundity compared with migratory trout. Large eggs were also found in a brown trout population living in sympatry with alpine bullhead Cottus poecilopus (Heckel) compared with allopatric populations living above a waterfall (Olsen & Vøllestad, 2001). Mean dry weights of the

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207

eggs were 26.84 and 20.80 mg, respectively. Apparently, this evidence lends support to the hypothesis that there is selection for large egg size in freshwater resident populations where the offspring meet strong competition from sea trout or another competing species. How can large eggs improve the competitive ability of the offspring of resident trout? In partially migratory populations, resident females spawn relatively late during the season compared with most sea trout females (Elliott & Hurley, 1998; Jonsson & Jonsson, 1999). Because of this asynchrony in spawning time, the offspring of resident ﬁsh will probably hatch later in spring when the water temperature is higher. Because of poorer yolk conversion efﬁciency to body tissue at high water temperatures (Fleming & Gross, 1990; Beacham & Murray, 1993), large eggs may be favourable for late spawning trout using their yolk at high water temperatures. This effect may be strengthened by the offspring of anadromous trout starting to feed earlier so that they may have a competitive size advantage over offspring of the later hatching eggs of resident females, as found for offspring of Atlantic salmon (Einum & Fleming, 2000). This advantage appears compensated by the large egg size in resident spawners (Jonsson & Jonsson, 1999; Einum & Fleming, 2002; Einum, 2003). This may also explain why the energy density of the eggs of resident females is higher than that of anadromous females caught on the same spawning ground (Jonsson & Jonsson, 1997). Thus, large egg mass and high energy density of the eggs of resident trout may be an adaptation favourable for trout hatching late at high and increasing water temperatures, improving the competitive ability towards earlier hatching sea trout offspring.

Life-history variation in sea trout Growth Whereas the size increment of resident trout usually starts levelling off at smolt size, approximately 15–20 cm in total length, sea trout continue to grow after entering sea water. Mean length increment of immature trout during the ﬁrst year at sea is between 7 and 22 cm. In Norway, it decreases with water temperature at sea and increasing latitude (L’Abée-Lund et al., 1989), but this trend disappears as more southern European populations are included (Jonsson & L’Abée-Lund, 1993). During the second year at sea, the length increment of immature sea trout may be similar or decrease relative to that of the ﬁrst year at sea. It declines during the third sea year, as it does even more, if the trout stay immature longer. At maturity, the length increment drops abruptly to about 50% of that of immature sea trout having spent the same number of years in the river and at sea (Jonsson, 1985). Hence, the ﬁrst period at sea is characterised by a high speciﬁc growth rate which levels off with time at sea. Furthermore, post-smolt growth is lower for ﬁsh entering sea water as larger than as smaller smolts (Jonsson & L’Abée-Lund, 1993). The physiological basis for this is not well understood but may be a combination of a reduction in relative oxygen and food uptake with increasing ﬁsh size and higher reproductive energy demands in larger than in smaller individual ﬁsh (Wootton, 1998). Growth rate varies within and among sea trout populations. A part of this variation is developmentally plastic. Water temperature, energy intake and ﬁsh size are the chief factors determining growth performance of the species (Elliott et al., 1995; Elliott &

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Hurley, 2001) and there is little evidence for genetic variation in temperature for optimal growth among populations of brown trout (Elliott, 1994). Although this view has been challenged a number of times in experiments using ﬁsh originating from different thermal environments, no clear-cut variation in temperature tolerance among populations has been found. If thermal adaptation exists, the best candidates appear to be populations inhabiting very cold environments as indicated by Jensen et al. (2000) and Nicola and Almodovar (2004). However, in Atlantic salmon, Gilbey et al. (1999) reported a relationship between size and genotype variation, possibly linked to habitat temperature. Furthermore, Rungruangsak-Torrissen et al. (1998) found that temperature during hatching and startfeeding of Atlantic salmon affected the subsequent expression of different trypsin isozymes. Trypsin isozymes are functionally sensitive to different temperatures and may affect the later temperature performance of the ﬁsh. There is not yet any knowledge about whether incubation temperature in brown trout will inﬂuence the subsequent scope for the growth of this species. In any case, the effect of this possible genetic and biochemical variation appears negligible compared with the variation directly caused by the ambient water temperature, food consumption and size of the ﬁsh (Elliott, 1994; Jensen et al., 2000). For experimental brown trout with very high food consumption, growth rate increases with increasing temperature from approximately 4◦ C to an optimum at approximately 17◦ C. Maximum growth rate decreases with increasing temperatures above this to zero at temperatures of approximately 25◦ C (Forseth & Jonsson, 1994; Elliott et al., 1995; Elliott & Hurley, 2000; Ojanguren et al., 2001). Optimal temperature for growth decreases at a lower food consumption and appears to be approximately 15◦ C for wild sea trout parr feeding in rivers (L’Abée-Lund et al., 1989; Jensen, 1990; Jensen et al., 2000). Speciﬁc growth rate decreases with reduced food rations (Elliott, 1975), as does the maximum temperature for growth. Trout may smolt as a response to decreased food consumption and growth (Thorpe & Metcalfe, 1998; Forseth et al., 1999; Morinville & Rasmussen, 2003), or mature sexually and adopt a non-anadromous life history (Jonsson, 1985; Dellefors & Faremo, 1988; Jonsson & Jonsson, 1993). Smolting Brown trout smolts are usually immature, and the age at smolting varies within and among populations (Table 14.1). Within populations, slow growers tend to be older and larger when they migrate to sea than more fast-growing individuals (Økland et al., 1993). Mean size of the oldest smolt cohort is larger than the youngest cohort, even 1 year before they smolt and migrate to sea. Variation in the food consumption rate relative to an inherited threshold level, may be the major reason for the variation (Thorpe & Metcalfe, 1998). This seems to hold for brown trout as well as other salmonids such as Atlantic salmon (Thorpe et al., 1998). The threshold age and size for smolting appear to be determined by the expected beneﬁts and costs at sea which are growth dependent. It is more energy-consuming to be a fast grower, whereas large ﬁsh can escape predation more easily. The metabolic rate of fast-growing parr is high, and they meet energetic constraints (because of low food availability) in fresh water earlier and at a smaller size than do more

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209

slow-growing parr from the same population (Forseth et al., 1999; Gowan & Fausch, 2002; Morinville & Rasmussen, 2003). Therefore, they are probably gaining more by moving to richer feeding habitat early in life, compared with more slow-growing individuals. The within-population variation in smolt age is probably chieﬂy phenotypic whereas the amongpopulation variation may to a large extent be inherited as it is not accompanied by a change in smolt size. Smolt size may be adapted to the conditions at sea, where water temperature, feeding opportunities and predator presence, are selecting factors. These are variables not experienced by the ﬁsh at the time of smolting. Thus, geographical variation in size at smolting appears to be inﬂuenced by local adaptation. Among populations, the smolt size tends to increase with the growth rate of the parr. Mean smolt size is approximately 20 cm in rivers where mean length increment during the second year is 10 cm, and 16 cm where they increase 4 cm during the same period (Jonsson & L’Abée-Lund, 1993). Growth rate of brown trout parr increases with decreasing latitude in rivers along the western coast of Europe and smolt age, but not size decreases from north to south (Jonsson & L’Abée-Lund, 1993). At approximately 70◦ N in Norway, brown trout smolt at 4–6 years of age, whereas smolt ages of 1–2 years are common in France and Spain at 43–48◦ N (Toledo et al., 1993; Euzenat et al., 1999) and typically 2–3 years in England, Wales, Scotland, Denmark and southern Norway (50–60◦ N). Within Norway, at 59–70◦ N, however, smolt size increases with latitude and decreasing water temperature (degree days) at sea near the river mouth, but not with the river temperatures. Thus, sea temperature appears important for smolt size. This may be because ionic regulation in salt water is more difﬁcult at low than high temperature and for smaller than larger trout (Hoar, 1976; Finstad et al., 1988, 1998). Sexual maturation Among populations, mean sea age at maturity varies between one summer and 4 years, and appears inﬂuenced by growth rate at sea. Within populations, immature trout tend to be larger than similar aged mature trout (Jonsson, 1985). This may be both because fast growers mature relatively old and because the migratory and reproductive costs are large (Jonsson, 1985). Mature ovaries constitute about 30% of the total energy of adult females and the male energetic costs of reproduction are of similar magnitude as that of females (Elliott, 1994; Jonsson & Jonsson, 1997). Within-population variation in age at sexual maturity may be largely phenotypic, and related to variation in feeding and growth. Among Norwegian sea trout populations, there is no signiﬁcant correlation between speciﬁc growth rate at sea and age at sexual maturity. However, ﬁsh from populations with high immature growth rate in the second relative to the ﬁrst year at sea mature at an older age than those with lower growth rate in the second relative to the ﬁrst year (Fig. 14.1). Furthermore, ﬁsh which smolt at a young age tend to stay for a longer time at sea before attaining maturity than those smolting at a higher age, which may be related to differences in parr growth (Jonsson et al., 2001). Water temperature may also inﬂuence age at maturity, probably, though it has an effect on its longevity. Fish from warm rivers appear to mature younger than those from colder

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Age of maturity

4 3 2 1

0.8

1.0

1.2

1.4

1.6

G1/G2 Fig. 14.1 Relationship between mean females sea age at maturity (A) and mean length increment during ﬁrst relative to second year at sea (G1 /G2 ): A = 4.509–1.919 G1 /G2 , r2 = 0.52, d.f. = 18, P < 0.01. From L’Abée-Lund et al. (1989).

rivers (Jonsson, et al., 1991). Mean sea age at maturity is found to be 3–4 years in the north Norwegian populations versus 1–2 years in France and Spain (L’Abée-Lund et al., 1989; Toledo et al., 1993; Euzenat et al., 1999). There appears to be no trend in adult size with latitude as it was for age, and selective factors such as local feeding opportunities, predator presence and environmental conditions on the spawning grounds, may be more important than latitude for variation in growth rate and temperature. Thus, genetic adaptation may be important among population variation in age and size at maturity. This has been found for Atlantic salmon (Gjerde et al., 1994) and rainbow trout Oncorhynchus mykiss (Walbaum) (Martyniuk et al., 2003), and should also hold for brown trout. Longevity Longevity of sea trout decreases with increasing water temperature, growth rate and latitude in Europe in parallel with age at maturity (Jonsson, B. et al., 1991). Most trout die at 5 years or younger in southern Europe at 43–50◦ N. At 68–70◦ N in the north, the ﬁsh live about 10 years (Jonsson & L’Abée-Lund, 1993). The longer lifespan of brown trout in northern than southern populations, may be chieﬂy related to temperature. The lower the temperature, the lower the metabolism and the longer the energy reserves and cells may last. A comparable decrease in longevity and water temperature has been observed in American shad Alosa sapidissima (Wilson) (Glebe & Leggett, 1981). In that case, ﬁsh tended to survive spawning in cold but not warm rivers along the east coast of North America. Longevity increases with decreasing growth rate, as also found in other salmonids such as cutthroat trout Oncorhynchus clarki (Richardson) (Jonsson et al., 1984) and many other less closely related species (Pauly, 1980; Metcalfe & Monaghen, 2003). This may be linked to higher metabolic rate in fast-growing than more slow-growing individuals. On the other hand, small sea trout are more short-lived than larger ones (Jonsson, B. et al., 1991). This may be at least partly a cost of early reproduction. Furthermore, mass speciﬁc metabolic rate decreases with increasing size of the ﬁsh which may be associated with longevity. In addition to temperature, variation in predation rate may inﬂuence longevity of sea trout. There is no systematic study on the effects of predation on life-history traits, but high

Life History of Anadromous Trout

211

predation rate in northern waters might explain why repeat breeding in sea trout decrease towards the north (Jonsson & L’Abée-Lund, 1993), contrasting with the effect expected from the low temperature in the north. Habitat constraints The intensity of natural selection is density dependent and should be strongest during critical periods in the life history (Elliott, 1994), and should be most evident in populations inhabiting extreme environments. Extreme habitats with a strong density effect may be trout streams with low water level and strong competition for habitat between parr and/or spawners. In small streams, habitat constraints may select for small body size, as seen in Arctic charr and Atlantic salmon (Jonsson et al., 1988; Jonsson, N. et al., 1991). For instance, a particularly small form of Arctic charr is adapted to life in the interstitial spaces in the lava bottom of Thingvallavatn, Iceland (Jonsson et al., 1988; Jonasson et al., 1998). Furthermore, body size and sea age at sexual maturity of Atlantic salmon decrease with decreasing ﬂow rate in Norwegian rivers with annual mean through ﬂow less than 20 m3 /s, (Jonsson, N. et al., 1991). Thus, there is reason to believe that habitat constraints inﬂuence the life history of brown trout in small streams. In southern Norwegian trout streams with annual mean through ﬂow 0.2 m3 /s, mean smolt age (MSA) and size decrease with decreasing water discharge (Jonsson et al., 2001). MSA was between 1 and 2 years and mean total smolt length was 6–8 cm in streams with annual mean ﬂow of approximately 0.05 m3 /s, whereas it was between 2 and 3 years and 12–16 cm in length in nursery brooks with annual mean ﬂow of 0.2 m3 /s and higher. Also elsewhere in Scandinavia, very young and small smolts have been found in small streams (Borgstrøm & Heggenes, 1988; Titus & Mosegaard, 1989), indicating that young sea trout may abandon their nursery stream and move to salt water very early in life when born in extremely small trout streams. They extend their feeding habitat and avoid periods of drought by moving early to sea. In Norwegian trout streams, about 1 m wide and with an annual mean ﬂow rate around 0.05 m3 /s, there is often no resident trout above the ﬁrst waterfall impassable for trout, as is the case in large streams. When parr leave fresh water during their ﬁrst year of life, they appear to need brackish feeding areas at the outlet where they can feed and survive. Brown trout from small streams on the Baltic Island of Gotland, sometimes move to sea soon after emergence (Landergren, 2001), and some ﬁsh may even spawn in the brackish Baltic Sea near stream outlets, as judged from the chemistry of their otoliths (Limburg et al., 2001). Eggs and fry may survive if salinity is below 4 ppt (Landergren & Vallin, 1998). The tendency for early emigration from the stream appears most common in very short streams. Also pink salmon Oncorhynchus gorbuscha (Walbaum) use intertidal spawning ground (Murphy et al., 1999), but few if any other trout or salmon species appears to use this habitat for spawning. In small streams, size and age at sexual maturity decrease with decreasing mean annual ﬂow rate of the stream. Mean ﬁsh length at maturity is often between 40 and 45 cm in streams where annual mean ﬂow rate in the spawning area is approximately 1 m3 /s and they are 20–30% shorter in streams where the annual mean ﬂow rate is approximately

212

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0.05 m3 /s (Jonsson et al., 2001). This may be because of genetic differences among populations, as reported for Atlantic salmon (Gjerde et al., 1994). Anadromous trout are genetically differentiated among streams, and the difference increases with distance between the populations, indicating an isolation by distance effect (Knutsen et al., 2001a). Recent tagging experiments in the same streams have shown a straying rate in the order of 2%, and the frequency of ﬁsh spawning in an unfamiliar brook should consequently be lower than that as some of the strays may leave without breeding (Jonsson et al., 2004). Stream length and altitude of the spawning grounds may also inﬂuence life-history decisions (Bohlin et al., 2001). L’Abée-Lund (1991) reported increased mean adult size with altitude and migratory distance of the spawning grounds from the sea, indicating that the cost of migration may inﬂuence the body size of the adult phenotype. Small sea trout may disappear if the costs of migration are large, either because it is relatively more energyconsuming or difﬁcult for smaller than larger ﬁsh to move far upstream through rapids and possible waterfalls. All in all, sea trout originating from very small streams appear to exhibit special adaptations in age and size at smolting and sexual maturity. By early emigration, populations may survive even if the stream dries out during summer, if there are brackish areas outside the stream mouth. When water level is low during autumn, small spawners may do better than large ones, and such habitat differences may have resulted in smaller sea trout in smaller than in larger streams. Furthermore, in long streams at high elevations, migratory costs may be large, decreasing the success of anadromous spawners, and only large ﬁsh may have a ﬁtness advantage by performing the migration.

Sexual size dimorphism Adult females are on average larger, older and less heterogeneous in size than adult males as exempliﬁed by observations of sea trout from Vangsvatnet (Fig. 14.2), and a similar sexual size dimorphism is also found elsewhere (Jonsson et al., 2001). Proximate reasons are that females, more than males, smolt and migrate to sea for feeding and to a lesser extent occupy territories in the nursery stream, and that age at sexual maturity differs between sexes with females attaining maturity at an older age than males (Table 14.2). For instance, 20

0 10

20

30

40 Length (cm)

50

60

Fig. 14.2 Length distribution (%) of male and female brown trout Salmo trutta in the Vangsvatnet lake. From Jonsson (1989). The solid line is for male ﬁsh and the dashed line for female.

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Table 14.2 Male and female life-history traits of sea trout in populations of sympatric anadromous and freshwater resident brown trout. Variable

Females

Male/female ratio, smolts Male/female ratio, adults Age at maturity (years, min.–max.) Age at maturity (years, population mean) Size at maturity (cm, min.–max.) Size at maturity (cm, population means) Length increase ﬁrst year at sea Duration of the sea-sojourn (days) Longevity (years) Gonadal energy (kJ g−1 ) GSI (energy, kJ %) (mass %)

Males

References

0.33–0.72 0.64–0.71 1–13

1–9

Jonsson, 1985; Euzenat et al., 1999 Alm, 1959; Campbell, 1977; Jonsson, 1981 L’Abée-Lund et al., 1989

2.3–5.2

1.6–6.3

L’Abée-Lund et al., 1989

24–90

20–95

L’Abée-Lund et al., 1989

30.7–56.6

19.9–75.8

8.5

7.4

L’Abée-Lund et al., 1989; Jonsson et al., 2001 Berg & Jonsson, 1990

69

66

Berg & Berg, 1989

5.0–11.1 779.7

4.7–11.0 567.3

Jonsson, B. et al., 1991 Jonsson & Jonsson, 1997 Jonsson & Jonsson. 1997

33.96 21.52

3.09 2.79

it has been found that 58% of the smolts were females in the Norwegian River Vosso, as was approximately 60% of the returning adults (Jonsson, 1985) and similar ﬁgures were reported by Campbell (1977) and Pemberton (1976b) from Scotland. At sea, females tend to stay longer and grow more than males (Berg & Jonsson, 1990). Males dominate numerically among the resident part of the stock (Table 14.1). Campbell (1977) found that 85% of the resident spawners in Kirk Burn, a tributary to the River Tweed, were males. Similar ﬁgures were reported from the Norwegian Lake Vangsvatnet (Jonsson, 1985). In the latter case, many of the resident females fed in the lake, whereas many males fed until maturity in tributaries to the lake (Jonsson & Gravem, 1985; Haraldstad et al., 1987). In systems with no lakes, more than 96% of the resident spawners were males (Jonsson et al., 2001). The higher tendency of juvenile males than females to occupy feeding territories in running water may be at least partly linked to their higher level of aggression (Johnsson et al., 2001), as aggressiveness does not give the same advantage for ﬁsh feeding in lakes and at sea as it does in a river where food patches are monopolised more easily (Haraldstad et al., 1987; Schei & Jonsson, 1989). Thus, the advantages of feeding in the nursery river, lake and open sea will differ between the sexes. Trout feeding in different habitats grow at different rates, with a gradually higher rate from tributaries to lakes and the coastal sea (Jonsson, 1985; L’Abée-Lund et al., 1989). Females grow larger than males because more of them move to sea, as illustrated by trout from Vangsvatnet, Norway (Fig. 14.2) and elsewhere (Gross, 1987; Jonsson et al., 2001). But there is no evidence that males and females occupying the same feeding habitat grow

214

Sea Trout

at different rates (L’Abée-Lund et al., 1989; Johnsson et al., 2001). Therefore, the partial difference in spatial distribution of the sexes is partly causing the sexual size dimorphism in brown trout (Jonsson, 1989). Females attain maturity at an older age than males. This is partly because residents mature at a younger age than anadromous ﬁsh which are mostly females, and partly because males mature at a younger age than females within each of the morphs (Jonsson, 1985; Jonsson et al., 2001). This trend appears to occur throughout the distribution area (L’Abée-Lund et al., 1989; Toledo et al., 1994). L’Abée-Lund (1994) found that smolt age, sex and environment accounted for 30% of the variation in age at ﬁrst maturity. However, males matured at a younger age even when the two sexes were experimentally reared in tanks and grew at the same rate (Jonsson, 1989), indicating an inherited difference in the maturity age between sexes. Females live, on average, longer than males (Jonsson, B. et al., 1991), but maximum longevity is similar (Table 14.2). Higher mortality in males than in females is probably because of decreased male predator avoidance because of higher aggressiveness relative to females (Magurran & Seghers, 1994; Johnsson et al., 2001), and they may be also more often seriously injured during spawning because of intra-sexual conﬂicts (Fleming, 1996). Furthermore, in both sexes, adult mortality is high and usually less than 50% spawn more than once (L’Abée-Lund et al., 1989). As a last point, female longevity may be higher than that of males because they attain maturity at an older age, as spawners can die as a consequence of high energy use during spawning. However, the proportion of the total energy used for spawning is probably similar for males and females (Lien, 1978; Jonsson & Jonsson, 1997). In brook charr (Hutchings et al., 1999) and small Atlantic salmon (Jonsson & Jonsson, 2003), it is similar or higher for males than females. Together, these conditions decrease the longevity of adult males more than that of adult females. The ultimate reason why males are more variable and have often a smaller mean size than females is probably because of a stronger link between size and reproductive success in females than males. Large females can better acquire and defend nesting sites which may increase the survival of their eggs as seen in Paciﬁc salmon Oncorhynchus spp. (van den Berghe & Gross, 1989; Foote, 1990), bury their eggs deeper in the gravel which may protect them from disturbance by other females and by ﬂoods and have higher fecundity (Elliott & Hurley, 1998; Fleming, 1996). Large females may also be preferred partners for males (Foote, 1989; Petersson et al., 1999). The reproductive potential of a female is determined by the number and quality of their eggs. Large eggs give large offspring which grow and compete well for food resources (Elliott, 1994, 1995; Einum & Fleming, 1999). Therefore, it is a trade off between egg size and egg number (Jonsson & Jonsson, 1999). Female spawning success is determined by the number of ﬁsh eggs which give highly viable offspring, strongly linked to ﬁsh body size as explained above. Therefore, reproductive success of females increases with body size (Dannewitz et al., 2003). During spawning, males compete for female access, and large males appear more successful in becoming primary partners than smaller ones (Jones & Ball, 1954). Small males, however, can spawn through successive fertilisation by adult suitors (Garcia-Vazquez

Life History of Anadromous Trout

215

et al., 2001; Largiander et al., 2001) and as sneakers at the nest (Gross, 1985; Jonsson, 1985). Thus, both large and small males may be successful in fertilising eggs of large females. Furthermore, small males may be successful as some of them appear to be the primary partner of small females by spawning in separate areas (Bagliniére et al., 1989; Skaala & Nævdal, 1989) and through assortative mating (Jonsson, 1985; Hindar et al., 1991). In males, the beneﬁt from large body size and migration appears smaller, and Dannewitz et al. (2003) were unable to establish a signiﬁcant association between body size and reproductive success in an experimental stream for males. Thus, there is reason to assume that there is generally stronger selection for large body size in females than males (Fleming, 1996).

Research needs The diverse, habitat-adapted life histories of anadromous and resident brown trout have been established, but we need better knowledge about vital life-history traits to be able to estimate the ﬁtness of these two competing life histories, mainly determined by the product of survival rate and the number of highly viable offspring produced (Jonsson et al., 1984; Stearns, 1992). To do that, we also need good estimates of the survival rate of the two sympatric forms, of which sound estimates for the non-anadromous part have been especially difﬁcult to obtain as each cohort smolts over a number of years. To understand differences in anadromy between populations, growth conditions (L’AbéeLund et al., 1990; Baum et al., 2004) and migratory costs in fresh water (Bohlin et al., 2001) have been compared. Although the theory indicates that growth conditions at sea are also important (Gross et al., 1988), there is as yet no investigation of how marine feeding conditions inﬂuence migratory tendencies of trout. There is as yet little knowledge about brown trout performance (e.g. growth and survival) in fresh and salt water. Will ﬁsh which perform well in rivers do poorly at sea and vice versa, as reported for salmon (Einum et al., 2002)? This might be the case, as for instance effects of sea lice infections vary between anadromous and freshwater resident trout (Glover et al., 2001) and smolting is a partly heritable trait (Boula et al., 2002). The understanding of the relationship between water temperature and growth performance is well established for brown trout in fresh water (Elliott & Hurley, 2001), but we lack knowledge on these relationships for brown trout at sea. Furthermore, is there a link between water temperature during early development and later performance such as sea growth and sexual maturation, as indicated for Atlantic salmon (Rungruangsak-Torrissen et al., 1998; Angilletta et al., 2003). To what extent does this hold for trout? This relationship is of particular interest if one wishes to model life-history consequences of climate change scenarios. The spawning behaviour of brown trout is well known (Jones & Ball, 1954; Petersson & Järvi, 1997), but there are few observations of spawning in partly migratory populations, and no measurements of the relative ﬁtness of the morphs or sexes depending on size, frequency, density or habitat characteristics. This is important information to understand life-history variations of the species. Furthermore, this knowledge would be of help when managing species and spawning habitats.

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Because of the relationship between parr growth, sexual maturation and residency, sea trout have been difﬁcult to sea ranch when reared to smolt size (Jonsson et al., 1994b). For instance, is there a particular time of the year when high food consumption and rapid growth should be avoided to suppress early maturity and residency (Thorpe et al., 1998), or is it the energy density of the young which inﬂuences maturation (Rowe et al., 1991), and to what extent will such relationships differ among populations, and can this be effectively manipulated under hatchery conditions? This, and related research will improve our ability to predict variation in the life history of sea trout, and help us when managing stocks and interpreting possible effects of climatic variation, now considered as one of the most pronounced changes in the future environment of brown trout.

Acknowledgement We thank Prof. J.M. Elliott for critically commenting on a draft of this chapter.

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Boula, D., Castric, V., Bernatchez, L. & Audet, C. (2002). Physiological, endocrine, and genetic bases of anadromy in the brook charr, Salvelinus fontinalis, of the Laval River (Quebec, Canada). Environmental Biology of Fishes, 64, 229–42. Campbell, J.S. (1977). Spawning characteristics of brown trout and sea trout Salmo trutta L. in Kirk Burn, river Tweed, Scotland. Journal of Fish Biology, 11, 217–29. Champigneulle, A., Buttiker, B., Durand, P. & Melhaoui, M. (1999). Main characteristics of the biology of the trout (Salmo trutta L.) in Lake Léman (Lake Geneva) and some of its tributaries. In: Biology and Ecology of the Brown Trout and Sea Trout (Baglinière, J.L. & Maisse, G., Eds). Series in Aquaculture and Fisheries, Springer–Praxis, Berlin, pp. 147–74. Claireaux, G. & Audet, C. (2000). Seasonal changes in the hypo-osmoregulatory ability of brook charr: the role of environmental factors. Journal of Fish Biology, 56, 347–73. Coutant, C.C. & Chen, C.H. (1993). Strontium microstructure in scales of freshwater and estuarine striped bass (Morone saxatilis) detected by laser-ablation mass-spectrometry. Canadian Journal of Fisheries and Aquatic Sciences, 50, 1318–23. Cross, T.F., Mills, C.P.R. & DeCourcy Williams, M. (1992). An intensive study of allozyme variation in freshwater resident and anadromous trout, Salmo trutta L., in western Ireland. Journal of Fish Biology, 40, 25–32. Dannewitz, J., Petersson, E., Prestegaard, T. & Järvi, T. (2003). Effects of sea-ranching and family background on ﬁtness traits in brown trout Salmo trutta reared under near-natural conditions. Journal of Applied Ecology, 40, 241–50. Dellefors, C. & Faremo, U. (1988). Early sexual maturation in males of wild sea trout, Salmo trutta L., inhibits smoltiﬁcation. Journal of Fish Biology, 33, 741–49. Dutil, J.D. (1986). Energetic constraints and spawning interval in the anadromous Arctic charr (Salvelinus alpinus). Copeia, 1986, 945–55. Eek, E. & Bohlin, T. (1997). Strontium in scales veriﬁes that sympatric sea-run and stream-resident brown trout can be distinguished by coloration. Journal of Fish Biology, 51, 659–61. Einum, S. (2003). Atlantic salmon growth in strongly food-limited environments: effects of egg size and parental phenotype? Environmental Biology of Fishes, 67, 263–68. Einum, S. & Fleming, I.A. (1999). Maternal effects of egg size in brown trout (Salmo trutta): norms of reaction to environmental quality. Proceedings of the Royal Society of London, Series B, 266, 2095–100. Einum, S. & Fleming, I.A. (2000). Selection against late emergence and small offspring in Atlantic salmon (Salmo salar). Evolution, 54, 628–39. Einum, S. & Fleming, I.A. (2002). Does within-population variation in ﬁsh egg size reﬂect maternal inﬂuences on optimal values? American Naturalist, 160, 756–65. Einum, S., Thorstad, E. & Næsje, T.F. (2002). Growth rate correlations across life-stages in female Atlantic salmon. Journal of Fish Biology, 60, 780–84. Elliott, J.M. (1975). The growth rate of brown trout (Salmo trutta L.) fed on reduced rations. Journal of Animal Ecology, 45, 823–42. Elliott, J.M. (1976). Body composition of brown trout (Salmo trutta L.) in relation to temperature and ration size. Journal of Animal Ecology, 45, 273–89. Elliott, J.M. (1984). Numerical changes and population regulation in young migratory trout Salmo trutta in a Lake District stream, 1966–1983. Journal of Animal Ecology, 53, 327–50. Elliott, J.M. (1988). Growth, size, biomass and production in contrasting populations of trout Salmo trutta in two Lake District streams. Journal of Animal Ecology, 57, 49–60. Elliott, J.M. (1989). Growth and size variation in contrasting populations of trout Salmo trutta: an experimental study on the role of natural selection. Journal of Animal Ecology, 58, 45–58. Elliott, J.M. (1994). Quantitative Ecology and the Brown Trout. Oxford Series in Ecology and Evolution, Oxford University Press, Oxford. Elliott, J.M. (1995). Fecundity and egg density in the redd of sea-trout. Journal of Fish Biology, 47, 893–901. Elliott, J.M. (1997). Stomach contents of adult sea trout caught in six English rivers. Journal of Fish Biology, 50, 1129–32. Elliott, J.M. & Chambers, S. (1996). A guide to the interpretation of sea-trout scales. National Rivers Authority, R&D Report 22, 53 pp. Elliott, J.M. & Hurley, M.A. (1998). An individual-based model for predicting the emergence period of sea trout fry in a Lake District stream. Journal of Fish Biology, 53, 414–33.

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Chapter 15

Migration as a Life-History Strategy for the Sea Trout D.J. Solomon Foundry Farm, Kiln Lane, Redlynch, Salisbury, Wiltshire, SP5 2HT, UK Abstract: Stocks of brown/sea trout exhibit a wide range of migratory tendencies, from the whole stock being sedentary, to the whole stock being sea-going, with a continuum between the two. There is a greater tendency for females to be migratory, presumably because the greater growth opportunity in the sea allows a greater production of eggs. However, the fact that not all ﬁsh become sea-going, even where access to the sea is possible, suggests that there are balancing disadvantages to a migratory lifestyle. This chapter explores possible factors contributing to this balance and develops the concept of mean lifetime egg deposition as a way of comparing the different life histories. Variations in the pattern of sea migrations, in space and time, are also explored. Keywords: Sea trout, migration strategies, lifetime egg production, residency factors, migration pattern.

Introduction Salmo trutta L. is a remarkably adaptable species with a considerable range of life-history variations, both across and within populations. This involves migration to a varying degree, within fresh water and between fresh water and the sea. The aim of this chapter is to consider why certain stocks adopt particular migratory strategies. People often wonder why sea trout do not enter and spawn in certain rivers to any great extent – examples in the UK include the Rivers Usk, Severn and Exe (Solomon, 1995). Knowing what we do of the homing ﬁdelity of spawning sea trout the correct way of posing this question is to ask why the trout stocks of these particular rivers do not adopt a migratory lifestyle, while those in other neighbouring rivers do. Even more intriguing, as many trout populations contain both migratory and resident individuals in a freely interbreeding stock (Solomon, 1982; Jonsson, 1985), is to consider what mechanisms operate to maintain this apparently stable polymorphism. In a consideration of the evolution of diadromy in ﬁsh, Gross (1987) suggested that migration increases the evolutionary ﬁtness of those that migrate relative to those that do not, and that ﬁtness can be quantiﬁed by the summation of an individual ﬁsh’ possibility of surviving to reproduce at any age multiplied by its fecundity (or male fertility) and breeding success at that age. How does this hypothesis ﬁt the complex variations in migratory behaviour across and within populations?

Migration for enhanced growth and reproductive potential The most likely potential advantage for trout adopting a migratory habit is the opportunity to access better feeding and growing conditions to enhance growth, and thus fecundity 224

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500 Tay sea trout Findhu Glen Burn brown trout

400

Length (mm)

River Tummel brown trout 300

200

100

0 0+

1+

2+

3+ 4+ Age group

5+

6+

7+

Fig. 15.1 Length at age for trout from three stocks in the Tay catchment. Data from Walker (1987) and Nall (1930).

Length (mm)

800 700

Tweed sea trout first spawning after 1 SW

600

Kirk Burn brown trout

500 400 300 200 100 0 0+

1+

2+

3+ 4+ Age group

5+

6+

7+

Fig. 15.2 Length at age for brown trout from the Kirk Burn (Tweed tributary) and of sea trout captured in the Tweed estuary which ﬁrst spawned after 1 year at sea. Kirk Burn data from Campbell (1977); Tweed data from Solomon (1995).

and ‘evolutionary ﬁtness’. The increased growth at sea associated with a migratory habit is indicated in Figs 15.1 and 15.2. In Fig. 15.1 the mean length of various age classes of trout from three populations in the Tay catchment are shown (brown trout data from Walker [1987]; sea trout data from Nall [1930]). The Findhu Glen Burn is a small, rapidly ﬂowing stream in which both brown trout and sea trout spawn; the lengths shown here are for

226

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non-anadromous brown trout only. The River Tummel is a large tributary with a population of trout that do not migrate to sea; growth conditions in the river are generally favourable. The third group are sea trout caught by nets in the Tay estuary; these are adults returning to spawn. Most Tay sea trout migrate to sea as smolts at an age of 2 years. In Fig. 15.2 two populations from the Tweed catchment are compared. The Kirk Burn is used by both brown trout and sea trout as a spawning and nursery area; only non-anadromous brown trout are included here (data from Campbell [1977]). Many of these ﬁsh have spent much of their adult life in the main river, and are returning to the Burn to spawn. The ﬁgures for sea trout are for returning adult ﬁsh caught in nets in the estuary (data from Solomon [1995]); again, most Tweed sea trout migrate to sea as smolts of 2 years of age. In both the Tay and Tweed examples the mean length of the sea trout at age 2+ is lower than that of the resident trout; this may be a meaningful indication that the migratory ﬁsh had thus far experienced poorer feeding and slower growth than the residents, but there is too much uncertainty regarding the seasonal timing of the different sets of observations to be conﬁdent that these differences are real. There is mixing of stocks of sea trout in estuaries, and thus a likelihood that some of the sea trout recorded in the Tay and Tweed samples discussed above did not originate from these rivers. However, their size is indicative of the growth of sea trout in these areas and thus a valid input to this consideration. It will be noted that the growth of the ﬁsh sampled in the Tweed estuary is rather greater than that for the Tay samples. In most trout populations in the UK that exhibit a migratory fraction, females signiﬁcantly outnumber males among the migrants (Solomon, 1995; Walker, 2004). The probable advantage of large size for females is in increasing the number, and perhaps the size, of eggs produced, thereby increasing reproductive potential. The length–fecundity relationship varies among stocks of trout (Solomon, 1997) but a mean relationship for the British Isles is described by the formula: Log N = 2.754 Log L − 4.0721 where N is the fecundity (number of eggs) and L is the length of the ﬁsh in millimetres. Larger trout produce many more mature eggs than smaller ones; based upon the above formula the expected fecundity of age 4+ ﬁsh adopting the different migratory lifestyles in Fig. 15.1 is: Findhu Glen Burn (258 mm) 371 eggs; River Tummel (328 mm) 719 eggs and Tay sea trout (459 mm) 1814 eggs.

Disadvantages of migration While there is doubtless some advantage to males being of larger size (see below), this is likely to be less marked than for females; even the smallest mature male can produce enough milt to fertilise all the eggs of the largest female, and small males do often participate in spawning with much larger ﬁsh. Equally, however, there would appear to be no disadvantage to large size for a male, so why do most of them not go to sea even in populations where virtually all the females do? And why do not all trout in populations that are not actually landlocked go to sea to gain the advantage of greater growth and reproductive potential?

Migration as a Life-History Strategy

227

As suggested by Gross (1987), the answer must be that there is also some cost or disadvantage in going to sea compared with remaining in the river, which is in many situ-ations balanced or exceeded by the advantages, especially for females. Again the most likely explanation is the increased risk of mortality associated with migration and life in the sea.

Mean lifetime egg production On the assumption that the main advantage of migration for females is increasing the number of eggs we can start to gain some insight into the balance of advantage by considering the mean lifetime egg production (MLEP) by ﬁsh of smolt size adopting different life-cycle strategies. The MLEP is calculated by multiplying the mean survival to each age (spawning opportunity) beyond smolt size by the fecundity at that age/size. The data to do this are not available for many stocks but those that are available cover a wide range of habitats and migratory behaviours. Derivation of the MLEP for four stocks is shown in Table 15.1. Brown trout in the Candover Brook, an upper tributary of the River Itchen, do not go to sea at all, even though there are sea trout further downstream in the Itchen catchment. Solomon & Paterson (1979, 1980) presented a quantitative life history of a typical year class of ﬁsh in a 3 km reach, starting with 912 ﬁsh at 6 months of age and ending with just two 6+ ﬁsh. At the time of the study this was an unusual chalk stream in that it was not artiﬁcially stocked and the population was virtually natural. Based upon an interpolation of 700 ﬁsh at age 1 (the assumed age at which such fast-growing ﬁsh would have migrated to sea as smolts) and the generalised British Isles fecundity relationship (see above) indicates that each Candover smolt-size female resident could expect to produce on average 648 eggs in its lifetime, that is, it has an MLEP of 648. As none of the Candover trout migrate to sea it must be assumed that this MLEP is greater than would be achieved by ﬁsh from this tributary adopting a migratory habit. Given the benign conditions and good growth and survival of chalkstream brown trout this is likely to be towards the ‘top end’ of the range of MLEP for non-migratory populations. Three Dubs Tarn (Frost & Brown, 1967) is a small moorland lake in northern England, where there is no access to the sea and the ﬁsh grow to a maximum of about 28 cm at an age of 8 years. Here the MLEP of smolt-size ﬁsh (2 years, 180 mm) is 135 eggs. Were a migratory habit to be viable here it is likely that it would dominate. We will now consider the situation of sea trout in the St Neot River, a tributary of the River Fowey in Cornwall. The ﬁsh populations here were studied over a number of years by Hugh Sambrook, as part of the Colliford Reservoir scheme investigation. The ﬁgures we need are available from Solomon (1995), though a number of assumptions are necessary which means that the results are indicative rather than absolute. The MLEP for smolts is 609 eggs. By UK standards, the Fowey sea trout are relatively slow growing and short-lived, but mature at a low sea age; their MLEP is thus likely to be well within the range for UK populations. Another migratory trout example is provided by the Burrishoole River in Ireland; the appropriate data for an assessment based on the years 1983–86 are presented by Mills et al. (1989). Here the MLEP for a female smolt was 215 eggs (Table 15.1); it should be

228

Sea Trout

Table 15.1

Calculation of MLEP for four populations.

Stock

Age

Candover Candover Candover Candover Candover Candover Candover

1 1+ 2+ 3+ 4+ 5+ 6+

Three Dubs Three Dubs Three Dubs Three Dubs Three Dubs Three Dubs Three Dubs

2 3+ 4+ 5+ 6+ 7+ 8+

St Neot St Neot St Neot St Neot St Neot St Neot St Neot St Neot St Neot St Neot St Neot St Neot St Neot

smolt .+ .sm+ .2sm+ .3sm+ .4sm+ .5sm+ .1+ .1+sm+ .1 + 2sm+ .1 + 3sm+ .1 + 4sm+ .1 + 5sm+

Burrishoole Burrishoole Burrishoole Burrishoole Burrishoole

smolt .+ .1+ .2+ .3+

Length (mm)

Fecundity

Start number

231 286 329 369 401 455

0 493 725 994 1250 1770

700 700 700 700 700 700 700

216 241 259 269 274 277

0 309 375 417 439 451

1000 1000 1000 1000 1000 1000 1000

307 372 424 471 523 496 408 459 496 537 542 599

599 1017 1458 1947 2598 2245 1311 1814 2245 2794 2866 3775

n

Eggs

MLEP

0.464286 0.338571 0.144286 0.02 0.002857

0 0 100 100 100 100 100

0 228.8323 245.4188 143.4544 25.00285 5.058192

648

500 265 84.8 39 9 3.6

0.5 0.265 0.0848 0.039 0.009 0.0036

0 0 100 100 100 100 100

0 81.79161 31.83412 16.2768 3.954609 1.622509

135

1108 1108 1108 1108 1108 1108 1108 1108 1108 1108 1108 1108 1108

163 130 63 15 7 1 114 48 20 4 1 2

0.147112 0.117329 0.056859 0.013538 0.006318 0.000903 0.102888 0.043321 0.018051 0.00361 0.000903 0.001805

0 100 100 100 100 100 100 100 100 100 100 100 100

88.13064 119.2831 82.88317 26.36034 16.41391 2.026361 134.9035 78.56581 40.52722 10.08717 2.586987 6.814339

609

1000 1000 1000 1000 1000

140 131 33 3

0.140 0.131 0.033 0.003

0 30 100 100 100

0 31.7 134.6 43.5 5.5

213

538 325 237 101 14 2

Survival

Mature (%)

1. Starting point is smolts or the age (April, a whole number of years) at which smolting would be likely if the stock were migratory. Ages given with + are around spawning time. For explanation of the St Neot sea-age classes see Solomon (1995). The start numbers are arbitrary. 2. For Burrishoole stock fecundity ﬁgures are taken direct from the source paper. For other stocks generalised formula given in the text is used. Number of eggs at age is calculated from survival × fecundity × proportion of females that are mature at that time. 3. For St Neot stock all sea-age classes are shown separately. For Burrishoole stock sea ages are combined (e.g. 1+ includes both maiden ﬁsh and second time spawners that ﬁrst returned at .+).

noted that the situation being considered here was before the major decline in sea trout in this area described by Poole et al. (2006). This is likely to be towards the lower end of the range of potential egg depositions for British Isles sea trout stocks by virtue of the relatively slow growth and low tendency for multiple spawning exhibited by western Ireland stocks; ‘poor condition Atlantic growth’ and ‘short-lived stock’ characteristics as described by Fahy (1985). It is likely however that less productive stocks occur further north where

Migration as a Life-History Strategy

229

sea conditions, and indeed the freshwater conditions that drive the tendency to a migratory habit, are harsher. These are just four examples of a wide range of life-history strategies and potential lifetime egg production rates. These ﬁgures alone do not prove that the tendency to migrate to sea is driven predominantly by increased MLEP, but they are indicative. Where appropriate data become available it would be of interest to calculate the MLEP for more stocks including situations where life-history data are available for migratory and non-migratory stocks occurring together (where it might be hypothesised that the MLEP for the two life-history types might be rather similar) and for stocks above and below impassable falls adopting different life cycles, where there would be no reason to assume that the MLEP would be similar as a migratory habit would not be available to the landlocked population. Although the four populations considered in Table 15.1 cover a wide range of situations, they probably do not include the extremes. The lowest MLEP might be expected in a resident population in a harsh environment above impassable falls. A migratory habit is not a viable option (any ﬁsh that showed a tendency to migrate downstream would be effectively removed from the population) and the MLEP of ﬁsh reaching potential smolt size may be just a matter of a few tens of eggs. The other extreme might be represented by a migratory stock from a highly productive fresh water such as the lower Itchen or one of the French chalk streams (where very large sea trout occur). Euzenat et al. (2006) present some ﬁgures for the annual range of smolt runs (2500–9500) and total egg deposition (2–7.5 million) for the River Bresle in Normandy which suggest an MLEP of around 800 if all smolts are female, and rather more if part of the smolt run comprises males.

Factors affecting the residency–migration balance Although at any one location there may be a single MLEP ﬁgure which represents the balance of advantage between migratory and non-migratory life history, the ﬁgure is likely to vary markedly across different rivers, and indeed within catchments, with different conditions. For example, in the case of the Itchen, the ﬁsh from an upper tributary (the Candover Brook, see above) do not adopt a migratory habit, whereas some trout in the lower catchment do; as growth conditions are probably similar in both locations, this is likely to be mediated through the relative disadvantage of the migratory habit. It may be simple as a matter of the distance from the sea and the added costs and dangers of a long migration within the river system for the ﬁsh from the upper catchment. There is a wide range of factors that may inﬂuence the balance across life-history patterns; some of the factors that may be expected to inﬂuence the situation are listed in Table 15.2. The balance of advantages is less clear cut for the males. While large males are likely to dominate in spawning, smaller males may participate in a number of ways. Campbell (1977) and Jonsson (1985) report that adult female sea trout are often attended by several males while spawning, and that while a large (migratory) male is generally dominant if present, a smaller (non-migratory) male may be the primary mate in the absence of larger ﬁsh. Further, smaller attendant males will participate in spawning while the larger dominant male is distracted by chasing off other males (Campbell, 1977). This range of behaviour is

230

Sea Trout

Table 15.2 List of environmental factors that are likely to inﬂuence tendency towards residency or migratory habit. Factors likely to favour migratory tendency

Factors likely to favour residency

Poor growth conditions in the river Good spawning and nursery conditions leading to high juvenile populations Good safe route to sea (e.g. lower reaches of river system, few predators on route) and easy return

Good growth conditions within the river Poor spawning and nursery conditions leading to low juvenile populations Dangerous route to sea (e.g. upper reaches of river system, many predators in lower reaches) or difﬁcult/impossible return Stream habitat with stable ﬂow Abundance of good habitat for feeding growing adult ﬁsh Long growing season of optimal temperature Indifferent marine conditions for feeding and growth Little interspeciﬁc competition Low riverine predator population

Stream habitat vulnerable to drought Lack of good habitat for feeding growing adult ﬁsh Low temperature – short growing season Good marine conditions for feeding and growth Competition from other species High riverine predator population

rather different to the ‘sneak’ mating by precocious male salmon parr, which hide beneath the bodies of spawning adults, as Jones (1959) demonstrated that adult female salmon would not spawn when only precocious males were present. Surviving to spawn at all is more likely to ‘win’ as a choice as, in contrast to females, there would appear to be little absolute advantage in large size for males. This reduced advantage of large size is likely to explain why a much greater proportion of males remain resident in most populations. What is less clear is the mechanism that prevents all males from remaining resident. So what mechanisms might be involved in maintaining the apparent balance in the range of migratory behaviour within a population, and prevent individual populations from being entirely migratory or non-migratory? The difference in the advantages of large size between males and females can help to explain why the situation is different between the sexes. Inter-annual variation in the conditions within the river and at sea, and in population density and thus competition for space and food, means that the relative advantage of the two strategies may shift across years. However, the fact that the polymorphism is maintained in the long term in the majority of populations with good access to the sea suggests the presence of a feedback mechanism that prevents total domination of one or another pattern of behaviour. A possible mechanism may be an increase in the potential advantage of each pattern whenever it is displayed by only a minor part of the population. Whenever the migratory fraction becomes the great majority, those few ﬁsh that remain resident are likely to enjoy reduced competition for food and space. For males, when the great majority of ﬁsh are small residents, the occasional large (migratory) male is likely to succeed competing for mates, spawning with several females; on the other hand, when the great majority of males are migratory and, thus of large size, the occasional small (resident) male may be more successful as a sneak mate in the absence of many other small ﬁsh. Ferguson (2006) suggests that sea migration is a threshold quantitative trait controlled by multiple genes and by environmental inﬂuences. Walker (1987) hypothesised that, while

Migration as a Life-History Strategy

231

there may be a strong genetic tendency in many stocks towards resident or migratory patterns of life cycle, in some populations it may be under short-term environmental control with the proportion migrating varying from year to year. However I have found no evidence for such an inter-annual variation, with the balance in most populations apparently being rather stable over quite wide ranges of population density. Clearly, however, longer-term shifts in balance do occur. The sea trout populations in Patagonia (Leitch, 1991) and in the Falkland Islands (Arrowsmith & Pentelow, 1965) were derived from European stocks that had shown a resident pattern for many generations. It is also not generally known that one of the UK’s foremost sea trout rivers, the Teiﬁ, has not always been so. Sea trout were rare on the river in the late nineteenth and early twentieth centuries, and started to appear in numbers in about 1920 (Solomon, 1995). What was responsible for this change is not clear; did the Teiﬁ brown trout develop a migratory habit, or was the river colonised by straying migratory trout from other rivers? Non-migratory populations can be derived from migratory parents too. Walker (2006) describes how a non-migratory population was produced above an impassable waterfall in a previously trout-free tributary of the Findhu Glen Burn by the introduction of ﬁn-clipped ﬁsh from migratory parents. Although marked migratory and brown trout were found below the falls in the early years following the introduction, the 1 : 1 sex ratio in the population above the falls after 20 years indicates that the migratory tendency had been virtually eradicated in the new stock. It is also interesting to consider how a stock might respond if the balance of advantage of residency versus migration changes on a longer-term basis. Examples might include development of a signiﬁcant net ﬁshery in the estuary or lethal levels of sea lice occurring in inshore waters. Poole et al. (2006) report on a situation of reduced marine survival of sea trout post-smolts in western Ireland associated with heavy infestations of sea lice. Adult runs and resultant smolt output are both signiﬁcantly reduced. How long can such a situation be maintained? As the spawning contribution of the migratory fraction of the population will be much less than it was in the recent past it is likely, in many populations at least where the balance of advantage is fairly ﬁne, that the stock will retrench to a sedentary habit and may lose the migratory ability, in the short to medium term at least.

Patterns of migration The distance travelled by migratory trout is a continuum from a matter of hundreds of metres for ‘resident’ ﬁsh to hundreds of kilometres for some of the stocks on the North Sea coast of Britain. Basically the ﬁsh go just as far as they need to gain the maximum beneﬁt from migrating. On a chalkstream such as the Candover Brook, where good conditions for spawning, juvenile rearing and adult growing occur throughout much of the stream, most migrations are limited to a matter of hundreds of metres (Solomon & Templeton, 1976). There are many examples of stocks that migrate signiﬁcant distances within fresh water, for example between lakes and inﬂowing streams (Frost & Brown, 1967) and between main rivers and smaller streams (Walker, 1987). For stocks going to salt water there is a range of strategies including estuary residence, local coastal movements and extensive open sea migration (Nall, 1930; Solomon, 1995).

232

Sea Trout

Most sea trout stocks migrate to sea as smolts at an age of two or more years at a size of 12–25 cm, and once they start to return they are annual spawners. However, there are variations on this pattern where conditions allow or dictate. In areas of relatively low salinity such as in the Baltic the ﬁsh may migrate to sea at 1 year at a length of 60–80 mm (Titus & Mosegaard, 1989) or even as fry (Jarvi et al., 1996). This allows the use of ephemeral watercourses as spawning grounds. Limburg et al. (2001) present evidence of trout completing their life cycle entirely in brackish water. Pemberton (1976) noted extensive migration of parr into salt water in the autumn, typically at an age of 6 months younger than the spring smolt migration, from rivers draining to sea lochs of reduced salinity. There is evidence of alternate year spawning (as opposed to annual) in some stocks with long distance migrations. Nall (1930) observed many sets of scales that indicated such a pattern among Tweed stocks, which in any event have a long sea absence before ﬁrst spawning (Solomon, 1995). Alternate year spawning also appears likely among the Black Sea stock of S. trutta; in this case high inshore water temperatures preclude river entry at any time other than the spring, very soon after the ﬁsh have left the river as kelts. Their phenomenal growth between spawning visits suggests that the inter-spawning marine period is of the order of more than 1 year rather than the month or two that annual spawning would dictate (D.J. Solomon, unpublished data).

Conclusion There is a wide range of migratory activity available to trout populations and the life-history strategy adopted is likely to be the one that, over the long term, maximises reproductive potential under the conditions prevailing for that population. Stocks can and do change their migratory habits, especially when introduced to new areas or when a major environmental factor changes, but there seems to be considerable short-term stability in the patterns.

References Arrowsmith, E. & Pentelow, F.T.K. (1965). The introduction of trout and salmon to the Falkland Islands. Salmon and Trout Magazine, 174, 119–29. Campbell, J.S. (1977). Spawning characteristics of brown trout and sea trout Salmo trutta L. in the Kirk Burn, river Tweed, Scotland. Journal of Fish Biology, 11, 217–29. Euzenat, G., Fournel, F. & Fagard, J.-L. (2006). Population dynamics and stock–recruitment relationship of sea trout in the River Bresle, Upper Normandy, France. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 307–23. Fahy, E. (1985). A Child of the Tides. Glendale Press, Dublin, 188 pp. Ferguson, A. (2006). Genetics of Sea Trout, with Particular Reference to Britain and Ireland. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 157–82. Frost, W.E. & Brown, M.E. (1967). The Trout. New Naturalist Series. Collins, London, 286 pp. Gross, M.R. (1987). Evolution of diadromy in ﬁshes. In: Common Strategies of Anadromous and Catadromous Fishes (Dadswell, M.J., Ed.). American Fisheries Society Symposium 1, pp. 14–25. Jarvi, T., Holmgren, K., Rubin, J.-F., Petersson, E., Lundberg, S. & Glimsater, C. (1996). Newly emerged Salmo trutta fry that migrate to the sea – an alternative choice of feeding habitat. Nordic Journal of Freshwater Research, 72, 52–62.

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Jones, J.W. (1959). The Salmon. New Naturalist Series, Collins, London. Jonsson, B. (1985). Life history patterns of freshwater resident and sea-run migrant brown trout in Norway. Transactions of the American Fisheries Society, 114, 182–94. Leitch, W.C. (1991). Argentine Trout Fishing. Frank Amato Publications, Portland, OR, 192 pp. Limburg, K.E., Landergren, P., Westin, L., Elfman, M. & Kristinsson, P. (2001). Flexible modes of anadromy in Baltic sea trout; making the most of marginal spawning streams. Journal of Fish Biology, 59, 682–95. Nall, G.H. (1930). The Life of the Sea Trout. Seeley Service and Co, London, 335 pp. Pemberton, R. (1976). Sea trout in North Argyll sea lochs; population, distribution and movements. Journal of Fish Biology, 9, 157–79. Poole, W.R., Dillane, M., DeEyto, E., Rogan, G., McGinnity, P. & Whelan, K. (2006). Characteristics of the Burrishoole sea trout population: census, marine survival, enhancement and stock–recruitment, 1971–2003. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 279–306. Solomon, D.J. (1982). Migration of juvenile brown and sea trout. In: Proceedings of the Salmon and Trout Migratory Behavior Symposium (Brannon, E.L. & Salo, E.O., Eds). University of Washington, Washington, DC, pp. 136–45. Solomon, D.J. (1995). Sea trout stocks in England and Wales. R&D Report 25, National Rivers Authority, Bristol, 102 pp. Solomon, D.J. (1997). Review of sea trout fecundity. R&D Technical Report W60, Environment Agency, Bristol, 22 pp. Solomon, D.J. & Paterson, D. (1979). Fisheries investigations, pp. 129–39. In: Itchen groundwater regulation scheme. Final Report on Candover Pilot Scheme. Worthing, Southern Water Authority, 165 pp. Solomon, D.J. & Paterson, D. (1980). Inﬂuence of natural and regulated streamﬂow on survival of brown trout (Salmo trutta L.) in a chalkstream. Environmental Biology of Fish, 5(4), 379–82. Solomon, D.J. & Templeton, R.G. (1976). Movements of brown trout Salmo trutta L. in a chalk stream. Journal of Fish Biology, 9, 411–23. Titus, R.G. & Mosegaard, H. (1989) Smolting at age 1 and its adaptive signiﬁcance for migratory trout, Salmo trutta L., in a small Baltic-coast stream. Journal of Fish Biology, 35(Suppl. A), 351–3. Walker, A.F. (1987). The sea trout and brown trout of the river Tay. In: The Sea Trout in Scotland (Picken, M.J. & Shearer, W.M., Eds). Scottish Marine Biological Association and Department of Agriculture and Fisheries for Scotland, Oban, 102 pp. Walker, A.F. (2004). Sex ratio in diadromous and freshwater-resident trout (Salmo trutta L.) populations in Scotland. Poster presented at the Ist International Sea Trout Symposium, University of Cardiff, Wales, 6–8 July 2004. Walker, A.F. (2006). The rapid establishment of a resident brown trout population from sea trout progeny stocked in a ﬁshless stream. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 389–400.

Chapter 16

Life History of a Sea Trout (Salmo trutta L.) Population from the North-West Iberian Peninsula (River Ulla, Galicia, Spain) P. Caballero1 , F. Cobo2 and M.A. González2 1

Centro de Investigaciones Ambientales de Lourizán, Apartado 127, 36080, Pontevedra, Spain 2 Laboratorio de Hidrobiología, Departamento de Biología Animal, Facultad de Biología, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain

Abstract: The life-history characteristics of sea trout population of the Ulla River (Iberian Peninsula) are described, based mainly on trapping data over 10 years. Sea trout in the Ulla spawned from December to mid-February, had an average relative fecundity of 2051 ova/kg, ova diameter 5.1 mm and a sex ratio of 2.2 females per male. After living in fresh water for between 1 and 4 years (mean smolt age [MSA] = 2.2), they migrated to the sea in spring (mainly in April) at an average fork length of 215 mm and a weight of 100 g. Smolt migration timing did not differ between the main river and the tributary, but signiﬁcant differences were detected in biometry and age structure in this stage, between both rivers. Recaptures in the sea, indicated that sea trout do not go far from the Galician coast, but made movements of at least 200 km from their natal river. Sea trout returned to fresh water with an average length of 400 mm and a weight of 900 g, most of them to spawn the same year of the smolt migration, as ﬁnnock. This migration started in May and most of the adults were caught between June and July, but adult sea trout in the Liñares (tributary) were caught mainly in autumn. Kelts returned to the sea from December to May, remaining less time in the tributary than in the Ulla. They showed a relatively high survival rate to a second return to fresh water as indicated by the 25% of multiple spawners found among adults. Keywords: Salmo trutta L., life history, Galicia, run-timing, migration, adult returns.

Introduction The anadromous brown trout (Salmo trutta L.), commonly known as sea trout, occupy rivers in the north-western quadrant of the Iberian Peninsula, which is considered to be the southern limit of its natural distribution. However, there is very limited scientiﬁc literature on these populations, and only one reference can be found on the Iberian stock’s characteristics (Toledo et al., 1993). Therefore, there is a major requirement for information and knowledge. This is especially important in Galician rivers, where sea trout are ubiquitous and represent an important resource. The severe decline of sympatric Atlantic salmon (Salmo salar L.), which is nearing the southern limit of its range in Galicia, has highlighted the need to urgently learn more about sea trout populations in order to inform its conservation and 234

Life History of the Iberian Sea Trout

235

management. Characteristics of the Ulla sea trout populations, reported here are considered to be representative of populations from the Iberian rivers which drain into the Atlantic Ocean (north-west Iberian Peninsula), but may be less representative of the Cantabric rivers in the north.

Study area The Ulla River passes through the middle of Galicia, and its mouth is located at 42◦ 40 41 north latitude. It is the second largest Galician river after the Miño, both in main river length (132 km) and in mean ﬂow (79.3 m3 /s) and also in catchment surface area (2800 km2 ). It drains into the Atlantic Ocean in the Arosa Ria in the south-west direction. Geologically, the Ulla catchment is mainly formed of schist and granitic rocks, representing a lowland river with clay substrate and a low level of mineralisation. Two ﬁsh traps at the Ulla river system (Fig. 16.1) provided information about sea trout population in this river. One (Ximonde), is located in a dam of the main river, 16 km upstream of the tidal limit. The other is in a tributary (the Liñares), with its conﬂuence 500 m downstream from the Ximonde trap, that is 17 km long, but natural falls limiting accessibility to migratory ﬁsh is restricted to 3.1 km. The catchment surface area of this tributary is 104 km2 and the annual mean discharge is 3 m3 /s. The accessibility for migratory ﬁsh in the Ulla is limited by a hydroelectric power

R. Ulla

Ximonde Trap

N

Ulla

10 km

R.

Liñares Trap are Liñ

Arosa Ría

. sR

Fig. 16.1

Location of the study area.

236

Sea Trout

station situated 80 km from the mouth, which causes severe ﬂow regulation and habitat disturbance in the downstream stretches. In the tributaries the passable sections are even more limited because of a great number of artiﬁcial obstacles and some waterfalls. Fish species present in this catchment are: brown trout (Salmo trutta L.) in both resident and migratory forms; Atlantic salmon (Salmo salar L.); sea Lamprey (Petromyzoon marinus L.); European eel (Anguilla anguilla L.); twaite shad (Alosa fallax, Lacépede) (Twaite shad); three-spined stickleback (Gasterosteus gymnurus, Cuvier)); Duero’s nase (Chondrostoma duriense, Coelho) and Iberian roach (Chondrostoma arcasii, Steindachner). Water temperature was measured manually twice a day at Ulla ﬁsh traps from 1994 to 2002, manually (twice a day) and from 1998 hourly with a temperature logger. Minimum and maximum temperatures were between 2.2◦ C and 23.4◦ C. The coldest month was January with an average of 11◦ C in the Liñares and 8.8◦ C in the Ulla, in August mean temperatures were 17.3◦ C and 19◦ C, respectively.

Material and methods Ximonde trap facilities include two ladders, an old pool type and a Denil one built in 1994. This trap also has a partial smolt trap in the channel of an old watermill. The Liñares trap, located 300 m upstream of this river’s conﬂuence with the Ulla, has an adult trap in a Denil ladder and a total smolt trap that is overtopped in big ﬂoods (Fig. 16.2). Both traps are designed to minimise ﬁsh damage and are sampled three times per day every day of the year. Downstream catch efﬁciency was estimated each spring since 1998 in both traps, using an upstream release of a proportion of tagged smolts. Ximonde smolt trap efﬁciency is low (8–12% range over 5 years), but the Liñares trap has a high efﬁciency almost the 100% Ximonde trap N

Smolt trap Hatchery

aR Ull

.

Denil fish pass (Adult trap) Pool type fish pass (Adult trap)

Liñares trap Smolt trap

ar Liñ

es

R.

Denil fish pass (Adult trap)

Fig. 16.2

Location and facilities of the Ulla ﬁsh traps.

0

100 m

Life History of the Iberian Sea Trout

237

recapture, except in 2 years, when owing to a big ﬂood recaptures were between 60% and 68%. Upstream catch efﬁciency in Liñares trap is considered to be 100%. Ximonde adult trap has an unknown efﬁciency in most years, because it is dependent upon water level, but in a few years it has been calculated from the proportion of marked and non-marked kelts caught. An increase in catch was detected at Ximonde in 1995, when the Denil ﬁsh pass started to work. For that reason the information used in this study in some cases (e.g. the timing of return to river) includes only data since that year and not the whole series. Although the series of sea trout catches at Ulla traps ﬁnished in 2003, age determination from adult’s scales was only possible up to 2002. All the trout were measured (fork length, to the nearest mm), weighed (nearest 0.1 g), and scales taken from between the posterior margin of the dorsal ﬁn and the lateral line (Ombredane & Richard, 1990). A coloration index and the sea lice (Lepeophtheirus salmonis, Krøyer, 1837) presence were recorded. Smolts were marked with Floy tags under the dorsal ﬁn, ﬁnnock and adults were additionally marked individually with Alcian blue (Hart & Pitcher, 1969). Trout were classiﬁed as alevin, fry, parr, adult resident trout, smolt, sea trout (ﬁnnock and adults) or kelts (Allan & Ritter, 1977). The sex ratio in the Ulla sea trout population was based on secondary sexual characters of the autumn ﬁsh only, because it is considered that sex determination is more reliable in this season (Richard, 1986). Fecundity and egg size information was obtained between 1999 and 2001, from artiﬁcially stripped ﬁsh and spawning times recorded at a spring fed hatchery located at Ximonde trap. Eggs were counted with 500 hole (diameter = 6 mm) methacrylate plate and measured in groups of 10, to the nearest 0.1 mm. Ages and spawning history were determined by scale reading (Richard & Baglinière, 1990; Elliott & Chambers, 1996). Mean values were estimated for smolt age, marine age, ﬁrst spawning age and the number of breeding years. The means of variables obtained from different groups (Ulla versus Liñares, marine age classes or years of the study series) were compared using analysis of variance (ANOVA). When the assumption of normality was not met, the non-parametric test of Kruskal–Wallis (K–W) was used, in both tests signiﬁcance levels below 0.05 indicate that the means of the different groups differ. Statistical relationships between sea trout length and ova number, and length–ova size, were described using linear regression analysis and the coefﬁcient of determination was used to describe the strength of that relationship. All analyses were made using the statistical program SPSS v.11.0.

Results The number of the different stages of sea trout (smolts, adults and kelts, not corrected for trap efﬁciency) caught during the period 1993–2003 at Ximonde and Liñares traps are shown in Table 16.1. Reproductive parameters The sex ratio (females per male) of the Ulla sea trout population sampled in the autumn between 1993 and 2003 (n = 837) was 2.2 (range = 1.6–3.2). Spawning took place between

238

Sea Trout Table 16.1 Number of sea trout (smolts, adults and kelts) caught at Ulla traps (1993–2003). Year

Ximonde trap Smolts

Liñares trap

Adults

Kelts

Smolts

Adults

Kelts

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

28 18 34 27 9 35 206 78 42 82 63

90 82 438 340 184 163 672 257 358 307 123

8 9 17 5 6 26 10 10 23 15 11

— 19 81 22 2 21 29 12 8 27 15

— 4 18 20 9 5 10 4 27 16 16

— 2 8 11 2 2 4 1 5 4 2

Total

622

3014

140

236

129

41

December and February. The total number of stripped females in three seasons (1999– 2000, 2000–2001 and 2001–2002) at Ximonde hatchery was 85. The highest activity period was the ﬁrst January fortnight and 75% of the reproductions were completed by the end of this month. Mean fork length of the females was 374 mm, their mean absolute and relative fecundities were 1174 ova and 2051 ova/kg, respectively. Mean ova diameter was 5.13 mm. A positive relation was found between length of the sea trout and ova number (y = 0.081(±0.06)x + 275.33(±7.66); R2 = 0.67, n = 85), and also between ova size and length of the sea trout stripped when a linear regression was ﬁtted (y = 106.03(±15.04)x − 159.57(±76.76); R2 = 0.54, n = 43). Finnock mean fecundity was 938.2 ova (n = 27, SD = 258.8) with a mean weight of 466.9 g (n = 10, SD = 85.2) 1+ sea winter (SW) had a mean fecundity of 1408 ova (n = 5, SD = 347.1) with a mean weight of 792.6 g (SD = 163.9) and for 2+SW mean fecundity was 2118 ova (SD = 330.6) and mean weight was 1131 g (SD = 176.4). Ulla sea trout reproduce at three different marine ages after they have migrated as smolts, the same or the next two winters after that migration. Most (75%) spawn ﬁrst during the ﬁrst winter (sea age 0+), 24% as 1+ SW and 1% as 2+ SW. The 24% of the sea trout caught at the Ulla traps had spawned at least once. Most (79%) of the repeat spawners showed 1 spawning mark on their scales, 16% had spawned twice, 4% had spawned three times and 1% had spawned four times.

Smolts The size and weight of the 858 smolts caught between 1993 and 2003 at the Ulla traps, varied between 13–30 cm and 26–272 g, respectively. Three smolt age groups (1+, 2+ and 3+) were detected. Freshwater age determination on sea trout adults returned to the Ulla revealed the presence of 4-year-old smolts, but in a very low proportion (<1%), this

Life History of the Iberian Sea Trout

239

Table 16.2 Characteristics (length and ages) of sea trout smolts caught at Ulla’s traps (1993–2003). Trap

Smolt age class 1

2

3

Total

FL (mm)

n

%

FL (mm)

n

%

FL (mm)

n

%

FL (mm)

N

MSA

Liñares Ximonde

146.0 164.9

4 16

1.7 2.6

203.4 216.2

141 493

59.7 79.3

220.6 233.7

91 113

38.6 18.2

209.1 218.1

236 622

2.4 2.2

Total

161.1

20

2.3

213.4

634

73.9

227.9

204

23.8

215.6

858

2.2

may have been attributable to the much larger sample size for adults (2891) than for smolts (858). Downstream smolt migration occurred in spring, between March and May. Trap operation throughout the year demonstrated that in the Ulla catchment autumn migration did not occur, although in some years a pre-smolt migration was seen before the spring. Peak migration was always in April. There were no detectable differences between the timing of migration in the main river (Ulla, Ximonde trap) and the tributary (Liñares), but their biological characteristics were different (Table 16.2). Ulla smolts of all age classes were almost 1 cm longer than those from the Liñares. In both rivers the dominant smolt age class was 2+, but more so in the Ulla (80%) than in the Liñares (60%), because of the presence of 3+ smolts in the latter. Consequently, mean smolt age (MSA) was higher in the tributary (2.4 years) than in the main river (2.2 years). Of the 858 smolts in which the age was determined, 72.2% demonstrated ‘B’ type smolts (Fahy, 1998), that is, growth just before spring smoltiﬁcation. A similar proportion was found in both rivers, but signiﬁcant differences were detected between the main age classes. For both rivers, 75.5% of the 2-year-old smolts were ‘B’ type compared with 58.9% of 3-year-old smolts. These differences between both groups were signiﬁcant (K–W, d.f. = 1, P < 0.001). Marine migration Almost 2000 adults and 210 smolts were Floy-tagged between 1998 and 2003, but only 8 adults and 3 smolts were recaptured outside the River Ulla. Most of the recaptures were in the Arosa Ría (the Ulla estuary), but some came from adjacent estuaries. Two recaptures were recorded in two more distant estuaries (Miño and Anllons) almost 200 km far from the Ulla mouth (Fig. 16.3). At Ulla traps, 150 adults and 7 smolts were recaptured. Most recaptures outside the Ulla, were reported by anglers and fewer by ﬁshermen associations. The prohibition to ﬁsh salmonids in saltwater for anglers and net ﬁshermen, probably reduces the number of reported recaptures, and unfortunately does not allow an estimate of the unreported recaptures, but the few returns do give a picture of the Ulla trout movements in the sea.

240

Sea Trout

Ulla traps

Fig. 16.3 Recaptures location of tagged sea trout at smolt ( ) and adult (•) stages (only recaptures outside of the Ulla are shown).

Return migration to fresh water The 9-year catch pattern of ﬁnnock and sea trout adults (1995–2003) differed substantially between the traps (Fig. 16.4, Table 16.3). Sea trout catches at Ximonde trap (Ulla) lasted from April to February, with the main peak in June and July and a smaller peak in the autumn. In contrast, migration in the Liñares occurred between June and January, with a peak near the spawning season in December. This pattern of catches has remained stable during the nine study years. At Ximonde trap, the spring and summer adult catch represented between 61% and 80% of the annual catches; but in the Liñares was between 11% and 21%. In the Ulla between 1993 and 2003 adult sea trout size and weight varied between 22–71 cm and 120–4300 g, respectively. Five sea groups were recorded from the Ximonde trap over the 10-year period 1993–2003, being: 0+ (52%), 1+ (36%), 2+ (8%), 3+ (2%) and 4% (>1%). In 6 of the 10 years ﬁnnock was the most abundant class, while in the other 4 years the main class was 1+ (Fig. 16.6). The proportion of ﬁnnock varied between 27% and 85%. The length and weight of each group varied from 346 mm and 508 g in ﬁnnock

Life History of the Iberian Sea Trout

241

Percentage of cumulative frequency

100

50 Ulla (main river) Liñares (tributary)

0 Apr

May

June

July

Aug

Sep

Oct

Nov

Dec

Jan

Feb

Month

Fig. 16.4

Monthly catches of sea trout adults at Ulla traps (1995–2003).

Table 16.3 Monthly adult sea trout catches at Ulla traps (1995–2003). Month

Ximonde

Liñares

n

%

n

%

March April May June July August September October November December January February Spring–summer Autumn–winter

0 13 103 565 919 474 184 258 209 106 7 4 2074 768

— 0.5 3.6 19.9 32.3 16.7 6.5 9.1 7.4 3.7 0.2 0.14 73.0 27.0

0 0 0 5 13 8 1 7 40 49 6 0 29 100

— — — 3.9 10.1 6.2 0.8 5.4 31.0 38.0 4.6 0 22.5 77.5

Total

2842

—

129

—

to 606 mm and almost 2.7 kg in 4+ sea trout. The length and weight averages of the ﬁve age classes combined were 403 mm and 890 g. The migration pattern of the different sea-age classes during the year varied markedly. Older sea trout migrated before the younger ones; 0.1+ and 0.2+ movement started in spring, achieving 50% of the total annual by June; whilst ﬁnnock did not start to move until June, achieving 50% 1 month later in August (Fig. 16.5).

242

Sea Trout Table 16.4 Sea-age structure and biometric characteristics of adult sea trout (Ximonde trap 1993–2002). Sea-age class

FL (mm)

Weight (g)

Condition factor (K)

N

%

SD

Mean

SD

Mean

SD

Mean

0.0+ 0.1+ 0.2+ 0.3+ 0.4+

1419 981 232 66 14

52.3 36.2 8.5 2.4 0.5

30.2 36.1 41.9 40.2 52.4

346.0 444.2 512.0 578.9 606.4

140.0 312.9 438.9 700.9 692.3

508.3 1115.6 1670.1 2467.4 2695.9

0.178 0.245 0.249 0.309 0.121

1.193 1.221 1.225 1.246 1.174

Total

2712

—

73.8

402.8

550.9

891.5

0.215

1.207

Percentage of cumulative frequency

100

50 0+ 1+ 2+

0 Apr

Fig. 16.5

May

June

July

Aug

Sep Month

Oct

Nov

Dec

Jan

Feb

Migration pattern of different sea-age classes (Ximonde trap 1995–2003).

Mean sea age differed signiﬁcantly across the 10 years (1993–2002) (K–W, d.f. = 8, P < 0.001), (Fig. 16.6). Mean condition factor (K = 100 W/L3 ) for all sea-age classes was 1.207, but there were signiﬁcant differences between age classes for K (ANOVA, d.f. = 4, P < 0.001) in spring and summer ﬁsh. There were also signiﬁcant (K–W, d.f. = 11, P < 0.001) monthly differences in K. The condition factor is higher for all age classes in spring and early summer but beginning with autumn K decreased until the spawning season (Fig. 16.7). The extent of weight loss between tagging and recapture varied amongst the 56 trap recaptures of sea trout obtained more than 1 month after tagging. The mean proportion of weight loss relative to their body weight, was −12.8% g (range, −1.1% to −31.6%), and the mean of weight loss per day was −2 g (range, −0.1 to −6.6). There were signiﬁcant differences (K–W, d.f. = 1, P < 0.0001) between the mean daily weight loss in ﬁnnock and 1 or more sea winter (SW) ﬁsh, being −0.6 g (95% CL = −1.2, +0.1) −2.6 g (95% CL = −2.8, −2.1), respectively.

Life History of the Iberian Sea Trout

243

1.4 1.2

Mean sea age

1 0.8 0.6 0.4 0.2 0 1993

Fig. 16.6

1994

1995

1996

1997 1998 Month

1999

2000

2001

2002

Mean sea-age evolution of the Ulla sea trout (Ximonde trap, 1993–2002).

K

1.4

0+ 1+ 2+

1.2

1

May

June

July

Aug

Sep

Oct

Nov

Dec

Month

Fig. 16.7

Monthly variation of sea trout condition factor for 3 sea year classes (Ximonde trap).

Kelt stage Downstream migration of kelts started in December (Fig. 16.8), but Liñares kelts migration was completed earlier, achieving 50% by January, and ending in April, compared with the Ximonde in which the 50% of kelts are caught by March and the migration ﬁnished in May.

Discussion The mean relative fecundity found in this population is similar, but slightly lower than sea trout in the Bresle and Orne stocks in Normandy-Picardy (Euzenat et al., 1991). In the

244

Sea Trout

Percentage of cumulative frequency

100

50

Ulla Liñares

0

Dec

Jan

Feb

Mar

Apr

May

Month

Fig. 16.8

Migration of kelts at Ulla traps.

Ulla, as in many other sea trout populations studied (Alm, 1950; Svardson & Anheden, 1962; Jensen, 1968; Skrochowska, 1969; Harris, 1970; Campbell, 1977; Paterson, 1973; Pemberton, 1976; Pratten & Shearer, 1983; Le Cren, 1984; Jonsson, 1985; Richard, 1986; Euzenat et al., 1991), the sex ratio was clearly biased towards the females. Three maiden sea ages at the ﬁrst return to fresh water were detected in the Ulla. However, because most (75%) of the Ulla sea trout spawn as ﬁnnock, the mean age of ﬁrst spawning was the one of the lowest so far reported (Alm, 1950; Went, 1962; Chelkowski, 1969; Harris, 1970; Khalturin, 1970; Zarnecki, 1973; Fahy, 1978; Le Cren, 1984; Jonsson, 1985; Richard, 1986). In general, sea trout parr become smolts at an older age than salmon parr in the same stream (Elliott et al., 1992). In the Ulla, salmon smolts are on average 1 year younger than the sea trout smolts (Caballero et al., 2002). Variations in MSA are related to parr growth rates, such that faster-growing parr become smolt at an earlier age than slower-growing siblings (Elliott et al., 1992). The latitude of the stream can inﬂuence MSA (Fahy, 1978; L’Abée-Lund et al., 1989; Jonsson & L’Abée-Lund, 1993), because growth is related to water temperature and the length of the growing season (Elliott, 1975). MSA of the Ulla sea trout population (2.2) was similar to some Welsh, English and Irish populations (Harris, 1970; Fahy, 1978), but rather high in comparison with some other southern European rivers. For example Euzenat et al. (1991) found most Normandy smolts were 1 year old. It is likely that the observed growth in the Ulla parr during their ﬁrst year of life is not enough to achieve the size that would allow them to smolt. Other Iberian sea trout populations with published data (Toledo et al., 1993) from two Asturian rivers, show similar MSA to that reported here for the Ulla population. Although smolt age varies systematically with latitude, smolt size does not (Jonsson & L’Abée-Lund, 1993). Size of the 2-year-old Ulla smolts is slightly lower than those found in most of the Normandy-Picardy rivers (Euzenat et al., 1991). There were signiﬁcant

Life History of the Iberian Sea Trout

245

differences in the size of each smolt age class between the Ulla and the tributary as has been noted in some French rivers (Euzenat et al., 1991). Smolt run timing takes place in spring, usually April and May (Elliott et al., 1992). Migration timing in the Ulla does not differ from the British, Irish or French rivers, but in Scandinavian rivers the migration period extends until June (Rassmussen, 1986). No autumn smolt run was found on the Ulla, as has been described in some other populations (Le Cren, 1984). Information on marine movements of sea trout is sparse. However, limited studies from Norwegian, Irish, British and French populations (Went, 1962; Pemberton, 1976; Pratten & Shearer, 1983; Berg & Berg, 1987; Euzenat et al., 1991) have indicated that migrations in the sea are relatively short and mostly comprise movements along the coast to adjacent estuaries. Displacement as far as 600 km has been reported, but such distances are rare. The Ulla sea trout appeared to conform to this pattern, but much more work is required, as elsewhere. In common with several other European rivers (Le Cren, 1984), ﬁnnock in the Ulla usually ran into fresh water later in the year than older sea trout, generally beginning in June. But unusually, in the Ulla population, the upstream movements in some years continued until February, although with a low number in the ﬁrst and second months of the year. Differences in the timing of upstream migration between the Ulla and the Liñares, suggested that the tributaries are not used until the spawning season is close. The reported proportion of ﬁnnock is very variable amongst European rivers, but the value for the Ulla (52%) was high compared with other rivers (Went, 1962; Fahy, 1978; Le Cren, 1984; Toledo et al., 1993). The mean sizes achieved among the sea-age groups of the Ulla sea trout (mean fork length for 1+ SW = 44 cm), indicated that in this region they experienced a medium marine growth compared with other European regions. It was lower than some Baltic populations in which 1SW length varied between 55 and 65 cm (Alm, 1950; Chelkowski, 1969; Sych, 1970; Svardson & Fagerstrom, 1982; Bornecka, 2001), or Normandy-Picardy in which length

5 4.5

Mean smolt age

4 3.5 3 2.5 2 1.5 1 40

45

50

55

60

65

70

75

Latitude°N

Fig. 16.9 Relation between mean smolt age (MSA) and latitude, including Iberian sea trout populations.

246

Sea Trout

was 46–53 cm (Euzenat et al., 1991). However, it was higher than some Scottish or Irish stocks in which mean 1SW length has been reported as 38 and 37 cm, respectively (Fahy, 1978; Pratten & Shearer, 1985). There are several reports of sea trout feeding on return to fresh water (Nall, 1926; Harris, 1971; Toledo et al., 1983; Elliott, 1997). Detailed feeding studies were not carried out in the Ulla, but weight loss occurred during their freshwater residence prior to spawning, as also reported for northern French rivers (Euzenat et al., 1991). The timing of kelt migration was also similar between the Ulla and the Normandy-Picardy rivers (Euzenat et al., 1991). Among sea trout populations, growth rate in fresh water, mean smolt age, mean sea age at maturation, proportion of repeat spawners and length of adult life span, exhibit latitudinal clines (Jonsson & L’Abée-Lund, 1993). The Ulla population has the expected characteristics of a southern stock, except that MSA is higher than that found in northern rivers (Fig. 16.9). The variation found in MSA between French and Iberian populations may be inﬂuenced by environmental factors such as productivity, which is particularly high in some Normandy rivers (Richard, 1986).

Acknowledgement Part of this work was carried out at the ‘Encoro do Con’ Hidrobiological Field Station of the University of Santiago de Compostela.

References Allan, J.R.H. & Ritter, J.A. (1977). Salmonid terminology. Journal du Conseil International pour l’Exploration de la Mer, 37(3), 293–9. Alm, G. (1950). The sea trout population in the Ava stream. Report of the Institute of Freshwater Research, Drottningholm, 31, 26–51. Berg, O.K. & Berg. M. (1987). Migrations of sea trout, Salmo trutta L., from the Vardnes River in northern Norway. Journal of Fish Biology, 31, 113–21. Bornecka, I. (2001). Growth of Vistula sea trout (Salmo trutta m. trutta L.) based on adults caught in Vistula River prior to the construction of the dam in Wloclawek. Bulletin of the Sea Fisheries Institute, 2(153), 3–12. Caballero, P., García Rego, M. & García de Leániz, C. (2002). Atlantic salmon and brown trout migratory strategies in the Ulla river system (Galicia): implications for the conservation of these populations. In: The Salmon: A Treasure of Our Rivers (García de Leániz, C., Serdio, A. & Consuegra, S., Eds) (In Spanish with English abstracts), Proceedings of the III Symposium on Atlantic salmon in the Iberian Peninsula, 2001, Santarder, pp. 155–71. Campbell, J.S. (1977). Spawning characteristics of brown trout and sea trout Salmo trutta L., in Kirk Burn, river Tweed, Scotland. Journal of Fish Biology, 11, 217–30. Chelkowski, Z. (1969). The sea trout, Salmo trutta m. trutta L., of the Pomeranian coastal rivers. Przeglad Zoologica, 13(1), 72–91. Elliott, J.M. (1975). The growth rate of brown trout (Salmo trutta L.) fed on maximum ratios. Journal of Animal Ecology, 44, 805–21. Elliott, J.M. (1997). Stomach contents of adult sea trout caught in six English rivers. Journal of Fish Biology, 50(5), 1129. Elliott, J.M. & Chambers, S. (1996). A guide to the interpretation of sea trout scales. National Rivers Authority, Bristol, R & D Report, Vol. 22, 53 pp. Elliott, J.M., Crisp, D.T., Mann, R.H.K. et al. (1992). Sea trout literature review and bibliography. National Rivers Authority, Fisheries Technical Reports.

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Euzenat, G., Fournel, F. & Richard, A. (1991). La truite de mer (Salmo trutta L.) en Normandie-Picardie. In: La Truite Biologie et Écologie (Baglinière, J.L. & Maisse, G., Eds), INRA-ENSA, Paris. Fahy, E. (1998). Variation in some biological characteristics of British sea trout, Salmo trutta L. Journal of Fish Biology, 13, 123–38. Harris, G.S. (1970). Some aspects of the biology of Welsh sea trout (Salmo trutta L.). PhD Thesis, University of Liverpool. Harris, G.S. (1971). The freshwater feeding of adult sea trout in the Afon Dyﬁ. Journal of the Institute of Fisheries Management, 2(1), 20–3. Hart, P.J.B. & Pitcher, T.J. (1969). Field trials of ﬁsh marking using a jet inoculator. Journal of Fish Biology, 1, 383–5. Jensen, K.W. (1968). Sea trout (Salmo trutta L.) of the river Istra, western Norway. Report of the Institute of Freshwater Research, Drottningholm, 48, 187–213. Jonsson B. (1985). Life history pattern of freshwater resident and sea-run migrant brown trout in Norway. Transactions of the American Fisheries Society, 114, 182–94. Jonsson, B. & L’Abée Lund, J.H. (1993). Latitudinal clines in life-history variables of anadromous brown trout in Europe. Journal of Fish Biology, 43 (Suppl. A), 1–16. Khalturin, D.K. (1970). A study of the biology of the brown trout (Salmo trutta L.) of the Kareliam Isthmus. Journal of Ichthyology, 10(2), 218–28. L’Abée Lund, J.H., Jonsson B., Jensen, A.J. et al. (1989). Latitudinal variation in life-history characteristics of sea-run migrant brown trout Salmo trutta. Journal of Animal Ecology, 58, 525–42. Le Cren, E.D. (Ed.) (1984). The Biology of Sea Trout. Atlantic Salmon Trust. Nall, G.H. (1926). The sea trout of the river Ewe and Loch Maree. Plas Menai, Wales, UK, 24–26, October 1984. Salmon Fisheries, Edinburgh. No. 1, pp. 1–41. Ombredane, D. & Richard, A. (1990). Dètermination de la zone optimale de prélévement d’écailles chez les smolts de truite de mer. Bulletin Francaise de la pêche et Pisciculture, 319(4), 224–38. Paterson, D. (1973). Observations on the sea trout (Salmo trutta L.) spawning populations from light Tweed tributaries. BSc Thesis, University of Edinburgh, 110 pp. Pemberton, R. (1976). Sea trout in North Argyll sea lochs, population, distribution and movements. Journal of Fish Biology, 9, 157–79. Pratten, D.J. & Shearer, W.M. (1983). The migrations of North Esk sea trout. Fisheries Management, 14(3), 99–113. Rassmussen, G. (1986). The population dymamics of brown trout (Salmo trutta L.) in relation to year-class size. Polskie Archiwum Hydrobiology, 33(3–4), 489–508. Richard, A. (1986). Les populations de truite de mer (Salmo trutta L.) de L’orne et de la Touques (BasseNormandie): scalimetrie, sexage, caracteristiques biometriques, demographiques et migratoires. These, Universite de Rennes. Richard, A. & Baglinière, J.L. (1990). Description et interprétation des écailles de truite de mer (Salmo trutta L.) de dux rivières de Basse-Normandie: L’Orne et la Touques. Bulletin FranÇais de la Pêche et de la Pisciculture, 319(4), 239–57. Skrochowska, S. (1969). Migrations of the sea trout (Salmo trutta L.), brown trout (Salmo trutta M. fario L.) and their crosses. Part IV – General discussion of results. Polish Archives of Hydrobiology., 16, 181–92. Svardson, G. & Anheden, H. (1963). Konskvöt och utvandring ho, verkeans Örig. Svensk Fiskerl Tidesskrift, 72, 165–69. Svardson, G. & Fagerstrom, A. (1982). Adaptative differences in the long distance migration of some trout (Salmo trutta L.) stocks. Swedish Board of Fisheries, 60, 51–80. Sych, R. (1970). Some comparison of the background of an eleven-year study on the growth of sea trout (Salmo trutta L.). Acta Hydrobiology, 12(2–3), 225–49. Toledo, M.M., Lemaire, A.L., Baglinière, J.L. & Braña, F. (1993). Caractéristiques biologiques de la truite de mer (Salmo trutta L.) au nord de l’Espagne, dans deux rivières des Asturies. Bulletin Francaise de la Pêche et Pisciculture, 330, 295–306. Went, A.E.J. (1962). Irish sea trout, a review of investigations to date. Scientiﬁc Proceedings of the Royal Dublin Society, 1A(10), 265–96. Zarnecki, S. (1973). Differentiation of Atlantic salmon (Salmo salar L.), sea trout and brown trout originating in the Vistula. ICES ANACAT. Fisheries Committee, 128, 1–3.

Chapter 17

Review and Perspectives on Molecular Genetic Approaches to Sea Trout Biology M.W. Bruford School of Biosciences, Cardiff University, Cardiff CF10 3TL, Wales, UK Abstract: A review of the literature of molecular studies in Salmo trutta L. in comparison with Salmo salar L. gives the unavoidable impression of the sea/brown trout as the less-favoured ‘little brother’ of the highly studied salmonids in Eurasia and the Atlantic. Regardless of which way one approaches the large body of publications that have accumulated over the past 15 years, Atlantic salmon studies are twice as numerous as those of brown and sea trout combined. Clearly such a large difference must reﬂect the relative economic importance of these species but as pointed out elsewhere in this volume, sea trout can also have major regional economic signiﬁcance and it could be argued that their biology is at least interesting as their bigger brother, if not more so. It is clear that many of the conservation issues surrounding these two species are similar; many of those issues are clearly outlined in the contributions to this Symposium, and herein especially by the contributions of Ferguson and Lundqvist et al. and will not be repeated here. It may be worth speculating, however, on what the future may bring for sea trout genetics, and whether the above-mentioned disparity of academic effort between the two species is, in scientiﬁc terms, merited. Here, I attempt to summarise the results presented by those contributors to the genetics session of the Symposium and summarise potential future avenues for research from the industry, academic and funding point of view. Keywords: Sea trout, molecular genetics, demography, life history, conservation, stocking, genetic markers.

Basic biology: demography and life history One of the key aspects that molecular genetics has brought to salmonid biology, is a greater appreciation of the evolution, ecology and basic life history of these exploited species. Studies have progressed from basic descriptions of regional genetic diversity (e.g. Giuffra et al., 1994), small-scale geographical differentiation (e.g. Estoup et al., 1998), temporal changes in the population structure in response to the environment (Ostergaard et al., 2003) to the detection of sex-speciﬁc dispersal patterns (Bekkevold et al., 2004). The contributions outlined below ﬁrst summarise much of this work, then place the work in the context of what we need to know, in order to manage populations more efﬁciently and how new methodologies may help. However, they also highlight how our lack of knowledge impinges on our ability to rationally manage these sources at a local and regional level. Ferguson’s keynote address on the Origin and Signiﬁcance of Sea Trout in Britain and Ireland is excellently summarised (Ferguson, 2006). However, a few key points emerge which perhaps bear repetition. Ferguson focused on four important issues: (1) the origins of anadromous and non-anadromous forms; (2) the genetic basic of anadromy; (3) genetic 248

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differentiation and gene ﬂow and (4) the impact of stocking. The existence of four main lineages and ﬁve main genetic groups of sea trout in western Eurasia was emphasised and has clear management implications. It is the colonisation processes of these anadromous forms and their subsequent interbreeding, adaptation and demography which have shaped the high levels of ‘natural’ genetic diversity in sea trout stocks today: diversity which is not as obvious in purely freshwater trout. The increasing evidence for a genetic component in anadromy was discussed at length in the context of physiological adaptation requirements (discussed also in Giger et al., 2006) and the need for more experimental studies from diverse crosses with environmental parameters controlled was stressed. The applicability of different molecular tools (mostly genetic ‘markers’ not seen by natural selection) was discussed, especially their application to discriminate between sea and brown trout, the results of which, Ferguson concluded, merely reﬂected natural differences that occurred among all trout populations, regardless of anadromy status, within river systems, and deﬁnitive studies should ideally be carried out between anadromous and non-anadromous ﬁsh within the same cohort. Ferguson then went on to describe studies of UK sea and brown trout populations which conﬁrmed general features including multiple colonisation events in river systems, limited gene ﬂow among systems (genetic differentiation often being twice as high as in Atlantic salmon) and a general lack of correlation between geographical and genetic distance across river populations. The interesting phenomenon of hybridisation between trout and salmon and the use of molecular markers was also discussed, and comparisons of hybridisation rates across rivers (which can be as high as 17%!) were made and their implications for ﬁsh identiﬁcation discussed. Finally, Ferguson described the various management issues implicated in the basic biological characteristics of sea trout described above, especially the problems of stocking with domesticated trout and its impacts on the recruitment rates for wild trout in stocked rivers. He concluded that supportive breeding and the use of brood stock from appropriate rivers including genetic analysis for veriﬁcation could be potentially crucial for successful management in the future. Lundqvist and colleagues (see Lundqvist et al., this volume) took the discussion forward by asking the important general question of whether hatchery-reared brown trout pose a threat to wild stocks, using a long-term example study from Sweden. The authors summarised key life-history differences between anadromous and resident ﬁsh (e.g. spawning, morphology, growth rates) and placed this in the context of physiological and life-history costs and beneﬁts of migration. They then used the example of comparisons of migration and recapture rates for hatchery-reared ﬁsh released above and below dams, including for smolts released of different sizes and placed these data in the context of the performance of wild ﬁsh analysed over 23 years (Lundqvist et al., 2006). The authors identiﬁed some key future research needs for the development of optimal stocking strategies, including ﬁtness estimates for competing life histories; ascertaining the inﬂuence of marine feeding on migratory tendency, a better understanding of the relationship between performance in fresh and salt water; ascertaining marine growth performance at different temperatures; evaluating the relationship between temperature during early development compared with that during later growth and understanding spawning behaviour and alternative mating strategies in migratory populations.

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Cucherousset et al. presented interesting evidence for a continuum of life-history strategies in brown trout using data collected by PIT tagging. The aim was to use PIT tags to follow individuals throughout their early riverine life in the Oir river system (France, studied since 1994) where microsatellite analysis has failed to detect any genetic differentiation between anadromous and resident morphs. Nearly 6000 juveniles were tagged between 1995 and 1999 and were monitored until 2002 by electroﬁshing, trapping and ﬂat bed antennae to detect PITs. Data was presented for 3000 capture/recaptures (approximately 1300 ﬁsh until migration, 400 until reproduction with 12 life-history traits measured). Five main growth patterns emerged, with various combinations of growth strategies in years 1 and 2, including a general low juvenile growth in brook dwelling males; intermediate ﬁrst year, followed by high second year growth for males and females in the Oir; with the opposite pattern being found in the Selune River. The authors concluded that the complex patterns identiﬁed related to factors such as selection for age of maturity and its interaction with sex in different environments and identiﬁed a continuum of characteristics between freshwater trout and sea trout in both males and females, governed by individual metabolic requirements. Many traits were found to be phenotypically plastic (i.e. environmentally mediated interactions with a particular genotype), which the authors identiﬁed as the main reason leading to the observed continuum. Caballero et al. presented life-history data for a sea trout population from the northeastern Iberian Peninsula in Spain, focusing on the Ulla River where from 1993 there have been two permanent traps; one in the main river and the other in a tributary, operating at an efﬁciency of 10–20%. They found juvenile migration patterns to be similar in both the river and its tributary (March–April smolt migration), but that adult return migration differed, with a slower, longer period, migration in the main river. They also presented evidence that age composition and size differed between the river and tributary. At a more local level, Solomon asked why sea trout do not enter the Wye and Usk catchments, or why resident ﬁsh from these rivers do not adopt a migratory phase and exploit, for instance, an opportunity for increased growth. He discussed a number of factors likely to favour or act against migration (e.g. general river conditions, spawning conditions, safety of routes to sea, temperature, marine conditions, competition from other species, predation and parasite loads). He concluded that these factors must ultimately be less important in males than in females and explored the potential for short-term variation in migration frequency caused by environmental ﬂuctuations.

Conservation: management and stocking If there is one area where molecular studies have had a major impact in sea and brown trout biology, it is in the area of stocking and management – especially introgression between native and stocked ﬁsh (e.g. Largiader & Scholl, 1996). For example, in the journal Molecular Ecology, there have been 15 papers published on the subject Salmo trutta since the journal’s inception in 1994, with many more in the specialist literature (e.g. Ruzzante et al., 2004). Ferguson highlights much of this work in his keynote paper, but Carvalho and Hauser (below) add to this debate by emphasising the role of studies of long-term effective

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population size in the context of adaptive potential, and additional contributions reinforce the role that molecular methods (including chemical ecology) can play. Carvalho (and Hauser) discussed the concept of effective population size into the population biology and conservation of structured ﬁsh populations. Starting by asking what the effective population size (Ne ) is (broadly speaking the number of breeding individuals in a population at a given point in time), they went on to ask how Ne can be estimated (at a given point in time, or incorporating changes across generations), how the evolutionary signal of changes in effective size can be detected and the implications of these patterns and associated genetic structure for ﬁsheries management. Carvalho then outlined the application of molecular markers to Ne estimation for estimating the number of breeders and pointed out how most estimators make unrealistic assumptions about population structure. He then described temporal methods for estimating Ne (using the change in marker allele frequencies over time). The evolutionary signiﬁcance of Ne was also discussed from the standpoint of the genetic properties of a population, including ‘standing levels’ of genetic variation, a population’s ability to withstand ﬂuctuations in size and the effects of migration. He cited the example of Palm et al. (2003) who carried out temporal estimates of effective population size in brown trout to look at stability of populations. He then discussed emergent patterns (e.g. evidence for isolation-by-distance in northern Europe and Scandinavia) and how this can be confounded by the huge range in Ne found across river systems. The potential for local adaptation is linked to Ne (e.g. Hansen et al., 2002) who assessed long-term stability in Danish sea trout populations. The take-home message is that local adaptation in small populations only occurs by chance at a regional scale, that maintaining connectivity across populations is essential for gene ﬂow in small populations (and heritability of migratory ability) and that the value of Ne can provide an indication of local adaptation and enable predictions about short and long-term population survival. Charles et al. discussed the interesting use of genetic and isotopic tools to investigate population dynamics and genetics of the progeny of sympatric forms of anadromous and resident trout in the Oir River. The authors carried out an evaluation of the females of the two forms up to fry stage, using stable isotopes. The two forms occur in sympatry in this river system and were sampled March–April in two successive years; when 69 and 65 emerging fry were electroﬁshed and analysed genetically and for stable carbon and nitrogen isotopes. The basis of this analysis is that each organism and tissue potentially has its own isotopic signature in relation with its diet and metabolism because of different maternal feeding areas (i.e. sea or river). The authors found good evidence for differences across the forms using isotope analysis and this approach could be used to quantify the contribution of anadromous females to fry. However, neutral molecular markers (microsatellites) were not able to discriminate the forms possibly because of overlapping spawning times and behaviour and a possible absence of reproductive barriers resulting in gene ﬂow among them. The isotopic approach is limited, however, to the point at which the chemical signature of the yolk sac has become exhausted. Ostergren et al. discussed the spawning, migration and genetic status of wild sea trout in two northern Swedish rivers (the same system as described in Lundqvist et al., 2006). During the attempted restoration of this damaged ecosystem (logging waste has been a major

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factor in riverine degradation), the focus of the study is to identify spawning sites and ask whether anadromous trout belong to one or more populations. The study involved telemetry tracking of migration (by car and plane) of 29 (31) tagged ﬁsh and 5 (15), respectively from the two rivers in 2002 and 2003 using mitochondrial and microsatellite genetic analysis. Microstellite allele diversity was similar across rivers with no differences between hatchery and wild ﬁsh. There was large variation in individual migration patterns (females showed the greatest variation). This study is ongoing.

Perspectives for detecting selection – could there be a sea trout ‘marker’? Perhaps the most exciting area in population genetics currently is the integration of genomic, transcriptomic and (potentially) functional genomic approaches to identify genes or genomic regions under selection. These methods are no longer necessarily conﬁned to ‘model’ organisms (Luikart et al., 2003; Vasemagi & Primmer, 2005) although it is clear (as in Giger et al., 2006) that Atlantic salmon genomic tools will be applicable, at least to some degree and with sufﬁcient rigour, to sea and brown trout. Indeed, given the lack of genetic differentiation consistently detected using ‘neutral’ genetic markers, such as microsatellites, between sympatric anadromous and resident stocks of S. trutta, that analysis of markers under selection are by far the route most likely to succeed towards detecting a ‘sea trout marker’. Population genomic studies are in their infancy in trout, but Atlantic salmon studies are already progressing (e.g. Vasemagi et al., 2005). In this study the authors looked for the occurrence of highly polymorphic microsatellites in the untranslated regions of Atlantic salmon expressed sequence tags or ESTs as a source of gene-associated polymorphisms. (ESTs are DNA sequences made available through genome projects and are the result of the partial sequencing of a large number of randomly selected cDNA clones from a particular tissue. cDNA EST databases describe the kinds of genes and expression levels in that particular tissue.) They identiﬁed nine microsatellites which showed evidence for divergent selection and showed similar trends in population samples from different environments (saltwater, brackish and freshwater habitats) and sea areas (here, Barents versus White Sea). The authors considered these ESTs as candidate loci likely to have been affected by divergent selection, and these could allow the location of promising genes associated with adaptive divergence in Atlantic salmon. An alternative approach to the one described above involves examining expression levels in populations inhabiting different environments. This approach (transcriptomics) again uses cDNA but in this case, known gene sequences are spotted in minute amounts onto a substrate (ﬁlter or glass slide) and the RNA from a given tissue is hybridised to the cDNA genes. (cDNA is a strand of DNA synthesised to complement a given strand of messenger RNA. Complementary DNA represents the parts of a gene that are expressed in a cell to produce a protein.) The more intense the hybridisation is, the higher the levels of expression there are for that particular gene in that particular tissue. These results can be tissue, individual, population, taxon and environment speciﬁc. The challenge is managing

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this level of diversity and analysing the variation (there can be 10 000 genes on a single slide!). The power of this study is obviously enormous, however, and Giger et al. (2006) give a ﬁrst glimpse of its application in sea trout. In the meeting, Giger et al. described differences in global gene expression levels between sedentary and migratory forms of brown trout using a cDNA microarray assessment. Measuring gene expression levels is a potentially important new approach to understanding adaptive variation in ﬁsh because it allows us to measure gene expression at the population level (where selection acts on the phenotype) since selection is only possible with heritable phenotypic variability and because, for most phenotypes, the relationship between phenotypic variation and genomic (DNA) polymorphisms is poorly understood. A cDNA microarray study can be used to analyse thousands of genes in a single step. This approach has been used to study gene expression changes in migratory versus non-migratory trout (see Giger et al., 2006) and help identify genes with differential expression in individuals which have undergone smoltiﬁcation.

Acknowledgement I would like to thank Dr Joanne Cable for providing minutes from the genetic session, which aided greatly in the preparation of this chapter and in my ability to Chair the session.

References Bekkevold, D., Hansen, M.M. & Mensberg, K.L.D. (2004). Genetic detection of sex-speciﬁc dispersal in historical and contemporary populations of anadromous brown trout Salmo trutta. Molecular Ecology, 13, 1707–12. Estoup, A., Rousset, F., Michalakis, Y., Cornuet, J.M., Adriamanga, M. & Guyomard, R. (1998). Comparative analysis of microsatellite and allozyme markers: a case study investigating microgeographic differentiation in brown trout (Salmo trutta). Molecular Ecology, 7, 339–53. Ferguson, A. (2006). Genetics of sea trout, with particular reference to Britain and Ireland. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publications, Oxford, pp. 157–82. Giger, T., Amstutz, U., Excofﬁer, L. et al. (2006). The genetic basis of smoltiﬁcation: functional genomics tools facilitate the search for the needle in the haystack. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 159–82. Giuffra, E., Bernatchez, L. & Guyomard, R. (1994). Mitochondrial control region and protein-coding gene sequence variation among phenotypic forms of brown trout Salmo trutta from northern Italy. Molecular Ecology, 3, 161–71. Hansen, M.M., Ruzzante, D.E., Nielsen, E.E., Bekkevold, D. & Mensberg, K.L.D. (2002). Long-term effective population sizes, temporal stability of genetic composition and potential for local adaptation in anadromous brown trout (Salmo trutta) populations. Molecular Ecology, 11, 2523–35. Largiader, C.R. & Scholl, A. (1996). Genetic introgression between native and introduced brown trout Salmo trutta L. populations in the Rhone River Basin. Molecular Ecology, 5, 417–26. Luikart, G., England, P.R., Tallmon, D., Jordan, S. & Taberlet, P. (2003). The power and promise of population genomics: from genotyping to genome typing. Nature Reviews Genetics, 4, 981–94. Lundqvist, H., McKinnell, S.M., Jonsson, S. & Östergren, J. (2006). Are reared anadromous brown trout compatible with the conservation of wild trout? In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 356–71.

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Ostergaard, S., Hansen, M.M., Loeschcke, V. & Nielsen, E.E. (2003). Long-term temporal changes of genetic composition in brown trout (Salmo trutta L.) populations inhabiting an unstable environment. Molecular Ecology, 12, 3123–35. Palm, S., Laikre, L., Jorde, P.E. & Ryman, N. (2003). Effective population size and temporal genetic change in stream resident brown trout (Salmo trutta L.). Conservation Genetics, 4, 249–64. Ruzzante, D.E., Hansen, M.M., Meldrup, D. & Ebert, K.M. (2004). Stocking impact and migration pattern in an anadromous brown trout (Salmo trutta) complex: where have all the stocked spawning sea trout gone? Molecular Ecology, 13, 1433–45. Vasemagi, A. & Primmer, C.R. (2005). Challenges for identifying functionally important genetic variation: the promise of combining complementary research strategies. Molecular Ecology, 14, 3623–42. Vasemagi, A., Nilsson, J. & Primmer, C.R. (2005). Expressed sequence tag-linked microsatellites as a source of gene-associated polymorphisms for detecting signatures of divergent selection in Atlantic salmon (Salmo salar L.). Molecular Biology and Evolution, 22, 1067–76.

Section 3

POPULATION DYNAMICS, ECOLOGY AND BEHAVIOUR

Chapter 18

A 35-Year Study of Stock–Recruitment Relationships in a Small Population of Sea Trout: Assumptions, Implications and Limitations for Predicting Targets J.M. Elliott1 and J.A. Elliott2 1

Freshwater Biological Association, The Ferry House, Far Sawrey, Ambleside, Cumbria LA22 OLP, UK 2 NERC Centre for Ecology and Hydrology: Lancaster Environment Centre, Library Avenue, Bailrigg, Lancaster LA1 4AP, UK

Abstract: In a study of sea trout in Black Brows Beck (north-west England) from 1966 to 2000, survivor density for different life stages was strongly dependent on egg density, the relationship being well described by the Ricker model. Exceptions were all for year classes affected by summer droughts, 1+ parr being the most vulnerable life stage, rather than 0+ parr. Key-factor analysis revealed that population density throughout the life cycle was regulated by density-dependent survival in the early life stages, especially when the fry emerged from the gravel and started to feed. The assumptions, implications and limitations of three commonly used stock–recruitment models are discussed. Data from Black Brows Beck are used to illustrate how the models predict different values for three spawning targets: the equilibrium density for replacement, the stock density for maximum recruitment and the stock density for maximum surplus yield. Ricker-type models describe a wide range of dynamics, including cyclic behaviour, but models such as a power-function or an asymptotic curve cannot do this, and are limited in their ability to predict spawning targets. Spatial scale is also important, that is the area of stream or river used in the estimates. Time-series analysis of inter-generation ﬂuctuations in density showed that the pattern differed markedly between four time series. A hypothetical example illustrated the problem of detecting a gradual decrease in the equilibrium density, which might eventually lead to population extinction. Although the data from Black Brows Beck were for a relatively stable population in a relatively benign environment, density dependence accounted for only about half of the inter-generation variability in recruitment. As the environment becomes less benign, moderation of variability in recruitment by density-dependent factors will decrease markedly. Sea trout populations in harsh environments will be inherently unstable with large variations in density between year classes, chieﬂy because of densityindependent factors. It is impossible to ﬁt stock–recruitment models to these latter populations and therefore difﬁcult to predict spawning targets. Keywords: Density dependence, key-factor analysis, Salmo trutta L., stock–recruitment models.

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Introduction Many short-term ecological studies attempt to detect a change in population density because of natural or anthropogenic factors. They usually test the null hypothesis of ‘no change’, but such an approach is of little value because it is difﬁcult to assess the signiﬁcance of any population change when little is known about the ‘baseline’ from which the change occurred. It is naive to assume a constant baseline without any dynamic changes in population density. Impact effects must be tested against a null hypothesis error term representing betweenyears variation in the absence of the impact, that is, the temporal variation in the baseline. Therefore, the major objectives of long-term population studies are to provide reliable estimates of baseline variation, to detect long-term trends in the mean level of the baseline, to assess the impact of rare events and to provide information for meaningful, testable, hypotheses (Likens, 1989; Elliott, 1990, 1994). It is not always easy to deﬁne ‘long term’. One approach is to use generation time, but such a unit does not take into account environmental changes which usually follow an annual pattern. Therefore, whatever the generation time, the basic unit for long-term studies should be a year. Although long-term studies on vertebrate species are scarce because many species have long generation times (Sinclair, 1989), there are some excellent studies on salmonid species. Density-dependent models have been ﬁtted to data sets for Paciﬁc salmon, Oncorhynchus spp. (e.g. Hilborn & Waters, 1992; Achord et al., 2003) and Atlantic salmon, Salmo salar L. (e.g. Kennedy & Crozier, 1993, 1995; Jonsson et al., 1998; Dumas & Prouzet, 2003). There is a paucity of similar work on brown trout, Salmo trutta L., apart from this study. Density-dependent processes are also discussed in reviews of the life histories of Atlantic salmon and brown trout (Klemetsen et al., 2003). A long-term study of a sea trout population in Black Brows Beck started with redd counts in November and December 1966. These were used later to estimate egg density at the start of the 1967 year class, deﬁned as the year in which the eggs hatched. The study ended with redd counts in November and December 2000. Results were presented in 19 publications summarised in a book (Elliott, 1994). Later publications described the relationships between female size and fecundity or egg density in the redd (Elliott, 1995), and between smolt density and fry density (Elliott, 1996), the natural removal of dead trout fry (Elliott, 1997), the effects of droughts on the population (Elliott et al., 1997), individual-based models for predicting the emergence period and size of fry (Elliott & Hurley, 1998a, b), the correlation between the fry emergence period and the North Atlantic Oscillation (Elliott et al., 2000) and shadow competition between juveniles (Elliott, 2002). This review summarises variations in the population-density relationships over 35 years, and considers the assumptions, implications and limitations of stock–recruitment models for predicting spawning targets. The data were also examined for the ﬁrst time as an inter-generation time series.

Study area and ﬁeld sampling methods As these have been described in detail by Elliott (1994), only a summary of the essential points is provided here. Black Brows Beck is a small stream (length 512 m, mean width 0.8 m) in north-west England, and serves as a nursery for the progeny of sea trout.

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Water temperature usually ranged annually from 0.1◦ C to 18◦ C with a mean of about 9◦ C. Water velocity increased with discharge but never exceeded 60 cm/s at high discharges because the stream overﬂowed its banks and ﬂooded the adjacent ﬁeld. The stream bed was therefore never subjected to high water velocities and remained stable, this being important for the maximum survival of eggs, alevins and fry. The conductivity of the stream was low at about 100 μS/cm (k25 ) with a low calcium concentration of about 0.4 m eq/l and a pH range of 6.7–6.9. Black Brows Beck is one of several tributaries of Dale Park Beck which is a major tributary of Russland Pool (actually a river in spite of its name). The total length of mainstream and all tributaries is 27 km and it discharges into the estuary of the River Leven. Spawning occurred in November and December, eggs usually hatched in February/early March and the alevin stage ended in late April/early May when the fry dispersed from the redds. A two-way trap across the mouth of the stream between January and October indicated that the parr stage lasted about 2 years and most trout started to migrate downstream to the estuary in May at the start of their third year (age 2+ years) (Elliott, 1986). Very few fry and 0+ parr emigrated out of the stream and, therefore, population losses in the ﬁrst spring, summer and winter of the life cycle were chieﬂy because of mortality rather than emigration. Although few 1+ parr emigrated, their trap catches were higher than those for 0+ parr and, therefore, losses for 1+ parr were a result of both mortality and migration. Therefore, the term ‘loss rate’ was used instead of mortality rate because a decrease in density could be attributable to mortality or migration or both. The trap also showed that there was no immigration of immature trout into the stream. Two hundred and sixteen adults were caught by netting or electroﬁshing during this study and these provided information on the sex, ages and sizes of spawning trout (for methods, see Elliott, 1984, 1985). Some mature males in their third year returned from the sea/estuary in the year of their smolt migration after spending only one summer away from fresh water (i.e. they were ﬁnnock). Most spawners were males and females in their fourth year (age 2/1+ years, i.e. 2 years in fresh water and over 1 year in the sea/estuary). A very small number of females in their ﬁfth year spent two winters at sea before spawning for the ﬁrst time. Repeat female spawners were very rare and only three have been recognised from their scales during the whole study. All female spawners were anadromous. Spawning adults were assumed to be returning to their natal stream and this assumption is supported by genetic evidence for salmonids in other streams (Youngson et al., 2003). Electroﬁshing was used to catch trout from known areas of stream at the end of May or early June and at the end of August or early September (1967–2000). The study section of 60 m2 was divided by block nets into six subsections, each with a surface area of 10 m2 . All ﬁsh were removed, counted and returned live to the stream. Additional samples were taken in some years to answer speciﬁc questions. Direct observations on spawners indicated that there were usually two to four males attending each female, but only one male fertilised the eggs; a female was never seen to construct more than one redd and there was never more than one female per redd (Elliott, 1984). The female left the stream soon after spawning whilst the males remained, waiting for the next female. Since November 1966, an annual census was made of the number of redds and hence the number of females returning to spawn within a study section of

260

Sea Trout

120 m2 which included the section used to sample the juveniles. Excavations of 70 redds elsewhere in the stream provided information over 30 years on the mean number of eggs per redd (±2 SEs): 1553 (±36), 1034 (±27) and 722 (±24) for early (2–13 November), middle (14–25 November) and late (26 November–7 December) spawners, respectively (Elliott & Hurley, 1998a). For 29 of these redds, the female constructing the redd was netted immediately after she had laid her eggs. It was thus possible to quantify the relationships between female size and the number of eggs per redd and the mean weight of an egg in each redd (Elliott & Hurley, 1998b). Catches of adults showed that early spawners were always the largest females (fork length 401–449 mm) and late spawners the smallest (241–336 mm) with the middle group (300–398 mm) between these two extremes. It was thus possible to estimate egg density in each year from the number of redds and the time of spawning. Although most redds were in the downstream half of the study section, the fry soon dispersed throughout the study section, as shown by a detailed study of their behavioural movements (Elliott, 1986).

Statistical methods The six most frequently used density-dependent stock–recruitment models were tested on the population-density data (Elliott, 1994). The following two-parameter model (also called the stock–recruitment curve of Ricker, 1954) provided signiﬁcant ﬁts for all life stages: R = aS exp(−bS)

(18.1)

where R = number of survivors at different stages in the life cycle, S = number of eggs at the start of each year class, a and b were parameters estimated by non-linear least squares. For each year class, there was an estimate of initial egg density (S eggs per 60 m2 ) and trout density (R ﬁsh per 60 m2 ) for ﬁve life stages: parr aged 0+ years sampled in late May or early June (R1 ) and late August or early September (R2 ); parr aged 1+ years sampled in late May or early June (R3 ) and late August or early September (R4 ) and spawning females (R5 ). Data for parr aged 2+ years sampled in May or June were excluded from the analyses because most of these ﬁsh had already migrated downstream as smolts (Elliott, 1986). Only 13 2+ parr were taken in all the August/September samples, indicating that smolt migration occurred chieﬂy in spring. A few extreme outliers that adversely affected the ﬁt of the model were identiﬁed by a stepwise procedure based on analyses of standardised residuals, using the GENSTAT Program (Genstat 5 Committee, 1987). These outliers were all for year classes in which part of the freshwater stage of the life cycle occurred in a summer drought, and were excluded from the ﬁnal ﬁtting of the stock–recruitment relationships (year classes given under R2 , R3 , R4 , R5 and total eggs in Table 18.1). Although the concept of a drought is well recognised, there is no general agreement on a deﬁnition. A realistic approach is to consider precipitation over a 2 or 3-month sequence (Marsh & Lees, 1985). Therefore, periods of 3 months were used: spring (March–May), summer (June–August), autumn (September–November) and winter (December–February). A severe drought was deﬁned as a season in which total precipitation was less than 50% of the mean value for all the years

Stock–Recruitment Relationships

261

Table 18.1 Stock–recruitment relationships for different life stages of sea trout in Black Brows Beck. Stage

Year classes (n)

a (SE)

b (SE)

r2

R1 : 0+ parr (May/June)

34 30

0.000394 (0.0000104) 0.000276 (0.0000126) 0.000262 (0.0000224) 0.000318 (0.0000246) 0.000356 (0.000039) 0.000350 (0.000042)

0.87∗∗∗

R2 : 0+ parr (Aug/Sept) exc. 83,84,89,95 R3 : 1+ parr (May/June) exc. 83,84,95 R4 : 1+ parr (Aug/Sept) exc. 68,83,84,94,95 R5 : 3+ females (Nov/Dec) exc. 75,82,83,88,89,92,94 Total eggs produced exc. 75,82,83,88,89,92,94

0.437 (0.0208) 0.0860 (0.00568) 0.0326 (0.00286) 0.0341 (0.00286) 0.00513 (0.000602) 5.101 (0.806)

30 28 24 24

0.72∗∗∗ 0.56∗∗∗ 0.58∗∗∗ 0.56∗∗∗ 0.47∗∗∗

Parameters a and b in Equation (18.1) are estimated by non-linear least squares; n is the number of year classes used in each analysis and year classes excluded from an analysis are given after each stage; and coefﬁcients of determination (r 2 ) with signiﬁcance levels for F -values (∗∗∗ P < 0.001).

in the study, and a less severe drought as a season in which total precipitation was less than 80% of the mean value (Elliott et al., 1997).

Results Trout density at different stages in the life cycle varied considerably between year classes (Fig. 18.1). For the duration of the study, the overall mean density of trout per 60 m2 was 317 (range = 132–460) in the ﬁrst spring of the life cycle, 90 (range = 15–127) at the end of the ﬁrst summer, 35 (range = 12–53) in the second spring, 28 (range = 2–47) at the end of the second summer and 3.5 (range = 0.5–7) for spawning females. The variation between year classes was lowest in the ﬁrst year of the life cycle (CV = 29% for R1 and 27.5% for R2 ), higher in the second year (CV = 33% for R3 and 41% for R4 ) and highest for spawning females (CV = 55% for R5 ). The Ricker stock–recruitment model (Equation (18.1)) was a signiﬁcant ﬁt (P < 0.01) to all ﬁve life stages (Fig. 18.2, Table 18.1). Two hypotheses were tested: (1) survivor density was not dependent on stock density and varied randomly around a constant value (Ho : R = mean R); (2) constant proportionate survival (p) occurred (Ho : R = pS). Both null hypotheses were rejected (P < 0.01), and it was concluded that survivor density at different stages in the life cycle was dependent on egg density at the start of each year class. These analyses, therefore, provided evidence for density-dependent population regulation of the trout in Black Brows Beck. Egg density also varied considerably between year classes (Fig. 18.3a) with an overall mean density per 60 m2 of 3655 (range 517–7958). There was a signiﬁcant density-dependent relationship between total egg production (progeny as R eggs per 60 m2 )

262

Sea Trout 0+ May/early June R1

Survivors (R fish per 60 m2)

500

400

300

200

100

R fish per 60 m2

0

67

69

71

73

75

77

79

81

83

85

87

89

91

93

95

97

99

97

99

97

99

97

99

0+ Aug/early Sept R2

120 80 40

R fish per 60 m2

60

R fish per 60 m2

0

60

67

69

71

73

75

77

79

81

83

85

87

89

91

93

95

1+ May/early June R3

40 20 0

67

69

71

73

75

77

79

81

83

85

87

89

91

93

95

1+ Aug/early Sept R4

40

R fish per 60 m2

20 0

67

69

71

73

75

77

79

81

83

85

87

89

91

93

95

Female spawners R5

8 6 4 2 0

67

69

71

73

75

77

79

81 83 85 Year class

87

89

91

93

95

97

Fig. 18.1 Density (R ﬁsh per 60 m2 ) of trout in Black Brows Beck for each year class 1967–97 as parr aged 0+ years sampled in late May or early June (R1 ) and late August or early September (R2 ); as parr aged 1+ years sampled in late May or early June (R3 ) and late August or early September (R4 ) and spawning females (R5 ).

Stock–Recruitment Relationships

263

Survivors (R fish per 60 m2)

500

0+ May/early June R1

400

300

200

100

0 0

1000

2000

3000

4000

5000

6000

7000

8000

−2

Egg density (S eggs 60 m )

Survivors (R fish per 60 m2)

140

0+ Aug/early Sept R2

120 100

1+ May/early June R3

40

80

95 89

60

20

83

40

84 20

83

95

84

0

0 0

2000

4000

6000

8000

0

2000

4000

6000

8000

7

60

Survivors (R fish per 60 m2)

60

1+ Aug/early Sept R4

Female spawners R5

6 5

40

4 3 20

94 84

94

2

68

88

95

1 92

83 0

83

75

89

0 0

2000

4000

6000

Egg density (S eggs per 60 m2)

8000

0

2000

4000

6000

82 8000

Egg density (S eggs per 60 m2)

Fig. 18.2 Relationship between egg density (S eggs per 60 m2 ) and trout density (R ﬁsh per 60 m2 ) as parr aged 0+ years sampled in late May or early June (R1 ) and late August or early September (R2 ); parr aged 1+ years sampled in late May or early June (R3 ) and late August or early September (R4 ) and spawning females (R5 ). Curves were estimated from Equation (18.1), using the parameter estimates given in Table 18.1 and the broken curves indicate 95% conﬁdence limits. Year classes are indicated for values of R2 , R3 , R4 and R5 excluded from the analyses (Table 18.1).

264

Sea Trout

Progeny (R eggs per 60 m2)

8000

(a)

6000

4000

2000

0

67

69

71

73

75

77

79

81 83 85 Year class

87

89

91

93

9000

95

97

(b) 78

8000

Line of equality (R = S )

70 71

Progeny (R eggs per 60 m2)

7000

6000

84

72 85

97 5000

95

73

91 4000

81

3000

68

87

93

76

69

96 86

94

79 2000

74

80

1000

0

77

67

90

0

1000

88

83

92

2000

3000

75

89

4000

5000

82 6000

7000

8000

Parent stock (S eggs per 60 m2) Fig. 18.3 (a) Density of progeny (R eggs per 60 m2 ) in each year class; (b) relationship between total eggs produced by each year class (progeny as R eggs per 60 m2 ) and the parent stock at the start of the year class (S eggs per 60 m2 ); curve estimated from Equation (18.1), broken curves indicate 95% conﬁdence limits, year class is given for each point which is the mean ±2 SE; parameter estimates and year classes excluded from the analyses are provided in Table 18.1. Straight broken line indicates equality (R = S) and is often termed the replacement line (RL).

Stock–Recruitment Relationships

265

and egg density at the start of each year class (Fig. 18.3b, Table 18.1). As approximately 4 years separated the two values represented by each point, many factors other than initial egg density must have affected the relationship. It is therefore remarkable that Equation (18.1) was such a good ﬁt to the (selected) data. Trout densities often lay close to or within the 95% conﬁdence limits (CL) for the curve for each life stage (broken lines for curve in Figs 18.2, 18.3b), but as trout age increased (from R1 to R5 and eggs), progressively fewer points lay close to each curve. The coefﬁcient of determination (r 2 ) indicated that variation in egg density between year classes could account for 87% of the variation in the density of 0+ parr in May or June (R1 in Table 18.1). Subsequent values were, however, lower: 72% for 0+ parr in August or September (R2 ), 56% for 1+ parr in May or June (R3 ), 58% for 1+ parr in August or September (R4 ), 56% for spawning females (R5 ) and 47% for the total number of eggs produced. Winter droughts had no signiﬁcant effects on trout density, and spring and autumn droughts were important only when they preceded or followed a summer drought. There were seven summer droughts in the 35 years of this study: severe droughts in 1976, 1983, 1984 (also a severe spring drought) and 1995, and less severe droughts in summer but followed by autumn droughts in 1969, 1989, 1993. The ﬁrst life stage was unaffected by any droughts (R1 in Fig. 18.2) and drought effects on other life stages were very variable (R2 –R5 in Fig. 18.2, eggs in Fig. 18.3b). The 1969 drought continued with an autumn drought and reduced the density of 1+ parr (1968 year class), but appeared to have no longterm effects on spawning female, or egg, densities. The 1983, 1984 (plus spring drought) and 1989 droughts reduced 0+ (1983, 1984, 1989 year classes) and 1+ (1983 year class) parr densities, and led to low densities of returning females and their eggs from the 1982, 1983, 1988 and 1989 year classes. The 1976 summer drought and 1993 summer and autumn droughts had little effect on parr densities, but in the long term produced low densities of returning females and their eggs from the 1975 and 1992 year classes. The 1995 drought reduced 0+ (1995 year class) and 1+ (1994 year class) parr densities, and the female and egg densities from the 1994, but not the 1995, year class. Therefore, long-term effects of droughts on female and egg densities were most marked for seven year classes: 1975 (1976 drought), 1982, 1983 (1983 and 1984 droughts), 1988, 1989 (both 1989 drought), 1992 (1993 drought) and 1994 (1995 drought). In most of these year classes, the losses were associated with the 1+ parr being subjected to a drought. This was the most vulnerable life stage, rather than the 0+ parr. Loss rates across life stages were compared by ‘key-factor analysis’ in which population density was expressed on a logarithmic scale so that the total loss rate was the sum of the loss rates between successive life stages (see Elliott, 1994). Actual values for total loss rates (K) between the egg stage (S) and 3+ female spawners (R5 ) were calculated directly from the data, assuming a sex ratio close of 1 : 1 (K = ln(0.5 S/R5 )). This assumption was justiﬁed by the catches of 216 adults taken during the study. Estimated values of K were calculated from a simple model for loss rates across all intermediate life stages (k1 to k5 in Fig. 18.4): K = k1 + k2 + k3 + k4 + k5

(18.2a)

266

Sea Trout

Loss rates in the ﬁrst spring of the life cycle (k1 ) provided the highest contribution to K in most year classes. There was a positive relationship between k1 and initial egg density (S), and a negative relationship between k2 and k1 , but no similar relationships were obtained for k3 , k4 and k5 . These latter values remained fairly constant apart from some year classes affected by summer droughts (Fig. 18.4). Therefore, using mean values for k3 , k4 and k5 (k3 + k4 + k5 + 2.64) and the linear regressions between k2 and k1 (k2 = 1.88 − 0.28 k1 ), and between k1 and S (k1 = 0.85 + 0.000395S), Equation (18.2a) was simpliﬁed to: K = a + bS

(18.2b)

where a = 5.129 and b = 0.000283. Most estimated values of K from this simple model (× in Fig. 18.4) were very similar to the actual values ( in Fig. 18.4), the exceptions being associated with the summer droughts. Therefore, population density throughout the life cycle was regulated by density-dependent survival in the early life stages, the key factor being the spring losses soon after the young trout emerged from the gravel nest and started to feed.

Discussion Assumptions, implications and limitations of stock–recruitment models for predicting spawning targets It may be assumed that any persistent salmonid population must be regulated by negative density-dependent factors and that the signiﬁcant ﬁt of a stock–recruitment model provides strong evidence for this regulation. Whilst such regulation is most frequently found in the early life stages, it may also occur during spawning, but will probably never account for more than about half of the inter-generation variation in recruitment, the remaining variation being attributable to positive density-dependent and density-independent factors (Elliott, 1994). A stock–recruitment model should not be used blindly without ﬁrst testing a suite of models. For example, Ricker-type models will describe a wide range of dynamics, including cyclic and chaotic behaviour, whereas other models such as a power function (e.g. Cushing, 1969) or asymptotic type (e.g. Beverton & Holt, 1957) cannot do this. The choice of model may also affect derived relationships, for example smolt–fry relationships (Elliott, 1996). It is important to recognise the limitations of stock–recruitment models for predicting spawning targets. Anadromous salmonids are essentially K-strategists and therefore negative density-dependent factors are more likely to exercise stronger regulation than in many freshwater and marine ﬁsh that are r-strategists. Three types of two-parameter models commonly used in stock–recruitment relationships were compared: the Ricker model, the asymptotic model of Beverton & Holt (1957) and the power-law model of Cushing (1969) (Fig. 18.5). Equations for the two latter models can be found in Elliott (1994). The two-parameter Ricker curve was an excellent ﬁt to most of the data from Black Brows Beck, with 47% of the variability in egg production between year classes being explained by variation in initial egg density (Fig. 18.5a). The relationship can be also described by rewriting Equation (18.1) in terms of the equilibrium density for replacement (S*), that is the value at which the number of eggs produced equals the initial

Stock–Recruitment Relationships Total loss rate

8

Loss rate

267

6

K

4 2nd winter and sea 2

k5 2nd summer

k4 0 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99

Loss rate

1st winter 2

k3

1 0

67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99

Loss rate

4 3

1st summer

2

k2

1 0

67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99

5

Loss rate

4

1st spring

3 2

k1

1 0

67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 Year class

Fig. 18.4 Relationship of various loss rates (k1 to k5 ) to the total loss rate (K) for the decrease in numbers between the egg stage (0.5 S eggs per 60 m2 ) and 3+ female spawners (R5 ﬁsh per 60 m2 ) in the different year classes; loss rates were between: egg and 0+ parr stage in May/June (k1 = ln(0.5S/0.5R1 ) = ln(S/R1 )); 0+ parr stages in May/June and August/September (k2 = ln(R1 /R2 )); 0+ parr stage in August/September and 1+ parr stage in May/June (k3 = ln(R2 /R3 )); 1+ parr stages in May/June and August/September (k4 = ln(R3 /R4 )); and 1+ parr stage in August/September and 3+ females in November/December (k5 = ln(0.5R4 /R5 )); for total loss rates (K), actual values () are compared with values (x) estimated from Equation (18.2b).

268

Sea Trout

Progeny (R eggs per 60 m2)

Ricker R=S 6000 RMAX

S * = 4656 RMAX = 5362 SMR = 2857 RMSY = 3090 SMSY = 1804

S* 4000

RMSY

2000

0 0

SMSY

2000

SMR

4000

6000

8000 Beverton and Holt

Progeny (R eggs per 60 m2)

(a)

8000

(b)

8000

6000

S * = 4570 RMAX = 4836 SMR = infinity RMSY = 2833 SMSY = 868

4000

2000

0

0

2000

4000

6000

8000

Progeny (R eggs per 60 m2)

Cushing (c) 8000

6000 S * = 4353 RMAX = infinity SMR = infinity RMSY = c.4000 SMSY = c.300

4000

2000

0

0

2000 4000 6000 Parent stock (S eggs per 60 m2)

8000

Fig. 18.5 Three stock–recruitment models ﬁtted to the data for the relationship between total eggs produced by each year class (progeny as R eggs per 60 m2 ) and the parent stock at the start of the year class (S eggs per 60 m2 ): (a) Ricker curve; (b) Beverton and Holt curve and (c) Cushing curve. For each curve, values are given for the equilibrium density for replacement (S∗ where R = S), the density at maximum recruitment (RMAX ), the parent stock density (SMR ) providing the density at maximum recruitment, the maximum surplus yield of recruits (RMSY ) and the parent stock density (SMSY ) providing the maximum surplus yield of recruits. Straight broken line indicates equality (R = S).

Stock–Recruitment Relationships

269

density at the start of a year class (S ∗ = S = R): R = S exp[r(1 − S/S ∗ )]

(18.3)

where r = ln a and S ∗ = (ln a)/b. The parameter r serves as an index of long-term population stability because the slope of the curve in the neighbourhood of S ∗ is given by 1 − ln a = 1 − r. The equilibrium point (S ∗ ) is stable for r < 2, there are stable cycles of different periods for r > 2 but <2.692, and chaotic behaviour for r > 2.692 (see reviews by May & Oster, 1976; May, 1981). Using the same data as those used to ﬁt Equation (18.1), S ∗ was 4656 eggs per 60 m2 and r was 1.63, indicating that the equilibrium density was stable. The stock density (SMR ) for maximum egg production (RMAX ) was 2857 eggs per 60 m2 and the stock density (SMSY ) for maximum surplus yield of recruits above the initial stock density (RMSY ) was 1804 eggs per 60 m2 (Fig. 18.5a). The Beverton–Holt model produced a slightly lower value for S∗but RMAX must be higher than S ∗ in this model and occurs at an inﬁnitely high stock density (Fig. 18.5b). The value of SMSY was much lower than that for the Ricker curve. The Cushing model produced an even lower value of S ∗ with the values of both RMAX and SMR at inﬁnity and a very low value for SMSY (Fig. 18.5c). Note also the very different shapes for the three relationships. Of the two null hypotheses mentioned earlier, the second (R = pS) was rejected for all three models, but the ﬁrst (R varies randomly around a mean value) was rejected only for the Ricker model. This means that the other two models were no better than ﬁtting a horizontal line with a value of mean R through the data. Such a relationship provides only one value for target prediction and this will be close to the value for S ∗ in the third model. Therefore, the Beverton–Holt and Cushing models could be used to estimate S ∗ with similar values, but their use to estimate SMSY would be erroneous with dangerously low values. The Ricker model is a better ﬁt and this two-parameter model is the simplest of a large family of such models that offer great ﬂexibility (see Schnute, 1985; Elliott, 1994). For example, Jonsson et al. (1998) ﬁtted the three-parameter model of Shepherd (1982) to their data on juvenile Atlantic salmon. Most studies mentioned in the Chapter Introduction used the Ricker curve to model density-dependent relationships. Therefore, it is worth considering further the use of the Ricker model to determine three possible targets for the sea trout population in Black Brows Beck (Fig. 18.5a): S ∗ , SMR , SMSY . The lowest target (SMSY ) is the minimum acceptable level (Prévost & Chaput, 2001; Milner et al., 2003), but estimating such a value from a stock–recruitment model assumes that all variation in recruitment is because of density dependence. As noted earlier, this is clearly incorrect; the density-dependent model will never account for more than about half of the inter-generation variation in recruitment when the latter is measured in terms of egg production (e.g. 47% in Black Brows Beck). As environmental conditions deteriorate, and stock densities decrease, it may be tempting to set SMSY at progressively lower values, but such a procedure would be a recipe for disaster. Setting a target at S ∗ would be ideal from the point of view of population stability, but would restrict a ﬁshery to a low catch level, for example this value was exceeded in only nine out of the 31 year classes (Figs 18.3b, 18.5a). Note that as S ∗ is the equilibrium density, it does not allow for any surplus yield. A compromise is SMR which would probably buffer the effects of density-independent factors, the

270

Sea Trout Table 18.2 Estimates of the mean density of eggs (per 100 m2 ) for the equilibrium density (S∗ ), maximum recruitment (SMR ) and maximum surplus yield of recruits (SMSY ) at different spatial scales: (1) within the spawning area of Black Brows Beck (150 m2 = 37% of total stream area); (2) for the whole of Black Brows Beck (410 m2 with length = 512 m and mean width = 0.8 m) and (3) for the whole river system (∼20 000 m2 with spawning area of approximately 3000 m2 = 15% of total stream area); and estimates of the total number of females returning to the river for each spawning target, with separate values for early, middle and late spawners. Mean number of eggs (±2 SEs) and length ranges for spawning females: early, 1553 (±36) and 401–449 mm; middle, 1034 (±27) and 300–398 mm; late, 722 (±24) and 241–336 mm. S* eggs per 100 m2

SMR eggs per 100 m2

SMSY eggs per 100 m2

Spawning area

7760

4762

3007

Whole stream

2839

1742

1100

Whole river

1164

714

451

150

92

58

Middle

225

138

87

Late

322

198

125

Spawning females Early

latter accounting for at least half of the inter-generation variability. One additional property of the model is that the stability index r in Equation (18.3) is also equal to the ratio S ∗ /SMR , and therefore an increase in this ratio because of a change in the value of S ∗ or SMR would convert an essentially stable population to one that exhibited cyclic or even chaotic dynamics. Another problem in predicting spawning targets, the area of stream or river used in the estimates, is illustrated by the data for Black Brows Beck. To facilitate comparisons, values for parent stock were converted to the number of eggs per 100 m2 for the spawning area of the stream where the original estimates were made. Values for S ∗ , SMR and SMSY were 7760, 4762, 3007 respectively (Table 18.2). Observations over 35 years indicated that the maximum spawning area was 150 m2 or 37% of the total stream area of 410 m2 . Therefore, when the whole of Black Brows Beck was considered and not just the spawning area, the mean density of eggs per 100 m2 for the three targets decreased to 37% of the original values. Finally, when the whole river system was considered, surveys on ten occasions showed that the spawning area used by sea trout throughout the catchment was about 15% of the total stream area. Therefore, the mean density of eggs per 100 m2 for the whole river system with an area of approximately 20 000 m2 was much lower than that for Black Brows Beck. For the sea trout population of the whole river, the equilibrium density for replacement abundance was about 12 eggs per m2 , with about 7 eggs per m2 for maximum recruitment and about 4.5 eggs per m2 for maximum surplus yield. It is

Stock–Recruitment Relationships

271

estimated that it would require 150 early female spawners or 322 late spawners to attain the equilibrium density target. In reality, a mixture of early, middle and late spawners would be returning to the river in most years. For example, if equal numbers of early, middle and late spawners returned to the river, then the total number of females required to meet the spawning targets would be 211 for S ∗ , 129 for SMR and 82 for SMSY . All these estimates assume that the egg densities in Black Brows Beck were typical of those occurring in other spawning areas throughout the river system. This assumption was not checked. Therefore, the different spawning targets presented in Table 18.2 illustrate the importance of spatial scale in choosing a suitable value for each of the three targets, but the values for the whole river system must be treated with caution in the absence of information on egg densities in other spawning areas. Inter-generation ﬂuctuations in density as a time series The average generation time between year classes was 4 years for egg production because most females were just under 4 years when they spawned. Genetic isolation between year classes was prevented by the presence of spawning males from two age groups. The length of this study allowed inter-generation ﬂuctuations to be examined for successive generations in spite of the long generation time (Fig. 18.6). The 1967, 1968, 1969 and 1970 year classes were the starting points for each time series and the Ricker model enabled predictions to be made into the future. For example, the series covering 32 years from the 1967 year class started near the equilibrium density before being perturbed by the 1976 drought affecting the 1+ ﬁsh of the 1975 year class. Because of other droughts, it took until 1995 before this series attained the equilibrium density (Fig. 18.6a). Perturbations for the other time series were also chieﬂy attributable to drought effects, with an eventual return to values close to the equilibrium density, apart from the cyclic pattern in the ﬁnal series starting with the 1970 year class (Fig. 18.6d). In the absence of perturbations, the population would remain fairly stable, as indicated by the predicted values from the Ricker model (broken lines in Fig. 18.6). However, perturbations such as droughts are a natural feature of the environment and their effects can produce very different ﬂuctuations in density, as illustrated by the population in Black Brows Beck. The most striking feature of these comparisons is the markedly different patterns shown by the observed values for the four series. The equilibrium density provides a measure of the average carrying capacity of the stream, measured in terms of egg production. Carrying capacity is not a constant value over time, but will vary, depending on annual and seasonal changes in environmental conditions. The maximum carrying capacity is probably closer to the equilibrium density than the minimum carrying capacity, below which the population would soon be extinct. There is strong evidence for the assumption of a constant equilibrium density for this population, but there may be other populations in which the equilibrium density decreases over time. In a hypothetical example, the initial stock was 8000 eggs per m2 , the maximum density recorded in this study, and the Ricker model was used to predict inter-generation ﬂuctuations in density with a constant equilibrium density (Fig. 18.7a). After six generations and 24 years, the population eventually returned to the equilibrium density. In the second example, the

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Fig. 18.6 Long-term changes in egg density (S eggs per 60 m2 ) for successive generations (generation time = 4 years), starting with year classes: (a) 1967; (b) 1968; (c) 1969 and (d) 1970 year class. Horizontal line is the equilibrium density (S∗ = 4656 eggs); solid line links actual values and broken line links simulated values.

Stock–Recruitment Relationships 8000

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Environment Fig. 18.7 Simulations of long-term changes in parent stock (S eggs per m2 ) for successive generations; generation time is 4 years and the broken line is the equilibrium density (S∗ ): (a) return to S∗ from an initial density higher than S∗ ; (b) effects of a long-term trend with S∗ decreasing by 10% per generation (note that initial density is the same as in (a)). (c) Hypothetical relationship for the proportional effect of density-dependent factors on population density in relation to population stability and environmental changes from harsh to benign.

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equilibrium density decreased by 10% per generation (Fig. 18.7b). The population density was often higher than the equilibrium density and therefore it could be assumed erroneously that there were no problems. The population was still present after nine generations and 36 years, but this population would eventually go extinct unless something could be done to increase the equilibrium density. There are many possible reasons for such a decrease in equilibrium density, and the most obvious are: a decrease in the carrying capacity of the stream, an increase in mortality at sea with fewer returning females, an increase in mortality in fresh water with fewer smolts migrating to the sea, and a decrease in female fecundity, often associated with a decrease in female size. The gradual decrease in equilibrium density would be very difﬁcult to detect in a wild population unless long-term data were available. Unfortunately, such data are rare for most salmonid populations. An additional problem is the paucity of knowledge on the population dynamics of sea trout at sea. In Black Brows Beck, the number of returning females was directly proportional to estimates of the number of smolt emigrants, indicating no density dependence at sea (Elliott, 1994). This direct relationship also supports the earlier assumption that the spawning adults were returning to their natal stream. Similarly, observations on Atlantic salmon in the River Imsa, Norway, showed that marine survival was density independent (Jonsson et al., 1998). This lack of density dependence may be because the carrying capacity of the marine habitat is not limiting for anadromous salmonids. More research is required on marine population dynamics of sea trout to establish when, where and how mortality occurs (Milner et al., 2003). Although migration may increase the risk of predation, there are advantages in the richer food supply at sea and hence the faster growth and larger adult size, especially for females which have a much higher fecundity and larger eggs than resident-trout females (Elliott, 1994). Juveniles also beneﬁt. A migratory strategy ensures that larger trout do not remain in fresh water to use valuable resources such as food and space. Therefore, juveniles dominate trout biomass and production in fresh water. In contrast, it is older, larger trout that form most of the biomass in a resident-trout population, whilst contributing little to annual production (Elliott, 1994). Finally, it must be emphasised that the data from Black Brows Beck were for a relatively stable population in a relatively benign environment. Negative density-dependent relationships accounted for 47% of the inter-generation variation in recruitment, the remaining variation being a result of positive density-dependent factors (e.g. predation, disease) and density-independent factors (e.g. environmental factors). As the environment becomes less benign and the population less stable, the importance of density-dependent factors will decrease (hypothetical curve in Fig. 18.7c). In the least favourable areas with harsh environments, density-dependent factors probably account for a small amount of the variations in population density. These populations will be inherently unstable with large variations in density between year classes, and may be expected to have much lower densities than more stable populations in benign environments. However, it is possible to have a stable, low density population in a benign environment with a small carrying capacity, for example a small trout stream with little temporal variation in environmental variables. Stable populations would be expected to be more common in pristine habitats, and habitat-based models for such habitats indicate that about 30–45% of the variability in

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juvenile salmonid density could be attributed to density-independent factors (Elliott, 2001; Armstrong et al., 2003). This range complements that of no more than 50% for densitydependent factors. The remaining variation in population density could be a result of variations in ﬁsh growth, for example 17% of the variation in production between year classes was attributed to variation in growth in Black Brows Beck (Elliott, 1994). Therefore, 47% of the inter-generation variation in recruitment in Back Brows Beck was explained by density-dependent mortality in the early life stages, 17% could be explained by variations in growth and the remaining 36% was probably because of density-independent factors, chieﬂy environmental factors affecting carrying capacity. Haldane (1956) hypothesised that changes in population density will be largely because of negative density-dependent factors in favourable areas where population densities are fairly stable, and both positive density-dependent and density-independent factors in unfavourable areas where population densities are highly variable and therefore unstable, and that natural selection will be for different genotypes in the two types of habitat. Clearly, most populations will lie somewhere between these two extremes. One important result of this hypothesis is that isolated populations of the same species should be genetically different with genotypes that are best suited to the long-term characteristics of their particular habitat. Genetic evidence is accumulating that the brown trout, including sea trout, occurs as genetically distinct populations or ‘stocks’ (Youngson et al., 2003). It is therefore probable that trout in stable populations in more benign environments have been selected for traits linked to density-dependent mechanisms, whilst trout in unstable populations in harsh environments have been selected for traits related to the effects of environmental factors. It is impossible to ﬁt predictive stock–recruitment models which do not account for environmental effects to these latter populations. One possibility would be to use habitat-based models (Armstrong et al., 2003) to predict targets, especially as density-independent factors will predominate, but such methods have yet to be tested for spawning targets in sea trout.

General conclusions This review has illustrated the value of long-term studies. Perhaps the simplest justiﬁcation for such studies is that they prevent ill-considered conclusions being drawn from short-term studies! The analysis of the data from Black Brows Beck has shown that the population is self-regulated through density-dependent mechanisms, the key factor being the early life stage when the young fry emerge from the gravel nest and start to feed. More detailed studies during this critical period for survival have shown that the acquisition of a feeding territory is essential for survival (Elliott, 1994) and that shadow competition occurs within groups of territorial fry (Elliott, 2002). This study also showed how long-term data can be used to answer more general questions. The data from Black Brows Beck were used to explore the strengths and weaknesses of different stock–recruitment models. Dome-shaped models of the Ricker type are much more versatile than power-function or asymptotic models, especially when estimates of spawning targets are required. It was also possible to illustrate the problems of estimating spawning targets for a whole river system and the importance of spatial scale in making such estimates.

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The length of the study facilitated for the ﬁrst time a time-series analysis of intergeneration ﬂuctuations in density. This revealed marked differences in the patterns shown by the four series. Once a model has been ﬁtted, it can be used to answer ‘what if’ scenarios. A hypothetical example illustrated the difﬁculty of detecting a gradual decrease in the equilibrium density, leading eventually to population extinction. Finally, caution is advised before applying the results from Black Brows Beck to other sea trout populations. Although the data are for a relatively stable population in a relatively benign environment, negative density-dependent relationships accounted for only about half of the inter-generation variability in recruitment. Density-dependent factors may be less important in less-benign environments, and probably account for a small amount of the variations in density for a population in a harsh environment. It is unreasonable to use stock–recruitment models to predict spawning targets for these inherently unstable populations, unless they take account of environmental factors. Such populations may become more frequent if the environmental conditions for sea trout are allowed to deteriorate in the future. Alternative approaches may have to be used, such as those derived from habitat-based models, especially as density-independent factors will predominate.

Acknowledgements Parts of this work were ﬁnanced by the Freshwater Biological Association, Natural Environment Research Council, Ministry of Agriculture, Fisheries and Food, Atlantic Salmon Trust and the Environment Agency (North-West Region). The authors are grateful to all those who assisted with the sampling, especially Derek Allonby, Toby Carrick, Judith Elliott and Paula Tullett.

References Achord, S., Levin, P.S. & Zabel, R.W. (2003). Density-dependent mortality in Paciﬁc salmon: the ghost of impacts past? Ecology Letters, 6, 335–42. Armstrong, J.D., Kemp, P.S., Kennedy, G.J.A., Ladle, M. & Milner, N.J. (2003). Habitat requirements of Atlantic salmon and brown trout in rivers and streams. Fisheries Research, 62, 143–70. Beverton, R.J.H. and Holt, S.J. (1957). On the dynamics of exploited ﬁsh populations. Fishery Investigations, London Series 2, 19, 1–533. Cushing, D.H. (1969). The ﬂuctuation of year classes and the regulation of ﬁsheries. FiskDir. Skr. Serie Havundersøkelser, 15, 368–79. Dumas, J. & Prouzet, P. (2003). Variability of demographic parameters and population dynamics of Atlantic salmon (Salmo salar L.) in a southwest French river. ICES Journal of Marine Science, 60, 356–70. Elliott, J.M. (1984). Numerical changes and population regulation in young migratory trout Salmo trutta in a Lake District stream, 1966–83. Journal of Animal Ecology, 53, 327–50. Elliott, J.M. (1985). Population regulation for different life-stages of migratory trout Salmo trutta in a Lake District stream, 1966–83. Journal of Animal Ecology, 54, 617–38. Elliott, J.M. (1986). Spatial distribution and behavioural movements of migratory trout Salmo trutta in a Lake District stream. Journal of Animal Ecology, 55, 907–22.

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Elliott, J.M. (1990). The need for long-term investigations in ecology and the contribution of the Freshwater Biological Association. Freshwater Biology, 23, 1–5. Elliott, J.M. (1994). Quantitative Ecology and the Brown Trout. Oxford University Press, Oxford, xi + 286 pp. Elliott, J.M. (1995). Fecundity and egg density in the redd for sea-trout. Journal of Fish Biology, 47, 893–901. Elliott, J.M. (1996). The relationship between smolt density and fry density in salmonids. Journal of Fish Biology, 48, 1030–32. Elliott, J.M. (1997). An experimental study on the natural removal of dead trout fry in a Lake District stream. Journal of Fish Biology, 50, 870–77. Elliott, J.M. (2001). The relative role of density in the stock–recruitment relationship of salmonids. In: Stock, Recruitment and Reference Points. Assessment and Management of Atlantic Salmon (Prévost, E. & Chaput, G., Eds). INRA, Paris, pp. 25–66. Elliott, J.M. (2002). Shadow competition in wild juvenile sea-trout. Journal of Fish Biology, 61, 1268–81. Elliott, J.M. & Hurley, M.A. (1998a). An individual-based model for predicting the emergence period of sea-trout fry in a Lake District stream. Journal of Fish Biology, 53, 414–33. Elliott, J.M. & Hurley, M.A. (1998b). Predicting ﬂuctuations in the size of newly emerged sea-trout fry in a Lake District stream. Journal of Fish Biology, 53, 1120–33. Elliott, J.M., Hurley, M.A. & Elliott, J.A. (1997). Variable effects of droughts on the density of a sea-trout Salmo trutta population over 30 years. Journal of Applied Ecology, 34, 1229–38. Elliott, J.M., Hurley, M.A. & Maberly, S.C. (2000). The emergence period of sea trout fry in a Lake District stream correlates with the North Atlantic Oscillation. Journal of Fish Biology, 56, 208–10. Genstat 5 Committee (1987). GENSTAT 5 Reference Manual. Oxford University Press, Oxford. Haldane, J.B.S. (1956). The relation between density regulation and natural selection. Proceedings of the Royal Society B, 145, 306–8. Hilborn, R. & Walters, C.J. (1992). Quantitative Fisheries Stock Assessment: Choice, Dynamics and Uncertainty. Chapman & Hall, New York, 570 pp. Jonsson, N., Jonsson, B. & Hansen, L.P. (1998). The relative role of density-dependent and density-independent survival in the life cycle of Atlantic salmon Salmo salar. Journal of Animal Ecology, 67, 751–62. Kennedy, G.J.A. & Crozier, W.W. (1993). Juvenile Atlantic salmon (Salmo salar) – production and prediction. In: Production of Juvenile Atlantic Salmon, Salmo salar, in Natural Waters (Gibson, R.J. & Cutting, R.E., Eds). Canadian Special Publication of Fisheries and Aquatic Sciences, 118, 179–87. Kennedy, G.J.A. & Crozier, W.W. (1995). Factors affecting recruitment success in salmonids. In: The Ecological Basis for River Management (Harper, D.M. & Ferguson, A.J.D., Eds). Wiley, Chichester, pp. 349–62. Klemetsen, A., Amundsen, P.A., Dempson, J.B. et al. (2003). Atlantic salmon Salmo salar L., brown trout Salmo trutta L. and Arctic charr Salvelinus alpinus (L.): a review of aspects of their life histories. Ecology of Freshwater Fish, 12, 1–59. Likens, G.E. (1989). Long-Term Studies in Ecology: Approaches and Alternatives. Springer-Verlag, New York, 230 pp. Marsh, T.J. & Lees, M.L. (1985). The 1984 Drought. Institute of Hydrology, Wallingford. May, R.M. (1981). Models for single populations. In: Theoretical Ecology: Principles and Applications s. 2nd edn. (May, R.M., Ed.). Blackwell Scientiﬁc Publications, Oxford, pp. 5–29. May, R.M. & Oster, G.F. (1976). Bifurcations and dynamic complexity in simple ecological models. American Naturalist, 110, 573–99. Milner, N.J., Elliott, J.M., Armstrong, J.D., Gardiner, R., Welton, J.S. & Ladle, M. (2003). The natural control of salmon and trout populations in streams. Fisheries Research, 62, 111–25. Prévost, E. & Chaput, G. (2001). Stock, Recruitment and Reference Points, Assessment and Management of Atlantic Salmon. INRA, Paris, 223 pp. Ricker, W.E. (1954). Stock and recruitment. Journal of the Fisheries Research Board of Canada, 11, 559–623. Schnute, J. (1985). A general theory for analysis of catch and effort data. Canadian Journal of Fisheries and Aquatic Sciences, 42, 414–29. Shepherd, J.G. (1982). A versatile new stock–recruitment relationship for ﬁsheries, and the construction of sustainable yield curves. Journal du Conseil. Conseil Permanent International pour l’Exploration de la Mer, 40, 67–75.

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Sinclair, A.E.G. (1989). The regulation of animal populations. In: Ecological Concepts (Cherrett, M., Ed.). British Ecological Society Symposium. Blackwell Scientiﬁc Publications, Oxford, pp. 197–241. Youngson, A.F., Jordan, W.C., Verspoor, E., McGinnity, P., Cross, T. & Ferguson, A. (2003). Management of salmonid ﬁsheries in the British Isles: towards a practical approach based on population genetics. Fisheries Research, 62, 193–209.

Chapter 19

Characteristics of the Burrishoole Sea Trout Population: Census, Marine Survival, Enhancement and Stock–Recruitment Relationship, 1971–2003 W.R. Poole, M. Dillane, E. DeEyto, G. Rogan, P. McGinnity and K. Whelan Aquaculture & Catchment Management Services, Marine Institute, Newport, Co. Mayo, Ireland Abstract: This chapter examines the characteristics of the Burrishoole sea trout, Salmo trutta L., population, which has been monitored through ﬁsh trapping since 1958. These are the only available data that allow a fully quantitative examination of a sea trout population in a single Irish catchment. The Burrishoole system is located in the mid-west where a severe sea trout population collapse was evident between 1988 and 1990. The percentage of smolts that returned as ﬁnnock in the same year ranged from 11.4% to 32.4% over the period 1971 to 1987 with a mean of 21%. In 1988, this return rate decreased to 8.5% and in 1989 to 1.5%. This was followed by ﬁnnock return rates ﬂuctuating around a mean of 6.8% until 1999, when it rose to 16.7% – the highest rate since 1986. Returns of older searun ﬁsh followed the same pattern. The total migratory trout stock included silvered and unsilvered migrants, from which the estimated number of ova deposited annually ranged between 27.5 thousand to 1.61 million. Recruitment to the sea of downstream migrants from these ova was determined by trapping 0+ and 1+ autumn migrating juveniles and 2+ and 3+ spring migrating smolts. Total recruitment (four year classes) per annual spawning cohort ranged from 1157 to 8457 and smolt output from 632 to 5813. The 1989 spawning stock collapse signiﬁcantly reduced both the total number of ova deposited and subsequent levels of recruitment. These observed changes in the structure of the sea trout population and the reduction in survival suggested that this problem was related to marine conditions. Monitoring of these events facilitated the testing of the stock–recruitment relationship for the ﬁrst time in a lacustrine sea trout population undergoing wide ﬂuctuations in abundance, and provides some insight into the relationship between anadromous and resident brown trout populations. Keywords: Sea trout, Salmo trutta L., Burrishoole, marine survival, population, stock, recruitment, enhancement.

Introduction In Ireland, sea trout (Salmo trutta L.) has long been a highly valued angling species, inhabiting many river and lake systems around the coast where there is access to the sea. 279

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Sea trout tend to inhabit acidic nutrient-poor fresh waters where growth rates are slow and survival is a challenge (Fahy, 1985). It is well known that sea trout stocks ﬂuctuate and that recruitment to and from the marine environment is regulated by a number of interacting environmental parameters (Whelan, 1991; Byrne et al., 2004). A decline in sea trout catches in the mid-western region of Ireland (from Galway City to Achill Head) in the late 1980s culminated in a collapse in rod catch in 1989 and 1990 in all major sea trout ﬁsheries in the region. This was subsequently conﬁrmed as reﬂecting a collapse in the spawning stocks (Anon, 1994; Poole et al., 1996; Gargan et al., 2006). Previous studies of the sea trout population in the Burrishoole system have examined age and growth (Piggins, 1961), stock production, survival rates, life history (Piggins, 1975, 1984) and ova production (O’Flynn, 1988). There has been a complete population census since 1970 (Mills et al., 1990; Poole et al., 1996), and these data allow a full examination of the ﬂuctuations and collapse of the Burrishoole sea trout stock over a 33-year period. One goal of ﬁsheries management is the determination of the relationship between stock and recruitment (Hilborn & Walters, 1992). There is strong evidence that trout and salmon Salmo salar L. populations are regulated by density-dependent mortality during the freshwater stages (Ricker, 1954; Beverton & Holt, 1957; Elliott 1984a, b, 1985a; Solomon, 1985). However, few studies have described the relationships between stock and recruitment, and most of these involve Atlantic salmon (i.e. Gee et al., 1978; Buck & Hay, 1984; Gardiner & Shackley, 1991). With the exception of the comprehensive study on migratory trout in Black Brows Beck (see Elliott, 1994; Elliot & Elliot, 2006), stock–recruitment relationships for brown trout (Salmo trutta L.) populations that include the sea-run form remain virtually unexamined. The complexity of life-history patterns of S. trutta, their aptitude for multiple spawning and, in particular, the lack of understanding regarding the relationships between resident and anadromous brown trout stocks within the same catchment, have hampered attempts to establish realistic stock–recruitment relationships. This chapter provides an update of Poole et al. (1996), examines the symptoms of the sea trout stock collapse and its effects on the population though the relationship between spawning escapement and subsequent recruitment of juveniles in a population of cohabiting resident and anadromous trout from a largely lacustrine catchment.

Study site and methods Study site The Burrishoole system (53◦ 57 N 009◦ 35 W) occupies a valley of glacial origin in the Nephin Beg mountain range and is drained by at least 45 km of shallow streams which are oligotrophic and poorly buffered (Whelan et al., 1998). Approximately 30% of the catchment is under active coniferous forestry management. The geology is principally composed of Dalradian schists and quartzites, with some outcropping of sandstone and limestone (Parker, 1977). These streams discharge to the north-east corner of Clew Bay, on the mid-west coast of Ireland, through a chain of three main lakes: Lough Furnace (brackish lake, 172 ha), and two freshwater lakes, Lough Feeagh (410 ha) and Bunaveela Lough (46 ha) (Fig. 19.1). Bunaveela Lough, at the northern upper end of the

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catchment, has a maximum depth of 23 m and lies at an altitude of 150 m. pH values range from 6.1 to 6.9 and conductivity values are around 60–120 μS/cm. Bunaveela L. is accessible to migrating salmonids, salmon and sea trout, and contains resident brown trout, eel (Anguilla anguilla L.) and charr (Salvelinus alpinus L.). Lough Feeagh has a maximum depth of 43 m, lies at an altitude of 14 m and has water which is soft and distinctly coloured, though transparency is only moderate. pH values range from 6.3 to 6.9 and conductivity values are around 80–90 μS/cm. Surface water temperature is generally within the range 2–20◦ C. Lough Feeagh contains salmon, resident and migratory trout, eel and three-spined stickleback (Gasterosteus aculeatus L.). Lough Furnace lies at the southern lower end of the catchment in a cryptodepression bounded by moraine and drumlin countryside, has an area of 141 ha, a maximum depth

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of 21.5 m and is tidal. The Lough may be considered as an oligotrophic lake, typical in many respects of other bog lakes in the nature of its surface waters, but radically different in the deeper areas as a consequence of meromictic stratiﬁcation attributable to high salinity levels (Parker, 1977). This results in the development of a thermocline and permanently anoxic conditions in the deeper areas. L. Furnace supports salmon, trout, eel, three-spined stickleback, ﬂounder (Platichthys ﬂesus L.), grey mullet (Chelon labrosus Risso) and other incidental estuarine and marine species. Trout stock Brown trout occur as freshwater resident and anadromous forms in the Burrishoole system, where sea trout smolts run to sea from March to June and downstream migrations of 0+ and 1+ unsilvered trout also occur in autumn and early winter. Some ﬁsh return to fresh water in the same summer as their ﬁrst migration to sea, known in Ireland as ﬁnnock. A further (proportionately small) component may not return to the natal river for at least 1 year. In addition to the true sea trout, some migratory ﬁsh return as unsilvered adults, or ‘slob’ trout. Trout migrate back to sea after the winter, both as spawned kelts and ﬁsh that have over-wintered without spawning, and may return to spawn in successive years. Enhancement Between 1962 and 1988, the Salmon Research Trust of Ireland (SRTI) carried out sea trout enhancement, releasing nearly 27 000 reared sea trout ‘smolts’ into the Burrishoole system (Mills et al., 1985, 1990). Freshwater rearing was often problematic and, in some years, the entire stock was lost during the early stages. Various river stocks were used, including Swedish, Polish and Welsh sea trout. Following the stock collapse in the late 1980s, a programme of sea trout kelt reconditioning and juvenile enhancement using only Burrishoole stock was initiated by the Salmon Research Agency (Poole et al., 1994, 2002). This involved the release of 50 144 adipose clipped 1+ parr into L. Feeagh between 1993 and 1998. Of these ﬁsh, 15 697 were microtagged and 34 447 had either elastomer or alphanumeric visible implant tags. Tag loss rates at release varied from 3% to 5.7% (Byrne et al., 2002). Fish census Fish trapping has taken place in Burrishoole since 1958, with a full census of all trout movements upstream and downstream since 1971. Upstream and downstream Wolf-type traps, situated on two short rivers that join L. Feeagh to L. Furnace (Fig. 19.1), were monitored daily and more often during periods of peak migration. The data presented in this chapter represent the best available collation of all the census data, from the trap records and the historical Annual Reports of the Salmon Research Trust and Agency, and update Mills et al. (1990) and Poole et al. (1996). Upstream ﬁnnock trapping has always been problematic and, between 1980 and 1984, estimates of ﬁnnock moving upstream were raised according to the subsequent downstream count, which was considered to represent 66% of

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the probable upstream ﬁnnock run, based on overwintering survival rate of ﬁnnock derived from tagging studies (Anon, 1970–2003 – Ann. Rep. SRTI, 1985). Collections of scales, taken from between the dorsal ﬁn and the lateral line and stored dry in paper envelopes, have been made over the study period and these were used to determine size–age classes of trout using standard procedures (e.g. Elliott & Chambers, 1996). The Burrishoole rod ﬁshery, managed by the Marine Institute, retains a full record of all rod-caught salmon and sea trout since 1970, and the angling effort involved since 1980 (Anon, 1970–2003). However, since 1990, the Conservation of Sea Trout Bye-Law (No. 657) requires all sea trout caught to be released alive, and this may have reduced the accuracy of the rod catch records, although considerable efforts are made to maintain the accuracy of these data. The spawning escapement was determined by subtracting the number of rod-caught sea trout from the total upstream migration through the traps. Additional natural mortality was assumed to have been relatively constant between years and was not taken into account when estimating spawning escapement. Analysis Spawning escapement of sea trout was classiﬁed into three sea-age categories: ﬁnnock (0+), one sea winter (SW) (1+) and older ﬁsh (>1+). Between 1971 and 1989, numbers of 1SW spawners were as estimated by Mills et al. (1990) and post-1989 they were determined by scale reading and length distributions. Sex ratios and mean fecundities were estimated from the historical trap and rod catch data (Anon, 1970–2003; O’Flynn, 1988; Mills et al., 1990; Poole et al., 1996), from which an estimate of the number of ova deposited each year was made. Fecundity was determined using ovaries removed from 102 rod-caught females between 1984 and 1987 (O’Flynn, 1988; Mills et al., 1990). Eggs were counted in cross sections cut through the ovaries and these were veriﬁed by counting sub-samples. Mean fecundities were calculated for each age class. In this study, fecundity of unsilvered migrants was assumed to be the same as that of the silvered sea-run ﬁsh. The proportion of maturing ﬁnnock, determined by dissection of rod-caught ﬁsh, electroﬁshing of spawning beds and external examination of downstream migrants, ranged from 25% to 48%. Between 25% and 30% of female ﬁnnock mature and spawn (Mills et al., 1990), and a ﬁgure of 30% was used in this study. The following table gives the parameters used in the calculation of the sea trout ova deposition each year:

Length (cm)

Av. weight (g)

Finnock (0+) ≤31.9 250 1 SW 32.0–39.9 530 Older adults ≥40.0 875

Fecundity ova/ﬁsh

Sex ratio f:m

754 1027 1600

1 : 0.8 1 : 0.6 1:1

284

Sea Trout

The relationships between spawning stock, as estimated ova deposition, and recruitment, as (a) spring smolt; (b) 1+ juvenile autumn trout and smolt output and (c) total recruitment (0+ & 1+ autumn trout and 2- and 3-year smolt) were then described by ﬁtting the Beverton–Holt and the Ricker models (Elliott, 1985a, 2006; Hilborn & Walters, 1992) to the Burrishoole sea trout dataset. Each model conforms to the general expression: R = aSf (S) where R is the number of recruits (downstream migrants), S is the stock (estimated number of ova deposited), a is a parameter that determines the degree of density-independent mortality and f (S) is a density-dependent function that relates proportionate survival to density (Elliott, 1985a). Non-linear least-squares regression analysis, using the Fletcher iterative method (Datadesk® 6.0. Data Description Inc., Ithaca, NY) to minimise the residual sum of squares, were used to ﬁt curves through the data and to provide estimates of the equation parameters. Goodness of ﬁt was assessed by comparing the residual sum of squares and the correlation coefﬁcient (r 2 ).

Results Upstream migration Numbers of sea trout migrating upstream through the Burrishoole traps from 1971 to 2003 are given in Table 19.1. The annual number of returning ﬁnnock and older sea trout reached a maximum of 3348 in 1975, declined through the 1980s (note amendment to ﬁnnock counts 1980–84), and more rapidly in 1987 and 1988, followed by a collapse in all sea ages in 1989 (Fig. 19.2). Prior to 1990, relatively few unsilvered ﬁsh were included in the upstream ‘sea trout’ count (Piggins, pers. comm.). Each year since 1990, between 40 and 167 unsilvered trout have been recorded migrating upstream (Fig. 19.2). The majority (62–88%) of these ﬁsh fall into the 0+ ‘sea-age’ class (Table 19.1), and mark-recapture information has indicated that Table 19.1 Mean data for upstream migration for silvered and unsilvered trout, including the proportion of 0+ sea age and the spawning escapement. Migration year

Total silver

No. of ﬁnnock

% 0+ sea age

1970–74 1975–79 1980–84 1985–89 1990–94 1995–99 2000–2003

2130 (351) 2624 (302) 1719 (98) 978 (222) 206 (36) 177 (27) 98 (9)

1065 (176) 868 (192) 740 (65) 455 (119) 124 (24) 111 (24) 57 (10)

50.0 31.6 43.5 42.6 60.0 62.0 63.5

Standard errors (SE) are given in parentheses.

Unsilvered

74 (6) 112 (20) 56 (6)

% 0+ sea age

Total migration

Spawning escapement

73.4 75.3 71.6

2130 (351) 2624 (302) 1719 (98) 978 (222) 280 (34) 289 (37) 155 (15)

1812 (299) 2369 (327) 1622 (83) 906 (196) 279 (34) 289 (37) 155 (15)

Features of Burrishoole Trout Population 4000

Unsilver Silver

3500 Total upstream count

285

3000 2500 2000 1500 1000 500 2002

2000

1998

1996

1994

1992

1990

1988

1986

1984

1982

1980

1978

1976

1974

1972

1970

0

Year

Fig. 19.2 Annual numbers of upstream migrating sea trout through the Burrishoole ﬁsh traps for 1970–2003, showing silvered and unsilvered migrants separately since 1990.

many of these originated from both downstream smolt and autumn-migrating trout (Poole, pers. comm.). The increase in the proportion of 0+ ‘sea age’ unsilvered ﬁsh in the 1990s may be due to the premature return to fresh water of post-smolts, leading to reduced time in the sea and residence in estuarine or brackish water. Angling and rod catch Between 1980 and 1986 the mean catch per unit effort (CPUE) (effort normalised to eighthour rod-days, between June and September) was 0.84 for L. Furnace and 0.56 for L. Feeagh. There was a marked reduction in CPUE in both lakes between 1985 and 1990 (Fig. 19.3), and it has since remained low. These data indicate that the collapse in the sea trout catch between 1988 and 1990 was not related to reduced angling effort, which increased throughout the late 1980s and the 1990s. Table 19.1 presents the spawning escapement, which is the total upstream count of migrating trout less the rod catch in L. Feeagh. The proportion of the stock entering L. Feeagh that was subsequently captured by angling has varied between 4% and 19% over the period 1971 to 1996 (mean: 10.5%), with a high of 32% in 1993. Population structure and marine survival Length distributions for the L. Furnace rod catch (Fig. 19.4) and for upstream migrants through the traps (Fig. 19.5) show that the Burrishoole sea trout belong to a relatively shortlived population dominated by 0+ and 1+ sea-age classes (Table 19.2). In 1985, 1986 and 1987, when detailed samples were measured, a good population of 0+, 1+ and some 2+ and 3+ sea-age classes were present in the L. Furnace and in the upstream migration (1987) (Table 19.2). Since 1989, both the numbers and proportions of sea-age classes have changed (Table 19.2; Fig. 19.5), with low returns of ﬁnnock being associated with lower proportions of 1+ sea-age and older age classes in subsequent years. Using the total trap

286

Sea Trout Furnace Rod days

CPUE

1600

1.40

1400

1.20 1.00

1000

0.80

800 0.60

600

CPUE

Rod days

1200

0.40

400

0.20

200

0.00

0 1980 1983 1986 1989 1992 1995 1998 2001 Year Feeagh Rod days

CPUE

600

0.80 0.70

500

0.50

300

0.40

CPUE

Rod days

0.60 400

0.30

200

0.20 100

0.10

0

0.00 1980 1983 1986 1989 1992 1995 1998 2001 Year

Fig. 19.3 Rod effort data and CPUE Loughs Furnace and Feeagh, Burrishoole over the period 1980–2003.

dataset (Table 19.6), the overall change in the proportion of age classes indicates fewer ﬁsh in the older age classes as follows:

1970–88 1989–2003

0+ (%)

1+ (%)

>1+ (%)

43 63

38 31

19 7

The proportion of smolts that returned as ﬁnnock prior to 1988 ranged from 11.4% to 32.4% (Fig. 19.6), with a mean of 21%. In 1988 this decreased to 8.5% and in 1989 to

Features of Burrishoole Trout Population 50 1985

September August

40

July

30

20

10

24

28

32

36

40

44

48

52

56

1986 40

Number

30

20

10

24

28

32

36

40

44

48

52

56

28

32

36

40

44

48

52

56

28

32

36

40

44

48

52

56

1987 20

10

24 1989 10

0 24

Length interval (cm)

Fig. 19.4 Length–frequency distributions for L. Furnace rod catch, 1985, 1986, 1987 and 1989 (taken from Poole et al., 1996).

287

Sea Trout 50

1990

50

1995

50

30

30

20

20

20

10

10

10

0

0

0

40

1991

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52

30

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52

40

50

50 40

1996

50

30

20

20

10

10

10

0

0

0

50 1992

40

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52

30

20

40

50 1997

30

20

20

20

10

10

10

0

0

0

1993

50 40

1998

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52

30

40

50

30

20

20

10

10

10

0

0

0

50 40 30

20

20

10

10

0

0 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52

30

1999

Silver Unsilver

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52

1994

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52

30

20

40

2003

40

30

50

2002

40

30

50

2001

40

30

50

2000

40

40

18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52

288

Fig. 19.5 Length–frequency distributions for upstream migrating silvered and unsilvered trout from 1990 to 2003.

a low of 1.5%. There has been a saw-tooth pattern of ﬁnnock return rates in the 1990s, increasing to 16.7% in 1999 – the highest return rate since 1986. The mean for the 1990s, excluding 1999, was 6.8%, three times lower than the historical average. Only in 1996 and 1997 were there two consecutive years of improved marine return and this was reﬂected in a higher survival of older age classes (Fig. 19.5). Downstream migration There was considerable variation in the annual number of smolts counted downstream between 1970 and 1990 (Fig. 19.7; Table 19.3), from a maximum of 6710 in 1981 to a minimum of 530 in 2001. Before 1991, there was no signiﬁcant trend (P > 0.05) in annual smolt numbers, but since 1991 there has been a signiﬁcant reduction in smolt output (Mann–Whitney U; P < 0.005). The age composition of the smolt run was similar in 1958–60 and 1980–84 and averaged 68% 2+ and 32% 3+ years (Table 19.4). No samples were taken between

Features of Burrishoole Trout Population

289

Table 19.2 Stock composition derived from length measurements of rod-caught (L. Feeagh 1986) and upstream migrants through the traps in Burrishoole, 1985, 1987 and 1990–2003. Year

n

1985 1986a 1987 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Percentage sea age

n/a n/a 920 210 370 211 207 248 235 262 271 191 382 166 129 170 114

0+

1+

2+

3+

50.0 56.0 56.0 82.9 71.6 60.2 79.7 62.9 59.6 64.9 64.2 68.1 77.2 60.8 53.5 74.1 80.7

33.0 39.0 33.0 10.5 27.8 32.7 15.9 32.7 37.4 28.2 29.5 21.5 19.1 33.7 39.5 19.4 17.5

15.0 5.0 11.0 5.7 0.5 7.1 2.9 4.4 3.0 5.3 5.5 8.9 3.1 4.8 4.7 5.3 0.9

2.4 0.0 0.5 1.0 0.0 0.0 1.4 0.0 0.0 1.5 0.7 1.6 0.5 0.6 2.3 1.2 0.9

a L. Feeagh rod catch sample. 1985–86 data from Mills et al., 1990, 1987 data from Ann. Rep. SRTI (1987).

% Return as finnock 35.00

% Return

30.00 25.00 20.00 15.00 10.00 5.00 2003

2000

1997

1994

1991

1988

1985

1982

1979

1976

1973

1970

0.00

Year

Fig. 19.6

Annual percentage return of smolts returning to the Burrishoole traps as ﬁnnock.

1984 and 1990. The age composition of smolts in 1990 showed a slight increase in the number of 3-year-old smolts on the 1980–84 average; but a signiﬁcant change (χ 2 = 175.7; P < 0.0001) in the proportions of ages was found in 1991, with 43.2% 2+, 49.4% 3+ and 7.4% 4+ year-old smolts (Poole et al., 1996). Sampling in 1992 showed a continuing, but not signiﬁcant change, towards the older age groups, which may have been a result of

290

Sea Trout 8000

Smolt number

7000 6000 5000 4000 3000 2000 1000 2003

2000

1997

1994

1991

1988

1985

1982

1979

1976

1973

1970

0

Year Fig. 19.7

Annual numbers of smolts counted migrating downstream, 1971–2003.

Table 19.3 Summary data for number of smolts and juvenile autumn trout counted migrating downstream through the traps. Migration year

Smolt

Autumn trout

Percentage 0+ autumn trout

Total recruitment

1970–74 1975–79 1980–84 1985–89b 1990–94 1995–99 2000–03

4450 (607) 4305 (484) 4038 (820) 3814 (193) 1873 (228) 1361 (171) 840 (153)

2836 (253) 3116 (327) 1907 (228) 1765 (185) 491 (134) 739 (93) 1162 (377)

35.0a 35.0 37.7 33.9 30.9 30.7 46.2

5943 (841) 6373 (530) 5413 (874) 4877 (165) 2149 (286) 1860 (174) 1455 (320)

Total recruitment is the total of smolts and 1+ age autumn trout from the preceding year. Standard errors are given in parentheses. a Autumn trout were not categorised into age class in the traps between 1970 and 1979. An average from 1980 to 2003 was used as an estimate to calculate 1+ recruitment. b No autumn trout count in 1989.

Table 19.4 smolts.

Percentage age composition of Burrishoole sea trout

Year

1958–60 1980–84 1990 1991 1992 1993 1994

Percentage age class 2+

3+

4+

68–70 68.0 60.0 43.2 36.9 54.9 74.6

28–31 32.0 38.7 49.4 53.1 42.5 25.4

1–2 — 1.3 7.4 10.0 2.6 —

n

Source

599 — 75 419 211 195 122

Piggins (1961) Mills et al. (1990) Poole et al. (1996) Poole et al. (1996) Poole et al. (1996) Poole et al. (1996) Poole et al. (1996)

Features of Burrishoole Trout Population

291

the spawning collapse in 1988 and 1989. In 1993 and 1994, the proportions of 2+ and 3+ smolts reverted back to the ratios observed prior to 1990 where up to 70% of the run was 2-year-old smolts. There was a signiﬁcant downward trend in numbers of unsilvered juvenile trout migrating in autumn over the entire study period (y = −132.7x + 12899.7; r = −0.88, P < 0.0001) (Table 19.3). This contrasts with smolt abundance, which has shown a decline only since 1991. The age composition of the autumn trout ranged from 0+ to 3+ years, the percentage of 0+ trout varied from 16.1% to 60.9% in the period 1982–2003. It is not known if the 0+ age ﬁsh are true migrants or if they are displaced downstream as a result of population pressure or, possibly, ﬂoods. Whilst 0+ trout are not old enough to become sea trout smolts in the following spring, tagging studies (Mills et al., 1990; Poole et al., 1996) show that the remainder, predominantly 1+ age ﬁsh, could contribute to the overall recruitment of smolts in the following year.

Enhancement According to Mills et al. (1990), the majority of reared sea trout released into the Burrishoole River system between 1962 and 1988 were 2+ smolts, and they gave return rates of 0–10%, averaging 4%, compared with means of 22% and 16% for wild smolt returning as ﬁnnock in the 1970s and 1980s, respectively (Table 19.5a). Most ﬁrst time sea-run recaptures in the

Table 19.5a Details of sea trout enhancement programmes from 1964 to 2002, showing the numbers of ﬁsh released, recaptured, their age class at recapture and an estimate of possible ova deposition from surviving adults. Year

No. released

No. recaptured

10% estimate rod catch

% 0+

No. 0+

No. older

Total ova

1964 1970 1971 1974 1982 1983 1984 1985 1986

485 500 1 517 1 949 2 844 1 300 2 761 3 527 12 030

5 0 3 3 188 8 235 38 149

4.5 0 2.7 2.7 169.2 7.2 211.5 34.2 134.1

42.5 0 42.5 42.5 34.9 42.5 33.4 42.5 59.2

2 0 1 1 66 3 78 16 88

3 0 2 2 122 5 157 22 61

1 267 1 267 86 803 3 380 110 323 16 054 50 106

1994 1995 1996 1997 1998 1999 2000 2001 2002

976 1 532 3 170 1 950 1 690 465 58 0 0

101 92 103 272 136 64 6 2 0

0 0 0 0 0 0 0 0 0

68.9 44.6 74.8 72.7 90.3 51.6 16.7 0.0 0.0

70 41 77 198 123 33 1 0 0

31 51 26 74 13 31 5 0 0

28 907 37 871 26 815 75 438 24 845 24 033 3 966 1 600 0

2 112

292

Sea Trout

Table 19.5b Number and age of smolts produced from 1+ parr stocked into L. Feeagh along with the percentage return as silver ﬁnnock, percentage total return as 0+ age (silver and brown) and total return of trout from stocked progeny between 1993 and 2000. Smolt migration year

Number of parr released

Total no. of autumn trout the previous year

No. of smolt

Total recruitment

% age 1/2/3/4 year olds

% Return as 0+ sea age silver

% Return as 0+ sea age total

Total return of trout all ages

1993 1994 1995 1996 1997 1998 1999 2000

6 463 9 234 23 914 4 705 4 250 1 578 — —

— 64 74 418 453 515 246 48

202 912 1458 2752 1497 1175 219 10

202 976 1532 3170 1950 1690 465 58

100/0/0/0 2/98/0/0 46/54/0/0 5/88/7/0 28/37/35/0 17/65/12/6 0/77/16/7 0/0/80/20

0.5 7.2–7.7 1.3–1.4 2.3–2.7 5.8–7.5 2.0–2.9 2.4–5.0 1.7–10.5

1.0 9.4–10.1 3.4–3.6 3.4–3.9 11.2–14.6 4.0–5.8 6.6–14 1.7–10.5

— 101 92 103 272 136 64 6

angling catch in L. Furnace and in the upstream traps were made as ﬁnnock within nine months of release. Though these ﬁsh were the progeny of sea trout, a large proportion of recaptures (e.g. 64% in 1984) had not migrated to sea, but had adopted a brown trout-like existence in L. Furnace and the estuary. Between 1994 and 2000, a total of 8225 marked spring smolts migrated seaward through the traps (Table 19.5b). This represented a mean smolting rate from released parr to 2+ and 3+ smolt of 16.4%. As a total of 8702 wild smolt also migrated through the traps during the same period, this programme effectively doubled the smolt output from the catchment. Similar to the wild ﬁsh, a downstream migration of marked trout was also recorded in the autumn. These were predominantly immature ﬁsh up to 3+ age, although some older maturing ﬁsh were also recorded. The total numbers recorded have been included in Table 19.5 and, when added to the smolt run in the following year, give an estimate of total recruitment downstream. From these releases, a total of 324 reared trout, or 3.9% of the smolts, returned as silvered ﬁnnock, compared with 9% for wild smolt, and a further 249 as unsilvered trout, giving a total return of 7.0% of the smolts, compared with 16% for their wild counterparts. The corresponding return rates for the total recruitment were 3.2% and 5.7% respectively. A total of 774 adult trout counted through the traps between 1994 and 2000 originated from the released parr.

Stock and recruitment Table 19.6 gives the main parameters used in the construction of the stock and recruitment relationship, including spawning escapement and percentage age class of upstream migrating trout, estimated ova deposition and subsequent downstream recruitment as 0+

Table 19.6 Spawning escapement, sea-age class and number of ova deposited along with the subsequent number of recruits as autumn-migrating trout (Aut) and smolt for individual year classes of the Burrishoole sea trout stock, 1971–2003. Usptream migration (stock) Year Spawn % 0+ % 1+ % >1+ Total ova Escape sea age sea age sea age deposited

1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

1249 1883 2391 2519 3118 3117 1898 1486 2226 1846 1794 1519 1531 1422 1308 1274 905 820 224 249 412 223 236 276 249 265 278 220 431 174 143 183 118

50.0 50.0 50.0 50.0 33.0 44.9 32.7 21.0 26.6 31.0 44.8 48.4 56.5 36.9 46.8 50.7 48.8 42.4 24.6 75.3 62.4 54.1 71.1 56.1 56.9 61.8 65.9 71.3 77.4 60.4 54.8 70.8 79.7

44.0 38.0 38.0 38.0 38.0 38.0 38.0 35.9 38.0 28.5 25.6 26.2 23.3 55.5 46.8 43.4 44.0 50.6 63.8 11.2 37.1 36.0 22.5 38.4 40.6 30.6 27.6 21.8 18.7 34.1 37.7 20.9 18.6

6.0 12.0 12.0 12.0 29.0 17.1 29.2 43.1 35.4 40.5 29.6 25.4 20.2 7.6 6.4 5.9 7.2 7.0 11.6 13.4 0.5 9.9 6.4 5.5 2.4 7.6 6.5 6.9 3.8 5.5 7.5 8.2 1.7

Downstream migration (recruitment) Est. ova Tot. ova dep. by enhanced

490 891.6 1 267 758 588.8 962 928.1 1 014 229.5 1 267 1 613 761.5 1 363 536.9 985 380.8 894 172.4 1 247 436.6 1 007 884.3 820 925.7 656 584.8 86 803 585 091.4 3 380 658 895.9 110 323 537 135.3 16 054 496 329.5 50 106 362 965.1 355 833.8 119 499.8 68 244.9 132 190.1 84 431.4 67 221.7 99 614.0 28 907 87 584.5 37 871 88 745.1 26 815 86 655.7 75 438 62 657.2 24 845 106 968.9 24 033 58 921.3 3 966 52 999.1 1 600 52 927.7 0 27 533.9 0

492 158.6 758 588.8 962 928.1 1 015 496.5 1 613 761.5 1 363 536.9 985 380.8 894 172.4 1 247 436.6 1 007 884.3 820 925.7 743 387.8 588 471.4 769 218.9 553 189.3 546 435.5 362 965.1 355 833.8 119 499.8 68 244.9 132 190.1 84 431.4 67 221.7 128 520.9 125 455.9 115 559.8 162 093.3 87 502.6 131 001.7 62 887.2 54 599.1 52 927.7 27 533.9

No. 0+ No. 1+ No. 2+ No. 3+ Aut Smolt Smolt & 1+ Tot. Recruit. Aut Aut Smolt Smolt Equivalent Equivalent Equivalent Equivalent

1231 722 886 919 1418 1002 1192 885 799 895 430 1109 374 773 640 331 518 866 236 356 82 124 175 57 114 264 165 325 259 200 1174 476 408

2389 1402 1720 1784 2753 1945 2314 1718 1552 1736 1300 1109 1200 611 1472 1726 949 556 634 655 234 183 306 282 336 513 717 644 358 218 910 976 426

2013 3716 4128 3078 2439 3541 2645 2154 3860 1589 4563 2657 3299 1620 2882 2349 2292 2917 2529 1238 1260 716 946 845 1382 611 664 1144 896 547 186 282 438

948 1749 1943 1449 1148 1666 1244 1013 1816 748 2147 1250 1553 763 1356 1105 1079 1373 1190 825 1260 1220 774 282 439 678 153 464 364 222 345 990 349

2442 2670 3672 3363 3316 2910 2437 2536 2195 1539 2309 985 2245 2366 1280 1074 1500 891 590 265 430 457 393 627 981 809 683 477 1110 2150 902

4226 4105 4785 3658 3970 4608 3736 5813 4209 4062 2977 3987 3427 3665 4107 3354 2498 2480 1490 1228 1284 2060 764 1128 1508 1118 892 1175 632

5946 5889 7538 5603 6284 6326 5288 7550 5509 5171 4177 4598 4899 5391 5056 3910 3132 3135 1724 1411 1590 2342 1100 1641 2225 1762 1250 1393 1542

6668 6775 8457 7021 7286 7518 6173 8349 6404 5601 5286 4972 5672 6031 5387 4428 3998 3371 2080 1493 1714 2517 1157 1755 2489 1927 1575 1652 1742

294

Sea Trout

and 1+ age juvenile trout and 2+ and 3+ smolt. The following text box indicates the sources of the data or the basis on which estimates were made.

Sources of data used in Table 19.6. Spawning escapement from trap and rod ﬁshery records, summarised in Table 19.1. No attempt was made to quantify natural mortality. % sea age: 1971–88 from Mills et al. (1990). 1989–2003 from trap records based on lengths, veriﬁed by scale reading. Numbers of ova deposited, see Section Materials and methods. Estimated number of ova deposited by enhanced ﬁsh. 1971–89 fecundity from Mills et al. (1985), and using estimates of exploitation (10%) and sea age. Information on enhanced ﬁsh was incomplete and considered to be overestimates. 1990–2003 similar criteria used for wild ﬁsh were applied to returns from enhanced ﬁsh, but this was considered to be an overestimate as many ﬁsh were trapped migrating downstream before spawning, were immature or may not have spawned effectively. 0+ & 1+ autumn trout aged by length, conﬁrmed by sporadic scale reading; 0+ <10 cm Smolt age 1958–60 Piggins (1961) 1980–84 Mills et al. (1990). Average of 68% 2+ & 32% 3+ applied to 1970–89 smolt data. 1990–94 Poole et al. (1996) 1995–2003 Estimated by length measurements, using the 1990–94 length at age data.

Total wild migratory trout spawning escapement before 1987 ranged from 1249 to 3118 individuals with an overall downward trend since the early 1980s. The stock collapsed in 1989 and remained low thereafter, decreasing to a minimum of 118 trout in 2003 (Table 19.6). From 1979 to 1980, there was a corresponding decline in the estimated number of ova deposited by wild migratory trout in the catchment, estimated to range between 0.49 and 1.61 million before 1987, decreasing to a low of 27 500 in 2003. Estimates of possible ova deposition by surviving mature adults returning from the enhancement programme are also included in Table 19.6. The population was largely ﬁnnock dominated, few migrated to sea and the rod catch and upstream migration recaptures were reported together, making it impossible to calculate either age classes or spawning escapement (Mills et al., 1985). There is evidence that these ﬁsh may not spawn successfully, and the ova ﬁgures for enhanced ﬁsh in the 1980s are probably grossly overestimated.

Features of Burrishoole Trout Population

295

Recruitment per annum and recruitment per spawning cohort are also presented in Table 19.6. Recruitment per spawning cohort, shown as equivalents in Table 19.6, is the sum of the relevant age class of juveniles and is tabulated opposite the spawning year from which they were derived. Hence, the stock–recruitment data series is only complete up to 1999. Between 1971 and 1988, the percentage output for ova to smolt (equivalent) averaged 0.53% with a range of 0.25–0.86% (Fig. 19.8; Table 19.7). After 1988, survival rates 3.00

Smolt

% Output

2.50 2.00 1.50 1.00 0.50 1998

1995

1992

1989

1986

1983

1980

1977

1974

1971

0.00

Ova year 3.00

Smolt & 1+

% Output

2.50 2.00 1.50 1.00 0.50 1989

1992

1995

1998

1989

1992

1995

1998

1986

1983

1980

1977

1974

1971

0.00

Ova year 3.50

Total Recruit

3.00 % Output

2.50 2.00 1.50 1.00 0.50 1986

1983

1980

1977

1974

1971

0.00

Ova year Fig. 19.8 Percentage output of wild only sea trout ova to the smolt stage, as total recruitment of smolt and 1+ autumn trout and for total recruitment including smolt, 0+ and 1+ autumn trout.

296

Sea Trout Table 19.7 Average percentage output of wild sea trout ova, and total ova including a maximum estimated enhanced contribution, to the smolt stage, as total recruitment of smolt and 1+ autumn trout and for total recruitment including smolt, 0+ and 1+ autumn trout. Year

Smolt

Smolt and 1+ trout

Total recruitment

Wild ova only 1971–88 0.53 1989–99 1.38

0.71 1.85

0.82 2.07

Maximum total ova 1971–88 0.52 1989–99 1.18

0.70 1.57

0.80 1.74

Table 19.8 Estimates of parameters for Beverton–Holt and Ricker models for wild ova deposited in the Burrishoole system plotted against equivalent smolt, smolt plus 1+ autumn trout and total recruitment output. Recruitment stage

Model parameters

1/b

Replacement abundance S* R=S

a

b

Residual sum of squares

r2

Beverton–Holt Wild ova Smolt Smolt &1+ Total recruit Total ova Total recruit

0.021 0.021 0.023

0.00000419 0.00000266 0.00000241

9 501 044.0 16 152 618.9 17 529 377.4

0.70 0.87 0.93

238 663 375 940 476 190

233 652 368 045 405 394

0.021

0.00000216

18 855 340.8

0.86

462 963

453 241

Ricker Wild ova Smolt Smolt &1+ Total recruit Total ova Total recruit

0.019 0.016 0.021

0.00000154 0.00000104 0.00000116

20 607 635.5 19 794 593.4 28 190 663.5

0.85 0.89 0.88

649 351 961 538 862 069

2 573 582 3 976 122 3 330 373

0.02

0.00000108

27 258 033.8

0.86

925 925

3 622 246

The replacement abundances are also shown. Also shown are values for total ova, including enhanced ova, related to total recruitment.

increased signiﬁcantly (Mann–Whitney U; P < 0.0005) to an average value of 1.38% ranging from 0.6% to 2.4%. This pattern is also seen for recruitment as smolt and 1+ autumn trout and as total recruitment, with an average of 0.82% ova to total recruitment (1971–88) increasing to 2.07% between 1989 and 1999 (Table 19.8). The pattern was similar when total ova, including estimates of ova deposited by enhanced ﬁsh, although the inclusion of enhanced ova had a much greater impact on ova-smolt survival after 1989.

Features of Burrishoole Trout Population

297

7000 78

Smolt equivalent

6000

73

5000 4000 86 3000 92 2000

80 82 72 77 74 83 84 81

75

87

500.0

1000.0 1500.0 Ova deposited (x1000)

8000

2000.0

73 78

7000

76 71

6000

72

74

84 85 83 82

5000

86

4000 3000

79

88

95 89 91 1000 97 93 99 0 0.0

Smolt & 1+ autumn trout equivalent

76

71 85

88

77

75

79

80 81

87

92 95 2000 96 89 90 91 99 1000 98 97 93 0 0.0

500.0

1000.0 1500.0 Ova deposited (x1000)

2000.0

Fig. 19.9 The number of smolts, and total recruitment of smolt & 1+ autumn trout derived from each cohort of wild ova deposited. Year of spawning is shown on graph.

S/R relationship Figure 19.9 shows that ova deposition rates of more than 0.5 million gave rise to equivalent smolt recruitments in the range 3427–5813, recruitments of smolts and 1+ autumn trout of 4598–7550 and total recruitments of 4972–8457, 3 and 4 years later. A marked decrease in recruitment was observed when ova deposition rates decreased below 0.5 million from 1987 onwards, when the observed relationship appeared to be tending towards zero. There was little evidence of non-stationarity, as demonstrated by the spread of recruitments in Fig. 19.9. The clumping of low stock levels in the latter years is because of changes in marine mortality (as evidenced by smolt-ﬁnnock return rates) and does not violate the assumptions for stock and recruitment relationship.

298

Sea Trout

As spawning stock, or number of ova deposited, increase, recruitment can theoretically either increase to an asymptotic limit or follow a dome-shaped curve. The asymptotic Beverton–Holt relationship ﬁtted the Burrishoole sea trout data better than the Ricker model, for all levels of recruitment (Fig. 19.10) and including the estimates of ova derived from returning enhanced ﬁsh (Fig. 19.11), particularly at low stock levels, as demonstrated by the lower SSR (Table 19.8), and the derived relationships of proportionate survival and mortality rate (Fig. 19.12). Table 19.8 gives the parameters and correlation coefﬁcients for both the Beverton–Holt and Ricker models and also the inﬂection points (1/b) and replacement abundances. The parameter a has the dimension recruits per unit stock and represents the slope of the curve at the origin. Parameter b has the dimension 1/S, where S is the stock size above which density dependence dominates over density independence, or the point at which maximum recruitment per unit stock occurs. For the Burrishoole migratory trout stock, S approximates to 240 000 ova in the smolt relationship increasing to 476 000 in the total recruitment relationship. These levels of ova deposition have not been achieved since 1988.

Discussion Monitoring of the sea trout population of the Burrishoole catchment since 1970 has enabled us to quantify the scale of the sea trout stock collapse in the late 1980s in the west of Ireland (Whelan, 1991; Poole et al., 1996; Whelan & Poole, 1996). It is clear that Burrishoole sea trout stocks had been declining since the mid-1970s, and that there was a strong reduction in marine survival (as evidenced by smolt returns) and spawning stock between 1988 and 1990. Similar long-term records are not available for stocks in other Irish systems. In general, the Connemara ﬁsheries, as well as the Delphi, Erriff and Currane, enjoyed good rod catches into the mid-1980s (Anon., 1994; Gargan et al., 2006). Mills et al. (1986a) demonstrated that ﬁshing effort was the single most important determinant of catch, with a weaker, but signiﬁcant, relationship between catch and stock. A relatively high catchability of sea trout at low stock levels makes the relationship more difﬁcult to interpret following the collapse in catch between 1988 and 1989. Historically, the study of adult sea trout in many ﬁsheries, including Burrishoole, was based largely on rod-caught ﬁsh. Sampling from the L. Furnace rod ﬁshery in 1985 and 1986 showed that around half the population measured less than 32 cm (mostly ﬁnnock) and the remainder were predominantly 1+ and 2+ sea ages with a few older ﬁsh. The majority of west of Ireland sea trout populations are dominated by ﬁnnock (Fahy, 1985). In 1989, the ﬁnnock component of the stock in Burrishoole decreased to 25%, a level previously only observed in 1978 and 1979, and noted in the respective SRTI Annual Reports at the time as being unusual. The stock collapse affected both sea run ﬁnnock and older adults from 1987 to 1990 leaving a spawning stock of sea trout of only 249 ﬁsh, including 187 ﬁnnock, in 1990. The stock has failed to recover in subsequent years. Although the smolt output had not changed signiﬁcantly prior to 1990, the percentage return of smolts decreased rapidly between 1987 and 1989 indicating a marine problem. Higher marine mortality of predominantly

Features of Burrishoole Trout Population 7000 Smolt

Smolt equivalent

6000 5000 4000 3000 2000 1000 0 0

400 000

800 000 Ova deposited

1 200 000

1 600 000

800 000 Ova deposited

1 200 000

1 600 000

800 000

1 200 000

1 600 000

8000 Smolt & 1+ autumn trout

Smolt & 1+ equivalent

7000 6000 5000 4000 3000 2000 1000 0 0

400 000

Total recruitment equivalent

9000 Total recruitment

8000 7000 6000 5000 4000 3000 2000 1000 0 0

400 000

Ova deposited Fig. 19.10 Beverton–Holt (heavy line) and Ricker curves (light line) ﬁtted to the wild ova against smolt, smolt & 1+ autumn trout and total recruitment outputs.

299

300

Sea Trout 9000 Total recruitment

Total recruitment equivalent

8000 7000 6000 5000 4000 3000 2000 1000 0 0

400 000

800 000 1 200 000 Total ova deposited

1 600 000

Fig. 19.11 Beverton–Holt (heavy line) and Ricker curves (light line) ﬁtted to the total ova deposited by wild and enhanced trout combined against total recruitment.

smolts and 1+ sea age ﬁsh from 1990 to 1992 was paralleled by poor growth of the surviving ﬁsh (Poole et al., 1996). Sea lice, Lepeophtheirus salmonis (Krøyer), emanating from sea farms, were implicated in this additional mortality (Tully & Whelan, 1993; Anon., 1994; Gargan et al., 2003). A number of enhancement trials have been conducted in the Burrishoole system since the 1960s (Mills et al., 1985, 1990; Poole et al., 1994, 2002; Byrne et al., 2002). In the 1970s and 1980s, low freshwater survival rates, a tendency not to migrate and the main return of ﬁsh as 0+ sea age were the main problems associated with enhancement programme (using a mixture of non-local-origin and Burrishoole stocks) and, in the 1990s, while the smolt output was almost doubled, the return of sea-run migrants remained low. Reared ﬁsh have been shown to have lower spawning success in the wild (Fleming et al., 1995), and it has been concluded that ranched hatchery salmon made little or no contribution to wild smolt production in the Burrishoole (McGinnity, 1997; Crozier et al., 2003). Consequently, the estimates of sea trout ova potentially produced by enhanced ﬁsh, are probably vastly overestimated and probably had little positive effect on the levels of recruitment. Although there are numerous studies on production, densities and downstream migration of juvenile trout, the only other long-term investigation into the relationships between spawning stock and subsequent recruitment in sea trout are for an exclusively migratory stream-dwelling populations (Elliott, 1985a; Elliott & Elliott, 2006; Euzenat et al., 2006). The Burrishoole study examines these relationships for a population of stream and lake cohabiting migratory and resident trout. The marked decrease in spawning escapement through the 1980s, followed by the stock collapse in 1989–90, resulted in a serious reduction in ova deposition by the migratory form, both sea run and unsilvered, and a consequent reduction in smolt output. While the contribution of ova spawned by resident trout to the seaward recruitment in Burrishoole is unknown, the stock–recruitment model suggests that

Features of Burrishoole Trout Population 8

0.03 0.025

Smolt Mortality rate [ln(S/R)

Proportionate survival (R/S)

Smolt

0.02 0.015 0.01 0.005 0

6

4

2 0

400 000

0

800 000 1 200 000 1 600 000

400 000

1 200 000 1 600 000

8

0.03 Smolt & 1+ autumn trout 0.025

Smolt & 1+ autumn trout Mortality rate [in(S/R)

Proportionate survival (R/S)

800 000

Ova deposited

Ova deposited

0.02 0.015 0.01 0.005 0

6

4

2 0

400 000

0

800 000 1 200 000 1 600 000

400 000

800 000

1 200 000 1 600 000

Ova deposited

Ova deposited 8

0.035 Total recruitment 0.03

Total recruitment Mortality rate [in(S/R)

Proportionate survival (R/S)

301

0.025 0.02 0.015 0.01 0.005 0

6

4

2 0

400 000

800 000 1 200 000 1 600 000 Ova deposited

0

400 000

800 000

1 200 000 1 600 000

Ova deposited

Fig. 19.12 Proportionate survival (%R/S) and mortality rates [ln(S/R)] plotted against number of wild ova deposited with Beverton–Holt (heavy line) and Ricker (light line) curves ﬁtted for smolt, smolt & 1+ autumn trout & total recruitment.

this is probably low. Clearly, the life history of ‘slob’ or unsilvered migrant trout and their relationship to sea trout needs to be further investigated. Hilborn & Walters (1992) warn that stock and recruitment models should not be mistaken for the true relationship between stock size and recruitment, as such models infer predictability in a stochastic process and an implicit stability of the key processes. There are indications that some changes may have occurred which could have affected the stationarity of the Burrishoole salmon S/R relationship, possibly as a result of environmental degradation, a reduction in competition from trout in the 1990s and climate change (Crozier et al., 2003). The sea trout S/R relationship would appear to be more stable over time than that for salmon and non-stationarity may not be a problem. Nevertheless, the application of these historical observations to biological reference points (BRPs) should be treated with caution and warrant further investigation, including the interrelationships in S/R between salmon and sea trout.

302

Sea Trout

The asymptotic Beverton–Holt model provided the best ﬁt over the full range of stock levels in the Burrishoole sea trout population. Given that the Burrishoole study encompasses an entire catchment, including considerable lake area, and that the stock may have been below the system’s carrying capacity for much of the time series, it is perhaps not surprising that the dome-shaped model (Ricker) was rejected. Previous evidence of density-dependent mortality giving rise to dome-shaped relationships in salmon and trout populations (Alm, 1950; Mortensen, 1977; Elliott & Elliot, 2006) came from studies carried out in relatively small sections of spawning habitat where competition and density-dependent effects may be more intense. In such areas, over-cutting of redds may occasionally lead to high ova mortality (Alm, 1950) and localised crowding following emergence from redds may lead to high initial losses of fry prior to their subsequent dispersal (Elliott, 1985b). Solomon (1985) concluded there was little evidence to support the existence of dome-shaped stock–recruitment relationships in salmonids when analysing data for a catchment where a wide range of optimal and sub-optimal habitats was available over the entire recruitment period. In ﬁsheries management terms, it is relatively unimportant whether the stock–recruitment data describes an asymptotic or dome-shaped curve, and it is uncommon to see situations where one form ‘signiﬁcantly’ outperforms the other (Walters & Korman, 2001). The inﬂection point, where the curve begins to ﬂatten, indicates the ova deposition limit (stock) below which recruitment and adult return are most strongly reduced. For the Burrishoole migratory trout population, it is apparent that an annual deposition of at least 376 000 ova is necessary to maintain total sea trout recruitment at approximately 4000 smolts and 1+ autumn trout per annum, or 476 000 ova for total trout recruitment. This equates to approximately 800–1008 ova ha−1 (in 472 ha productive habitat) and a recruitment of 6.3– 7.0 smolt ha−1 or total recruitment of 9.1–10.4 trout ha−1 . It may be important to achieve greater ova deposition rates than these targets in order to ensure that stock variability is sufﬁcient to cope with a wide range in survival rates and natural ﬂuctuations in the environment. Given its depressed state (<60 000 eggs since 2000), it may now be difﬁcult to achieve a natural recovery of the sea trout spawning stock in Burrishoole and, at best, this recovery will be slow. The increased variability in recruitment and ova to recruit survival at low spawning stock sizes (Fig. 19.8) is probably because of the relaxation of density-dependent population regulation pressures, which would have been more apparent before the 1989 stock collapse. Another possible cause could be a change in the relative importance of speciﬁc densityindependent processes, such as temperature, predation or competition with salmon. Salmon smolt output and ova deposition in the Burrishoole both decreased signiﬁcantly between 1970 and 1985, but there was no difference between the 1980s and 1990s and no obvious trend in ova to smolt survival (Crozier et al., 2003). This would support the observation that the recent increase in survival of sea trout ova to smolt, and total recruit, was a real phenomenon. Fragmentation of spawning effort may also have occurred at these low escapement levels. Walters & Korman (2001) suggest that, given ‘patchy’ habitat use and following a period of low abundance and extinction of some spatial or behavioural stock components, sudden increases in spawning escapement may result in locally intense competition within those

Features of Burrishoole Trout Population

303

spawning areas that are still used by migratory trout. Furthermore, they suggest that it may take several generations of dispersal events to re-establish the most productive use of spawning habitat. Migrating smolts originating from the Burrishoole trout population (assuming a randomly interbreeding population of both resident and anadromous forms following Jonsson [1985]) have experienced persistently high (>90% per annum) levels of marine mortality for an extended period (1989–2004). This has caused dramatic changes in the trout population, particularly the sea migrating component, with a major reduction in spawning population size and in the abundance of older age classes. It is interesting to speculate as to what extent adaptive or evolutionary changes may have also occurred as a result of the high marine mortality and to what extent this exceptional event might provide some insight into the genetic basis of anadromy in trout. The decline in adult sea trout numbers between the mid-1970s and the mid-1980s was not matched by a change in smolt output, until the spawning escapement collapse between 1987 and 1989. Subsequently, there followed a rapid decline in smolt output, as demonstrated by the downstream census and the S/R relationship. This outcome could be interpreted in two ways. First, production of sea trout smolts in the Burrishoole system may require adult sea trout phenotypes to spawn successfully, if the propensity of ﬁsh to go to sea, or remain as resident trout, is under strong genetic control. In this case, the Burrishoole trout population would not be phenotypically plastic and the occurrence of anadromous phenotypes and resident phenotypes will be tightly controlled. As a consequence, one would conclude that, because of the continued high mortality at sea, the Burrishoole population has undergone signiﬁcant evolutionary change resulting in a reduction in the sea-run genetic component. Alternatively, the reduction in trout smolt numbers could be a response of the Burrishoole trout population to new environmental conditions where competition has been substantially decreased (1.6 million sea trout ova reduced to <60 000), thus reducing the necessity for a marine phase to the life cycle. This would be perpetually reinforced by the failure to recruit sea trout eggs to the system. Consequently, there would be negligible pressure for evolutionary change and the resident population would retain the capacity for anadromy. The hypothesis that the propensity for marine migration is a strongly selected inheritable trait is supported by the rapid decline in the stock–recruitment curve, driven by a sudden change in the sea-going population structure, leading to a collapse in spawning capacity. It was estimated that, in 1985, migratory trout contributed less than 20% of the total trout ova deposition in the Burrishoole catchment (Mills et al., 1986) and, subsequent to the collapse in adult sea trout, juvenile trout stocks in the streams appear to have remained unchanged (Marine Institute data, unpublished). It would also appear that production of juvenile salmon has beneﬁted, particularly in L. Feeagh, where numbers of 0+ and 1+ salmon parr have increased (Matthews et al., 1997). It seems, therefore, less likely that the observed reduction in migratory recruitment is a plastic adaptive response towards residency as a result of some freshwater environmental or density-dependent change. Whilst resident trout may produce smolts and thus buffer the collapse in ova contributed by migratory trout, it would appear that, in the Burrishoole system, this buffering capacity

304

Sea Trout

may be low, or absent. The stock–recruitment relationship suggests that the production of smolts, or juvenile recruits, is closely related to the level of ova deposited by migratory trout, supporting the hypothesis discussed earlier that the propensity for marine migration is under strong genetic control. Without a marked and sustained improvement in marine survival for the remaining ﬁsh migrating to sea, the future of sea trout stocks in catchments on the west coast of Ireland is uncertain, and urgent management action must take place to protect sustainable production and genetic biodiversity.

Acknowledgements We would like to thank the staff of the Salmon Research Agency and the Marine Institute, past and present, for their dedication in operating the ﬁsh trapping facilities and making historical data available. We would also like to thank Mike Pawson and two referees for their valuable contributions to improve the text.

References Alm, G. (1950). The sea trout population in the Ava stream. Report of the Institute of Freshwater Research, Drottningholm, 31, 26–56. Anon. (1970–2003). Annual reports of the Salmon Research Trust. Salmon Research Agency and the Marine Institute, Newport, Co. Mayo. Anon. (1994). Report of the Sea Trout Task Force. Department of the Marine, Dublin, 80 pp. Beverton, R.J.H. & Holt, S.J. (1957). On the dynamics of exploited ﬁsh populations. Fishery Investigations, London (Series 2), 19, 1–533. Buck, R.J.G. & Hay, D.W. (1984). The relation between stock size and progeny of Atlantic salmon, Salmo salar L., in a Scottish stream. Journal of Fish Biology, 23, 1–11. Byrne, C.J., Poole, W.R., Dillane, M.G. & Whelan, K. (2002). The Irish sea trout enhancement programme: an assessment of the parr stocking programme into the Burrishoole catchment. Fisheries Management & Ecology, 9(6), 329–41. Byrne, C.J., Poole, R., Dillane, M., Rogan, G. & Whelan, K.F. (2004). Temporal and environmental inﬂuences on the variation in sea trout (Salmo trutta L.) smolt migration in the Burrishoole system in the west of Ireland from 1971 to 2000. Fisheries Research, 66(1), 85–94. Crozier, W.W., Potter, E.C.E., Prévost, E., Schön, P.-J. & Ó’Maoiléidigh, N. (Eds) (2003). A co-ordinated approach towards the development of a scientiﬁc basis for the management of wild Atlantic salmon in the North-East Atlantic (SALMODEL). Queen’s University of Belfast, Belfast, 431 pp. Elliott, J.M. (1984a). Numerical changes and population regulation in young migratory trout Salmo trutta in a Lake District stream, 1966–83. Journal of Animal Ecology, 53, 327–50. Elliott, J.M. (1984b). Growth, size, biomass and production of young migratory trout Salmo trutta in a Lake District stream, 1966–83. Journal of Animal Ecology, 53, 979–94. Elliott, J.M. (1985a). The choice of a stock–recruitment model for migratory trout, Salmo trutta, in an English Lake District stream. Archiv fur Hydrobiologie, 104(1), 145–68. Elliott, J.M. (1985b). Population regulation for different life-strategies of migratory trout Salmo trutta in a Lake District stream, 1966–1983. Journal of Animal Ecology, 54, 617–38. Elliott, J.M. (1994). Quantitative Ecology and the Brown Trout. Oxford University Press, Oxford, New York, 286 pp. Elliott, J.M. & Chambers, S. (1996). A guide to the interpretation of sea trout scales. R&D Report No. 22, National Rivers Authority, Bristol, UK, 54 pp. Elliott, J.M. & Elliott, J.A. (2006). A 35-year study of stock–recruitment relationships in a small population of sea trout: assumptions, implications and limitations for predicting targets. In: Sea Trout: Biology, Conservation

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and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 257–78. Euzenat, G., Fournel, F. & Fagard, J.-L. (2006). Population dynamics and stock–recruitment relationship of sea trout in the River Bresle, Upper Normandy, France. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 307–23. Fahy, E. (1985) Child of the Tides. The Glendale Press, Dublin, 188 pp. Fleming, I.A., Jonsson, B., Gross, M.R. & Lamberg, A. (1995). An experimental study of the reproductive behaviour and success of farmed and wild Atlantic salmon (Salmo salar). Journal of Animal Ecology, 33, 893–905. Gargan, P.G., Tully, O. & Poole, W.R. (2003). Relationship between sea lice infestation, sea lice production and sea trout survival in Ireland, 1992–2001. In: Salmon at the Edge (Mills, D., Ed.). Proceedings of the Conference organised by the Atlantic Salmon Trust. Blackwell Publishing, Oxford, pp. 119–35. Gargan, P.G., Roche, W.K., Forde, G. & Ferguson, A. (2006). Characteristics of the sea trout (Salmo trutta L.) stocks in the Owengowla and Invermore Fisheries, western Ireland, and recent trends in marine survival. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 60–77. Gardiner, R. & Shackley, P. (1991). Stock and recruitment and inversely density-dependent growth of salmon, Salmo salar L., in a Scottish stream. Journal of Fish Biology, 38, 691–6. Gee, A.S., Milner, N.J. & Hemsworth, R.J. (1978). The effect of density on mortality in juvenile Atlantic salmon (Salmo salar). Journal of Animal Ecology, 47, 497–505. Hilborn, R. & Walters, C.J. (1992). Quantitative Fisheries Stock Assessment: Choice, Dynamics and Uncertainty. Chapman & Hall, London, 570 pp. Jonsson, B. (1985). Life history patterns of freshwater resident and sea-run migrant brown trout in Norway. Transactions of the North American Fisheries Society, 114, 182–94. Matthews, M.A., Poole, W.R., Dillane, M.G. & Whelan, K.F. (1997). Juvenile recruitment and smolt output of brown trout (Salmo trutta L.) and Atlantic salmon (Salmo salar L.) from a lacustrine system in western Ireland. Fisheries Research, 31, 19–37. McGinnity, P. (1997). The biological signiﬁcance of genetic variation in Atlantic salmon. PhD Thesis, Queen’s University, Belfast, 279 pp. Mills, C.P.R., Quigley, D.T. & Cross, T.F. (1985). Rearing and ranching of sea trout in the Burrishoole river system. In: Biology of Sea Trout (LeCren, E.D., Ed.). Summary of a symposium held at Plas Menai, October 1984. Atlantic Salmon Trust, Pitlochry, Scotland. Mills, C.P.R., Mahon, G.A.J. & Piggins, D.J. (1986a). The inﬂuence of stock levels, ﬁshing effort and environmental factors on angler’s catch of Atlantic salmon and sea trout. Aquaculture and Fisheries Management, 17, 289–97. Mills, C.P.R., O’Grady, M. & Macdonald, R.A. (1986b). A population estimate and some characteristics of the resident brown trout (Salmo trutta L.) population in L. Feeagh. Appendix I, Annual Report of the Salmon Research Trust of Ireland, XXXI, 81–9. Mills, C.P.R., Piggins, D.J. & Cross, T.F. (1990). Burrishoole sea trout – a twenty year study. Institute of Fisheries Management. 20th Annual Study Course Proceedings, pp. 61–78. Mortensen, E. (1977). Population survival, growth and production of trout Salmo trutta in a Danish stream. Oikos, 28, 9–15. O’Flynn, F.M. (1988). Investigation on the fecundity of sea trout (Salmo trutta) from the Burrishoole River system, Co. Mayo. BSc Thesis, University College of Cork, 77 pp. Parker, M.M. (1977). Lough Furnace, County Mayo; physical and chemical studies of an Irish saline lake, with reference to the biology of Neomysis integer. PhD Thesis, Dublin University. Piggins, D.J. (1961). The age and growth of sea trout of the Burrishoole River. Appendix. II, Annual Report of the Salmon Research Trust of Ireland, VI, 21–7. Piggins, D.J. (1975). Stock production, survival rates and life history of sea trout of the Burrishoole River system. Appendix. I, Annual Report of the Salmon Research Trust of Ireland, XX, 45–57. Piggins, D.J. (1984) Sea trout in the Burrishoole system. In: Biology of Sea Trout (LeCren, E.D., Ed.). Summary of a symposium held at Plas Menai, October 1984. Atlantic Salmon Trust, Pitlochry, Scotland.

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Poole, W.R., Dillane, M.G. & Whelan, K.F. (1994). Artiﬁcial reconditioning of wild sea trout (Salmo trutta L.) as an enhancement option; initial results on growth and spawning success. Fisheries Management and Ecology, 1(3), 179–92. Poole, W.R., Whelan, K.F., Dillane, M.G., Cooke, D.J. & Matthews, M. (1996). The performance of sea trout, Salmo trutta L., stocks from the Burrishoole system western Ireland, 1970–1994. Fisheries Management & Ecology, 3(1), 73–92. Poole, W.R., Byrne, C.J., Dillane, M.G., Whelan, K. & Gargan, P.G. (2002). The Irish sea trout enhancement programme: a review of the broodstock and ova production programmes. Fisheries Management & Ecology, 9(6), 315–28. Ricker, W.E. (1954). Stock and recruitment. Journal of Fisheries Research Board of Canada, 11, 559–623. Solomon, D.J. (1985). Salmon stock and recruitment, and stock enhancement. Journal of Fish Biology, 27 (Suppl. A), 45–57. Tully, O. & Whelan, K.F. (1993). Production of nauplii of Lepeophtheirus salmonis (Krøyer) (Copepoda: Caligidae) from farmed and wild salmon and its relation to the infestation of wild sea trout (Salmo trutta L.) off the west coast of Ireland in 1991. Fisheries Research, 17, 187–200. Walters, C. & Korman, J. (2001). Analysis of stock–recruitment data for deriving escapement reference points. In: Stock, Recruitment and Reference Points – Assessment and Management of Atlantic Salmon (Prévost, E. & Caput, G., Eds). Hydrobiologie et aquaculture, INRA, Paris, pp. 67–94. Whelan, K.F. (1991). Disappearing sea trout-decline or collapse? The Salmon Net, 23, 24–31. Whelan, K.F. & Poole, W.R. (1996). The sea trout stock collapse, 1989–1992. In: The Conservation of Aquatic Systems (Reynolds, J., Ed.). Royal Irish Academy. Proceedings of a Seminar held on 18–19 February 1993, pp. 101–10. Whelan, K.F., Poole, W.R., McGinnity, P., Rogan, R. & Cotter, D. (1998). The Burrishoole system. Chapter 11. In: Studies of Irish Rivers and Lakes (Moriarty, C., Ed.). Marine Institute, Dublin, 279 pp.

Chapter 20

Population Dynamics and Stock–Recruitment Relationship of Sea Trout in the River Bresle, Upper Normandy, France G. Euzenat, F. Fournel and J-L. Fagard Conseil Supérieur de la Pêche, Délégation Nord-Ouest, Station Salmonicole, rue des Fontaines, F-76260 EU, France

Abstract: The population dynamics of sea trout in the River Bresle, Upper Normandy, in northwestern France, have been studied since 1982 by continuous trapping of annual smolt and adult runs. The total numbers were estimated by double trapping and mark-recapture methods. Biological parameters and stock dynamics were derived from the census data. Over the 20-year period, adult runs ranged between 810 and 2850 ﬁsh, giving an estimated egg deposition of 1.7–7.2 million eggs, that is 650–2700 eggs per 100 m2 unit of accessible juvenile habitat. The average smolt run was over 6000 ﬁsh, ranging from 2200 to 10 000 ﬁsh annually. Resident brown trout contributed up to 12% of the downstream run. Most smolts were 1+ and most adults were 1+ sea winter (SW). 2+ smolts accounted for 17% of the population and 0SW and 2+SW ﬁsh contributed 6% and 5% respectively to the adult group. In an average year, previously spawned ﬁsh accounted for 16% of the run. The egg-to-smolt survival rate ranged between 0.05% and 0.40% (mean: 0.16% for smolt 1 and 0.20% for smolts 1 and 2), and smolt-to-adult survival (at ﬁrst trap return as 1+ SW ﬁsh) is 11% to 35% (mean: 20%). The survival rate of 2+SW ﬁsh can be up to 14% (mean: 6.7%). Ricker and Beverton–Holt models gave similar estimates of the stock–recruitment parameters. The recruitment maximum was close to 7000 smolts and the survival rate, at small stock sizes, was around 0.80%. The Ricker model gave a maximum spawning stock of 955 ﬁsh, equivalent to 2.4 million eggs, that is 875 eggs/100 m2 and a maximum production of 2.6 smolts per 100 m2 . At this level, the egg-smolt survival was 0.30%. The moderately strong relationship between survival rate and April rainfall suggests an adverse effect of silt-laden ﬂows at the critical period of fry emergence. River environmental factors appeared to moderate the development of the high potential of the stock and to buffer its variation, thus leading to an inter-annual balance achieved at the expense of egg survival. Implications for ﬁshery management and future research are brieﬂy considered. Keywords: Sea trout, population dynamics, runs, survival rate, return rate, stock, recruitment, S–R models, Ricker model, density-independence, catch.

Introduction Although the sea trout is relatively abundant in European rivers and supports important commercial and sport ﬁsheries, little detailed scientiﬁc information has been made available 307

308

Sea Trout

for this ﬁsh. The exceptions are the long-standing studies on the Black Brows Beck in Cumbria, England (Elliott & Elliott, 2006) and on the Burrishoole, Co. Mayo, Ireland (Poole et al., 2006). These studies were later complemented by studies on the North Esk, Scotland (Pratten & Shearer, 1983), the Welsh Dee (Davidson et al., 2000) and on the River Bresle, France (this chapter). Sea trout ﬁsheries have seldom been subject to speciﬁc management in comparison with salmon ﬁsheries. However, there is an increasing obligation to manage the stocks and their exploitation, and the recent development of restoration programmes is leading biologists and ﬁshery managers to call for a greater understanding of sea trout biology and better use of scientiﬁc knowledge and advice. Fish monitoring work on the River Bresle covers sea trout as well as salmon, and its purpose is to provide good estimates of the size of runs, up and down, to record rod and net catch statistics, and to relate these to each other. It has already provided scientiﬁc knowledge, upon which regional river restoration programmes and biological targets for salmon are based, to establish the stock–recruitment model and subsequent TAC for salmon ﬁshing in France (Prévost & Chaput, 1996; Prévost et al., 1996). It is now time to do the same for sea trout and this chapter presents the main ﬁndings of a 20-year study on the River Bresle, focused on the population dynamics of sea trout.

Study area The River Bresle (length = 72 km; slope = 0.26%; drainage area = 750 km2 ) ﬂows north-west through the Normandy-Picardy plateau and drains into the English Channel at Le Tréport (Fig. 20.1). The river lies within a chalky geological zone and is fed from alluvial deposits. Consequently, the water quality is generally fair, except for the occasional pollution by the glass frosting industry, and chronic agricultural run-off that probably affects the sensitive phases during incubation and fry emergence. The 20-year mean ﬂow is 7.8 m3 /s, with the highest ﬂow in February (mean = 9.4 m3 /s) and the lowest in September (mean = 6.4 m3 /s). Over the same period, the mean monthly water temperature has been 11◦ , ranging from 3.6◦ in January to 17.1◦ in July.

Methods Double trapping and marking recapture Sea trout runs were evaluated by double trapping, coupled with mark-recapture operations, using three trapping facilities: an upstream trap at Eu, 3 km from the sea; a main downstream trap at Beauchamps, 12 km from the sea and a secondary, smaller, downstream trap at Eu (Fig. 20.1). The traps have been run from 1982 (smolts) and 1984 (adults) to the present. Trapping takes place during the entire period of migration (December to April for kelts; late February to late May for smolts; March to January for upstream running adults) and the traps are checked each day. Over the 22-year period, smolt and adult trap data were missing for 5 years and 1 year, respectively. Trapping was impeded by high ﬂows in 2 years for the smolt run and once for the adult run, for which run estimations were impossible.

Population Dynamics and Stock–Recruitment

309

the Channel upstream trap and secondary downstream trap

Le Tréport

Eu

main downstream trap smolt and kelt trapping

upstream limit present

experiment years

marking

efficiency mean min-max

smolts

10/18

opercular punching

66% 28–81

adults

14/20

pelvic fin third cutting

58% 29–85

upstream limit potential

juvenile habitat

Fig. 20.1

River Bresle trapping facilities. Location of facilities and trapping efﬁciency.

Run estimation was usually by using Petersen mark-recapture methods. At the adult stage, all trapped fresh ﬁsh were marked by pelvic ﬁn clip. An average of 190 kelts (70–400 annually) were trapped during their downstream run, which was 20% on average of the trapped and marked upstream ﬁsh. For smolts, each ﬁsh caught was opercular punched, and 775 ﬁsh, or 17% of the total run, were trapped in the secondary trap on average for the year. First-trapping efﬁciency was estimated to be between 29% and 85% for adults (58% on average) and 28–81% for smolts (66% on average). For the years where mark-recapture estimation was not available, ﬁsh numbers were estimated using linear regression between R/C (the proportion of marked ﬁsh in the total recaptures taken in the second trap) and the April ﬂow. This relationship was selected after examining correlations for all months. The relationships between R/C and April ﬂow (Q) were described by: R/C (adults) = exp(0.610 − 0.143Q)

(n = 14, R2 = 0.88, SE = 0.124)

R/C (smolts) = normal(2811 − 1229 ln Q)

(n = 10, R2 = 0.60, SE = 0.286)

No corrections were made for direct or delayed mortality caused by trapping, handling and marking, the levels of which were assumed to be similar each year.

310

Sea Trout

Biological sampling Each adult ﬁsh was measured and weighed to the nearest millimetre and 10 g. Sex of adult ﬁsh was deﬁned using external criteria on the autumn run only. Scales samples were taken from an average of 60% of the ﬁsh. Fecundity, expressed as number of eggs per whole body weight, was estimated using 114 female ﬁsh caught by nets and rods. Ninety per cent of smolts were measured, and length–weight relationships and age distributions were estimated using selected samples. Smolt identiﬁcation During smolt trapping, ﬁsh were classiﬁed by external morphological characteristics: brightness, degree of silvering, ﬁnger mark burring, ﬁn lightening and blackening of the caudal ﬁn edge. However, there are differences in appraisal of these changing features of these ﬁsh between operators and the distinction between pre-smolts and resident trout can be problematic. To estimate the contribution of these ﬁsh to the run, a differential marking scheme was implemented for 6 years (from 1997 to 2000 and then 2003 and 2004), with ‘obvious’ pre-smolts and smolts being given a punch mark on the left operculum, whilst other ﬁsh (‘whitening’ trout, brown trout) were punched on the right operculum. Opercular marks were checked on ﬁsh taken in the second downstream trap, and the results used to correct the number of anadromous downstream migrants. Model ﬁtting The relationship between spawning stock and recruits was described using the stock– recruitment model equations of Ricker (1954, 1975) and Beverton & Holt (1957) ﬁtted to estimates of the numbers of smolts or recruits (R) that result from each year’s estimate of the numbers of spawners or stock (S). In both models, parameters a and b were used to estimate the survival rate and the recruitment maximum RM . Thus, using the Ricker model: RM = b, survival = exp(a)/2530, the latter number being the ‘average spawner’ fecundity, that is, 2530 eggs per spawner. With Beverton and Holt’s model: RM = a, and survival = a/b. Two reference points were derived from the stock-recruitment relationships: SM , deﬁning an exploitation level that maximises returning spawners and SG , deﬁning a spawning level that maximises the potential catch level (Crozier et al., 2003).

Results Life cycle and characteristics of the Bresle sea trout Most Bresle adult sea trout returned as 1.1 sea winter (SW) ﬁsh, of mean length 56 cm (range = 23–91 cm) and weight 2.25 kg (range = 0.14–9.3 kg) (Table 20.1, Fig. 20.2). Most returned to the river in early summer (70%) or autumn after a long distance migration in the North Sea (Fournel et al., 1987; Euzenat et al., 1991). Eighty-two per cent of the juvenile ﬁsh migrated downstream as 1+ smolt and the remainder migrate as 2+ ﬁsh. The average smolt size was 20 cm (range = 11–34 cm) (Table 20.1, Fig. 20.2) and 85 g (range = 15–390 g). Spawning runs were always dominated by 1SW ﬁsh (mean = 74%), with

Population Dynamics and Stock–Recruitment Table 20.1

Biological characteristics of sea trout in the River Bresle.

Years

Smolt

Adult

16

20

Mean length (all years) yearly average min. and max. (individual limits) Mean weight

19.8 cm 19.0–20.8 cm (11–34)

55.6 cm 53.1–58.8 cm (23–91)

85 g

Age structure (all years) (yearly min. and max %)

1 81.9% 70.5 88.5 2 17.5% 11.3 29.2 3 0.6 % 0.2 2 SMAa : 1.19 (1.08–1.30)

2.250 kg 2.070–2.750 0+ 5.7% 3.0 I+ 73.7% 61.8 II+ 16.9% 11.2 III+ 2.5% 1.2 IV+ and > 1.1% 0.4 F/M: 1.63 (1.41–2.58) F/Total: 0.62 (0.59–0.72) aver. F = 2032 eggs per kg log F = 2.715 log L − 3.84 (R2 = 83.4%) May to December 70% of numbers between May and August 30% of numbers between September and December

Sex ratio, F = females, M = malesb Fecundity (F), L = fork lengthc

Run timing

311

March/April/May peak in April (1 or 2 day per fortnight) 70% of numbers in April

8.8 79.6 27.4 5.1 2.6

a SMA: smolt mean age (Fahy, 1978) = (% S1 + (% S2 × 2) + (% S3 × 3))/100. b Sex ratio: only investigated on the autumn-running stock, afterwards the September 15th (1474 M – 2399 F). c Fecundity: based on 114 females taken by rod or in trap during autumn. Mean fork length = 56.1 cm (range 39.5–85.0 cm) and

mean weight = 2.460 kg (range 0.7–7.3 kg).

0+ (ﬁnnock) and 2SW maiden ﬁsh comprising around 6% and 5% of the run, respectively. On average, previous spawners comprised 16% of the run. Females outnumbered males by 1.6/1 among the autumn-running stock, and 2/1 when trapped as kelts from December to late April. Mean relative fecundity was 2030 eggs per kg (whole body weight in autumn). A small population of salmon lives sympatrically with the Bresle sea trout. From an average of 185 adults trapped yearly in the period 1980–90, the salmon run declined to an average of 85 ﬁsh after 1992. Multi-sea winter (MSW) salmon have disappeared, and 70% of the run now occurs after early September compared with 30% in the late 1980s. The available and accessible juvenile habitat for both species is 2700 units of 100 m2 , accounting for 38% of total accessible water surface area. Impassable dams prevent access to the upper third of the river and a current programme for ﬁsh passage restoration is expected to provide a gain of 1000 units.

Trap counts and estimated runs The mean annual trap catch of adults was 900 (range = 400–1445) and that of smolts was 4000 (range = 900–7700). Mean run estimates were 1600 adults (range = 800–2900) and 6200 smolts (range = 2200–10 000) and there was no statistical trend in the time series

312

Sea Trout

(a) 14 000

(b)

Number

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

12 000 10 000

Sea trout – Smolts Mean length: 198 mm

8000 6000 4000 2000

905

855

805

755

705

655

605

555

505

455

405

355

305

255

205

155

105

0

1400

Sea trout – Adults Mean length: 556 mm

800 600 400 200

Length classes

905

855

805

755

705

655

605

555

505

455

405

355

305

255

205

155

0 105

1

2 3 River-age classes

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

1200 1000

Percentage

0+

I+ II+ III+ IV+ Sea-age classes

Fig. 20.2 Length distribution (a) and age composition (b) of sea trout smolts, 1982–2004 (top) and adult returning sea trout, 1984–2003 (bottom). Shaded columns in age composition: low and upper percentage values.

(Table 20.2, Fig. 20.3). The smolt output was equivalent to a freshwater production of 2.3 smolts per 100 m2 . Contribution of resident trout Brown trout that have not apparently smoltiﬁed made up 18% in average of the total trout trapped in the main downstream facility, of which two-thirds were described as ‘whitening’ ﬁsh, that is to say 12% of the total run (Fig. 20.4). On average, 8.4% of these ‘resident’ trout were recaptured in the second facility 12 km downstream (range = 5–22% over the 6-year period). In comparison, 10.2% of the migratory trout were recaptured. The difference in ratios was not statistically signiﬁcant, which suggests ﬁsh designated as ‘resident’ migrated downstream as well as ﬁsh considered to be smolts. Forty per cent of these ‘resident’ trout recaptured in the lower facility were re-labelled as sea trout, whilst 3% of juvenile ‘sea trout’ were re-classiﬁed as resident ﬁsh. Though these ratios would have been applied to the numbers trapped in the main facility, this minimal correction was not made in view of the complexity of the situation and the need to have a clearer understanding. Egg deposition and river survival Mean annual egg deposition was 3.5 million, equivalent to 1300 eggs per 100 m2 unit of production habitat, and varied between 1.7 million eggs in 1994 and 4.7 million eggs in 1998 (Table 20.3).

Population Dynamics and Stock–Recruitment

313

Table 20.2 River Bresle: trapped and estimated numbers of adult and smolt sea trout, 95% conﬁdence limits and predicted values. Years

Adults Trapped numbers

1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Mean

Estimated numbers

Smolts Conﬁdence limits (95%)

1021 771 817 818 614 654 689 1250 1381 926 259 401 1138 1428 1444 1160 1079 (123) 491 798

1832 1094 1247 1394 1824a 1229a 1016a 1809 1755 1429a 811 1362 1337 1889 2067 2857 2787a

1620–2046 1015–1175 1123–1372 1246–1541 1360–2450 935–1635 775–1350 1809–2051 1542–1968 1070–1880 537–1085 1032–1691 1289–1384 1750–2029 1864–2271 2372–3343 2125–3690

1280 1746

1105–1456 1489–2004

902

1620

Trapped numbers 3703 4437 1357 4408 4310 1850 n.d. n.d. n.d. n.d. 2800 3192 2795 (1411) 5250 5700 6780 3900 893 n.d. (780) 3884 7712 3940

Estimated numbers

Conﬁdence limits (95%)

6284a 8083a 2212a 7318a 7295 3790a

4870–8810 6150–11 670 1740–3020 5725–10 020 6859–7730 2920–5285

3960a 4574 5815

3220–5185 4401–4747 5454–6175

6836 8091 9402 5655 2616

6580–7093 7626–8556 9091–9713 5356–5953 2266–2966

7617 10 000 6220

7024–8210 9727–10 273

n.d.: No data. (value) = incomplete count because of ﬂoods. a Predicted from April ﬂows.

For the period 1984–2002, the average egg-to-smolt survival for 1+ smolts was 0.16% (range 0.09–0.34%) and 0.20% for 1+ and 2+ smolts combined (range 0.05–0.40%). The lowest survival rates corresponded to the highest spawning stock, whilst the highest survival came from the lowest spawning stock and the second highest came from a medium stock. Unfortunately, data are lacking for the highest stock runs in 1999 and 2000. The equation of the best ﬁtted model (square root of Y ) is: survival = (−0.00023 stock + 0.75)2 (R2 = 0.53, SE = 0.0915). Relationship between river ﬂow and egg-to-smolt survival The correlation between river ﬂow and egg-to-smolt survival between mid-December and the end of April (the discharge associated with incubation and emergence) was not signiﬁcant. Using the monthly rainfall between December and May, only April rainfall showed an inverse relationship with juvenile survival and among the linear models ﬁtted,

314

Sea Trout 12 000

Annual number

10 000 Smolts 8000 6000 4000 ?

2000

?

0 82

84

86

88

90

92

94

96

98

00

02

04

02

04

3000 2500

Adults

2000 1500 1000 500

?

0 82

84

86

88

90

92

94

96

98

00

Fig. 20.3 Trapped and estimated numbers of adult and smolt sea trout. = no trapping, ? = no estimation, black columns = counted ﬁsh in traps, shaded = estimated ﬁsh.

(a)

(b) Left opercular punching

Right opercular punching

Main downstream trap marking

10.2%

8.4%

2d downstream trap recapture

pre-smolt and smolt sea trout marked or recaptured as:

‘full’ brown trout whiting brown trout

Fig. 20.4 Protocol for separating smolt and resident trout during mark-recapture experiments (1997–2004). (a) Left opercular punching, (b) Right opercular punching.

Population Dynamics and Stock–Recruitment Table 20.3 Upstream years

315

Egg-to-smolt survival rate (%). Adults estimated N

Rod catches

Spawners

1984 1985 1990 1991 1992 1994 1995 1996 1997 1998 2002

1832 1094 1016 1809 1755 811 1362 1337 1889 2067 1280

35 25 20 40 160 90 120 70 60 160 100

1800 1070 995 1770 1595 720 1240 1270 1830 1910 1180

Mean

1479

80

1400

Eggs

Smolts N

Survival (%)

S1

S2

S1

S1 + S2

4 055 209 2 639 536 2 479 568 4 388 612 4 329 836 1 732 577 3 386 641 2 973 065 4 583 444 4 687 705 2 866 336

6060 2686 3409 3866 5142 5846 6868 7611 4347 2129 8265

1090 n.d. 694 659 n.d. 1113 1668 1274 435 n.d.

0.15 0.10 0.14 0.09 0.12 0.34 0.20 0.26 0.09 0.05 0.29

0.18 (0.12) 0.17 0.10 (0.14) 0.40 0.25 0.30 0.10 (0.05) (0.34)

3 465 684

5112

990

0.16

0.20

Last column: ( ) smolt 1 + 2 survival, calculated on the basis of a mean 15% part of S2 in the run (full cohort) n.d.: no data.

the square-root model gave the best ﬁt: Survival = 0.6155–0.0034 April rainfall (R2 = 0.40, SE = 0.103, P = 0.036). Smolt output and sea survival Using the 12 downstream runs for which there were suitable data over the 20-year study, the average smolt to 1+ adult return rate was 21.7% and 24.3% for all maiden ﬁsh (ranges of 11–35% and 14–44%, respectively) (Table 20.4, Fig. 20.5). The two highest return rates correspond to small smolt runs whilst the three lowest return rates are for medium-to-large runs. The highest rates are given by runs with a high component of 2+ smolts (>20%). Stock–recruitment relationship The stock–recruitment data (Fig. 20.6) were not well matched to either the Ricker or Beverton and Holt models, though their parameter estimates were similar (Table 20.5). The Ricker model was considered to provide the best ﬁt to the data, and gave a spawning stock SM (for maximum recruitment) of 955 ﬁsh, equivalent to 2.4 million eggs laid, that is 875 eggs per 100 m2 and a maximum production RM of around 7000 smolts or 2.6 smolts per 100 m2 . At this level, the river survival was 0.30%. The stock required for replacement, SR , was 1550 spawners, laying 3.4 million eggs, whilst the stock level that provided maximum surplus production, SG , was 605 spawners (1.5 million eggs), which is 63% of SM . However, it is apparent that the Bresle sea trout stock shows a high variability without detracting from its stability. Numbers of returning adults were directly proportional to smolt output (Fig. 20.6b, d). The regression between adults and smolts is A = 0.22 S (R2 adjusted: 49%, SE: 448), giving a mean return rate of 22% (ranging 18–26%).

316

Sea Trout Table 20.4

Smolt-to-adult return rates (%).

Downstream years

Smolts estimated N

1983 1984 1985 1986 1987 1992 1993 1994 1996 1997 1998 1999 Mean

Adult numbers

Smolt return rate (%)

0SW

1SW

2SW+ maiden

All adults

1SW only

8083 2212 7318 7295 3790 3960 4574 5815 6836 8091 9402 5655

n.d. 161 88 71 88 55 54 58 79 79 114 177

1370 765 938 1004 1335 883 510 885 1487 1571 2114 1938

53 51 77 95 111 42 95 40 89 106 164 n.d.

44.2 15.1 16.0 40.5 24.7 14.4 16.9 24.2 21.7 25.4

16.9 34.6 12.8 13.8 35.2 22.3 11.1 15.2 21.8 19.4 22.5 34.3

6086

93

1233

84

21.7

24.3

30

40 %

Smolt-to-adult

Egg-to-smolt

n.d.: no data

0.20 0.30

%

0.4

22.3 22.0

0

0

10

20

Fig. 20.5 River and marine survival rates: Grey = annual and mean values; black = values given by Ricker model.

Discussion Simultaneous data on the biological characteristics of sea trout, adult and smolt run sizes and return rates are available for very few rivers. In this respect, the Bresle data set is very valuable, despite some gaps in the time series. The estimates of both smolts and spawners abundance are considered to be reliable, because the trapping was very intensive, trap efﬁciency was evaluated annually from mark-recapture results versus counts and the

Population Dynamics and Stock–Recruitment

8

4 adults (× 1000)

(b) 5

smolts (× 1000)

(a) 10

6 4

317

3 2 1

2

0

0 0

1

2

3

4

5

0

2

Spawners (× 1000)

(c)

4

6

8

10

smolts (× 1000)

10

smolts (× 1000)

8

6

4

2

0 0

(d)

1

2

3

4

5

Spawners (× 1000) 10

8 RM 6

4

2

0 0

SG SM 1

SR

2 3 Spawners (× 1000)

4

5

Fig. 20.6 Stock and recruitment relationships for Bresle sea trout. (a) Smolts versus spawners data; (b) adults versus smolts data; (c) Ricker (solid line) and Beverton and Holt (dashed line) ﬁtted curves and (d) BRPs; SM = 995, SG = 605, SR = 1550, RM = 7000.

318

Sea Trout Table 20.5 Parameters of Ricker and Beverton & Holt models. R = smolts, Se = eggs (estimated from run size times ‘average spawner’ fecundity, i.e. 2530 eggs per adult ﬁsh). Models

Ricker

smolts versus spawners

R = Se

a(1−S/b)

Beverton & Holt R = aSe /(1 + (a/b)Se )

a b R2 adjusted Standard error (SE) Survival s # slope at origin Recruitment (Rmax ) Spawning stock (Smax ) Egg deposition max

2.970 (2.090–3.850) 2 837 (1 873–3 801) 39.63% 2 245 0.79% (0.33–1.89%) 6 900 (2 665–17 050) 955 (897–987) 2 364 000

0.0084 (−0.035–0.052) 7 205 (–597–15 000) 40.65% 1 904 0.84% (…−5.17%) 7 200 ( …−15 000) ∞

correlation between April ﬂow and size run enabled estimates of the numbers of ﬁsh when the above methods failed. Smolt identiﬁcation and therefore the conﬁdence in the estimates of smolt numbers, requires further investigation. There are uncertainties in the association between morphological features and physiological smolt status, as shown for example by gill ATPase activity monitoring before and during the smolt run (Tanguy, 1993), though the large size of Bresle smolts does make selection easier. ‘Resident’ brown trout can contribute up to 12% to the ‘smolt’ run in the Bresle, similar to the 15% rate given by Ombredane et al. (1996) for a tributary of the River Touques (Lower-Normandy). It will be important to determine the true contribution of these ﬁsh to the downstream run, because they could affect the estimates of river survival and return rates, though only within the order of the distance of conﬁdence limits on numbers. All the same, the S–R relationship described here do not seem to have to be basically questioned. Although both Ricker and Beverton and Holt S/R models give parameter values that are consistent with those for other river systems and stocks (Prévost et al., 1996; Walters, 1996), the variability of recruitment (by factor of 3 for two sets of similar stock sizes) suggests that recruitment is independent of stock. However, there is a moderately strong inverse relationship between stock and river survival. Because of the small numbers of pairs for modelling, their median place in the possible range and the lack of contrast (limited range in egg deposition (Hilborn & Walters, 1992), the S/R relationship cannot be considered robust. Very low spawning sea trout stocks seem improbable in the Bresle, given the present stock size level. In this context, the case of salmon has not to be neglected. Census data show that the smolt and adult salmon are, on average, ×3 and ×13 lower than the sea trout runs (Euzenat & Fournel, unpublished data) and sea trout thus has the advantage of numbers over salmon. Any joint stock–recruitment relationship has yet to be investigated on the Bresle, but in such small chalk streams, it may be that interspeciﬁc interactions are more important than in larger rivers (see Milner et al., 2006) and are underestimated.

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Considering that there are few observations on the dynamics of sea trout and the great similarity of Bresle sea trout with grilse salmon, on the basis of their age structure, and long distance marine migration, it seems reasonable to compare their life-history straits and demographic characteristics. At 1285 eggs per 100 m2 of accessible habitat, the average sea trout egg deposition is near to that for grilse salmon, for example, 1100 eggs in the Nivelle (Dumas & Prouzet, 2003), 1400 eggs in the Oir (Prévost et al., 1996). Furthermore the estimated egg density that maximises sea trout smolt production in the Bresle ﬁts the range values for salmon: between 700 and 1000 eggs per 100 m2 unit (Gardiner & Shackley, 1991; Kennedy & Crozier, 1993). The smolt output of the Bresle is low, at 2.3 smolts per 100 m2 , similar to that of salmon observed on other monitored French rivers such as the Oir, Scorff and Nivelle (Baglinière & Champigneulle, 1986; Prévost et al., 1996; Dumas & Prouzet, 2003), but much lower than values in northern salmon rivers (Kennedy & Crozier, 1993; Chadwick, 1996). Adding the estimates of salmon yield (Euzenat & Fournel, unpublished data) the whole river capacity for migratory salmonids is nearer to three smolts per 100 m2 (range: 1.6–4.5). Considering the very bad river survival, and in comparison with the model results (SG and SM being 55% and 30% lower respectively), the sea trout egg deposition seems high and wastefully high. Egg-to-smolt survival of sea trout was indeed very low at 0.2% and below the mean rate observed for grilse salmon in the Bresle itself (around 0.6% on average, Euzenat & Fournel, unpublished data) and in other French and foreign rivers: 0.45% for the Nivelle (Dumas & Prouzet, 2003), 0.36% for the Oir (Prévost et al., 1996) and 0.36–0.62% for the Burrishoole in Ireland (Anon., 1998). It is less than the usual values of 1–5% for salmon populations in the world (Bley & Moring, 1998; Hutchings & Jones, 1998). It is not known why survival in freshwater was so low. The relatively high egg depositions, the always low survival rate, the inverse relation between some stocks–recruits pair data, and the inverse relation between stocks and survival rate suggest that compensatory densitydependence may be high on the Bresle. But random environmental factors, such as ﬂow regimes, as indicated by the inverse relation between survival and the rainfall during fry emergence, may also be important. However, Elliott (1985a, b, c) did not ﬁnd this adverse effect, and the impact of environmental factors on hatching success and fry to parr stage populations need to be investigated. The cause is evidently a mix of biological and physical regulation mechanisms, which are probably difﬁcult to distinguish. The low survival rate of juvenile sea trout appears to be compensated by high growth rate in the Bresle, as shown by the large 1+ smolt and the low MSA. Fahy (1978) recorded 6 year classes of smolts from the British Isles, with mean lengths ranging from 14 to 25 cm and MSA greater than 2. This good growth in the Bresle probably leads to better sea survival, as shown by the high return rate and the highest returns given by runs of a high component of 2+ smolts. Similar observations have been reported for Atlantic salmon (Peterson, 1971; Larsson, 1977; Chadwick, 1986), steelhead (Ward & Slaney, 1988) and coho salmon (Bilton et al., 1982). The low freshwater production is partly offset by the high return rates to the river (see also Fournel et al., 1990). The return rate of 1+ ﬁsh, is high at 24.3%, similar to the rate recorded for 1+ sea trout in the Irish River Burrishoole (21.2%) before the collapse in the

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late 1980s (Poole et al., 1996; Anon., 1998), or in the Vardnes river in Norway, 25% (Berg & Jonsson, 1990), and far higher than the common value for grilse in Bresle itself: 6.5% (Euzenat & Fournel, unpublished data) and other salmon rivers of the north-east Atlantic area: 6.2%, 7.2%, 8.4%, 9.6% and 12.6% in the Imsa (Norway), Ellidaar (Iceland), Corrib (Ireland), North Esk (Scotland) and Nivelle (France), respectively (Dumas & Prouzet, 2003; Anon., 2004). Although less drastic than the regulation within the river, regulation at sea is noticeable. A factor ×3 in the range of return rate leads to a spawning stock size variation equal to 60–150% of SM , which has a delayed effect on the regulation in the river and therefore on population dynamics. On the whole, it seems that the Bresle environment, through the limited spawning habitat and the probable adverse effect of ﬂow and water quality during the early life, acts against the development of the high reproductive potential of sea trout and buffers its variation. This leads to an apparent inter-annual balance as seen in the adult data series and in the proximity between the average adult run and the replacement stock SR , around 1550 ﬁsh. The balance is achieved at the expense of eggs, because half of the annual egg depositions seems to be ‘in excess’. The biological reference points (BRPs) of the Bresle model, SM and SG indicate that 600–950 ﬁsh would be harvestable, that being in agreement with the current catch situation. Mean annual catch of sea trout in the Bresle is 835 ﬁsh, in a ratio 5 : 1 between net and rod ﬁsheries. This catch is 38% of the home waters returns and is equivalent to 60% of the escapement (Fournel et al., 1999). Considering the apparent high level of the current stock in relation to the observed S/R relationship, some additional ﬁsh could be caught without reducing the stock’s sustainability: for example, by doubling the current rod catch which exploits only 10% of the run. The model must be used with caution, but it does suggest how to use the sea trout stock’s productivity better. Though a lot has been learnt about Bresle sea trout in this 20-year study, it must be continued in order to understand better and to explain the past as much as the future. Monitoring the runs has to be continued to make up for the interruptions in the time series and to strengthen the models. In particular, it will be instructive to follow the population’s response to the increase in juvenile habitat availability expected to result from the current programme of ﬁsh passage restoration. The high mortality in fresh water also needs to be investigated and comparisons with other studies made, as new data become available. In both cases, the question of anadromy versus residency is important, because the possible shifts in resident/anadromous habits in the population may throw light on the balance between environment and genetic factors in controlling migration. It will be informative to study the possible interactions between the healthy sea trout population and the low stock of salmon, the Bresle offering a rare opportunity to do that. Finally, the migrations, behaviour and survival of smolts and post-smolts in the sea are poorly understood, but are an essential part of understanding stock dynamics. This will aid management decisions on exploitation control and help to predict the consequence of environmental change in sea and in fresh water, given the links between freshwater growth/maturation balance and subsequent marine performance. International collaborative effort through marking experiments will be needed for this.

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Clearly, the long-term study on Bresle sea-trout population belongs to the very rare ‘club’ of index rivers where such studies are carried out, with the Burrishoole, North Esk, Black Brows Beck and Dee in Ireland, Scotland, England and Wales respectively (Piggins, 1976; Pratten & Shearer, 1983; Elliott, 1994; Poole et al., 1996; Davidson et al., 2000). Effort has to be made to protect and prolong these studies so that essential background data for the interpretation of short-term ﬂuctuations and trends are thus provided and made available.

Acknowledgements We thank the Head of CSP, at national and regional levels, for their administrative and ﬁnancial support during this long-term study, the Anglers Federation of the Department of Somme, owner of the secondary trap to let us to use this facility, the CSP agents and other local people for their assistance in the ﬁeld. Thanks also to Alain Bellido (senior lecturer, University of Rennes), Sébastien Delmotte (PhD student, University of Toulouse), Michel Larinier (hydraulic engineer R&D, CSP Toulouse) who provided advice in data processing.

References Anon. (1998). Annual report of the Salmon Research Agency of Ireland/Marine Institute, Furnace, Newport, No. 44, p. 66. Anon. (2004). Annual report of the Working Group on North Atlantic salmon. ICES, 2003, p. 99. Bagliniere, J.L. & Champigneulle, A. (1986). Population estimates of juvenile Atlantic salmon, Salmo salar, as indices of smolt production in R. Scorff, Brittany. Journal of Fish Biology, 29, 467–82. Bagliniere, J.-L. & Maisse, G. (1991). La Truite, Biologie et Écologie. INRA Edition, Paris, Serie Hydrobiologie et Aquaculture, 303 pp. Berg, O.K. & Jonsson, B. (1990). Growth and survival rates of the anadromous trout, Salmo trutta, from the Vardnes River, northern Norway. Environmental Biology of Fishes, 29(2), 145–54. Beverton, R.J.H. & Holt, S.J. (1957). On the dynamics of exploited ﬁsh populations. Fishery Investigations, London, Series 2, Vol. 19, pp. 1–553. Bilton, H.T., Alderdice, D.F. & Schnute, J.T. (1982). Inﬂuence of time and size at release of juvenile coho salmon (Onchorynchus kisutch) on returns at maturity. Canadian Journal of Fisheries and Aquatic Science, 39, 426–47. Bley, P. & Moring, J.R. (1988). Freshwater and ocean survival of Atlantic salmon and steelhead: a synopsis. US Fish and Wildlife Service, Biological report, Vol. 88(9), 22 p. Chadwick, E.M.P. (1986). Relation between Atlantic salmon smolts and adults in Canadian rivers. In: Atlantic Salmon: Planning for the Future (Mills, D. & Piggins, D., Eds). Proceedings of the Third International Atlantic salmon Symposium, 21–23 October 1986, Biarritz, France, pp. 301–324. Chadwick, E.M.P. (1996). Transportability of stock and recruitment relationships: theoretical constraints and practical recommendations. (Prévost, E. & Chaput, G., Eds). International Workshop on Spawning Targets for the Assessment and Management of Atlantic Salmon Stocks, Pont-Scorff. Crozier, W.W., Potter, E.C.E., Prevost, E., Schon, P.-J. & O’Maoileidigh, N. (Eds) (2003). A coordinated approach towards the development of a scientiﬁc basis for management of wild Atlantic salmon in the North East Atlantic (SALMODEL). Queen’s University of Belfast, Belfast, p. 431. Davidson, I.C., Wyatt, R.T. & Milner, N.J. (2000). Assessment of the effectiveness of byelaws in controlling salmon exploitation on the river Dee. In: Management and Ecology of River Fisheries (Cowx, I.G., Ed.). Fishing News Books, Blackwell Scientiﬁc Publications, Oxford, pp. 373–87. Dumas, J. & Proujet, P. (2003). Variability of demographic parameters and population dynamics of Atlantic salmon (Salmo salar L.) in a southwest French river. Journal of Marine Science, 60, 356–70. Elliott, J.M. (1985a). The choice of a stock–recruitment model for migratory trout, Salmo trutta, in an English Lake District stream. Archiv für Hydrobiologie, 104(1), 145–68.

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Elliott, J.M. (1985b). Population regulation for different life-stages of migratory trout, Salmo trutta, in a Lake District stream, 1966–83. Journal of Animal Ecology, 54(1), 617–38. Elliott, J.M. (1985c). Population dynamics of migratory trout, Salmo trutta, in a Lake District stream, 1966–83, and their implications for ﬁsheries management. Journal of Fish Biology, 27 (Suppl. A), 35–43. Elliott, J.M. (1994). Quantitative Ecology and the Brown Trout. Oxford Series in Ecology and Evolution, Oxford University Press, Oxford, 286 pp. Elliott, J.M. (2001). The relative role of density in the stock–recruitment relationship of salmonids. In: Stock, Recruitment and Reference Points, Assessment and Management of Atlantic salmon (Prévost, E. & Chaput, G., Eds). INRA edn., Paris, pp. 25–66. Elliott, J.M. & Elliot, J.A. (2006). A 35-year study of stock-recruitment relationships in a small population of sea trout: assumptions, implications and limitations for predicting targets. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 257–78. Elliott, J.M., Crisp, D.T., Mann, R.H.K. et al. (1992). Sea trout literature review and bibliography. Fisheries Technical Report, 3. NRA, 141 p. Euzenat, G., Fournel, F. & Fagard, J.L. (1991). La truite de mer en Normandie/Picardie. In: La Truite, Biologie et Écologie (Bagliniere, J.L. & Maisse, G., Eds). Vol. 303, Serie Hydrobiologie et Aquaculture, INRA, Paris, pp. 183–213. Fahy, E. (1978). Variation in some biological characteristics of British sea trout, Salmo trutta L. Journal of Fish Biology, 13, 123–38. Fournel, F., Euzenat, G. & Fagard, J.L. (1987). Rivières à truites de mer et à saumons de Haute-Normandie. Réalités et perspectives. Le cas de la Bresle. In: Restauration des Rivières à Saumons (Thibault, M. & Billard, R., Eds). Hydrobiologie et aquaculture, INRA, Paris, pp. 315–25. Fournel, F., Euzenat, G. & Fagard, J.L. (1990). Evaluation des taux de recapture et de retour de la truite de mer sur le bassin de la Bresle (Haute-Normandie/Picardie). Bulletin Français de la Pêche et de la Pisciculture, 318, 102–14. Fournel, F., Euzenat, G. & Fagard, J.L. (1999). La truite de mer et le saumon dans le Nord-Ouest. Stock, captures et échappement. Le point sur les programmes de restauration. Réunion du GRISAM, St Valéry-s/Somme, 10 p. Gardiner, R. & Shackley, P. (1991). Stock and recruitment and inversely density-dependent growth of salmon, Salmo salar L., in a Scottish stream. Journal of Fish Biology, 38, 691–6. Hilborn, R. & Walters, C.J. (1992). Quantitative Fisheries Stock Assessment: Choice, Dynamics and Uncertainty. Chapman & Hall, New York, 570 pp. Hutchings, J.A. & Jones, M.E.B. (1998). Life history, variation in growth rate thresholds for maturity in Atlantic salmon, Salmo salar. Canadian Journal of Fisheries and Aquatic Sciences, 55 (Suppl. 1), 22–47. Kennedy, G.J.A. & Crozier, W.W. (1993). Juvenile Atlantic salmon, Salmo salar, production and prediction. In: Production of Juvenile Atlantic Salmon, Salmo salar, in Natural Waters (Gibson, R.J. & Cutting, R.E., Eds). Canadian Special Publication on Fisheries and Aquatic Sciences, 118, pp. 178–87. Larsson, P.O. (1977). Size dependent mortality in salmon smolts plantings. ICES CM 1977/M, 43, p. 8. Milner, N.J., Karlsson, L., Degerman, E., Jholander, A., MacLean, J.C. & Hansen, L.-P. (2006). Sea trout (Salmo trutta L.) in Atlantic salmon (Salmo salar L.) rivers in Scandinavia and Europe. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 139–53. Ombredane, D., Siegler, L., Bagliniere, J.L. & Prunet, P. (1996). Migration et smoltiﬁcation des juvéniles de truite (Salmo trutta) dans deux cours d’eau de Basse-Normandie. Cybium, 20 (Suppl. 3), 27–42. Peterson, H. (1971). Smolt rearing methods, equipment and techniques used successfully in Sweden. In: Atlantic Salmon Workshop (Carter, W.M., Ed.). Manchester, New Hampshire. 25–26 March 1971, pp. 32–62. International Atlantic Salmon Foundation, p. 88. Piggins, D.J. (1976). Stock production, survival rates and life-history of sea trout of the Burrishoole river system. Salm. Res. Trust Irel. Inc., Ann. Report XX, pp. 45–57. Poole, W.R., Whelan, K.F., Dillane, M.G., Cooke, D.J. & Matthews, M. (1996). The performance of sea trout, Salmo trutta L., stocks from the Burrishoole system western Ireland, 1970–1994. Fisheries Management and Ecology, 3, 73–92. Poole, W.R., Dillane, M., DeEyto, E., Rogan, G., McGinnity, P. & Whelan, K. (2006). Characteristics of the Burrishoole sea trout population: census, marine survival, enhancement and stock recruitment, 1971–2003. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of

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the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 279–306. Pratten, D.J. & Shearer, W.M. (1983). Sea trout of the North Esk. Fisheries and Management, 14, 49–65. Prévost, E. & Chaput, G. (1996). Spawning Targets for the Assessment and Management of Atlantic Salmon Smolts. Determination, Precision, Transportability and Risks. International workshop, Pont-Scorff, France, 24–28 June, 1996. Prévost, E. & Chaput, G. (2001). Stock, Recruitment and Reference Points, Assessment and Management of Atlantic Salmon. INRA, Paris, 223 pp. Prévost, E., Bagliniere, J.-L., Maisse, G. & Nihouarn, A. (1996). Premiers éléments d’une relation stock– recrutement chez le saumon Atlantique ( Salmo salar) en France. Cybium, 20 (Suppl. 3), 7–26. Ricker, W.E. (1954). Stock and recruitment. Journal of the Fisheries Research Board of Canada, 11, 559–623. Ricker, W.E. (1975). Computation and interpretation of biological statistics of ﬁsh populations. Bulletin of the Fisheries Research Board of Canada, 2196. Solomon, D.J. (1995). Sea trout stocks in England and Wales. National Rivers Authority, R&D Report 25, 102 pp. Tanguy, J.M. (1993). La smoltiﬁcation de la truite de mer (Salmo trutta): caractérisation éco-physiologique des juvéniles en milieu contrôlé et en milieu naturel. Thèse de 3è cycle, ENSA Rennes, 107 pp. Walters, C. (1996). Analysis of stock and recruitment data for deriving spawning targets: models, ﬁtting, precision, diagnostics, pitfalls. In: International Workshop on Spawning Targets for the Assessment and Management of Atlantic Salmon Stocks. Determination, Precision, Transportability and Risks. (Prévost, E. & Chaput, G., Eds). Report of the Atlantic Salmon Spawning Target Workshop, Pont-Scorff, France, 24–28 June, 1996. Ward, B.R. & Slaney, P.A. (1988). Life history and smolt-to-adult survival of Keogh steelhead trout (Salmo gairdneri) and the relationship to smolt size. Canandian Journal of Fisheries and Aquatic Sciences, 45, 1110–22.

Section 4

MANAGING STOCKS AND FISHERIES

Chapter 21

The Spawning Habitat Requirements of Sea Trout: A Multi-Scale Approach A.M. Walker1 and B.D. Bayliss2 1 Centre

for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Pakeﬁeld Road, Lowestoft, Suffolk, NR33 0HT, UK 2 Environment Agency, Ghyll Mount, Gillan Way, Penrith 40 Business Park, Penrith, Cumbria, CA11 1BP, UK Abstract: The aim of this chapter is to further our understanding of the spawning habitat requirements of sea trout, Salmo trutta L., in order to facilitate the assessment of spawning habitat availability throughout catchments. Habitat data collected in the immediate vicinity of sea trout redds are presented from surveys in rivers of the southern, south-western and north-western regions of England and of central Wales, conducted over the period 1999–2003. In addition, the distribution of sea trout spawning throughout a catchment is assessed in relation to physical habitat characteristics, with data extracted from a GIS database and from paper maps. Keywords: GIS, redd, Salmo trutta L., sea trout, spawning habitat.

Introduction An understanding of the spawning habitat requirements of ﬁsh is vital for the effective and sustainable conservation of stocks and the management of associated ﬁsheries. This allows us to evaluate the potential impacts of habitat change/destruction, establish management priorities and provide baseline data for the calculation of biological reference points (BRPs) (e.g. conservation limits [CLs]). A large number of physical habitat characteristics associated with spawning sites (redds) chosen by various salmonid species have been examined, and the most common variables are grouped into three classes as: (1) hydraulic (depth, velocity); (2) spatial (redd dimensions, stream width, distance from the bank) and (3) substratum (particle size distribution, percentage of ﬁne materials and various related indices) (see Bardonnet & Bagliniere, 2000). In comparison with other species, however, there have been very few studies on sea trout spawning habitats. Crisp & Carling (1989) collected microhabitat data from redds constructed by Atlantic salmon (Salmo salar L.) and by sea and brown trout (Salmo trutta L.) from spate rivers and chalk-streams in England and Wales, but reported analyses using the combined data set. Elsewhere, the spawning habitat characteristics of sea trout have been reported from Norway (Heggberget et al., 1988) and France (Ingendahl et al., 1995). At present, therefore, our knowledge of the spawning habitat requirements of sea trout is insufﬁcient to include information on the availability and extent of suitable spawning 327

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habitat in models used to develop and set BRPs as management tools. Such knowledge is of particular importance to terrestrial and aquatic habitat management plans for the conservation of sea trout spawning habitats. The aim of this research was to investigate methods by which we can further both our knowledge of sea trout spawning habitat and develop tools to facilitate the assessment of spawning habitat availability and utilisation in relation to conservation and management. Here, we ﬁrst report the physical microhabitat characteristics, that is, in the immediate vicinity, of sea trout redds surveyed from rivers in England and Wales. The description of microhabitats associated with redds underpins the assessment of available spawning habitat, that it facilitates the identiﬁcation of suitable habitats. However, it is difﬁcult to apply microhabitat-based knowledge across a catchment, except where the catchment has been comprehensively mapped, and analytical methods are required which allow the availability of spawning habitat to be quantiﬁed using survey techniques applied at a much greater geographical scale. Such models have been developed to study the distributions of ﬁsh species and the abundances of salmonid juvenile populations at catchment (e.g. Beechie et al., 1994; Porter et al., 2000) and even regional scales (e.g. Kruse & Hubert, 1997; Argent et al., 2003) in the USA, but few studies have focused on the habitats used by spawning salmonids (Montgomery et al., 1999; Dauble & Geist, 2000). Therefore, second, we assessed the distribution of spawning sea trout throughout a catchment with respect to Geographic Information System (GIS) and other map-derived habitat features in order to explore the suitability of this method for the prediction of catchment-wide spawning habitat availability, and thus provide a method by which to prioritise areas for protection or restoration.

Methodology Microhabitat

Study sites Sea trout spawning habitats were investigated in ﬁve spate river catchments in England and Wales (Fig. 21.1, Table 21.1). The Rivers Blackwater and Beaulieu drain areas of the New Forest; although the Blackwater is a tributary of the chalk-stream River Test. The River Fowey drains central and southern Bodmin Moor and enters the sea from the south Cornish coast. The River Melindwr is an upper tributary of the Welsh River Rheidol. The Cumbrian River Kent is described in greater detail in the Catchment Scale Section.

Survey methods Reaches where sea trout were known to spawn were identiﬁed with the assistance of local Environment Agency staff and surveyed during the spawning seasons (typically October–January) between November 1999 and December 2003. Redds were surveyed within 24 h of the ﬁsh having completed spawning and then departed the redd site, and care was taken not to disturb spawning ﬁsh. Most sea trout spawned during the hours of darkness

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R.Kent

R.Melindwr (Rheidol)

R.Test R.Blackwater R.Beaulieu R.Fowey

Fig. 21.1 England and Wales with the locations of the ﬁve rivers in which sea trout spawning habitats were investigated.

Table 21.1 Location and physical catchment characteristics of rivers surveyed for spawning sea trout, the study period and numbers of redds sampled and water chemistry measured at the time redds were sampled.

Region Catchment area (km2 ) Mean daily ﬂow (m3 /s) Sampling period No. redds sampled Water temperature (◦ C) pH Conductivity (μS)

Blackwater

Beaulieu

Fowey

Melindwr

Kent

New Forest 105 0.85 Dec 1999 5 na na 282–380

New Forest na na Dec 2000 7 10–11 5.1–6.1 146–153

Cornwall 171 5.18 Nov 2000–Jan 2001 11 7–8 5.7–6.1 42–82

Mid-Wales 182 9.35 Nov 2001 11 9–11 6.1–6.6 63–67

Cumbria 209 8.94 Dec 2003 15 7 5.2–6.9 62–155

Note that catchment area and mean daily ﬂow statistics (1965–2001) are for the entire catchment, and for the River Rheidol of which the Melindwr is a tributary (EA data).

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or during spates when water clarity was poor. However, the lengths of 11 females observed spawning in the Rivers Fowey and Melindwr were visually estimated with reference to objects in or near the redd (total length range = 35–60 cm). The length and width of both the ‘pot’ and the ‘tail’ of redds were measured (cm) and total area (m2 ) calculated as if pot and tail were two ellipses (Ottaway et al., 1981). Depth (cm) and water velocity were measured over undisturbed substrate immediately upstream of the pot. Water velocity (m/s, averaged over 60 s) was measured at 0.6 of depth using a Valeport ‘Braystoke’ BFM002 current ﬂow meter. For the Rivers Melindwr and Kent, channel width and distance from mid-pot to the nearer-bank were measured to the nearest centimetre (cm), and cover was assessed in terms of (1) the depth of undercut bank (if any); (2) for the River Kent only, height above water surface of the bank nearer the redd (both to nearest cm) and (3) the proportion of sky directly above the redd that was occluded by vegetation (visual estimate). Redd substrate size was measured using two methods during the study. For the Rivers Blackwater, Beaulieu, Fowey and Melindwr, a substrate sample was collected from the side of the tail using a square fronted scoop with side 15 cm and depth 25 cm. This sample was then sorted through a stack of sieves with mesh diameters of 0.045, 0.063, 0.125, 0.25, 0.5, 1.0, 2.0, 4.0, 8.0, 16.0, 31.5 and 63.0 mm. The particles retained by each sieve were weighed and the cumulative percentage weight was plotted against particle size. A number of statistics can be used to characterise substrate composition (see Kondolf & Wolman, 1993), but here we report the median particle size. Based on the requirements for a comparative study of microhabitat selection between sea trout and Atlantic salmon (A.M. Walker, in preparation), the survey protocol was modiﬁed for the River Kent in order to facilitate sampling from a greater number of redds within the time restraints imposed by river conditions. A graduated (1 cm) quadrat, of side 10 cm, was placed on the surface of the redd tail and a digital video image recorded (after Geist et al., 2000). Still frames were subsequently captured from the video footage and substrate particles were measured within a 100 cm2 area (long-axis diameter and surface area) using image analysis software (Optimus). The particle size distribution for each sample was binned into six levels (longest axis <6, 6–25, 25–50, 50–75, 75–150 and >150 mm; after Geist et al., 2000) and a dominant size class derived according to percentage surface area per size bin. Particles that lay partially within the frame were measured and the area within the frame assigned to the appropriate bin.

Catchment scale Study site The River Kent catchment (∼209 km2 ) comprises the main River Kent (35 km in length and sourced at an elevation of 750 m) and two major tributaries, the River Sprint (19 km) and the River Mint (20 km), with numerous other smaller tributaries. Average annual rainfall (1968–2001) over the catchment was 1914 mm. The mean daily discharge measured near the estuary during the same period was 8.94 m3 /s, with Q10 and Q95 ﬂows

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of 21.08 and 1.123 m3 /s, respectively (Environment Agency data). The River Sprint ﬂows through the glacial valley of Long Sleddale, running south-east and largely parallel to the River Kent. Unlike the Kent valley, which widens and narrows several times over its length, the Sprint valley is relatively uniform in width and has an even gradient. The River Mint’s headwaters initially ﬂow in a south-easterly direction before turning south-west towards Kendal and its conﬂuence with the main River Kent. Land use in the upper reaches of the main stem and throughout the major tributaries is primarily agricultural with livestock grazing rough pasture. Land use further down the River Kent around Burneside becomes more mixed with increased urban characteristics affecting the nature of the river corridor. Much of the River Kent through Kendal has been modiﬁed as part of the River Kent Flood Prevention Scheme that was undertaken by North West Water between 1972 and 1977. The works involved channel widening and re-grading approximately 7 km of river, including lowering the level of the riverbed, bridge modiﬁcations and river wall construction. Downstream from Kendal, the river travels south through mainly agricultural land (improved pasture) with some steeply banked wooded areas and also passes through numerous deep limestone troughs before ﬂowing into Morecambe Bay. Sea trout and salmon populations Densities of juvenile trout and salmon are probably close to carrying capacity across the majority of the Kent catchment (based on regular juvenile monitoring surveys by the Environment Agency). The average annual run of sea trout past the resistivity counter at Sedgwick Weir, towards the mouth of the river, is 3957 ﬁsh for the period 1995–2004 (Anon., 2005). The average reported sea trout rod catch during the same period was 394 ﬁsh. The corresponding values for salmon are 2478 and 436 ﬁsh, respectively. Spawning distribution and physical habitat data The Environment Agency (north-west region) has historical records of the numbers and distribution of sea trout redds constructed throughout the River Kent catchment for a number of years. For the purposes of data recording, the entire catchment was split into reaches, typically bounded by landmarks (e.g. bridges, fences, weirs and overhead cables). Reach lengths varied between 0.24 and 4.46 km. Surveys were conducted by the same experienced EA ﬁsheries staff member who walked the riverbank during the hours of daylight over a representative period principally towards the end of the spawning season. The numbers of sea trout redds observed within each reach were recorded on Ordnance Survey (OS) 1 : 50 000 maps. At the commencement of the study, complete records were available for 5 years (seasons 1992–93, 1993–94, 1994–95, 1996–97 and 1998–99). Although the reach boundaries were consistent between years, data were not recorded for each reach every year, and adjoining reaches were combined in some years. Therefore, data were ﬁrst extracted from the 1993–94 dataset that included the maximum number of reaches (48), and data from other years were added to this spreadsheet, with reaches combined where necessary.

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Table 21.2 GIS-derived catchment scale habitat variables analysed and their deﬁnitions (after Dawson et al., 2002). Variable code

Deﬁnition

Altitude

Altitude of site (m) from the mean of altitude at the nearest points 500 m upstream and 500 m downstream of the sample point Gradient at site (m/km) from the difference in altitude between sites approximately 500 m upstream and downstream along the course of the river, divided by their actual distance apart Total length of watercourses upstream of the site (km) Catchment area above the site (km2 ) Distance from the site by the shortest route to the furthest point upstream (km) Distance from the site (km) by the shortest route to the deﬁned tidal limit Stream order according to Strahler (1957)

Gradient Upstream Catchment Source Tide Order

The 500 m interval is a feature of the CEH database and was chosen as the database was intended to underpin the River Habitat Survey method (Raven et al., 1997) for which 500 m is the survey unit.

The physical habitat characteristics associated with the lowermost boundary of each reach were derived either from GIS or map-based sources (Table 21.2). GIS data were extracted from the CEH River Network database (Dawson et al., 2002). Reach channel length (RL) and straight-line length (SL) were estimated from 1 : 10 000 scale OS maps and the sinuosity index was calculated as the quotient of SL and RL (Fukushima, 2001). Statistical analyses Analyses were conducted separately for two subsets of the data: either redds present/absent or, where redds were present, the density of redds per reach (log transformed). Habitat parameters were classed as continuous variables except for Order, which was treated as a category with six levels (1–6). The importance of each habitat variable in explaining the observed distribution of redds was assessed by a logistic (redds present/absent) or least squares (density of redds) regression model, but controlling for (1) the effects of sample year to account for differences in the numbers of redds observed at particular reaches between years and (2) for reach length in the presence/absence analysis alone. Multiple regression models were developed using a forward selection method and parameters retained if they improved the model ﬁt. Statistical signiﬁcance was assessed at α = 0.05 and all tests were conducted with JMP 5.1.1.

Results Redd microhabitat A total of 49 redds were sampled during the study (Table 21.1). The range and central tendencies of redd area, depth, water velocity and substrate size for each river, and for all redds combined, are presented in Table 21.3. Analysis of variances (ANOVA) indicated signiﬁcant differences between rivers for redd area (d.f. = 43, F = 2.86, P = 0.04), depth (d.f. = 42, F = 31.06, P < 0.0001), water velocity (d.f. = 42, F = 3.77, P = 0.02) and

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Table 21.3 Summary of spatial and physical habitat characteristics for sea trout redds from ﬁve rivers in England and Wales, and the data from all rivers combined. Blackwater Total redd area n 3 1.63–3.35 Range (m2 ) Mean ± 2.43 ± 0.87a,b SD Depth n 0 Range (cm) Mean ± SD Velocity n 0 Range (m · s−1 ) Mean ± SD Substrate n 5 Range 13.8–27.5 (mm) Mean ± 22.1 ± 5.7a,b SD Dominant class Range na (mm) Median na (mm)

Beaulieu

Fowey

Melindwr

Kent

Pooled

4 2.01–3.58

11 0.41–3.40

11 0.91–3.01

15 0.5–2.95

44 0.41–3.58

2.88 ± 0.65a

1.96 ± 0.95a,b

1.5 ± 0.58b

1.64 ± 0.84a,b

1.85 ± 0.87

6 22–41

11 26–51

11 8–31

15 12–23

43 8–51

32.5 ± 7.6a

37.9 ± 8.9a

17.0 ± 6.3b

16.0 ± 3.7b

24.2 ± 11.6

6 0.18–0.53

11 0.40–0.77

11 0.28–0.77

15 0.32–0.83

43 0.18–0.83

0.39 ± 0.12b

0.60 ± 0.11a,b

0.51 ± 0.17a,b

0.46 ± 0.14a,b

0.50 ± 0.15

7 11.6–27.5

11 3.7–39.6

9 18.4–42.7

15 na

32 3.7–42.7

18.6 ± 6.4b

20.6 ± 9.4a,b

30.0 ± 8.7a,b

na

23.0 ± 9.0

na

na

na

6–25 to 75–150

na

na

na

na

25–50

na

Means sharing the same letter code are not signiﬁcantly different at α = 0.05 (Tukey’s HSD test).

substrate (d.f. = 31, F = 3.22, P = 0.04). Post hoc tests split the rivers into two groups for depth, but the distribution was less clear for the other parameters (Table 21.3). As female spawner size was unknown for most redds and microhabitats were not fully surveyed at each spawning reach, it is not possible to determine whether differences in redd microhabitat parameters between rivers were a result of differential habitat selection by spawning ﬁsh, or whether the differences were in the habitats available to the ﬁsh. Redd tail length was not signiﬁcantly associated with water velocity (ANOVA: d.f. = 40, F = 0.46, P = 0.50), but was associated with median substrate size, though not signiﬁcantly so (ANOVA: d.f. = 29, F = 3.78, P = 0.06). Redds were located across the width of the channel (26 redds), distance of mid-pot from the bank as a proportion of channel width ranged from 6% to 50% (mid-point), but tended to be constructed towards the edges of the channel (channel position: mean ± SD = 28 ± 12%); even for those two sea trout that spawned in a relatively wide channel of 16–18 m (Fig. 21.2). An undercut bank was associated with three of 26 surveyed redds

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Sea Trout 10

Mid-pot to bank (m)

8

6

4

2

0

0

2

4

6

8

10

12

14

16

18

20

Channel width (m) Fig. 21.2 Channel position of sea trout redds from the Rivers Melindwr (open diamonds) and Kent (ﬁlled diamonds) in relation to channel width. Mid-channel is represented by the dashed line.

(range 22–32 cm deep). The median nearer-bank height was 90 cm (range 60–170 cm). Overhead cover was present above 8 of 15 redds surveyed on the River Kent, with a mean estimated sky occlusion of 53% (range 10–75%). Catchment scale

Spawning distribution A total of 3156 sea trout redds were recorded during the ﬁve spawning seasons from the 80.32 km of watercourse, partitioned into 42–48 reaches dependent on survey year. Sea trout redds were never found in two reaches: Kentmere Reservoir (0.46 km) and Sadgill (1.46 km), being the uppermost reaches of the Rivers Kent and Sprint, respectively. Overall, sea trout utilised 97.4% of the surveyed catchment for spawning. However, redds were not found in a number of other reaches in one or more years (range 23–75% of reaches).

Relationship between spawning distribution and habitat features When tested individually, all habitat variables except Gradient and Sinuosity had a signiﬁcant inﬂuence on the probability of predicting redd presence or absence (Table 21.4). When habitat variables were combined, the most statistically signiﬁcant ﬁt (d.f. = 7, χ 2 = 85.270, P < 0.001, r 2 = 0.278) was yielded by the model including distance from Source and Gradient (Table 21.5). The probability of sea trout spawning in a reach was associated with (1) increasing altitude and distance from tide; (2) decreasing channel length upstream; (3) catchment area and distance from source and (4) third-order

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Table 21.4 Stepwise model selection results from logistic regression of physical habitat data explaining the distribution of sea trout and salmon redds throughout the River Kent catchment, Cumbria: response variable is redds ‘present’ or ‘absent’. Term

Estimate

SE

χ2

P

Altitude Slope Upstream Catchment Source Order (1) Order (2) Order (3) Order (4) Order (5) Tide Sinuosity

0.013 −0.006 −0.007 −0.0002 −0.103 0.576 0.559 0.847 0.117 0.383 0.097 −0.351

0.003 0.019 0.001 <0.001 0.021 1.029 0.583 0.426 0.321 0.461 0.025 0.645

14.953 0.098 21.786 23.646 24.352 0.31 0.92 3.95 0.13 0.69 15.577 0.297

<0.001 0.757 <0.001 <0.001 <0.001 0.576 0.338 0.047 0.716 0.406 <0.001 0.586

Parameter codes are as in Table 21.2. The overall effect of Order was: d.f. = 5, χ 2 = 20.889, P < 0.001.

Table 21.5 Stepwise model selection results for multiple logistic regression of physical habitat data explaining the distribution of sea trout and salmon redds throughout the River Kent catchment, Cumbria: response variable is redds ‘present’ or ‘absent’. Term

Estimate

SE

χ2

P

Source Slope Intercept

−0.167 −0.113 2.756

0.029 0.031 0.705

33.43 13.03 15.29

<0.001 <0.001 <0.001

Parameter codes are as in Table 21.2.

streams (Table 21.4). This indicates that sea trout spawning tended to be associated with middle to upper areas of rivers and streams in the Kent catchment.

Relationship between redd density and habitat features The 5 years of data provided 122 sea trout redd densities greater than zero. These were signiﬁcantly associated with each of the habitat variables in isolation, except for Gradient and Sinuosity (Table 21.6). However, the best multivariate ﬁt of the data was achieved using all the habitat variables (ANOVA: d.f. = 16, F = 3.704, P < 0.001), and accounted for 36.1% of the variability in the data (Table 21.7). The relationships between density of sea trout redds in a reach and habitat parameters (Table 21.6) were similar to those reported for the presence of redds: redd densities tended to be greater towards the upper reaches of

336

Sea Trout Table 21.6 Stepwise model selection results from least squares regression of physical habitat data explaining the log transformed density of sea trout and salmon redds observed throughout the River Kent catchment, Cumbria. Parameter

Estimate

SE

Altitude Slope Upstream Catchment Source Order (1) Order (2) Order (3) Order (4) Order (5) Tide Sinuosity

0.003 0.013 −0.002 <−0.001 −0.019 −0.217 0.174 0.343 0.090 −0.212 0.022 0.265

<0.001 0.007 <0.001 <0.001 0.006 0.175 0.115 0.084 0.068 0.114 0.007 0.264

F

P

10.680 3.089 13.865 13.623 12.358 −1.24 1.52 4.07 1.32 −1.86 8.674 1.012

0.001 0.082 <0.001 <0.001 <0.001 0.217 0.132 <0.001 0.190 0.066 0.004 0.317

Parameter codes are as in Table 21.2. The overall effect of Order was: d.f. = 5, F = 4.362, P = 0.001.

Table 21.7 Stepwise model selection results from least squares multiple regression of physical habitat data explaining the log transformed density of sea trout and salmon redds observed throughout the River Kent catchment, Cumbria. Term

Estimate

SE

t

P

Order (1) Order (2) Order (3) Order (4) Order (5) Slope Sinuosity Tide Upstream Altitude Source Catchment Intercept

−1.017 0.103 0.300 0.125 −0.025 −0.030 0.539 −0.059 −0.005 0.003 −0.028 <0.001 −1.787

0.318 0.199 0.152 0.128 0.179 0.010 0.268 0.032 0.004 0.003 0.032 <0.001 0.662

−3.20 0.52 1.98 0.98 −0.14 −3.08 2.02 −1.85 −1.28 0.95 −0.86 0.42 −2.70

0.002 0.604 0.050 0.330 0.891 0.003 0.464 0.068 0.203 0.345 0.389 0.679 0.008

Habitat parameters are ranked in order of decreasing signiﬁcance and the codes are as in Table 21.2. The overall effect of Order was: d.f. = 5, F = 4.346, P = 0.001.

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the catchment, although there was a negative association with highest order streams from the multiple regression analysis (Table 21.7).

Discussion Microhabitat The range of values for hydraulic and substrate data collected for sea trout redds in this study are in general agreement with those reported in studies of salmonid (Ottaway et al., 1981; Crisp & Carling, 1989) or, more speciﬁcally, salmon (Moir et al., 1998) spawning in the UK: all of which reported considerable variation amongst the microhabitat conditions utilised. Although some ﬁndings are contradictory (see Crisp & Carling, 1989), it is generally considered that larger ﬁsh tend to spawn in areas of greater depth and water velocity and with larger mean substrate size (Kondolf & Wolman, 1993). Large ﬁsh are able to move larger substrate particles by way of the greater power of tail thrusts and because they can maintain station in higher velocity currents which, in turn, facilitate the downstream movement of particles from the cutting site (Kondolf & Wolman, 1993). Unlike previous studies of salmonid spawning in England and Wales (Ottaway et al., 1981; Crisp & Carling, 1989), we were unable to capture and measure spawning ﬁsh. Redd dimensions in salmonids are thought to reﬂect female size (Crisp & Carling, 1989) and might, therefore, provide a surrogate for ﬁsh size in analyses and explain some of the reported variability in spawning microhabitats. Although Crisp & Carling (1989) reported a signiﬁcant linear regression (r 2 = 0.71) between their ﬁsh length and redd dimensions data, the predicted lengths of sea trout from this study, and for salmon (A.M. Walker, in preparation), exceeded visually estimated lengths in 14 of the 16 cases, with an error of up to +107%. The relationship between ﬁsh size and redd dimensions may be complicated by the actions of more than one spawning female on a redd site, the construction of several redds by a single female, and by physical site characteristics such as substrate size distribution and instream ﬂow. Therefore, it was considered inappropriate to derive ﬁsh length estimates using the regression equation for those redd data where ﬁsh length was not visually estimated. Catchment scale Although microhabitat parameters in the immediate vicinity of redds indicate the type of habitat chosen by spawning sea trout, an assessment of habitat availability requires that the entire reach is surveyed at the same scale. While this may be practicable at the reach scale (101 –103 m), it is highly unlikely that time and resources will allow such surveys at the tributary or catchment scale. As an alternative approach, the results of our study indicate that the distribution of sea trout spawning throughout a catchment can be explained, in part, by variation in physical habitat data derived from GIS and other remote, map-based sources. However, we do not suggest that ﬁsh ‘choose’ spawning sites based directly on any of these reach-scale habitat characteristics, but rather that they are associated with (and thereby indicate the type) of microhabitat that is important to the ﬁsh in the choice of spawning site.

338

Sea Trout

For example, stream order is a categorical surrogate for stream size and often reﬂects the stream macrohabitat of a ﬁsh species (Sheldon, 1968). Water velocity will be inﬂuenced by gradient, catchment area and land use, the latter of which inﬂuences rate of run-off but is not included in the analyses because of its high variability in spate rivers. Water velocity is recognised as a key microhabitat feature of salmonid spawning sites, but it will also inﬂuence river bed substrate size distribution (Richards, 1982). Argent et al. (2003) assigned gradients into three categories: low (≤2%), medium (2–4%) and high (>4%) and stated that low-gradient streams (in Pennsylvania) are typically characterised by sand, silt and clay substrates while high gradient streams typically have cobble, boulder and rock (and bedrock) substrates and medium gradient streams occur between the two extremes and often have a mix of substrate types. Gradient and catchment area will also affect the form of the channel, that is its sinuosity and the ratio of rifﬂes to pools: both of which will inﬂuence the amount of suitable spawning habitat within a reach. Salmonids tend to spawn at the tails of pools, where intra-bed ﬂow is maximal and siltation low, thereby providing oxygenated water to the developing embryos and transporting waste materials away from the redd (McNeil, 1969). Fukushima (2001) found that the presence of Sakhalin taimen (Hucho perryi) redds was signiﬁcantly associated with channel sinuosity. Sources of error in the models The proportion of variability in the presence/absence and densities of sea trout redds explained by our models were similar to those reported by other studies linking salmonid abundance to site-speciﬁc (Rosenfeld et al., 2000: r 2 = 0.27) or reach-scale and watershedscale habitat variables (Pess et al., 2002: r 2 = 0.20–0.42). With almost any biological model, there will be a degree of error, or variation, that is not explained by the effects under study and may be attributed to bias inherent in the data collection. The surveys of the Kent catchment were biased against ﬁrst-order streams (i.e. width typically <0.5 m within the Kent catchment), perhaps because few, if any, sea trout were observed/expected to spawn in these smallest streams. This presumed bias was probably of relatively minor importance for the redd density models, because only the data for reaches where spawning was seen to occur were analysed. However, it might have contributed to the error associated with the models of the presence/absence of redds, and especially in the application of these models to other catchments, as it underestimated the number of reaches classed as ‘absent’. It is likely that the numbers of redds assigned to each reach were underestimated, either because they were obscured by shifts in the river bed substrates during high ﬂow events or were overcut by later spawning ﬁsh. Furthermore, some areas of the Kent catchment may contain prime spawning habitats, but have relatively low densities of redds because of historical factors that are not necessarily obvious today. For example, stock components in some catchments may have yet to recover after pollution events or the removal of barriers to migration. Finally, the variation across years in the distribution of sea trout redds throughout the catchment suggests that non-habitat factors may inﬂuence the inter-annual distribution of

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339

spawning sea trout. Although prolonged periods of high river levels would be expected in the autumn immediately before spawning, thereby theoretically providing access throughout the catchment, Moir et al. (1998) reported signiﬁcant positive relationships between median river and 95th percentile ﬂows and the median position and upstream limit, respectively of salmon spawning in the Girnock Burn. The ﬂow regimes encountered by ﬁsh will vary depending upon their time of entry into fresh water and the pattern of precipitation throughout the year. The physiological status of the ﬁsh during ﬁnal upstream migration will vary depending on the species, sex, state of maturation and period in fresh water. These factors, as well as ﬁsh size, will almost certainly inﬂuence a ﬁsh’s ability to reach some spawning grounds, and thereby inﬂuence the distribution of spawning ﬁsh. Future developments The redd observation data used in this study were not collected for any analytical purpose, but simply as a means of estimating annual spawning escapement and egg deposition. Similarly, the ‘automated extraction method’ was not developed by CEH with the physical habitat requirements of spawning salmonids in mind. Therefore, it is likely that the methods of data collection, both distribution of redds and physical habitat variables, could be improved considerably. Owing to the ad hoc nature of deﬁning reach boundaries, a wide range of reach lengths were used in this study (from 0.24 to 4.46 km). It is likely that future analyses would be more fruitful using a reach length that is more informative in relation to redd densities. However, the effects of some habitat variables may only be apparent within certain spatial scales, perhaps related to variation in the character assessed. For example, the relationship between the redd distribution of taimen and sinuosity was signiﬁcant when assessed at 50 m, but not at 100 or 200 m intervals (Fukushima, 2001). Therefore, future studies should compare models at different spatial scales. Such an exercise may well improve upon the survey design used in this study, and indicate the optimum scale to be used. The predictive power of these models might also be improved by the addition of reachscale habitat variables not included in these analyses. Channel width has been suggested as an explanatory habitat variable at microhabitat (Heggberget et al., 1988; Knapp & Preisler, 1999) and reach scales (Dauble & Geist, 2000). Channel width may be represented, in part, by stream order, although the categorical format of stream order will restrict its use as a surrogate. Although not incorporated in these analyses, the collection of summary reach width measurements is possible in a relatively short period of time using a laser distance ﬁnder. An additional use for stream width data, however, would be to provide in association with reach length, a measure of reach surface area (RSA), which might be superior to reach length as a covariant in the modelling procedure, as one would expect the density of redds to be more closely related to the former than to the latter.

Conclusions The range of microhabitats used by spawning sea trout, both within and between catchments, is such that while it is possible to identify those areas of a catchment where spawning would

340

Sea Trout

not be expected (e.g. where the river ﬂows over bedrock or over silt or sand), it is unlikely that analyses at such a scale will be suitable, or practicable, for the quantitative assessment of sea trout spawning habitat availability. The logistic regression models developed here demonstrate that the assessment of reach-scale habitat characteristics from GIS-based sources could provide a powerful diagnostic tool for identifying potential areas of salmon and sea trout spawning throughout a catchment. This could include those areas where spawning is limited or non-existent but could be improved through restorative actions, and thereby facilitate improvements in salmonid-orientated catchment management practices. However, we now need to test the predictive power of these models for further redd distribution data for the Kent catchment and their transferability to other catchments.

Acknowledgements Funding for this research was provided by the UK Department of Environment, Food and Rural Affairs, as part of R&D contract SFO231. We would like to thank the riparian owners who provided us with access to the various rivers and those EA staff whose guidance and assistance with habitat surveys was essential, including David Hunter, Richard Redsull, Mark Sidebottom, Andy Williams, Anthony Bevan, Gethyn Thomas, Graeme McKee, John Foster and Peter Evoy. We also thank those CEFAS staff who assisted in habitat surveys, substrate particle analyses and data processing, especially Bill Riley, Mark Ives, Robert Bush, Barry Bendall, Al Cook, Clare Mason and Alison Reeve. We are grateful to Jon Barry for having provided guidance on statistical analyses and to Mike Pawson for having improved the manuscript.

References Anon. (2005). Salmon and freshwater ﬁsheries statistics for England and Wales, 2004. Environment Agency, Cardiff, 58 pp. Argent, D.G., Bishop, J.A., Stauffer Jr, J.R., Carline, R.F. & Myers, W.L. (2003). Predicting freshwater ﬁsh distributions using landscape-level variables. Fisheries Research, 60, 17–32. Bardonnet, A. & Bagliniere, J.L. (2000). Freshwater habitat of Atlantic salmon (Salmo salar). Canadian Journal of Fisheries and Aquatic Sciences, 57, 497–506. Beechie, T., Beamer, E. & Wasserman, L. (1994). Estimating coho salmon rearing habitat and smolt production losses in a large river basin, and implications for habitat restoration. North American Journal of Fisheries Management, 14, 797–811. Crisp, D.T. & Carling, P.A. (1989). Observations on siting, dimensions and structure of salmonid redds. Journal of Fish Biology, 34, 119–34. Dauble, D.D. & Geist, D.R. (2000). Comparison of mainstem spawning habitats for two populations of fall chinook salmon in the Columbia River Basin. Regulated Rivers: Research & Management, 16, 345–61. Dawson, F.H., Hornby, D.D. & Hilton, J. (2002). A method for the automated extraction of environmental variables to help the classiﬁcation of rivers in Britain. Aquatic Conservation: Marine and Freshwater Ecosystems, 12, 391–403. Fukushima, M. (2001). Salmonid habitat-geomorphology relationships in low-gradient streams. Ecology, 82, 1238–46. Geist, D.R., Jones, J., Murray, C.J. & Dauble, D.D. (2000). Suitability criteria analyzed at the spatial scale of redd clusters improved estimates of fall chinook salmon (Oncorhynchus tshawytscha) spawning habitat use in the Hanford reach, Columbia River. Canadian Journal of Fisheries and Aquatic Science, 57, 1636–46.

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Heggberget, T.G., Haukebo, T., Mork, J. & Stahl, G. (1988). Temporal and spatial segregation of spawning in sympatric populations of Atlantic salmon, Salmo salar L, and brown trout, Salmo trutta L. Journal of Fish Biology, 33, 347–56. Ingendahl, D., Marty, A., Larinier, M. & Neumann, D. (1995). The characterisation of spawning locations of Atlantic salmon and sea trout in a French Pyrenean river. Limnologica, 25, 73–9. Knapp, R.A. & Preisler, H.K. (1999). Is it possible to predict habitat use by spawning salmonids? A test using California golden trout (Oncorhynchus mykiss aguabonita). Canadian Journal of Fisheries and Aquatic Sciences, 56, 1576–84. Kondolf, G.M. & Wolman, M.G. (1993). The sizes of salmonid spawning gravels. Water Resources Research, 29, 2275–85. Kruse, C.G. & Hubert, W.A. (1997). Geomorphic inﬂuences on the distribution of Yellowstone cutthroat trout in the Absaroka Mountains, Wyoming. Transactions of the American Fisheries Society, 126, 418–27. McNeil, W.J. (1969). Survival of pink and chum salmon eggs and alevins. In: Symposium on Salmon and Trout in Streams (Northcote, T.G., Ed.). University of British Columbia Press, Vancouver, British Columbia, Canada, pp. 101–17. Moir, H.J., Soulsby, C. & Youngson, A. (1998). Hydraulic and sedimentary characteristics of habitat utilized by Atlantic salmon for spawning in the Girnock Burn, Scotland. Fisheries Management and Ecology, 5, 241–54. Montgomery, D.R., Beamer, E.M., Pess, G.R. & Quinn, T.P. (1999). Channel type and salmonid spawning distribution and abundance. Canadian Journal of Fisheries and Aquatic Science, 56, 377–87. Ottaway, E.M., Carling, P.A., Clarke, A. & Reader, N.A. (1981). Observations on the structure of brown trout, Salmo trutta Linnaeus, redds. Journal of Fish Biology, 19, 593–607. Pess, G.R., Montgomery, D.R., Steel, E.A., Bilby, R.E., Feist, B.E. & Greenberg, H.M. (2002). Landscape characteristics, land use, and coho salmon (Oncorhynchus kisutch) abundance, Snohomish River, WA, USA. Canadian Journal of Fisheries and Aquatic Sciences, 59, 613–23. Porter, M.S., Rosenfeld, J. & Parkinson, E.A. (2000). Predictive models of ﬁsh species distribution in the Blackwater Drainage, British Columbia. North American Journal of Fisheries Management, 20, 349–59. Richards, K. (1982). Rivers: Form and Process in Alluvial Channels. Methuen, London. Rosenfeld, J., Porter, M. & Parkinson, E. (2000). Habitat factors affecting the abundance and distribution of juvenile cutthroat trout (Oncorhynchus clarki) and coho salmon (Oncorhynchus kisutch). Canadian Journal of Fisheries and Aquatic Sciences, 57, 766–74. Sheldon, A.L. (1968). Species diversity and longitudinal succession in stream ﬁshes. Ecology, 49, 193–8. Strahler, A.N. (1957). Quantitative analysis of watershed geomorphology. American Geophysical Union Transactions, 38, 913–20.

Chapter 22

Research Activities and Management of Brown Trout and Sea Trout (Salmo trutta L.) in Denmark G.H. Rasmussen Danish Institute for Fisheries Research, Department of Inland Fisheries, Vejlsøvej 39, DK – 8600 Silkeborg, Denmark

Abstract: This chapter describes the habitat problems and status of Danish trout streams and the research activities (e.g. monitoring, population dynamics of sea trout stocks, habitat demands of fry and ﬁngerlings, predation on seaward migrating smolt, telemetry studies in fresh and salt water and populations genetics) that take place at present and are used for management of brown trout and sea trout in Denmark. Trout stocking in inland and coastal waters requires permission from the relevant authorities. The ﬁsh are stocked in numbers appropriate to the given habitat and in accordance with stocking plans, which prescribe number, size/age and stocking position. The concept of carrying capacity and authenticity is the dominant guiding principle for all stockings. The ﬁsh used for stocking are offspring of either wild ﬁsh caught in nature or national domesticated ﬁsh. From 2006, only F1 offspring from wild ﬁsh may be stocked. Keywords: Salmo trutta L., habitat, restoration, stockings, stocking strategy, stocking schemes, population dynamics, monitoring, smolt migration, telemetry, predation, genetics.

Background Virtually all rivers, streams and lakes and brackish wetlands in Denmark are inﬂuenced by human activities. These activities have adversely affected most ﬁsh stocks and the main task is to gather information about the problems and ﬁnd solutions or take remedial action. This is primarily done by conducting ﬁeld and laboratory research projects. The environmental problems that occur can be summarised as follows: nutrient content is high, resulting in the extensive production of water weeds. Most of the streams have at some stage been regulated and channelled. Water diversions by numerous weirs and dams at rainbow trout ﬁsh farms, old mills and small hydropower stations create great problems for the freshwater fauna, especially the migratory species. During the past two decades restoration work in streams and rivers has been undertaken, many ﬁsh-ways have been established and very few streams are now polluted by wastewater. Most streams are small and shallow with abundant growth of water weeds necessitating frequent weed clearance. This weed clearance often results in a degradation of spawning and rearing habitat available for ﬁsh. The most important ﬁsh species for recreational ﬁshing are: brown trout, sea trout, 342

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Atlantic salmon, grayling, eel, pike, perch and the introduced species pikeperch (Rasmussen et al., 2002).

Management The original number of river systems in Denmark with wild trout was 887 with an estimated annual smolt production of 2.6 million, but this number was reduced to about 250 river systems in 1900 and the lowest number recorded was in 1960 when only 176 river systems contained spawning trout and smolt production was 0.177 million (Christensen et al., 1993). The present number of wild smolts produced today from about 360 river systems is estimated at 0.851 million. Legal size in fresh water is 30 cm and in salt water 40 cm. Large-scale national stocking programmes for trout covering all running waters in Denmark were initiated from 1987. The stocking programme (Rasmussen & GeertzHansen, 1998, 2001) implements stocking of trout in fresh and salt water according to stocking schemes that stipulate year classes, that is fry, half-yearlings, yearlings and smolts, numbers of trout and localities in cooperation with local ﬁshery associations. The numbers of stocked ﬁsh are determined from estimates of carrying capacity, based on descriptions of the physical habitats combined with monitoring of the local wild ﬁsh populations by electroﬁshing. In accordance with the national guidelines, an increasing number of stocked ﬁsh are offspring from wild ﬁsh. From 2006 only wild F1 ﬁsh will be used for brood stocks. Results from the monitoring programme, where data from about 7000 electroﬁshing stations covering all river systems are collected, are used to determine stocking practice. About 1000 stations are surveyed annually, thus giving a 7-year cycle. This provides information about the outcome of stocking in small and medium-sized streams, that is up to 6–7 m wide. Results from stockings in rivers and in salt water are monitored (e.g. Pedersen & Rasmussen, 2000; Pedersen et al., 2003; Ruzzante et al., 2004). A new project is running where wild smolts are compared with F1 reared smolts and the present results show that total yield is higher for wild smolts and that spawning stock mostly consists of wild sea trout. As a ‘guesstimate’, four F1 reared smolts equals one wild smolt. An internet-based handbook on ﬁsheries management (www.ﬁskepleje.dk) was established in 2002. This was produced to promote management activities including: ﬁsh biology, conservation genetic guidelines, administrative rules, procedures for catching wild ﬁsh for brood stock in accordance with genetic guidelines, dissemination of research results from scientists to ﬁshery associations and other stakeholders. The handbook provides excellent opportunities for swift interaction between management and various user groups. According to responses from users and numbers of visitors, the ‘home page’ has been a great success so far. In addition to the indirect web-based interaction with users, the institute has hosted public information meetings to increase the level of communication between management and stakeholders. Most of the researchers are regularly participating in meetings arranged by local associations, where they discuss new scientiﬁc discoveries and their potential impact on management.

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Research topics Many projects within this research area have concentrated on revealing key factors for the survival of stocked ﬁsh and new information about population dynamics of trout. Population dynamics of trout Distance in rivers from rearing area to the sea varies from under 1 km and up to 160 km. The annual biological production of trout in the streams is up to about 25 g wet wt/m2 indicating a food consumption up to about 180 kcal/m2 (Rasmussen, 1986). In Danish river systems a substantial part of the trout populations migrate to the sea as smolts during spring. The smolts (25% males and 75% females) leave fresh water in March to May as 1–4 year olds (10–25 cm). They are dominated by 2-year-old ﬁsh and the streams produce annually up to about 20 smolts per 100 m2 (Rasmussen, 1986; Christensen et al., 1993). Sexual maturity of non-migratory trout is usually attained at the age of 2 years (males) and 3 years (females) and sea trout predominantly mature after one or two summers in the sea. Spawning runs of the dominant younger year classes starts in October–December; older year classes may ascend as early as May (Christensen et al., 1993; Aarestrup & Jepsen, 1998). Many projects have concentrated on monitoring the effects of stocking and population data. Sea trout smolts and mature sea trout have been tagged with external Carlin and telemetric (i.e. acoustic and electronic) tags to compare survival between strains and between domesticated versus offspring from wild ﬁsh. Since the mid-1990s we have worked with the habitat requirements of brown trout in particular, that is fry, ﬁngerlings and older brown trout, in relation to depths, bank vegetation and cover, water weeds, roots, stones and water current, using telemetric and DST-tags (data storage tags), electroﬁshing, underwater video and snorkelling. The results are important when regulated streams are restored and managed by the local authorities and will be used in issuing regulations for land use close to rivers. We have worked with the effects and outcomes of restoration of salmonid spawning areas and the effects on brown trout and invertebrates from different methods of cutting water weeds. Population dynamics, that is numbers, growth, mortality, predation and smolt production of both resident and migratory brown trout have been monitored in ﬁrst-order and secondorder streams over several years. This is important for learning about natural variations in these parameters over a long-term scale in relation to monitoring and stocking, where electroﬁshing takes place every seventh year. These small streams are typically stocked with fry and ﬁngerlings, and do not necessarily represent the population dynamics of brown trout and sea trout in larger river systems. Therefore, we have estimated the same population parameters in a whole river system, but still comparatively little is known about production capacity in larger river systems. In order to estimate the total number of smolts reaching the sea and the derived potential for ﬁsheries we have focused on smolt migration (Aarestrup et al., 2002) and mortalities in rivers, lakes and man-made reservoirs through which the smolts have to pass using radio telemetry and smolt traps. The mortality in lakes and reservoirs is substantially higher than

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in rivers, and is caused by predation from ﬁsh and birds (Jepsen et al., 1998). Projects focusing on the behaviour of pikeperch and pike in rivers and reservoirs have used radio telemetry to demonstrate that these two predators cause high mortality of migrating trout smolts, especially by river inlets and outlets (Jepsen et al., 2000). This predation has a substantial effect on the total number of smolts entering the sea and thus can be detrimental to the whole population. In most rivers smolts must pass several weirs built for leading water through rainbow trout farms. Our studies of smolt migration have shown that there is a high mortality associated with passing these obstacles and that many smolts pass through the screens and into the ponds (Aarestrup & Koed, 2003; Aarestrup et al., 2003; Svendsen et al., 2004). Mortality rates for both hatchery and wild ﬁsh are alarmingly high and projects have been addressing these problems. Other projects have dealt with the physiological processes involved in smoltiﬁcation of seaward trout, and the results show a close connection between genetics, physiological status (i.e. gill-Na/K+ -ATPase activity) and migratory performance (Aarestrup et al., 2000; Nielsen et al., 2003, 2004). These results are used in the management of smolt stocking and in the construction of ﬁsh/fauna ways around obstacles in rivers as well as in projects aiming at establishing new wetlands to remove nutrients from rivers. Our knowledge about salmonid populations in rivers has increased during the past decade, but much less is known about the same species in the sea, even though many traditional tagging experiments with smolts of hatchery origin have provided useful data about growth, migration routes and exploitation rates. Therefore, a project has been started where wild smolts of sea trout are caught in a trap, tagged with acoustic transmitters and followed through the estuary by data-loggers at ﬁxed stations and manual tracking. The preliminary results are promising and the project will be continued and combined with feeding studies of post-smolts and DST-tagging of smolts and/or spent sea trout. The smolts are predated in estuaries by different species of birds and the impact can be important (Dieperink et al., 2001, 2002). New projects are addressing these questions where smolts and kelts are acoustic tagged and followed downstream within the river and through fjords. In most Danish river systems ﬂowing into the Kattegat and the North Sea, large numbers of immature sea trout migrate from sea to fresh water during the winter season where they are heavily ﬁshed for by anglers. Supposedly, those ﬁsh enter fresh water because of osmotic problems in cold, highly saline sea water. We do not know if they ‘home’ to certain rivers or mix with sea trout from other rivers. Thus, it is difﬁcult to optimise the management of these ﬁsh, which may constitute an important proportion of the sea trout population. A project including genetic analysis addresses these questions. Genetic structure and stocking impact in brown trout and sea trout During the 1980s large-scale stocking programmes were initiated in Denmark, involving releases of large numbers of hatchery-reared and, in most cases, domesticated trout. It was assumed that there were only a few remaining indigenous brown trout and sea trout

346

Sea Trout

populations left. However, in order to obtain more precise information, it was decided to initiate population genetics research aimed at salmonid ﬁshes (Hansen, 2003). We have created inventories of microsatellite variation in all commercial trout strains used for stocking. Combined with assignment tests, this has provided an invaluable tool for estimating stocking impact. Development of methods for analysing DNA from historical samples (50–90-years-old archived scale samples) has enabled the comparison of past (prior to stocking) and contemporary populations (Hansen et al., 2000, 2001; Hansen, 2002). We found that several populations subject to intensive stocking activity using domesticated strains remain partly or fully indigenous, even though there are also examples of heavily introgressed populations (Hansen et al., 2000). Clearly, stocked trout reared for several generations in captivity are subject to strong negative selection in the wild and disappear if stocking comes to an end. Nevertheless we have also found that stocked domesticated trout and wild trout do interbreed while stocking is ongoing, but interbreeding appears to be mainly mediated by stocked trout adopting a resident life history rather than by becoming anadromous. The ability to analyse historical samples has provided an understanding about the genetic population structure in trout. We have used time series of microsatellite data to estimate effective population sizes. These are high, at least 500, and by considering this, along with other demographic parameters, we have made predictions about the scale and extent of local adaptations.

Future perspectives In conclusion, today much is known about the initial life stages of brown trout, but less is known about the productivity of larger rivers and the biology of sea trout in the marine environment. This will be a focus area for many studies in the future. Most results from studies of both wild and hatchery smolts show that mortality associated with passing obstacles (weirs, dams, reservoirs) is a key factor for many populations of migratory trout in Danish rivers. Investigations on the habitat demand of brown trout in streams and rivers will be continued; including habitat modelling. Another major task for us will be to resolve the conﬂicts between conservation, the restoration of ﬁsh stocks and land use (aquaculture, hydropower, agriculture and forestry). Our future work will clearly be much focused on assessing the biological signiﬁcance of genetic differentiation, that is the presence of local adaptations (Hansen et al., 2002). This is both of basic scientiﬁc interest and of importance to management for reintroductions into rivers where the original populations have been extirpated. We have recently started analysing loci subject to selection (such as Major Histocompatibility Complex genes) and our future perspective is to identify QTL markers (Slate, 2005) linked to ecologically important traits. Our aim is to analyse differentiation at selected genes and QTLs on both a spatial and temporal scale (using historical samples), for example to estimate the importance of changes in selection regimes such as disease pressure and global warming. We also want to compare differentiation at selectively neutral molecular markers with the differentiation of quantitative traits.

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References Aarestrup, K. & Jepsen, N. (1998). Spawning migration of sea trout (Salmo trutta L.) in a Danish river. Hydrobiologia, 371/372, 275–81. Aarestrup, K. & Koed, A. (2003). Survival of migrating sea trout (Salmo trutta) and Atlantic salmon (Salmo salar) smolts negotiating weirs in small Danish rivers. Ecology of Freshwater Fish, 12, 169–76. Aarestrup, K., Nielsen, C. & Madsen, S.S. (2000). Relationship between gill Na+ , K+ ATPase activity and downstream movement in domesticated and ﬁrst generation offspring of wild anadromous brown trout (Salmo trutta). Canadian Journal of Fisheries and Aquatic Sciences, 57, 2086–95. Aarestrup, K., Nielsen, C. & Koed, A. (2002). Net ground speed of downstream migrating radio-tagged Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) smolts in relation to environmental factors. Hydrobiologia, 483, 95–102. Aarestrup, K., Lucas, M.C. & Hansen, J.A. (2003). Efﬁciency of a nature-like bypass channel for sea trout (Salmo trutta) ascending a small Danish stream studied by PIT telemetry. Ecology of Freshwater Fish, 12, 160–8. Christensen, O., Pedersen, S. & Rasmussen, G. (1993). Review of the Danish Stocks of Sea Trout (Salmo trutta). ICES C.M. 1993/M:22. Dieperink, C., Pedersen, S. & Pedersen, M.I. (2001). Estuarine predation on radiotagged wild and domesticated sea trout (Salmo trutta L.) smolts. Ecology of Freshwater Fish, 10, 177–83. Dieperink, C., Bak, B.D., Pedersen, L.-F., Pedersen, M.I. & Pedersen, S. (2002). Predation on Atlantic salmon and sea trout during their ﬁrst days as postsmolts. Journal of Fish Biology, 1, 848–52. Hansen, M.M. (2002). Estimating the long-term effects of stocking domesticated trout into wild brown trout (Salmo trutta) populations: an approach using microsatellite DNA analysis of historical and contemporary samples. Molecular Ecology, 11, 1003–15. Hansen, M.M. (2003). Application of molecular markers in population and conservation genetics, with special emphasis on ﬁshes. PhD Thesis, University of Aarhus. 315 pp. Hansen, M.M., Ruzzante, D.E., Nielsen, E.E. & Mensberg, K.-L.D. (2000). Microsatellite and mitochondrial DNA polymorphism reveals life-history dependent interbreeding between hatchery trout and wild brown trout (Salmo trutta L.). Molecular Ecology, 9, 583–94. Hansen, M.M., Ruzzante, D.E., Nielsen, E.E. & Mensberg, K.L.D. (2001). Brown trout (Salmo trutta) stocking impact assessment using microsatellite DNA markers. Ecological Applications, 11, 148–60. Hansen, M.M., Ruzzante, D.E., Nielsen, E.E., Bekkevold, D. & Mensberg, K.L.D. (2002). Long-term effective population sizes, temporal stability of genetic composition and potential for local adaptation in anadromous brown trout (Salmo trutta) populations. Molecular Ecology, 11, 2523–35. Jepsen, N., Aarestrup, K., Økland, F. & Rasmussen, G. (1998). Survival of radiotagged Atlantic salmon (Salmo salar L.) and trout (Salmo trutta L.) smolts passing a reservoir during seaward migration. Hydrobiologia, 371/372, 347–53. Jepsen, N., Pedersen, S. & Thorstad, E. (2000). Behavioural interactions between prey (trout smolts) and predators (pike and pikeperch) in an impounded river. Regulated Rivers: Research & Management, 16, 189–98. Nielsen, C., Aarestrup, K., Nørum, U. & Madsen, S.S. (2003). Pre-migratory differentiation of wild brown trout into migrant and resident individuals. Journal of Fish Biology, 63, 1184–96. Nielsen, C., Aarestrup, K., Nørum, U. & Madsen, S.S. (2004). Future migratory behaviour predicted from premigratory levels of gill Na+ /K+ -ATPase activity in individual wild brown trout (Salmo trutta). Journal of Experimental Biology, 207, 527–33. Pedersen, S.S. & Rasmussen, G.H. (2000). Survival of sea-water-adapted trout, Salmo trutta L. ranched in a Danish fjord. Fisheries Management and Ecology, 7, 295–303. Pedersen, S., Dieperink, C. & Geertz-Hansen, P. (2003). Fate of stocked trout Salmo trutta L. in Danish streams: survival and exploitation of stocked and wild trout by anglers. Ecohydrology and Hydrobiology, 3, 39–50. Rasmussen, G. (1986). The population dynamics of brown trout (Salmo trutta L.) in relation to year-class size. Polskie Archiwum Hydrobiologii, 33, 433–53. Rasmussen, G. & Geertz-Hansen, P. (1998). Stocking of ﬁsh in Denmark. In: Stocking and Introduction of Fish. (Cowx, I.G., Ed.). Blackwell Science Ltd, Fishing News Books, Oxford, pp. 14–21. Rasmussen, G. & Geertz-Hansen, P. (2001). Fisheries management in inland and coastal waters in Denmark from 1987 to 1999. Fisheries Management and Ecology, 8, 311–22. Rasmussen, G.H., Geertz-Hansen, P. & Jepsen, N. (2002). Management of recreational ﬁsheries in Denmark. Proceedings of the 3rd World Recreational Fishing Conference, 21–24 May 2002, Darwin, Northern Territory, Australia, pp. 157–9.

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Ruzzante, D.E., Hansen, M.M., Meldrup, D. & Ebert, K.M. (2004). Stocking impact and migration pattern in an anadromous brown trout (Salmo trutta) complex: where have all the stocked spawning sea trout gone? Molecular Ecology, 13, 1433–45. Slate, J. (2005). Quantitative trait locus mapping in natural populations: progress, caveats and future directions. Molecular Ecology, 14(2), 363–79. Svendsen, J.C., Koed, A. & Aarestrup, K. (2004). Factors inﬂuencing the spawning migration of female anadromous brown trout. Journal of Fish Biology, 64, 528–40.

Chapter 23

Stocking Sea Trout (Salmo trutta L.) in the River Shieldaig, Scotland D.W. Hay1 and M. Hatton-Ellis2 1 FRS

Freshwater Laboratory, Faskally, Pitlochry, Scotland Surveillance Ofﬁcer, Environmental Change Group, Countryside Council for Wales, Plas Penrhos, Ffordd Penrhos, Bangor, Gwynedd LL 57 2BQ

2 Environmental

Abstract: Sea trout populations have decreased substantially in many rivers in north-west Scotland over the past three decades. Habitat degradation and poor marine survival have been amongst the reasons cited for this decline. In the River Shieldaig, 25 000 adipose ﬁn-clipped trout fry have been stocked each year from 1998 to the present. After 2 or 3 years resident in the river, about 5% of the stocked fry survive to emigrate as sea trout smolts. Migrating smolts, of both wild and stocked origin, were captured and tagged in a permanent ﬁsh trap installed at the mouth of the river in spring 1999. The number of emigrants has risen each year since stocking commenced to a total of over 1900 smolts in spring 2003, of which more than 90% were of stocked origin. This productive capacity in fresh water demonstrates that the decline of the sea trout stock in this river was not caused by a reduction in the quality of the freshwater environment. Marine survival of the sea trout smolts remains low with inadequate returns to allow a self-sustaining population. Early indications suggest that the return rates of the stocked ﬁsh, although numerically higher, are proportionately inferior to those of wild ﬁsh. Consequently, returning stocked ﬁsh are not allowed above the trap in order to conserve the genetic integrity of the native stock. The original source of the stocked ﬁsh was Loch Coulin, 20 km from the River Shieldaig. A proportion of the wild smolts are now being retained and will be reared to maturity to provide eggs to allow future restocking to be carried out using ﬁsh of local origin. Keywords: Sea trout, stocking, smolt production, adult returns, genetic integrity.

Introduction The River Shieldaig is a short river draining a simple linear catchment, around 4 km in length, on the west coast of Scotland in Wester Ross. It is typical of many smaller Scottish west coast rivers, in that it drains a single valley and runs through a small freshwater loch, before draining into the sea at the head of a sea loch. The construction of a permanent ﬁsh trap at the mouth of the river in 1999 meant that all emigrating sea trout smolts, both stocked and wild, could be individually tagged and recaptured and examined for tags on their return to the adult trap. Various attempts have been made to stock the river with trout in the past but, as is frequently the case, none were monitored for success or failure. Cowx (1994) described the problem of stocking programmes being carried out without any evaluation of the potential or 349

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Sea Trout

actual success of the exercise. He proposed a protocol for appraising the need for stocking and suggested a strategy for planning a stocking exercise (modiﬁed from The Salmon Advisory Committee [SAC], 1991). This strategy was modiﬁed further by Fishery Research Services (FRS) (Anon., 2003). The stocking exercise described here had the combined objectives of mitigation and restoration. The mitigation element was to compensate for the current reduction in sea survival. Poole et al. (1996) recorded a collapse in the return rate of Burrishoole sea trout since 1989 and identiﬁed the problems of west of Ireland sea trout as being based in the marine habitat. Byrne et al. (2002) recorded a marine return rate of sea trout parr stocked into the Burrishoole in the range 2.8–8.0%. Jonsson et al. (2003) found that survival was signiﬁcantly higher for wild salmon smolts than for hatchery ﬁsh. McGinnity et al. (2003) showed that the offspring of farmed salmon and farmed wild crosses displayed reduced survival compared with wild salmon, but grew faster as juveniles and displaced wild parr, and that this competition could result in reduced wild smolt production. They also demonstrated that the estimated lifetime success of stocked farm and wild farm salmon was lower than for wild ﬁsh. Aprahamian et al. (2003) cautioned that the main concern regarding stocking relates to the impact on the genetic ﬁtness of the wild population. A continuing programme of work at Shieldaig has indicated that substantial quantities of larval sea lice can be found at the mouth of the river (McKibben & Hay, 2004) and that early returning post-smolt sea trout return to fresh water each year, with, in some years, substantial sea lice infestations. With the formation of the Torridon Area Management Group (AMG) in 2001, it was suggested that improved sea lice control at local ﬁsh farms might increase the sea survival of the local sea trout populations. It was hoped that the stocking programme, by introducing substantial numbers of sea trout smolts to the sea, could measure any changes in marine mortality, compare the return rates of wild stocked ﬁsh and act as a pump-priming exercise for any restoration programme.

Methods Early stocking attempts In autumn 1993, 5000 adipose-clipped summerling trout fry of Achiltibuie origin were stocked throughout the Shieldaig system by the proprietors in association with the Seaﬁeld Centre, Kishorn. In spring 1997, the proprietors above Loch Dughaill stocked a further 7000 similarly marked 1+ trout parr of mixed local origin. Fish of local Shieldaig origin were unavailable on both occasions, as sufﬁcient adult sea trout could not be obtained. Recent stocking and juvenile surveys From 1998 onwards, following the establishment of the FRS Shieldaig Sea Trout Project, a more consistent approach to stocking was adopted. Each year, in September, 25 000 trout fry of River Ewe (Coulin) origin were stocked throughout the system up to impassable falls. The ﬁsh were stocked in sites and at densities determined by a habitat survey supported

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by annual electro-ﬁshing surveys. Each August, electroﬁshing was repeated on the same 12 reference sites, but was supplemented by a more detailed survey every 5 years, when up to 40 sites are sampled. Marked and measured sites were ﬁshed three times and the total population calculated using a Zippin population estimate (Zippin, 1956). Densities of fry and parr were calculated and the age composition estimated subsequently from scale samples. The presence of trout fry in the electro-ﬁshing survey is only attributable to wild ﬁsh, because the stocked ﬁsh were not introduced into the river until September, after the electroﬁshing surveys had been completed. The stocked fry were kept in culture until large enough (∼60–70 mm) to be adipose ﬁn-clipped prior to release, allowing stocked and wild ﬁsh to be distinguished subsequently, both during electro-ﬁshing surveys and later as trap migrants. Smolt migration Since the completion of the ﬁsh trap in 1999, a reliable census of both stocked and wild sea trout smolts leaving the system was possible. Smolts caught in the trap were measured, examined for adipose ﬁn clips and scale samples taken from one in ten. Initially smolts were tagged with double Visible Implant (VI) tags, one in each adipose eyelid. Substantial problems with tag loss led to the phased introduction of Passive Integrated Transponder (PIT) tags during 2000. Since then all smolts have been PIT tagged. Adult returns Any returning ﬁnnock or adult sea trout caught in the upstream migrant section of the ﬁsh trap, had length and weight measurements taken and were examined for tags. Riverbed erosion downstream of the trap, soon after construction, made access to the adult trap difﬁcult except in high ﬂow conditions. This problem was addressed in summer 2000 by the construction of a small four-chambered ﬁsh pass leading to the adult trap allowing ﬁsh entry over a wider range of ﬂows.

Results The accelerated initial growth experienced by the stocked trout fry while still in culture was reﬂected in the length frequencies of stocked versus wild ﬁsh as recorded during the electro-ﬁshing surveys. This initial growth advantage was carried through to the smolt stage and the mean length of stocked smolts was slightly higher than that of wild smolts. This is shown for a typical year, 2002, in Fig. 23.1. As stocking was standardised at 25 000 fry per annum in 1998, the number of smolts leaving the system in the ﬁrst full year, 2000, exceeded 1000 and appeared to level off at around 1200 smolts in 2001 and 2002. This level of smolt production was thought to reﬂect the carrying capacity of the stream. However, in 2003, the number of stocked smolts rose to around 1800 (Fig. 23.2). The survival from fed fry to smolts between 2000 and 2003 has ranged from 4% to 7%. During this period the number of wild smolts, which had exceeded 600 in 1999, decreased to between 100 and 200 in subsequent years. However, the

352

Sea Trout 350 300

Fish number

250 200 Wild Stocked

150 100 50 0 100

120

140

160

180

200

220

Fish size (mm)

Fig. 23.1 Length–frequency distribution of wild and stocked sea trout smolts sampled in the Shieldaig trap in 2002.

2000 1800 1600

Fish number

1400 1200

Wild Stocked

1000 800 600 400 200 0 1998*

1999^

2000

2001

2002

2003

Year

Fig. 23.2

Migrant wild and stocked sea trout smolts, 1998–2003.

proportion of wild/stocked smolts has remained similar since stocking started in earnest. The smaller number of wild smolts at present is thought to be mainly controlled by the lack of returning wild adult spawners. At present the wild smolts must be mainly derived from the offspring of resident trout.

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353

14 12

Frequency

10 8 6 4 2 0 170

190

210

230

250

270

290

310

Sea trout length

Fig. 23.3

Length–frequency distribution of returning sea trout, 2002 (N = 49). 60

50

Trap catch

40 Sea trout Finnock

30

20

10

0 1999

Fig. 23.4

2000

2001 Year

2002

2003

Returning sea trout and ﬁnnock, 1999–2003.

The sea trout returning to the Shieldaig are small and return after a short absence at sea. The majority are ﬁnnock, immature ﬁsh returning during their ﬁrst season in the sea. Mature adult sea trout are now very scarce. The length–frequency distribution of returnees in a typical year, 2002, is shown in Fig. 23.3. The total numbers of returning sea trout and ﬁnnock between 1999 and 2003 are shown in Fig. 23.4. Fish were separated into ﬁnnock and

354

Sea Trout Table 23.1

Return rates of non-native stocked and wild sea trout smolts. Wild

Sea trout

1999 2000 2001 2002 2003

Stocked Finnock

Sea trout

Total

Finnock

Sea trout

Finnock

No.

%

No.

%

No.

%

No.

%

No.

%

No.

%

9 5 1 0 1

2.54a 0.74b 0.47 0.00 0.61

31 6 6 13 5

4.56b 2.80 4.65 7.88 4.31

0 0 0 1 0

0 0 0 0.1 0

0 20 7 35 12

0 1.91 0.58 2.83 0.67

9 5 1 1 1

2.54a 0.74b 0.47 0.07 0.07

31 26 13 48 17

4.56b 2.07 0.97 3.43 0.90

a Probable overestimate based on incomplete 1998 smolt numbers taken by fyke net. b Probable overestimate based on incomplete 1999 smolt numbers taken by fyke net and trap with spillage at high ﬂow.

Sea trout and ﬁnnock were split by age, or size (300 mm) or by known age through tagging.

sea trout on the basis of the period of absence in tagged ﬁsh, scale reading, size and sexual condition. Owing to early problems with VI tag loss, wild and stocked ﬁsh returning to the river were distinguished by their adipose ﬁn clips. All adipose-clipped ﬁsh were considered to be of stocked origin, whether they retained their tags or not. Emigrating smolts were captured by fyke net in 1998 and early 1999. This trapping method works well in low and intermediate ﬂows. However in high ﬂows, when most smolt migration takes place, the net could become overtopped and the numbers of smolts counted would be an underestimate. This would lead to a potential overestimate of the return rates of sea trout in 1999 and 2000 and of ﬁnnock in 1999. Table 23.1 shows the return rates of stocked and wild ﬁnnock and sea trout over the period 1999–2003. The returns to date suggest that the sea survival of stocked ﬁsh returning as ﬁnnock is consistently lower than that of wild smolts and that survival of both groups to mature sea trout is almost non-existent at present. A comparison of the return rates of wild and stocked ﬁnnock shows stocked ﬁnnock in the range 0–3% and wild ﬁnnock in the range 3–8%.

Discussion The initial purpose of the current stocking programme in the River Shieldaig was to introduce signiﬁcant numbers of ﬁsh to the sea each year in order to allow an accurate annual measurement of marine survival. At a time when the link between sea lice levels and sea trout survival was being examined, there were inadequate numbers of wild ﬁsh available to examine any relationship with the required precision. However, although extra stocked ﬁsh have been introduced to the system, once the poorer return rates of these ﬁsh, compared with wild ﬁsh, became evident, care was taken not to further compromise the genetic integrity of the remaining native stock by denying the returning stocked ﬁsh access above the trap to spawn from 2002 onwards. The present substantial reduction in sea trout catches in some areas of north-west Scotland does not appear to be primarily caused by poor conditions in fresh water as stocking can

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substantially increase the numbers of smolts produced. The survival rates of the stocked ﬁsh in fresh water are acceptable even though it is assumed that the stocked ﬁsh will be more naive to the risks of predators, at least initially. The survival rates of 4–7% to smolting are within the range 1.8–9.9% described by Aprahamian et al. (2004) for stocked salmon until the end of the second summer. The continued appearance of wild smolts, despite increased competition from stocked ﬁsh, means that the native genotype still survives, and is available as the basis for any subsequent improvements in the stocking operation. The relatively poor performance of the non-native stocked ﬁsh at sea mirrors the ﬁndings of McGinnity et al. (2003) who showed that, in salmon, non-native stocked ﬁsh could perform well in the short term, but were deﬁcient in terms of return rates and whole-life ﬁtness compared with native wild ﬁsh. If the sea survival of sea trout improves in the future, as a result of improved ﬁshery management, since the formation of an Area Management Group, the focus will move increasingly towards restoring a self-sustaining population of native sea trout in the River Shieldaig. It is intended, in view of the relatively poorer marine survival of the Coulin stock in the River Shieldaig, that future restoration stocking will be carried out using eggs produced from wild Shieldaig smolts, the ﬁrst of which have already been transferred into culture at two locations for rearing to maturity.

References Anon. (1991). The Salmon Advisory Committee. Assessment of Stocking as a Salmon Management Strategy. MAFF Publications, London, PB 0641, 18 pp. Anon. (2003). Salmon and Sea Trout To Stock or Not? Fishery Research Services. Scottish Fisheries Information Pamphlet No. 22, 24 pp. Aprahamian, M.W., Smith, K.M., McGinnity, P., McKelvey, S. & Taylor, J. (2003). Restocking salmonids – opportunities and limitations. Fishery Research, 62(2), 211–27. Aprahamian, M.W., Barnard, S. & Farooqi, M.A. (2004). Survival of stocked Atlantic salmon and coarse ﬁsh and an evaluation of costs. Fisheries Management and Ecology, 11, 153–63. Byrne, C.J., Poole, M.G., Dillane, K.F. & Whelan, K.F. (2002). The Irish sea trout enhancement programme: an assessment of the parr stocking programme into the Burrishoole catchment. Fisheries Management and Ecology, 9, 329–41. Cowx, I.G. (1994). Stocking strategies. Fisheries Management and Ecology, 1, 15–30. Jonsson, N., Jonsson, B. & Hansen, L.P. (2003). The marine survival and growth of wild and hatchery-reared Atlantic salmon. Journal of Applied Ecology, 40, 900–911. McGinnity, P., Prodohl, P., Ferguson, K. et al. (2003). Fitness reduction and potential extinction of wild populations of Atlantic salmon, Salmo salar, as a result of interactions with escaped farm salmon. Proceedings of the Royal Society of London Series B-Biological Sciences, 270(1532), 2443–50. McKibben, M.A. & Hay, D.W. (2004). Distributions of planktonic sea lice larvae Lepeophtheirus salmonis in the inter-tidal zone in Loch Torridon, western Scotland in relation to salmon farm production cycles. Aquaculture Research, 35, 742–50. Poole, W.R., Whelan, K.F., Dillane, M.G., Cooke, D.J. & Matthews, M. (1996). The performance of sea trout, Salmo trutta L., stocks from the Burrishoole system western Ireland, 1970–1994. Fisheries Management and Ecology, 3, 73–92. Zippin, C. (1956). An evaluation of the removal method of estimating animal populations. Biometrics, 12, 163–9.

Chapter 24

Is Stocking with Sea Trout Compatible with the Conservation of Wild Trout (Salmo trutta L.)? H. Lundqvist1 , S.M. McKinnell1,2 , S. Jonsson3 and J. Östergren1 1

Swedish University of Agricultural Sciences, Department of Aquaculture, SE-901 83 Umeå, Sweden 2 North Paciﬁc Marine Science Organization, c/o Institute of Ocean Sciences, P.O. Box 6000, Sidney, B.C., Canada 3 National Board of Fisheries, Fisheries Research Ofﬁce, Härnösand, Stora Torget 3, SE-871 30 Härnösand, Sweden

Abstract: Many wild sea trout (Salmo trutta L.) populations worldwide are threatened because human activities in the watershed hinder spawning migrations, destroy rearing habitats for juveniles or overexploit this natural resource. In Sweden, these problems are often met by restocking rivers with hatchery-reared trout to coexist with wild ﬁsh. Releases of 38 708 externally tagged hatchery sea trout occurred from 1973 to 1996 in the lower part of Umeälven and/or upstream in its largest tributary, Vindelälven. The mean length at tagging varied from 18.5 cm prior to 1990 to 23.3 cm thereafter. We analysed 3612 recoveries to assess exploitation patterns of hatchery ﬁsh over nearly three decades and compared these results with the annual returns of the wild and reared spawning stocks at a ﬁsh ladder. Hatchery-reared sea trout migrated relatively short distances with >95% of all recoveries occurring less than 200 km from the home river. Trout recovered beyond the home river (74% of 422 tag returns) were caught mainly during October, in a single large river to the south. The mean annual recapture rate of smolts released at upriver locations was 4.4% while that for releases downriver (below the hydroelectric dam) was 8.0%: that is there was a 50% loss when smolts were released above the dam. The mean recapture rate was higher for small smolts released prior to 1990 compared with the larger smolts released after 1990. Of all trout ascending the ﬁsh ladder from 1974 to 1997, 42.7% were of wild origin (intact adipose ﬁn: n = 717) with an annual average of 29 individuals/year (range 8–88). The average number of wild females ascending was approximately 16 per year (range: 4–34). The average date when 50% of the wild females passed through the ladder was 17 July over all years. The wild sea trout in the Umeälven and Vindelälven are near extinction and we recommend establishment of a conservation limit for the wild stock to secure their future existence and to establish limits on the ﬁshery of released hatchery trout. Keywords: Carlin tag, recovery rate, release location, growth, survival, Salmo trutta L.

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Introduction Many sea trout (Salmo trutta L.) stocks worldwide are extinct or are near extinction because human activities have destroyed spawning areas or interrupted spawning migrations in running waters (NRC Report, 1996; Laikre, 1999; Rivinoja et al., 2001; Kallio-Nyberg et al., 2003). There are relatively few reports on the juvenile biology (see Hearn, 1987 for review), abundance and migration of sea trout for any of the larger Swedish rivers (Järvi, 1940; Alm, 1950; Christensen & Johansson, 1975; Andersson, 1988; Karlsson, 1994). However, the plasticity of the life history of S. trutta is well known (Jonsson, 1985; Jonsson et al., 1991; Degerman et al., 2001). Although a number of studies on the upstream spawning migration and smolt downstream migration of sea trout have been made using telemetry (Evans, 1994; Aarestrup & Jepsen, 1998; Jepsen et al., 1998; Bagliniere & Maisse, 1999; Ostergren, 2003), we lack information on their post-smolt and adult feeding migration in coastal areas and the sea. The common approach to improve wild sea trout stocks in Sweden is by stocking smolts in hatchery release programmes (Lindroth, 1963, 1965; Ackefors et al., 1991). However, the release of hatchery ﬁsh into rivers where wild stocks exist has been questioned (Laikre, 1999; Dannewitz, 2003). Along with the risk of increased competition and genetic introgression, it typically generates a mixed-stock ﬁshery that may threaten the wild ﬁsh. There are also growing international concerns that hatchery programmes increase ﬁshing effort so wild stocks suffer (Hilborn & Eggers, 2001) (e.g. pink salmon in Prince William Sound, Alaska), although where hatchery ﬁsh are not exploited together with wild ﬁsh (Kodiak Island) the effects on the wild ﬁsh are minimal. In addition, there is also a concern that wild stocks of Baltic salmon have been overexploited in the intense mixed-stock ﬁshery for hatchery salmon in the Baltic region (McKinnell, 1998). The unique catch statistics in the regional ﬁsheries (offshore, coastal and in-river) from releases of tagged hatchery smolts of sea trout in our watershed, combined with statistics from the ladder on wild spawning migrants over a 23-year period, provides valuable information for the understanding of hatchery-reared and wild sea trout within the system. The present situation in the Umeälven is critical (Karlsson, 1994) and the wild sea trout are probably near extinction. In the tributary Vindelälven, the sea trout is known to have accomplished relatively long distance spawning migrations upriver to the mountain regions (Andersson, 1988; Östergren, 2003). The objective of this study is to develop a strategic management plan that would ensure genetic diversity and spawner recruitment from among the naturally produced trout. For these reasons we have analysed the ﬁshing mortality of annual releases of hatchery trout smolts in the drainage area over more than two decades and relate these ﬁndings to the variations in abundance and timing of wild spawners counted in an upstream ladder. We discuss management strategies to be undertaken if the ultimate goal is to conserve the wild stock.

Methods and materials The river system The Swedish River Umeälven and its largest tributary Vindelälven originate in the mountain region near the Norwegian border and ﬂow south-easterly for approximately 450 km

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in adjacent, parallel watersheds until entering the coastal area in the Bothnian Bay. Approximately 37 km upstream from the river-mouth the two rivers join; the River Umeälven is considered the mainstream which has several dams distributed along its entire length. The ﬂow of the River Vindelälven is impeded only by the terminal dam at Norrfors, located approximately 30 km upstream from the estuary. The construction of a power station at Norrfors began between 1924 and 1926, and ﬁsh migration was prevented from 1925. A ﬁsh ladder (280 m long) was ﬁnished in the Norrfors area, in the lower part of the River Umeälven, in 1935. Sea trouts are unable to ascend the Rivers Umeälven and Vindelälven without passing through the Norrfors ﬁsh ladder. At the top of the ladder a trapping unit makes it possible to collect and count all upstream migrating ﬁsh. An adipose ﬁn-clipping programme was started in 1971 and it was possible to distinguish returning wild (adipose ﬁn intact) from hatchery ﬁsh (adipose ﬁn removed) from 1974. Hatchery-reared ﬁsh and stocking practice The Norrfors hatchery was constructed to compensate for lost natural salmonid production in the River Umeälven drainage area because of power generation. Small releases of 2year-old sea trout smolts (our terminology follows Allan & Ritter, 1978) began in 1959 and continued for a few years. During the 1960s when the construction of a large power station at Stornorrfors was ﬁnished, the trout production in the lower part of the river was damaged and the hatchery production was ﬁxed at 22 000 trout smolts per year. Initially, most of the hatchery broodstock probably originated from rivers other than the River Umeälven. Approximately 35% of the releases from 1959 to 1969 were of unknown origin and from 1970 to 1978 about 45% were from the River Dalälven. Large returning hatchery-reared trout were selected as broodstock from 1987 to 1998. The mean weight of trout selected as broodstock (3.5 kg) was signiﬁcantly greater (ANOVA, P < 0.01) than the mean weight of all returning sea trout (2.7 kg). A similar pattern was observed for wild ﬁsh. During the years 1974–97, when relatively few returning ﬁsh entered the ladder, they were all used in the broodstock. Wild sea trout (adipose ﬁn intact) and returning hatchery ﬁsh not used for broodstock were all released to migrate upstream from the ladder after routine biological measurements were taken. Independent of their wild or hatchery-reared origin, all different types of mixed crossings took place in the hatchery. If the number of returning males were large, one male per female was used. If there were few males returning, one male was used to a maximum of three females. The rearing routines followed that of salmon (pers. comm. Åke Forssén, Norrfors hatchery, Umeå) and after 2 years in the hatchery, the trout smolts were released in early summer, usually in the last week of May, when ambient temperatures increased to 8◦ C. Trout juveniles are generally kept for a 2-year rearing cycle at ambient temperature before release. Less than 1% of the previously immature males showed early sexual maturation while in the hatchery phase (H. Lundqvist, unpublished data). Tag release groups in this study To estimate the contribution of hatchery-reared sea trout smolts from the Norrfors hatchery to the ﬁshery, a sub-sample of the release group is tagged with individually coded Carlin

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tags (Carlin, 1955) to identify their hatchery origin. Over the period under consideration, the number of uniquely tagged and released hatchery sea trout smolts per year has varied between 500 and nearly 4000; although in recent years it has remained rather steady at about 1000 per year. From 1973 to 1996, at least 38 708 tagged smolts were released in the Rivers Umeälven and Vindelälven. The true number released is probably slightly higher as some tags have been recovered without release data, that is tag release data for the 1979 release year were not located. Of all tagged smolts 71% were released below the dam at Norrfors and/or Baggböle. The rest of the smolts were transported and released in various years at different locations above the dam. Fish released below the dam were approximately 37 km from the river mouth while ﬁsh released above the dam had a varying distance to travel before they reached the estuary, from about 300 km (release location River Laisälven) to approximately 40 km above the dam (release location Långforsen). Prior to 1990, the mean size of smolts at tagging was 18.5 cm but it increased to an average of 23.3 cm thereafter. All Carlin-tagged ﬁsh for all years were released together with non-tagged ﬁsh to avoid increased predation on tagged ﬁsh.

Tag recoveries Tag recoveries rely on voluntary returns from ﬁshermen (in exchange for a small reward). Tag recovery data were handled by the former Salmon Research Institute, Älvkarleby (Carlin, 1971). All sea trout tag recoveries (N = 3612) from spring season releases to the Umeälven and its Vindelälven tributary were selected for this analysis. Most of the recaptures came from releases at the Norrfors hatchery (1581) or downstream at Baggböle (1465). In each of the years 1973–81 and 1984 tagged sea trout were released at both Baggböle and Norrfors. The percentage of tags subsequently recaptured over these 9 years was greater for releases at Baggböle than at Norrfors. The difference ranged from 0.5% to 9.5% annually and the mean values of 15.8% for Baggböle and 11.6% for Norrfors were signiﬁcantly different between sites (paired t-test, P < 0.006). There was no signiﬁcant interaction between these release locations and their recapture region (χ 2 , P = 0.92) and so the data were pooled for analyses that involved recovery location. In this case, recapture regions were deﬁned as (1) River Umeälven or its estuary (Carlin areas 028 and 231); (2) another Baltic river or (3) some other Gulf of Bothnia location. The ﬁshery legislation concerning sea trout over the 24-year period is difﬁcult to summarise. However, two major changes in the estuary and coastal areas in 1982 and 1993 respectively, can be identiﬁed. A restricted area was established in the estuary in 1982, and the ﬁshing effort in the coastal public ﬁshery was limited in 1993.

Fish ladder returns McKinnell et al. (1994) described the procedures for recording ladder returns. The ﬁsh ladder operates between 20 May and 30 September and provides an opportunity to sample all upstream migrants. The sex, type (hatchery/reared), weight and other characteristics were measured for each individual ﬁsh. As there are few distinguishing external features on

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immature sea trout, it was not possible to determine the sex of every individual ﬁsh passing through the ladder so these were recorded as being of unknown sex. Body weights of sea trout were determined with a measuring scale after 1986 while the weights of those handled before 1986 were estimated by skilled technicians. Data treatment SYSTAT Version 9 was used for all statistical analyses and plots (Wilkinson, 1990a, b). All tests of hypotheses were done at a statistical signiﬁcance level of P < 0.05. Analysis of covariance (ANCOVA) was used to determine whether the arrival day or the median day of the run were signiﬁcantly different by sex and rearing type. Any linear trends in biological characteristics of the return migration from 1974 to 1997 were resolved using return year as a covariate. Differences between mean length at tagging of all released sea trout smolts and the mean length of all recovered sea trout individuals at different time periods were tested using ANOVA. To compare differences in recapture rates (across tag groups, release locations, time periods) we computed means of tag recovery rates by various strata using size at tagging as a covariate. The LOWESS smoothing implementation in SYSTAT (Wilkinson, 1990a) was used frequently to assess trends.

Results Recapture rates The average number of tags returned each year from the Umeälven or its estuary was 191, an average recapture rate of 7.8%, for releases at Norrfors or Baggböle from 1974 to 1978. The average declined dramatically in 1979 to 46 per year thereafter. About half of this decline can be attributed to a reduction in the number of tagged sea trout released below the dam. In the 6 years (1981–83, 1986–88) when releases occurred both above and below the dam, there was a signiﬁcant interaction between release location and recapture region (contingency table, χ 2 , P < 0.001). Table 24.1 shows that the proportion of recaptures from release locations above the dam was greater within the Umeälven than was observed for releases below the dam. By further subdividing the recovery locations of tags released above the dam and subsequently recaptured within the river, it was observed that 22% of the tags recovered from sea trout released above the dam were also caught above the dam during the calendar year of release (see Table 24.2). When all years are combined, the percentage of tags recaptured above the dam is 34.4% (195 ﬁsh of a total of 566). As these recaptures continued to be reported for years after release, it must be concluded that a fraction of these releases never leave the river. Figure 24.1 shows, as might be expected, the growth of those sea trout caught above the dam was signiﬁcantly lower than that of the ﬁsh that had gone to sea. The mean recapture rate of all tagged releases above the dam was 5.7%, but when in-river recaptures, taken in the year of release, from above the dam are excluded, the mean annual recapture rate declines to 4.4%. In the same six release years, the mean annual recapture

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Table 24.1 Percentages of Carlin tag recaptures by release location and recovery location for release years 1973–96 and recovery years 1973–97. Recovery percentages at different locations are also shown for the total release. Recovery location

Ume/Vindeälven 0–20 km 20–50 km 50–100 km 100–200 km >200 km Other river Total (N)

Release location

Total

Above Norrfors dam

Below Norrfors dam

(%)

N

61.4 12.5 2.1 2.9 5.3 3.1 12.7 566

31.9 34.8 3.6 4.9 8.4 4.9 11.4 3046

36.7 31.2 3.4 4.6 7.9 4.6 11.6 100

1331 1132 122 167 287 151 422 3612

N = number of ﬁsh.

Table 24.2 Numbers of tags recaptured, by recovery location and years after release, for sea trout released at locations above the Norrfors dam. Recovery location

Above the dam Below the dam Beyond the river Total (N)

Number (N) of recaptures during different years (0–5) after release above the dam

Total (N)

0

1

2

3

4

5

123 4 37 164

44 56 107 207

11 38 66 115

12 30 22 64

2 5 3 10

3 2 1 6

195 135 236 566

rate for release locations below the dam was 8.0% so there appears to be a loss of at least 50% by releasing ﬁsh high in the river. Of the total recaptures of tagged sea trout released below the dam in all years, approximately two-thirds of the recoveries occurred within the Umeälven and/or within a 20 km distance from the river mouth (see Table 24.1). The number of recoveries decreased rapidly with increasing distance from the estuary. Of the 422 tags recovered in other rivers, 74% were returned from the River Ångermanälven (see Table 24.3) and most were taken during October. The age-structure of the Rivers Umeälven and Vindelälven sea trout taken in River Ångermanälven ﬁsheries was: 35% taken in the year of release, 48% after 1 year, 13% after 2 years and 4% after 3 years. Only 12 tags in total were recovered from as far south as the main basin of the Baltic Sea. Of the recaptures from releases below the dam, 28.6% were caught in the year of release, and the majority of these were taken in October beyond the river. There was no signiﬁcant difference between the mean length of approximately 33 cm for recaptured sea trout caught in Carlin areas 231 and 028 during the year of release. The mean size of 35 cm for these

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Sea Trout 100 90 80

Length (cm)

70 60 50 40 30 20 10

0

10

20

30

40

50

60

70

Months after release Fig. 24.1 Growth in mean length of sea trout released above the Norrfors dam and subsequently recaptured above the Norrfors dam (solid line); below the dam within the river (dashed line) or in the sea beyond the river (dotted line).

Table 24.3 Number of tagged sea trout recaptured in other rivers (n = 422) for smolt releases 1973–96 and recaptures to 1997. River

Number of recaptures (N)

Luleälven Byskeälven Skellefteälven Rickleån Öreälven Ångermanälven Indalsälven Ljungan Ljusnan Dalälven

1 2 12 1 1 311 78 3 9 4

young sea trout was also very similar to those found in the River Ångermanälven at the same time. Although the mean size at release had increased by about 5 cm since 1991 when compared with the average mean size of released brown trout for all previous years (ANOVA P < 0.001), there was no signiﬁcant change in recapture rate (see Fig. 24.2; ANOVA,

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50

Recapture rate (%)

40

30

20

10

0 100

150

200 250 Tagging length (mm)

300

350

Fig. 24.2 Recapture rate by length at tagging for smolts released at a smaller mean size before 1991 and at a larger mean size thereafter. As the number of tags applied in each length interval (0.5 cm classes) was unequal, length intervals with few released ﬁsh and recapture rates less than 0.5% or equal to zero were excluded. The trend lines were generated with a LOWESS smoother.

Recapture rate (%)

25 20

B

B N

N B

B

B B

15

B

N

10 5 0 1970

N

B

N N

B L N B L E N V N N N K R E L I I R

B S N S N

U A N E H D N N N T G

1980

1990

N

N N F N

2000

Release year Fig. 24.3 Recapture rate, by smolt release year, of tagged smolts released at various sites in the Umeälven, Vindelälven (A = Laisälven, B = Baggböle, D = Råstrand, E = Ekorrsele, F = Renforsen, G = Grundforsen, H = Handskforsen, I = Linaforsen, K = Ruskträsk, L = Långforsen, R = Rusksele, S = Selet, T = Storforsen, U = Beukaforsen, V = Vinafors, N = Norrfors). Plot symbols identify locations above (o) or below (•) the dam at Norrfors.

P = 0.27). In fact the mean recapture rate was lower when the smolts were larger during the 1990s than when they were smaller. Figure 24.3 shows that the difference between mean length at tagging of all released smolts and the mean length at tagging of all recovered smolts was signiﬁcantly different between these two periods (ANOVA, P = 0.02). The difference was 0.7 cm before 1991 and 0.4 cm thereafter. Recapture rates were persistently and signiﬁcantly higher (ANOVA, P = 0.03) before 1977 than afterwards.

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Sea Trout Table 24.4 Number of trout passing the Norrfors ﬁsh ladder by sex and rearing type from 1974 to 1997. Wild

Reared

Total

Female Male Unknown

400 188 129

475 437 55

875 625 184

Total

717

967

1684

300

Number

200

100

0 1970

1980

1990

2000

Year Fig. 24.4 Total number of sea trout ascending the ﬁsh ladder at Norrfors on the River Umeälven by year and rearing type. Symbols show hatchery sea trout (x) and wild sea trout (•).

Fish ladder returns In 23 years (1974–97), the total number of wild sea trout passing through the Norrfors ﬁsh ladder was 717 (see Table 24.4 and Fig. 24.4) and the annual average was 29 individuals per year (range = 8–88). The lowest abundance occurred in 1986 and 1994. The average number of wild females was about 16 per year (range = 4–34); but the exact number is unknown as it was not possible to determine the sex of approximately 17% of the run: most of this uncertainty occurred during the 1970s. Wild sea trout constituted 42.7% of the total number of sea trout passing through the ladder. The average number of hatchery-reared sea trout migrating through the Norrfors ﬁsh ladder was 37 individuals per year (range = 2–229). The largest return of reared sea trout occurred in 1998: a threefold increase on all previous years. The smallest runs occurred in 1981, 1982 and 1995. The annual number of wild sea trout ascending the ladder was positively correlated with the number of reared sea trout (r 2 = 0.52).

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Run timing Sex and rearing type have a signiﬁcant effect (ANOVA: P < 0.01) on the annual date when 50% of the sea trout have passed through the ladder. From 1974 to 1997, this is 17 July over all years; but it can vary from 25 June to 17 August. The average dates of passage are: 5 August (range 8 July to 20 September for wild males), 27 August (range 5 August to 14 September) for reared females and 26 August (range 22 July to 22 September) for reared males. There was no signiﬁcant difference between the mean dates of passage of reared males and females. Linear regression indicated that the median date of passage of wild female sea trout through the ladder has become signiﬁcantly late (Linear regression, P < 0.01) since the mid-1980s. While the linear trends were similar for the other strata, they were not statistically signiﬁcant. Body size The median length of all tagged sea trout caught during the year of release was about 34 cm. A linear regression of length against years at sea indicated that the annual increment was 9.2 cm per year. Of the 3021 tag recaptures that included body weight information, the median weight of a trout was: 0.4 kg during the year of release, 0.8 kg 1 year after release, 1.5 kg 2 years after release, 2.8 kg 3 years after release, 4.1 kg 4 years after release and 4.0 kg 5 years after release. From 1987 to 1998, the mean body weight of all trout ascending the Norrfors ﬁsh ladder was 3.1 kg. ANOVA indicates that there were signiﬁcant (P < 0.01) effects of rearing type, sex and year and their interactions on the mean weight of ascending sea trout. The mean size of wild males was signiﬁcantly larger (3.5 kg) than the mean size of wild females (2.7 kg) while the mean weights of reared males and females were not signiﬁcantly different. The mean weight of reared females (3.1 kg) was signiﬁcantly greater than the mean weight of wild females (2.7 kg) while the mean weights of reared (3.3 kg) and wild males (3.5 kg) were not signiﬁcantly different. The mean weights were signiﬁcantly different among years but there was no signiﬁcant temporal trend. The size frequencies suggest that there are multiple age-classes in the run. ANCOVA of the effects of year on ln (body weight) indicated that there is a signiﬁcant difference in mean weight among years after correcting for ln (length + 1) as a covariate. This suggests that the condition of the ﬁsh varies among years. The lowest values occurred in 1987 and 1995 and the highest in 1991 and 1994.

Discussion The migratory pattern of sea trout in the sea, their relatively high overall recapture rates close to 9% in the ﬁshery and the lack of a positive effect of increased smolt size using heated water on survival are three important ﬁndings in this study. Distribution maps of recoveries show that the trout released in their home river were found mainly within 200 km of the river mouth. This catch distribution over the whole release period demonstrates that sea running trout migrate over shorter distances than Baltic salmon (McKinnell, 1998) and agrees well with other studies (see Karlsson, 1994 for an overview). Most ﬁsh stayed in

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the Bothnian Bay and less than 1% migrated south to the main basin of the Baltic Sea. Sea trout seemed to conduct their feeding migration along the coast and showed a strong tendency to explore neighbouring rivers in late fall. It is not known if this straying behaviour is part of a reproductive strategy or if it represents normal overwintering behaviour to avoid harsh conditions in the sea as the deﬁnition of the word ‘straying’ involves migration of mature individuals to spawn in a stream other than the one in which they originated (Quinn, 1993). Interestingly, the large mountain Rivers Ångermanälven and Indalsälven attracted the highest number of sea trout found in any foreign rivers while the smaller forest rivers at closer distances to the home river attracted few ﬁsh. A majority of the ﬁsh ascending the Ångermanälven (∼75% of all ﬁsh caught in foreign rivers) were also captured after less than 1 year in the sea. These ﬁsh are probably caught as a by-catch to the whiteﬁsh ﬁshing allowed in the area (Karlsson, 1994). It thus seems unrealistic to discuss those ﬁsh entering foreign rivers as true strays as it is unclear whether they had reached maturity. Straying is considered as a survival mechanism that allows individuals in a population to colonise new areas. To our knowledge, no published reports have described the true straying rate for sea trout. The scientiﬁc literature on this topic seems to have focused only on salmon (Pratten & Shearer, 1983; Quinn, 1993; Jonsson et al., 2003). However, there is genetic evidence of high immigration rates between sea trout populations in small streams on the island of Bornholm in the southern Baltic (Oestergaard et al., 2003), where the authors suggest a ‘metapopulation’ of sea trout with possible immigration from other sea trout populations from the coast of Sweden. Also, in streams on the island of Gotland, southern Baltic, high genetic variability in small population is believed to persist over time because of immigration exchange between populations of sea trout (Laikre et al., 2002). Other genetic investigations show low immigration rates and local adaptations for sea trout (Hansen & Mensberg, 1998) and brown trout, even on a very small scale, within the same stream (Carlsson & Nilsson, 2000). There is currently no information whatsoever about the genetic structure of sea trout in the northern part of the Baltic. The overall recapture rate in the ﬁshery for trout released below the dam indicated a recapture rate that is about 50% greater than from ﬁsh released above the dam. This is not surprising since releases of trout above the dam produced two types of trout, that is typical seaward migrants and a substantial fraction of ﬁsh that did not migrate to sea. This is consistent with the complex life-history pattern of trout that both migrant and resident morphs often exist within the same breeding population (Jonsson, 1985). Pettersson et al. (2001) suggest that the choice of strategy in brown trout is based on both environmental conditions and genetic components. Näslund (1993) suggests that migration is mainly genetically determined, and that non-migrating trout populations rarely develop this life-cycle strategy. But, there are coexisting migratory and non-migratory trout without genetic differences which are believed to belong to the same population (Hindar et al., 1991; Pettersson et al., 2001). In our study, sea trout that remained in the river were caught above the dam for up to ﬁve years after release. As expected, the ‘choice’ to stay in the river signiﬁcantly reduced their lifetime growth compared with those that went to sea. One additional source of mortality for ﬁsh released above the dam might be that many seaward migrants were caught during the

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month of release close to their release site or when they passed the turbines at the Stornorrfors power station since Montén (1988) has shown an approximate 25% mortality for ﬁsh passing the turbines and the tailrace area. Moreover smolts suffer from high mortality from predation when passing through reservoirs and rivers with habitat suitable for predators such as pike (Esox lucius L.) (Jepsen et al., 1998, 2000). After passing through the Stornorrfors power station, these upriver smolt releases appeared to avoid the rod ﬁshery in the lower river on their seaward migration as only four tags were recovered from the lower river in the year of release. The coastal ﬁshermen caught more of these seaward migrants, but the largest catches in both locations occurred during the year after release in both the coastal and lower river ﬁsheries. One obvious stock enhancement problem observed in this study was the decline in overall tag recapture rates for all releases of sea trout in the river since the 1970s. This negative trend of fewer returning ﬁsh was also observed with the return of wild sea trout smolts (intact adipose ﬁn) passing through the ladder at Norrfors. The long-term negative trend in the abundance of wild adult sea trout, an average of 29 parents per year, has resulted in a critically low spawning population and this should be the most important conservation target to focus on in the future. Generally, the ﬁshing pressure needs to be reduced if our ultimate goal of conservation is to keep the wild stock healthy. This is also the conclusion from a recent investigation on the status of sea trout in Finland (Kallio-Nyberg et al., 2003). Another interesting observation (Fig. 24.5) concerns the apparent decline in the catch rate in the coastal area (Carlin area 231) when compared with the increased in-river catches (Carlin area 028). The declining proportion of tags recovered from the estuary and the increase in the proportion of tags recovered in the river is not easily explained. Because these tag returns are voluntary, and not part of a systematic catch sampling programme, the interpretation of these results is ambiguous. One possible explanation is that coastal ﬁshermen have reduced their willingness to return tags during the past 20 years. An 12

Recaptures (%)

10 8 6 4 2 0 1970

1980 1990 Release year

2000

Fig. 24.5 Annual number of Carlin tags returned to the Salmon Research Institute from catches in the Umeälven estuary (cross and solid line = Carlin area 231) or from within the river (ﬁlled circles and dashed line = Carlin area 028) for hatchery sea trout released below the Norrfors dam.

368

Sea Trout

alternative explanation is that sea trout catches in the coastal ﬁshery have been reduced by restrictions on ﬁshing, leaving more ﬁsh and tags available for the in-river ﬁshermen to catch as the sea trout migrate upstream. It is unknown if this change can be explained by the hatchery practice of releasing signiﬁcantly larger smolts from 1991 and onwards. Bergelin & Karlström (1985) related the decline of the sea trout to unrestricted setting of nets in the coastal ﬁshery beginning in the 1950s and the use of modern nets. The regulation of the commercial coastal ﬁshery that took place in the early 1980s was not sufﬁcient to protect the wild spawners (Karlsson, 1994). Since 1993 effort restrictions have been introduced to limit the number of nets set in coastal waters. One additional problem mentioned by Karlsson (1994), but not fully analysed in this study, is the relatively large number of catches of small trout taken as by-catch in the locally important whiteﬁsh ﬁshery. It is now well known that the recapture rate is positively related to the size of ﬁsh at release (Lundqvist et al., 1988, 1994). This positive relationship was found for trout smolts releases in the Umeälven prior to 1988 (Andersson, 1988); suggesting a hatchery strategy of producing trout smolts with a relatively larger size for releases during the 1990s. However, releases of relatively larger trout after 1988 did not increase the recapture rate to the ﬁsheries: even if the same positive effect of smolt size on catch was observed. This is additional support for the idea that relative size of the smolts can be of greater importance for survival than absolute size (McKinnell & Lundqvist, 2000). We conclude that our understanding of the interaction between growth and smoltiﬁcation in trout in their river release environment is still incomplete since we observed relatively low post-release survival among large smolts. The importance of time and size at release of smolts has been reported for many anadromous salmonid species (Peterson, 1973; Bilton et al., 1984; Lundqvist et al., 1994; McKinnell, 1998). In this study, all ﬁsh have been released at the end of May every year and the time-window for releases has varied within only a week. Peterson (1973) also stressed the importance of using a ﬁxed release date when releasing 2-year-old Baltic salmon smolts as he observed an optimal recapture rate from salmon smolts released at the same date during two successive years. We feel it is possible to compare recapture rates for different release groups between years even if Lundqvist et al. (1994) observed that the maximal rate of survival for releases during different years were at different dates. We have no previous information on when the natural seaward migration of trout smolts occur in the Rivers Umeälven and Vindelälven. Studies on natural salmon smolt runs in the vicinity of our study area (Österdahl, 1969) showed that the peak migration of wild salmon smolts was between mid-May to mid-June. We have no reasons to believe that the sea trout smolts would migrate at any other times of the year, if so probably 1 or 2 weeks earlier. However, the importance of the estuarine condition on the timing of smolt migration and post-smolt survival remains to be analysed. The declining wild sea trout populations (Karlsson, 1994) caused by human activities in the watershed and extensive ﬁshing mortality have been largely neglected because of the main focus and foundations for the rescue of the Baltic salmon: even though the situation for the sea trout, at least in the northern Baltic Sea, is even worse than for salmon (Kallio-Nyberg et al., 2003). In the future these sea trout stocks should be managed using a precautionary

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369

approach – as adopted by NASCO (National Atlantic Salmon Conservation Organization) for the Atlantic salmon. This will require establishing a conservation limit for the stock, that is the stock level at which recruitment or abundance start to decline, managers can set rules to avoid overexploitation and act to minimise habitat deterioration. In the drainage area where this study took place, the existence of hatchery populations will obviously increase the exploitation rate of trout by local ﬁshermen and cause a serious threat to the remaining wild ﬁsh that may lead to their local extinction. Another problem is that we have no knowledge of the genetic composition of sea trout populations in estuaries and along the coastline in northern Sweden. It is speculated that there is a mix of several populations in these areas and small local populations (if they exist) would suffer even more from ﬁshing in these ‘mixed-stock’ ﬁshery areas. For this reason, we consider releases of sea trout to be incompatible with the conservation of wild trout. It is important to be aware of this problem and set conservation rules that target the future existence of wild trout stocks. Some practical management actions for the sustainable management of the ﬁsheries are: (1) the development of ﬁn clipping programmes to enable hatchery ﬁsh to be distinguished from wild ﬁsh so that ﬁshing can be restricted to hatchery ﬁsh only; (2) the introduction of bag limits and catch and release; (3) the use of observers to monitor catch by commercial ﬁshermen; (4) the development of a reporting system for recording catches by sport ﬁshermen; (5) the introduction of closed seasons for nets and handheld gears in the river; (6) banning nets in the coastal area early and late in the season; (7) the creation of marine protection zones and (8) modiﬁcations to ﬁshing gear. However, in situations where wild stocks are absent and sea trout stocks are maintained solely by hatchery releases we see no reason to reduce exploitation because the life history of such stocks is already manipulated and long-term genetic changes can be controlled by other means.

Acknowledgements We wish to thank the staff at the Norrfors hatchery and the Vattenfall AB for their fruitful cooperation, the staff of the former Salmon Research Institute at Älvkarleby for providing tag return data and Ingemar Perä at the National Board of Fisheries (Luleå) for helpful comments on the chapter. This study was ﬁnanced by SLU, FORMAS and the European Union, Objective 1-LIP (to H.L.).

References Aarestrup, K. & Jepsen, N. (1998). Spawning migration of sea trout (Salmo trutta L.) in a Danish river. Hydrobiologia, 371/372(1–3), 275–81. Ackefors, N., Johansson, H. & Wahlberg, B. (1991). The Swedish compensatory program for salmon in the Baltic: an action plan with biological and economic implications. ICES Marine Scientiﬁc Symposium, 192, 109–19. Allan, I.R.H. & Ritter, J.A. (1978). Salmonid terminology. Journal du Consseil. Conseil Permanent International pour l’Exploration de la Mer, 37, 293–9. Alm, G. (1950). The sea-trout population in the Åva stream. Institute of Freshwater Research Drottningholm. Report, 31, 26–56 (In Swedish).

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Andersson, T. (1988). The sea trout in river Umeälven and river Vindelälven, after establishment of Water hydropower station Stornorrfors. Evaluations of compensatory releases in different river areas. Report May 1988. Swedish National Board of Fisheries, Härnösand (In Swedish). Bagliniere, J.L. & Maisse, G. (1999). Biology and Ecology of the Brown and Sea Trout. Springer, London (UK). Bergelin, U. & Karlström, Ö. (1985). The sea trout in tributaries to river Torneälven. Swedish National Board of Fishery, Luleå (In Swedish). Bilton, H.T., Morley, R.B., Coburn, A.S. & Van Tyne, J. (1984). The inﬂuence of time and size at release of juvenile coho salmon (Oncorhynchus kisutch) on returns at maturity, results of releases from Quinsam river hatchery, B.C. in 1980. Canadian Technical Reports of Fisheries and Aquatic Science 1306. Carlin, B. (1955). Tagging of salmon in the river Lagan. Institute of Freshwater Research, Drottningholm (Annual Report, 1954), 36, 57–74 (In Swedish). Carlin, B. (1971). Data processing in Swedish salmon tagging experiments. Salmon Research Institute Report. (Älvkarleby, Sweden) (In Swedish). Carlsson, J. & Nilsson, J. (2000). Population genetic structure of brown trout (Salmo trutta L.) within a northern boreal forest stream. Hereditas, 132, 173–81. Christensen, O. & Johansson, N. (1975). Reference report on Baltic salmon with additional information on Baltic sea trout. ICES Cooperation Research Report. No., 45, pp. 160. Dannewitz, J. (2003). Genetic and ecological consequences of ﬁsh releases. Dissertation. Faculty of Science and Technology, Uppsala University, Uppsala, Sweden. Degerman, E., Nyberg, P. & Sers, B. (Eds) (2001). Havsöringens ekologi. Finfo 2001:10, Fiskeriverkets Sötvattenslaboratorium, Örebro (In Swedish). Evans, D.M. (1994). Observations on the spawning behaviour of male and female adult sea trout, Salmo trutta L., using radio-telemetry. Fisheries Management and Ecology, 1, 91–105. Hansen, M.M. & Mensberg, K.Ld. (1998). Genetic differentiation and relationship between genetic and geographical distance in Danish sea trout (Salmo trutta L.) populations. Heredity, 81(5), 493–504. Hearn, W.E. (1987). Interspeciﬁc competition and habitat segregation among stream-dwelling trout and salmon: a review. Fisheries, 12, 24–31. Hilborn, R. & Eggers, D. (2001). A review of the Hatchery Programs for Pink Salmon in Prince William Sound and Kodiak Island, Alaska: response to comment. Transactions of the American Fisheries Society, 130, 712–20. Hindar, K., Jonsson, B., Ryman, N. & Staahl, G. (1991). Genetic relationships among landlocked, resident, and anadromous brown trout, Salmo trutta L. Heredity, 66(1), 83–91. Jepsen, N., Aarestrup, K., Oekland, F. & Rasmussen, G. (1998). Survival of radio-tagged Atlantic salmon (Salmo salar L.) and trout (Salmo trutta L.) smolts passing a reservoir during seaward migration. Hydrobiologia, 371/372(1–3), 347–53. Jepsen, N., Pedersen, S. & Thorstad, E. (2000). Behavioural interactions between prey (trout smolts) and predators (pike and pikeperch) in an impounded river. Regulated Rivers, 16, 189–98. Jonsson, B. (1985). Life history patterns of freshwater resident and sea-run migrant brown trout in Norway. Transaction of the American Fishery Society, 114, 182–94. Jonsson, B., L’Abee-Lund, J.H., Heggberget, T.G. et al. (1991). Longevity, body size and growth in anadromous brown trout (Salmo trutta). Canadian Journal of Fisheries and Aquatic Sciences, 48, 1838–45. Jonsson, B., Jonsson, N. & Hansen, L.P. (2003). Atlantic salmon straying from the river Imsa. Journal of Fish Biology, 62, 641–57. Järvi, T.H. (1940). Sea-trout in the Bothnian Bay. Acta Zoologica Fennica, 29, 1–28. Kallio-Nyberg, I., Jutila, E. & Saura, A. (Eds) (2003). Havsöringens tillstånd och havsöringsﬁsket i Bottniska viken. Fiskundersökningar 182B, Vilt-och ﬁskeriforskningsinstitutet, Helsingfors (In Swedish). Karlsson, L. (1994). Status of anadromous brown trout in Sweden. Working paper. International Council for Exploration of the Sea. GWBAST. Laikre, L. (Ed.) (1999). Conservation and genetic management of brown trout (Salmo trutta) in Europe. Report by the Concerted action on identiﬁcation, management and exploitation of genetic resources in the brown trout (Salmo trutta). Troutconcert; Eu fair CT97-38820. 91 pp. Laikre, L., Jaervi, T., Johansson, L. et al. (2002). Spatial and temporal population structure of sea trout at the Island of Gotland, Sweden, delineated from mitochondrial DNA. Journal of Fish Biology, 60, 49–71. Lindroth, A. (1963). Salmon conservation in Sweden. Transactions of the American Fisheries Society, 92, 286–91. Lindroth, A. (1965). The Baltic salmon stock: its natural and artiﬁcial regulation. Mitteilungen. International Vereinigung für Theoretische und Angewandte Limnologie, 13, 109–30.

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Lundqvist, H., Clarke, W.C. & Johansson, H. (1988). The inﬂuence of precocious sexual maturation on survival to adulthood of river stocked Baltic salmon, Salmo salar, smolts. Holarctic Ecology, 11, 60–9. Lundqvist, H., McKinnell, S., Fängstam, H. & Berglund, I. (1994). The effect of time, size and sex on recapture rates and yield after river releases of Salmo salar smolts. Aquaculture, 121, 245–57. McKinnell, S., Lundqvist, H. & Johansson, H. (1994). Biological characteristics of the upstream migration of naturally and hatchery-reared Baltic salmon (Salmo salar). Aquaculture and Fisheries Management, 25 (Suppl. 2), 46–63. McKinnell, S.M. (1998). Atlantic salmon (Salmo salar L.) life history variation: implications for the Baltic Sea ﬁshery. PhD thesis, Department of Aquaculture, Swedish University of Agricultural Science, SE 901 83 Umeå, Sweden. McKinnell, S.M. & Lundqvist, H. (2000). Unstable hatchery release optima in reared Baltic salmon (Salmo salar). Fisheries Management and Ecology, 7, 211–24. Montén, E. (1988). Compensatory hatcheries and hydroelectric power. Fiskodling och vattenkraft, Vattenfall, Sverige (In Swedish), p. 139. NRC (1996). Upstream: salmon and society in the Paciﬁc Northwest. National Academy Press Washington, DC. Näslund, I. (1993). Migratory behaviour of brown trout, Salmo trutta L.: importance of genetic and environmental inﬂuences. Ecology of Freshwater Fish, 2, 51–7. Oestergaard, S., Hansen, M., Loeschcke, V. & Nielsen, E.E. (2003). Long-term temporal changes of genetic composition in brown trout (Salmo trutta L.) populations inhabiting an unstable environment. Molecular Ecology, 12, 3123–35. Österdahl, L. (1969). The smolt run of a small Swedish river. In: Salmon and Trout in Streams (Northcote, T.G., Ed.). H.R. MacMillan lectures in ﬁsheries. University of British Columbia, Vancouver, pp. 205–15. Ostergren, J. (2003). Öringracet: Radiomärkning för att följa havsöringens lekvandring i Vindelälven och Piteälven. Arbetsrapport, Institutionen för Vattenbruk, SLU, Umeå, Sweden (In Swedish). Peterson, H.H. (1973). Adult returns to date from hatchery-reared one-year-old smolts. In: International Atlantic Salmon Symposium (Smith, M.V. & Carter, W.M., Eds). International Atlantic Salmon Foundation, New York, Vol. 4, pp. 219–26. Pettersson, J.C., Hansen, M.M. & Bohlin, T. (2001). Does dispersal from landlocked trout explain the coexistence of resident and migratory trout females in a small stream? Journal of Fish Biology, 58, 487–95. Pratten, D.J. & Shearer, W.M. (1983). The migration of North Esk sea trout. Fisheries Management, 14, 99–113. Quinn, T.P. (1993). A review of homing and straying of wild and hatchery-produced salmon. Fisheries Research, 18, 29–44. Rivinoja, P., McKinnell, S. & Lundqvist, H. (2001). Hindrances to upstream migration of Atlantic Salmon (Salmo salar) in a northern Swedish river caused by a hydroelectric power-station. Regulated rivers: Research & Management, 17, 101–15. Wilkinson, L. (1990a). SYGRAPH: The System for Graphics. Evanston, IL: SYSTAT, Inc. Wilkinson, L. (1990b). SYSTAT: The System for Statistics. Evanston, IL: SYSTAT, Inc.

Chapter 25

Sea Lice Lepeophtheirus salmonis Infestations of Post-Smolt Sea Trout in Loch Shieldaig, Wester Ross, 1999–2003 M. Hatton-Ellis1 , D.W. Hay2 , A.F. Walker3 and S.J. Northcott4 1

Environmental Surveillance Ofﬁcer, Environmental Change Group, Countryside Council for Wales, Macs-Ffynnon, Penrhosgarnedd, Bangor, Gwynedd 2 FRS Freshwater Laboratory, Faskally, Pitlochry, Scotland 3 Ellwyn, East Moulin Road, Pitlochry, Scotland 4 Fisheries Research Management Limited, Pitlochry, Scotland Abstract: Early returning post-smolt sea trout (Salmo trutta L.) are a phenomenon that has been studied since the late 1980s. In the River Shieldaig in western Scotland a sea trout trap has allowed this phenomenon to be studied in detail. The presence and absence of early returning post-smolts and the level of sea lice infection were examined over 5 years and the results related to the stage of the local ﬁsh farm production cycle. The results indicate that early returning sea trout smolts with high numbers of sea lice are primarily seen in the second year of the production cycle and numbers are lower in the ﬁrst year of production when ovigerous lice levels on local salmon farms are zero. As all sea trout smolts coming from the River Shieldaig were tagged, an estimate of infection levels over time could be achieved. A maximum of 977 sea lice were counted on a sea trout that could only have been at sea for a maximum of 15 days. This rapid infestation of sea trout smolts when ovigerous sea lice levels on local ﬁsh farms are elevated poses a threat to the sea trout stock. Keywords: Sea trout, salmon farms, sea lice, post-smolts.

Introduction Early returning sea trout, Salmo trutta L., sometimes seen within days of running to sea, have been recorded on the west coast of Ireland (Tully et al., 1993), Norway (Birkeland, 1996) and Scotland (Butler, 2002). In Ireland this phenomenon has been linked with salmon farming activities (Tully & Whelan, 1993; Tully et al., 1999). In Norway, the largest producer of farmed salmon, heavily infected wild sea trout post-smolts have been captured only in the areas where salmon are farmed (Birkeland & Jakobsen 1997; Bjørn et al., 2001) and in Ireland the highest sea lice levels on sea trout post-smolts have been recorded within 20 km of local salmon farms (Gargan et al., 2003). This study examined the sea lice levels on early returning post-smolt sea trout in the lower reaches of the River Shieldaig in relation to the production cycle of the local salmon farms in the Loch Torridon system, north-west Scotland. 372

Sea Lice Infestations of Post-Smolt Sea Trout Table 25.1

373

Post-smolt data from the River Shieldaig 1999–2003.

Year

Smolt migration

No. of samples

No. of post-smolts

No. of liced post-smolts

Prevalence of sea lice

1999 2000 2001 2002 2003

760 1257 1344 1400 1946

4 2 4 4 4

131 17 92 32 47

62 3 60 3 10

0.47 0.18 0.33 0.09 0.21

Table 25.2 Details of prematurely returning liced tagged post-smolts captured in the lower River Shieldaig showing the level of infection with sea lice and maximum time at sea. Fish no.

Date captured

Length (mm)

Max. no. of days at sea

Total sea lice present

1 2 3 4 5 6

27/05/99 27/05/99 27/05/99 27/05/99 27/05/99 30/05/03

178 140 205 201 199 181

5 13 5 15 15 30

20 544 436 977 30 100

Methods Sea trout smolts, trapped on their migration to sea, were individually marked using paired Visible Implant (VI) tags in 1999 and from 2000 onward with Passive Integrated Transponder (PIT) tags. The presence or absence of early returning post-smolts was examined by electroﬁshing for post-smolts in May and June from 1999 until 2003 (Table 25.1). An area of approximately 1100 m2 was ﬁshed, from above the high water mark to the ﬁsh trap, a distance of approximately 120 m. Only sea trout under 26 cm in length were included in the analysis, representing prematurely returning sea trout post-smolts. In this study the hypothesis that there was no signiﬁcant difference in early returning sea trout numbers between the ﬁrst and second year in the production cycle of the local ﬁsh farms was tested. Recaptured post-smolt sea trout were measured, weighed, photographed, had their lice burdens counted and tags read. From these recaptures, it was possible to determine the maximum number of days at sea. It is possible that some ﬁsh may occupy the fresh water zone below the trap for a variable period before or after seaward migration.

Results Sea trout smolt run data and the number of sea trout post-smolts recaptured in May and June from 1999 to 2003 are shown (Table 25.1). In 1999 a sample of post-smolts, captured at the river mouth at Shieldaig, was retained and the sea lice numbers counted. From 2000 to 2003 sea lice were counted on live ﬁsh.

374

Sea Trout Table 25.3 Average gravid sea lice by month on the nearest local ﬁsh farm. Month

2000

2001

2002

2003

March April May June

0 0 0 0

3.62 2.35 3.55 1.60

0 0 0 0

0.24 0.13 0 0

Data on sea lice infestation in relation to time spent at sea are shown in Table 25.2. The most extreme infestations recorded were on one sea trout smolt that had been tagged 5 days previously and had 436 lice and another having a total of 977 lice after being at sea just over 2 weeks. When comparing the prevalence of sea lice in production year one (2000 and 2002) with production year two (1999, 2001 and 2003) signiﬁcant differences were found (χ 2 = 12.07, d.f. = 1, P < 0.001). The data may have been inﬂuenced by the high number of smolts caught in 1999. Data obtained on the average gravid lice levels on the nearest local salmon farm (Table 25.3) showed no gravid female lice in any month in spring in the ﬁrst year of production (2000 and 2002) following fallowing and restocking with smolts. Gravid female sea lice were present from March to June in 2001 and from March to April in 2003. The decrease in lice levels in the second year of production from 2001 to 2003 was noted, following the introduction of an improved lice control strategy.

Conclusions Signiﬁcant differences (P < 0.001) were seen in the numbers of liced sea trout smolts sampled in the ﬁrst year of production (2000 and 2002) and those collected in the second year (1999, 2001 and 2003). Fewer ﬁsh were sampled in 2000 and 2002 (Table 25.1) and fewer of these ﬁsh were liced. Birkeland (1996) indicated that the catch rate of postsmolts in systems where the level of infestation is high is greater than in systems where the infestation rate is low. If sea lice induce ﬁsh to return prematurely to fresh water there should be a positive relationship between catch per unit effort (CPUE) sampling in estuaries and the level of infestation on those ﬁsh. The few heavily liced tagged sea trout recaptured give an indication of how quickly sea trout smolts can become infected (Table 25.2). However, although the numbers of recaptured sea trout were low and lice loading varied, this ﬁnding emphasises the need for strict lice control on ﬁsh farms during and after the wild smolt run. There was an improvement in gravid female lice levels on the nearest salmon farm by 2003; a change in ﬁsh farm ownership between 2001 and 2002 resulted in a much improved sea lice strategy which reduced sea lice levels (Table 25.3). The use of Slice treatment was also thought to be a major factor in reducing sea lice levels.

Sea Lice Infestations of Post-Smolt Sea Trout

375

C

River corrie

Upper loch torridon B Loch shieldaig

A Torridon river River balgy Salmon farm

Shieldaig river

N

Loch damph

Fish trap

Fig. 25.1 Loch Torridon. The nearest ﬁsh farm (Site B) is 4.6 km from the mouth of the River Shieldaig.

No. of sea trout caught

150

Year 1

Year 2

Year 1

Year 2

110 90

Liced

70

Unliced

50 30 10 –10

Fig. 25.2

Year 2

130

1999

2000

2001 Date

2002

2003

Early returning post-smolts to the lower part of the River Shieldaig 1999–2003.

The high numbers of liced sea trout caught in 1999, 2001 and 2003 in comparison with 2000 and 2002 indicate higher levels of infestation in the second year of production (Fig. 25.2). The results of this study compare well to plankton samples collected at the mouth of the River Shieldaig over the same time period which also showed an on/off pattern related to the year of production on the local salmon farms (McKibben & Hay, 2004).

376

Sea Trout

Acknowledgements We would like to thank J. Milton, C. Blyth and S. Buttle for assistance with sampling and the local ﬁsh farms for enabling access to their data. We would also like to thank A. Shanks for help with the statistics.

References Birkeland, K. (1996). Consequences of premature return by sea trout (Salmo trutta) infested with the salmon louse (Lepeophtheirus salmonis Krøyer): migration, growth and mortality. Canadian Journal of Fish and Aquatic Sciences, 53, 2808–13. Birkeland, K. & Jakobsen, P. (1997). Salmon lice, Lepeophtheirus salmonis, infestation as a causal agent of premature return to rivers and estuaries by sea trout Salmo trutta L., post smolts. Environmental Biology of Fishes, 49, 129–37. Bjørn, P.A., Finstad, B. & Kristoffersen, R. (2001). Salmon lice infection of wild sea trout and Arctic charr in marine and freshwaters: the effects of salmon farms. Aquaculture Research, 32, 947–62. Butler, J.A. (2002). Wild salmonids and sea louse infestations on the west coast of Scotland: sources of infection and implications for the management of marine salmon farms. Pest Management Science, 58, 595–608. Gargan, P.G., Tully, O. & Poole, R. (2003). The relationship between sea lice infestation, sea lice production and sea trout survival in Ireland, 1992–2001. In: Salmon at the Edge (Mills, D., Ed.). Proceedings of the 6th International Atlantic Salmon Symposium, Edinburgh, UK, July 2002, Chapter 10. Atlantic Salmon Trust/Atlantic Salmon Federation, pp. 119–35. McKibben, M.A. & Hay, D.W. (2004). Distributions of planktonic sea lice larvae Lepeophtheirus salmonis in the inter-tidal zone in Loch Torridon, western Scotland in relation to ﬁsh farm production cycles. Aquaculture Research, 35, 1–9. Tully, O. & Whelan, K.F. (1993). Production of nauplii of Lepeophtheirus salmonis (Krøyer) (Copepoda: Caligidae) from farmed and wild salmon and its relation to the infestation of wild sea trout (Salmon trutta L.) off the west coast of Ireland in 1991. Fisheries Research, 17, 187–200. Tully, O., Poole, W.R., Whelan, K.F. & Merigoux, S. (1993). Parameters and possible causes of epizootics of Lepeophtheirus salmonis (Krøyer) infesting sea trout (Salmo trutta L.) off the west coast of Ireland. In: Pathogens of Wild and Farmed Fish: Sea Lice (Boxshall, G.A. & Defaye, D., Eds). Ellis Horwood Ltd., London, pp. 202–13. Tully, O., Gargan, P., Poole, W.R. & Whelan, K.F. (1999). Spatial and temporal variation in the infestation of sea trout (Salmo trutta L.) by the caligid copepod Lepeophtheirus salmonis (Krøyer) in relation to sources of infection in Ireland. Parasitology, 119, 41–51.

Chapter 26

Comparison of Survival, Migration and Growth in Wild, Offspring from Wild (F1) and Domesticated Sea-Run Trout (Salmo trutta L.) S. Pedersen1 , R. Christiansen2 and H. Glüsing3 1 2 3

Danish Institute for Fisheries Research, Vejlsøvej 39, DK – 8600 Silkeborg, Denmark Viborg County, Skottenborg 26, DK – 8800 Viborg, Denmark Ringkjøbing County, Østergade 41, DK – 6950 Ringkjøbing, Denmark

Abstract: In order to evaluate the performance of trout (Salmo trutta L.) of different origin and early life history a tagging experiment was conducted in 1997–99 in a Danish lowland stream. Survival, migration and growth were compared between groups of Carlin-tagged trout released in a stream with an outlet into a fjord in the northern part of the country. Young trout of wild origin were tagged prior to or during the smolt-run (March–May) and adult wild trout tagged during the spawning season. At the time of natural smolt migration, F1 offspring from wild sea-run trout captured in the stream and raised to the smolting stage in a hatchery on the stream and smolt from a domesticated strain, were tagged and released close to the river mouth. Long-term survival (recapture date >200 days after release) was higher in wild smolt than in trout with a hatchery background. Migration patterns in the sea differed between ﬁsh of indigenous origin (wild and F1 smolt, wild adults) and the domesticated strain. All groups of indigenous origin in the stream migrated eastwards out of the fjord, and then southwards through the Kattegat, towards and into the south-western part of the Baltic Sea. Migration in the domesticated strain was more random. Wild trout from the river (both smolts and adults) were recaptured further upstream during their spawning migration into the river and closer to major natural spawning grounds, than both F1 offspring from wild and domesticated trout. The recapture pattern indicates that the precise return to spawning areas depends on early learning either prior to, or during smoltiﬁcation. Additionally, wild trout tagged during smoltiﬁcation exhibited a higher recapture rate in the home river during spawning than both domesticated and F1 offspring from wild trout. Keywords: Sea trout, stocking, migration, marine survival.

Introduction During the past century more than 90% of Danish streams have been subjected to regulation measures (canalisation, deepening and straightening) and many small streams have been replaced by drainage systems (Brookes, 1988). In addition to this, macrophyte growth is abundant and most larger streams are subjected to frequent cutting of weeds and virtually 377

378

Sea Trout

all streams are inﬂuenced by human activities (Rasmussen & Geertz-Hansen, 2001). A large number of weirs restrict access to upstream areas (Aarestrup et al., 2003), especially in smaller streams. The resulting habitat loss and restricted access to natural spawning areas, has resulted in a long history of stocking trout in streams in Denmark (Johansen & Løfting, 1919; Rasmussen & Geertz-Hansen, 1998). Initially the purpose of stocking in Denmark was to increase populations in order to increase catches, but in recent years emphasis has been on restoring and enhancing local populations. The source of material was originally mainly of hatchery origin, but in recent years has shifted towards offspring from wild spawners collected in the streams. From 2006 onwards, only offspring from wild trout are used as stocking material. In the streams, trout exhibit territorial behaviour and the number of trout is limited to the carrying capacity of the stream. As the productive capacity of many Danish streams has been reduced, stocking of 1 and 2-year-old smolts, normally in the lower parts of the streams, has been implemented in order to increase the number available, especially to the recreational ﬁshery (Rasmussen & Geertz-Hansen, 1998). Trout stocked in this way are expected to leave the stream soon after release, thus not competing for space with wild inhabitants of the stream. The survival of reared salmonids released in the wild has been found to be much lower than in wild conspeciﬁcs in several studies (Piggins & Mills, 1985; Jonsson, B. et al., 1991; Jonsson, N. et al., 2003; Poole et al., 2003). A number of reasons for this difference have been suggested, relating to both the pre-migratory and post-migratory stages, as well as the ability to home to the river of release. Pre-migratory survival may differ because of behavioural traits (Bachman, 1984; Metcalfe et al., 2003) and post-migratory survival in addition because of the ability to feed (Olla et al., 1998). Continuous sea ranching may inﬂuence life-history traits (sea age, size at maturity, condition factor) (Petersson et al., 1996) and it has been observed that the smolt ‘quality’ between wild and hatchery trout may differ, adversely affecting migration and sea survival in hatchery ﬁsh attributable to reduced physiological stress response and reduced gill Na+ K+ -ATPase activity (Sundell et al., 1998; Lepage et al., 2000). In Denmark it was found that stocked trout in general had low post-stocking survival after release in streams (Pedersen et al., 2003) and recent observations from mark recapture studies of reared and wild trout, strongly suggest that the survival of wild trout smolt is much higher than for ﬁsh of hatchery origin (Danish Institute for Fisheries Research, unpublished results). This chapter is an attempt to separate the effects of heritage and experience, as well as to add explanations to the observations in previous studies in the area.

Materials and methods The study was carried out in the River Karup, (56◦ 33.88 N, 9◦ 3.88 E) (Fig. 26.1). The River Karup is a lowland stream with a total length of 83 km. Average water ﬂow in the mid/lower part of the stream is approximately 6700 l/s. The section downstream of the town of Karup (56 km from the outlet) has an average width of approximately 9.5 m (6–15 m)

Comparison of Wild and Domesticated Trout

Limfjord

379

N

River Karup

Kattegat

North Sea

Baltic Sea 0

30

60

120 Kilometre

Skive N

Karup

0

Fig. 26.1

4

8

16 Kilometer

Denmark with River Karup and position of the towns Skive and Karup.

and average depth of approximately 1 m. At the town of Karup a partially impassable ﬁsh pass delayed, until recently, the passage of sea trout to the upper reaches of the stream. Natural spawning of sea trout is concentrated in the reach downstream the town of Karup and in some tributaries. River Karup has its outlet into the Limfjord at the town Skive. The Limfjord connects the North Sea and the Kattegat. Adult sea trout were captured in the stream by means of electroﬁshing during their spawning run. Fish were either transported to a hatchery for stripping and tagging and subsequently released in the lower part of the stream, or tagged on the riverbank and released immediately. Wild smolts were captured during their seaward migration in a smolt trap, 10 900 m from the outlet, tagged and released on the day of capture. In 1998 and 1999 about half of the tagged wild smolts were captured by electroﬁshing and tagged in upstream areas (between 26 600 and 55 200 m from the outlet), shortly prior to smolt migration. Only wild smolts with TL ≥ 15 cm were tagged. Offspring from wild sea trout (F1 wild) raised in a hatchery located on a tributary stream, were tagged at the hatchery 1–2 days prior to release. Hatchery strain smolts obtained from the Hårkjær hatchery (situated in

380

Sea Trout

Table 26.1

Number of tagged and recaptured trout released in River Karup 1997–99.

Type

Adults

Total adults F1 wild

Total F1 wild Hatchery

Total hatchery Wild

Total wild

Time/year

March 1997 December 1997 December 1998 December 1999 1997–99 April 1997 March–April 1998 April 1999 1997–99 April 1997 March 1998 March–April 1999 1997–99 April–May 1997 March–April 1998 March–April 1999 1997–99

Releases

Recaptures

Number tagged

Age

Average length (cm)

85 182 207 198 672 999 838 660 2497 998 975 1000 2973 33 1059 1096 2188

na na na na

66.3 73.2 69.6 65.6

2 1 2

18.5 16.1 18.2

1 1 1

20.0 18.1 18.2

1.8 (1–4) 2.0 (1–4) 2.2 (1–4)

16.9 17.1 18.9

Total

D < 200

D ≥ 200

N

%

N

%

N

%

6 17 11 11 45 31 5 5 41 30 11 15 56 0 18 20 38

7.1 9.3 5.3 5.6 6.7 3.1 0.6 0.8 1.6 3.0 1.1 1.5 1.9 0.0 1.7 1.8 1.7

4 8 8 3 23 17 2 3 22 25 7 13 45 0 2 7 9

4.7 4.4 3.9 1.5 3.4 1.7 0.2 0.5 0.9 2.5 0.7 1.3 1.5 0.0 0.2 0.6 0.4

2 4 2 4 12 14 3 2 19 5 4 2 11 0 14 13 27

2.4 2.2 1.0 2.0 1.8 1.4 0.4 0.3 0.8 0.5 0.4 0.2 0.4 0.0 1.3 1.2 1.2

Note: Recaptures are given as total number recaptured and recaptures sooner than 200 days (D < 200) and recaptures after 200 days or later (D ≥ 200). Recaptures do not include recaptures in smolt trap during outward migration. In bold the total number in each group is indicated. na = not available. Age of wild smolt is average (range) age determined by scale reading of samples collected in smolt trap.

south-western Jutland, formerly widely used for supplementary stocking purposes) were tagged at their home hatchery and released 1–2 days after tagging. Tagged smolts were all released (except pre-smolt tagged in the upper part of the stream) in the lower part of the river (10 900 m from the outlet, just downstream to the smolt trap). All ﬁsh were tagged with Carlin tags (Carlin, 1955). Data on the tagged trout are presented in Table 26.1. Recaptures were reported from the commercial, angling and recreational ﬁshery. In the home river some recaptures were also reported from the annual brood stock ﬁshery. Some recaptures were also recovered from the smolt trap. Relative survival of different groups was tested by comparing rate of recapture by χ 2 tests, for recaptures later than 200 days after release. This time limit was chosen because it ensures that trout tagged in the winter or spring, returning to the home river on spawning migration the following year, would return to the river after completion of their feeding migration. The position of recapture in the stream was reported for almost all tagged sea trout caught by anglers and for all sea trout recaptured by electroﬁshing. The average distance from the river outlet was calculated for each type of trout, using combined data for all years of release and ANOVA was applied to test for statistical differences. Growth was compared by comparing speciﬁc growth rates (G = (ln l2 − ln l1 )/(t2 − t1 )), where l is fork length in centimeters at time t2 and t1 respectively, t2 is day of recapture and t1 day of release (Bagenal, 1978) for the four groups of ﬁsh.

Comparison of Wild and Domesticated Trout

381

Results The overall recapture rates (with groups combined for the entire release period) were relatively low. They varied between 1.6% and 1.9% for the smolt compared with an average of 6.7% for the adult sea trout (Table 26.1). The difference between recapture rate of pooled groups of tagged smolt was not statistically signiﬁcant (χ 2 = 0.46, d.f. = 2, P = 0.79), while the recapture rate of adult tagged sea trout was much higher than that of smolts (χ 2 = 54.3, d.f. = 1, P < 0.001). Recapture rates within each type of released ﬁsh varied somewhat between years. However, it is believed that this is attributable to chance and to obtain a more comprehensive result, data from individual years were combined. Survival (frequency of recapture later than 200 days compared with recapture before 200 days after release) was higher in adults and wild smolt and lower in hatchery smolt (χ 2 = 19.45, d.f. = 3, P < 0.001) (Table 26.1). Long-term survival between wild and F1 wild was not signiﬁcantly different (P = 0.1). Individual positions of recapture in the sea are illustrated in Fig. 26.2. The vast majority of recaptures were in the central part of the Limfjord near the outlet from the home river. Numbers caught in different locations (Fig. 26.3) have been summarised in Table 26.2. Only four hatchery trout of the hatchery strain were recaptured in the sea, two of which were to the west in the Limfjord and two to the north of the outlet from the Limfjord into the Kattegat. This is in contrast to the indigenous River Karup trout, where recaptures, apart from recaptures in the local area, were predominantly either to the east in the Limfjord or to the south from the outlet of the Limfjord into the Kattegat. It appears that the normal route of migration for indigenous trout is to the east through the Limfjord and from here, to the

N

Symbols F1 Wild Hatchery Wild Adults

0

45

90

180 Kilometre

Fig. 26.2

Recapture positions in the sea.

382

Sea Trout

N North East West

South Central 0

Fig. 26.3

10

20

40

Kilometre

Deﬁnition of areas of recapture. The home river is included in the central area.

Table 26.2

Area of recapture, see Fig. 26.3 for area boundaries.

Type

Time/year

Recapture area Central

Adults

Total adults F1 wild

Total F1 wild Hatchery

Total hatchery Wild

Total wild

March 1997 December 1997 December 1998 December 1999 1997–99 April 1997 March–April 1998 April 1999 1997–99 April 1997 March 1998 March–April 1999 1997–99 April–May 1997 March–April 1998 March–April 1999 1997–99

4 15 11 10 40 25 0 7 32 26 11 15 52 0 30 32 62

West

East

North

South

Unknown/Other

0 0 0 0 0 0 1 0 1 2 0 0 2 0 0 0 0

0 1 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 2 0 0 2 0 0 0 0

2 0 0 1 3 5 1 1 7 0 0 0 0 0 6 4 10

0 1 0 0 1 1 0 0 1 0 0 0 0 0 1 0 1

The column Local includes recaptures in the home river. Recaptures of 35 tagged wild smolt in the smolt trap during outward migration are included. The total number from each group is indicated in bold.

south, through the Kattegat to the Danish Straits and the western parts of the Baltic Sea. One sea trout was reported caught as far away as Gotland in the central part of the Baltic proper. In contrast hatchery strain trout appear to have a more random pattern of migration. The frequency of return to the home river (i.e. numbers recaptured after at least 200 days) was highest for the trout tagged as adults (nearly 1.8%), followed by the wild smolts (nearly 1% of the number tagged) (Fig. 26.4). The total number of adults tagged was too low to allow statistical comparison. When data on smolt recaptures were grouped the difference between wild trout and trout with hatchery origin was highly signiﬁcant (χ 2 = 23.42, d.f. = 1, P < 0.001). Both types of reared trout (F1 wild and hatchery strain) yielded lower

Comparison of Wild and Domesticated Trout

383

2 1.8 % Recaptured

1.6 1.4 1.2

R. Karup Limfjord Other waters

1 0.8 0.6 0.4 0.2 0 Adults

F1 wild

Hatchery

Wild

Fig. 26.4 Recapture frequency (% recaptured after 200 days or more from all tagged ﬁsh) in River Karup, in the Limfjord or in other waters.

(a)

(b)

0

3.5

7

14

0

3.5

7

14

0

3.5

7

14

Kilometre

(c)

(d)

0

Fig. 26.5

Kilometre

3.5

7

14

Kilometre

Kilometre

Recapture positions in home stream. (a) F1 wild; (b) wild; (c) adults and (d) hatchery.

rates of recapture than trout of wild origin in the home stream. Homing ability combined with higher survival was highest in the trout tagged as adults, followed by the wild smolts. Locations of individual recaptures are illustrated in Fig. 26.5. The average distance from the outlet of the stream to the position of recapture varied signiﬁcantly between the release groups (ANOVA, P < 0.05) (Table 26.3). Trout of hatchery origin and F1 offspring from

384

Sea Trout Table 26.3 Average distance (m) from outlet to position of recapture in the home river. Type

Average distance to outlet (m)

SD

Difference

Hatchery F1 Adult Wild smolt

12 783 15 771 23 428 38 954

9002 6999 15 738 18 752

a a b c

SD: standard deviation. Difference: groups with same letter not statistically different (multiple range test, Fisher (LSD), P = 0.05).

Table 26.4 Speciﬁc growth rate of recaptured trout. Type Adult F1 wild Hatchery Wild

G

SD

N

0.000291 0.002098 0.001877 0.002074

0.000367 0.000682 0.001067 0.000964

28 21 25 27

SD: standard deviation.

wild trout were recaptured signiﬁcantly closer to the river outlet than the adult trout. These in turn were caught somewhat closer to the outlet than the wild trout, tagged as smolts. The speciﬁc growth rate was signiﬁcantly higher in the groups tagged as smolts (Table 26.4), compared with the adult sea trout (ANOVA, P < 0.0001), while there was no difference between smolt groups (ANOVA, P = 0.67).

Discussion Method Using recapture rate as an index for survival requires that all groups of trout have an equal probability of being recaptured. However, the size of the adult trout from the time of tagging makes it more likely that they are captured in size-selective tackle (e.g. gill nets) and they would also be more susceptible to angling mortality straight after tagging, than would the smolts. When long-term survival is considered, it is believed that this difference would level out, leaving only mortality related to tagging. It has been shown that larger salmonids survive Carlin tagging better than smaller ﬁsh (Strand et al., 2002) and it is generally found that larger smolts have better overall survival. Not all smolt groups were exactly the same size at tagging. Smolts were of different ages and the oldest smolts were those of wild origin (i.e. smolt age of the wild smolts sampled in the smolt trap was between 1 and 4 years reﬂecting the differences between natural production and hatchery conditions). The differences in average size was quite limited; however, and all tagged ﬁsh had a total length of at least

Comparison of Wild and Domesticated Trout

385

15 cm. Compared with the other groups the trout of hatchery origin (F1 wild and hatchery) were exposed to additional stress from handling and transportation; on the other hand, wild smolts did not have any recovery period prior to release. Given the uniform overall recapture rates of the smolt groups, it is believed that the minor differences in handling and size at release did not unduly inﬂuence the results. Survival The survival of the wild smolts was clearly higher than that for both the hatchery strain and F1 wild smolts, indicating a clear survival value of early learning. However, the wild ﬁsh will have undergone strong natural selection and more broadly based learning experiences. This may well have signiﬁcantly reduced the initial (pre-smolt) survival of this group. This ﬁnding is in accordance with ﬁndings in several other studies (Piggins & Mills, 1985; Jonsson et al., 1991; Poole et al., 2003). The adult sea trout and the wild smolt survived equally well, even though the adults theoretically would be more exposed to ﬁshing mortality. The reason for the high survival in the adult sea trout probably reﬂects the lower risk of being preyed upon, the value of additional learning and the very limited mortality attributable to tagging. Migration Only limited tagging experiments using wild sea trout have been carried out in Denmark, but preliminary results from tagging trout in streams on the east coast of Jutland (Danish Institute for Fisheries Research, unpublished results) suggest that the direction of migration resembles the pattern observed from this study: a movement towards the south of the Kattegat area, the Danish Sounds and in some cases into the western Baltic. In this area salinity varies between 20 and 10 ppt, reducing the osmotic burden on the ﬁsh, allowing for additional growth (Boeuf & Payan, 2001). Compared with other wild strains examined, the trout from the River Karup have a longer migration route to this area (>300 km). Unfortunately the number of recaptures of hatchery origin trout in the sea is too low to draw deﬁnite conclusions. However, it has been found in previous tagging experiments in this and in other streams in the area, where hatchery strain trout were used, that recaptures were distributed throughout the Limfjord and both to the north and south of the outlet of the Limfjord into the Kattegat (Kristiansen & Rasmussen, 1993). The results indicate that the sea migration of the indigenous River Karup trout is directed towards the southern part of Kattegat and in many cases also further to the south, irrespective of previous history. These traits are probably genetically inherited. In contrast to this, the migration of the hatchery strain ﬁsh appears to be more random. Return to home stream and recapture position in home stream The adult trout and the wild smolts returned to their home stream in higher frequencies than the trout of hatchery origin (i.e. F1 and domesticated), and in addition to this, they were recaptured further upstream. It is not certain that the trout of hatchery origin would not

386

Sea Trout

subsequently move to upstream areas, but it seems likely that a possible upstream migration would at least be signiﬁcantly delayed. The recapture localities appear to reﬂect the starting points at the onset of migration for the trout, as both hatchery groups were released in the lower parts of the stream. The higher return rate to the home river is likely to reﬂect both an advantage in survival and an ability to home correctly in the wild ﬁsh. Growth No difference in growth between smolt groups was observed. This may be inﬂuenced by the fact that many hatchery strain trout were recaptured soon after release; that is many trout of the hatchery strain did not leave the stream and consequently did not beneﬁt from the possibilities of enhanced growth in the sea. Management implications The results indicate that wild salmonids survive better than ﬁsh of hatchery origin. This result is in accordance with that of Cowx (1994) who advises that river production can be optimised by making full use of natural production, through habitat restoration and ensuring that access to natural spawning and nursery areas is not impeded. If stocking is judged to be required the use of indigenous wild parent trout is strongly preferred, because of the apparently natural inherited migratory traits of wild stocks and the genetic and other biological dangers of cross breeding between wild and reared trout stocks (e.g. growth rate, run timing, migration patterns, etc.) (Hansen & Loeschcke, 1994; McGinnity et al., 2003). The position of release should also take the subsequent return of spawners and possible participation in spawning into account. If collection of trout for stripping purposes always takes place in a given stretch of the stream and if the reared offspring from these trout do not return to the original area for spawning there are a range of inherent dangers. For example if these ﬁsh do not spawn successfully in this area, the continued collection and in effect removal of spawners could be potentially detrimental to the wild population. Continuous removal of ﬁsh for hatchery purposes could over time reduce the number of wild spawners, and also reduce genetic diversity and overall egg deposition rates in the stream. Previous studies in the River Karup have shown that the present trout is genetically very similar to the original population, in spite of massive stocking with hatchery origin trout in the 1970s and 1980s (Hansen, 2002). This may well have resulted from a combination of low survival and ﬁtness of the hatchery strain. These results offer some additional explanations. The suggested poor survival could be a result of both a low rate of emigration from the stream, and a low or at least delayed return to the spawning areas by hatchery trout. The low genetic impact from the stockings could also be because of post-stocking mortalities occurring prior to migration (Pedersen et al., 2003). In the Limfjord area, Ruzzante et al. (2004) found local variations in the proportion of locally assigned sea trout. The proportion of locally assigned trout increased in areas where stocking ceased in the 1990s indicating a low genetic impact from previous stockings.

Comparison of Wild and Domesticated Trout

387

Additionally, only low numbers of maturing adult sea trout, of hatchery origin, were observed. It was also suggested that hatchery strain trout suffered high mortalities in the sea. A reduced marine survival of hatchery strain trout cannot be ruled out; in fact it could be supported by the varying patterns of migration observed in hatchery strain trout.

Acknowledgements The authors wish to thank the technical staff at the Danish Institute for Fisheries Research, Department of Inland Fisheries, for practical assistance in the ﬁeld and during tagging. Anglers Associations at River Karup, especially Tonny Johansen and Mogens Thomassen, are thanked for practical ﬁeld help and facilities at their hatchery. We thank Dr Ken Whelan for valuable improvements of the chapter.

References Aarestrup, K., Lucas, M.C. & Hansen, J.A. (2003). Efﬁciency of a nature-like bypass channel for sea trout (Salmo trutta) ascending a small Danish stream studied by PIT telemetry. Ecology of Freshwater Fish, 12, 160–8. Bachman, R.A. (1984). Foraging behaviour of free-ranging wild and hatchery brown trout in a stream. Transactions of the American Fisheries Society, 113, 1–32. Bagenal, T.E. (1978). Methods for Assessment of Fish Production in Fresh Waters, IBP Handbook No. 3, Blackwell Scientiﬁc Publications Ltd. Boeuf, G. & Payan, P. (2001). How should salinity inﬂuence ﬁsh growth? Comparative Biochemistry and Physiology, C, 130, 411–23. Brookes, A. (1988). The distribution and management of channelled streams in Denmark. Regulated Rivers Research and Management, 1, 3–16. Carlin, B. (1955). Tagging of salmon smolts in the river Lagan. Report of the Institute of Freshwater Research, Drottningholm, No. 36, pp. 57–74. Cowx, I.G. (1994). Stocking strategies. Fisheries Management and Ecology, 1, 15–30. Hansen, M.M. (2002). Estimating the long-term effects of stocking domesticated trout into wild brown trout (Salmo trutta L.) populations: an approach using microsatellite DNA analysis of historical and contemporary samples. Molecular Ecology, 11, 1003–15. Hansen, M.M. & Loeschcke, V. (1994). Effects of releasing hatchery-reared brown trout to wild trout populations. In: Conservation Genetics (Loeschcke, V., Tomiuk, J. & Jain, S.K., Eds). Birkhäuser, Basel, pp. 273–89. Johansen, A.C. & Løfting, J.C. (1919). On the ﬁshes and ﬁsheries in the Gudenaa and Randers Fjord I (In Danish with an English summary). Skrifter udgivne af Kommisionen for Havundersøgelser no. 9, Bianco Lunas Bogtrykkeri, København. Jonsson, B., Jonsson, N. & Hansen, L.P. (1991). Differences in life-history and migratory behavior between wild and hatchery-reared Atlantic salmon in nature. Aquaculture, 98, 69–78. Jonsson, N., Jonsson, B. & Hansen, L.P. (2003). The marine survival and growth of wild and hatchery-reared Atlantic salmon. Journal of Applied Ecology, 40, 900–11. Kristiansen, H. & Rasmussen, G. (1993). Havørredens vandringsruter (Sea trout migration). IFF report No 23, 64 pp, DFU, Silkeborg Denmark (In Danish). Lepage, O., Oeverli, O., Petersson, E., Jaervi, T. & Winberg, S. (2000). Differential stress coping in wild and domesticated sea trout. Brain, Behavior and Evolution, 56, 259–68. McGinnity, P., Prodöhl, P., Ferguson, A. et al. (2003). Fitness reduction and potential extinction of wild populations of Atlantic salmon, Salmo salar, as a result of interactions with escaped farm salmon. Proceedings of the Royal Society of London B, 270, 2443–50. Metcalfe, N.B., Valdimarsson, S.K. & Morgan, I.J. (2003). The relative roles of domestication, rearing environment, prior residence and body size in deciding territorial contests between hatchery and wild juvenile salmon. Journal of Applied Ecology, 40, 535–44.

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Sea Trout

Olla, B.L., Davis, M.W. & Ryer, C.H. (1998). Understanding how the hatchery environment represses or promotes the development of behavioural survival skills. Bulletin of Marine Science, 62, 531–50. Pedersen, S., Dieperink, C. & Geertz-Hansen, P. (2003). Fate of stocked trout Salmo trutta L. in Danish streams: survival and exploitation of stocked and wild trout by anglers. Ecology and Hydrology, 3, 39–50. Petersson, E., Jaervi, T., Steffner, N.G. & Ragnarsson, B. (1996). The effect of domestication on some life history traits of sea trout and Atlantic salmon. Journal of Fish, 48, 776–91. Piggins, D.J. & Mills, C.P.R. (1985). Comparative aspects of the biology of naturally produced and hatchery-reared Atlantic salmon smolts (Salmo salar L). Aquaculture, 45, 321–33. Poole, W.R., Nolan, D.T., Wevers, T., Dillane, M., Cotter, D. & Tully, O. (2003). An ecophysiological comparison of wild and hatchery-raised Atlantic salmon (Salmo salar L.) smolts from the Burrishoole system, western Ireland. Aquaculture, 222, 301–14. Rasmussen, G. & Geertz-Hansen, P. (1998). Stocking of ﬁsh in Denmark. In: Stocking and Introduction of Fish (Cowx, I., Ed.). Fishing News Books, Oxford, pp. 14–21. Rasmussen, G. & Geertz-Hansen, P. (2001). Fisheries management in inland and coastal waters in Denmark from 1987 to 1999. Fisheries Management and Ecology, 8, 311–22. Ruzzante, D.E., Hansen, M.M., Meldrup, D. & Ebert, K.M. (2004). Stocking impact and migration pattern in an anadromous brown trout (Salmo trutta) complex: where have all the stocked spawning sea trout gone? Molecular Ecology, 13, 1433–45. Strand, R., Finstad, B., Lamberg, A. & Heggberget, T.G. (2002). The effect of Carlin tags on survival and growth of anadromous Arctic charr, Salvelinus alpinus. Environmental Biology of Fishes, 64, 275–80. Sundell, K., Dellefors, C. & Bjornsson, B.T. (1998). Wild and hatchery-reared brown trout, Salmo trutta, differ in smolt related characteristics during parr-smolt transformation. Aquaculture, 167, 53–65.

Chapter 27

The Rapid Establishment of a Resident Brown Trout Population from Sea Trout Progeny Stocked in a Fishless Stream A.F. Walker Fisheries Research Services Freshwater Laboratory, Pitlochry PH16 5LB, Scotland

Abstract: As part of a study of the diadromous trout population of the Findhu Glen Burn in eastern Scotland, unfed fry, the progeny of large, multispawning sea trout obtained from the main stream, were planted in spring in a ﬁshless area above waterfalls that are impassable to upstream migrants. In autumn, fry captured in this stretch by electroﬁshing were adipose-clipped (1982 and 1983), spraybranded with ﬂuorescent pigment (1982) and released. During autumn 1985, recaptures were made of stream-resident and river-migrant mature brown trout (mainly male ﬁsh) and sea trout (mainly females) below the falls. When the upper reaches of the Burn were revisited 20 years after the stocking of sea trout progeny, they were found to contain a small population of brown trout of mixed ages (0+ to 5+ winters). Above the falls, mature female brown trout were signiﬁcantly more common relative to males than in the main stream where they were normally very scarce. The results of this stocking trial are a clear expression of the large behavioural plasticity and rapid adaptive capacity of Salmo trutta L. when transplanted into new environments. Keywords: Salmo trutta L., sea trout, adaptive traits, stocking.

Introduction The highly varied trout (Salmo trutta L.) that inhabit the rivers of eastern Scotland are broadly grouped as brown trout and sea trout and are subject to different ﬁsheries legislation and management. Brown trout may be locally resident, or undergo variable levels of migration, but the essential fact is that they remain in fresh water, whereas sea trout migrate to feed in the sea. Both forms then return to their natal streams where they may spawn together, as exempliﬁed by studies on the River Tweed (Campbell, 1977) and River Tay systems (Walker, 1990), and in Scandinavia (Jonsson, 1985; Hindar et al., 1991). In the British Isles, the wide phenotypic and genetic variation that exists among present-day trout, and the divergence of the stocks into brown trout and sea trout, is believed largely to have evolved independently in the different rivers, after they were colonised by anadromous ancestral trout, during the retreat of the last period of glaciation (Ferguson, 2004). This chapter describes the present-day colonisation by trout of a previously ﬁshless headwater stream, the Allt Mor, a tributary of the Findhu Glen Burn, Perthshire, Scotland, 389

390

Sea Trout

that was stocked with progeny of indigenous sea trout in the early 1980s (Walker, 1990). The biological composition of the donor population in the main Burn also is described and simple population modelling is attempted. The stocking study in the Allt Mor, together with the observations about the main stem spawners, provides clear evidence of the plasticity which remains within S. trutta to adopt widely differing life-history traits, especially when challenged with new environmental circumstances. Better knowledge of the mechanisms which drive the complex relationship between brown trout and sea trout is a fundamental requirement for the management of ﬁsheries in eastern Scotland that depend on these biologically integrated stocks.

Materials and methods Site description The Allt Mor is a steep watercourse that ﬂows over impassable waterfalls into the main Findhu Glen Burn, which is situated in the upper River Earn system. The River Earn is a major lower tributary of the River Tay (Fig. 27.1). The Findhu Glen Burn is a substantial stream, more than 10 m wide in its lower reaches, which drains nearly 17 km2 of heather and grassy, sheep and deer-grazed moorland. The underlying rocks are sandstones and conglomerates, the gravels are rounded and the pH of the water tends to be neutral or slightly alkaline. The Burn is primarily used as spawning and nursery habitat by an early running stock of sea trout, of largely unknown marine migratory extent, that mainly enters fresh water in early summer (May/June) and spawns from mid-October to early November. Smaller numbers of mature brown trout found with the sea trout comprise a mixture of Burn residents and ﬁsh that have migrated up from the Water of Ruchill and the River Earn. Salmon (Salmo salar L.) only ascend the Burn for about 1 km and do not pass a complex waterfall. The sea trout and brown trout that migrate past this barrier spawn in a further 5 km of the main Burn and tributaries (Walker, 1990; Walker et al., 1998; Carlsson et al., 2004). They also ascend about 300 m of the lower reaches of the Allt Mor before being

River Tay Estuary

Loch Earn

Comrie

Crieff

Water of Ruchill main waterfall stocked section Allt Mr

Findhu Glen Burn

10 km

Fig. 27.1

rn

Ea River

Allt Mor and Findhu Glen Burn in the River Earn system.

Bridge of Earn

Establishment of Resident Brown Trout

391

barred by further waterfalls. The Allt Mor, like the main Burn, rises on the slopes of the neighbouring hills at an elevation of above 500 m. The stocking trial After ﬁsh were found to be absent in an initial survey of the Allt Mor, the upper reaches were used for a stocking experiment with local sea trout fry to determine whether they would adopt the range of life-history traits observed among the trout found spawning in the main Burn. In April 1982 and repeated in 1983, 20 000 unfed fry, obtained from strippings of several pairs of Findhu Glen sea trout, were evenly distributed at an approximate mean density of 10 m−2 over a distance of about 1500 m, between elevations of 250 and 350 m (Walker, 1990). In this moderately sloping section, bounded above and below by waterfalls impassable to upstream migrants, the Allt Mor comprises a succession of pools and rifﬂes, with occasional small riparian trees, mainly rowan (Sorbus spp.). Most of the stocked section, which averaged 1.3 m wide, was shallow enough to electroﬁsh using a backpack (Electrocatch). During September 1982, 1100 of the stocked fry remaining in the Burn were obtained by electroﬁshing. These ﬁsh were measured, and marked in the ﬁeld by adiposeclipping and spray-branded with ﬂuorescent pigment, visible under longwave UV light (Phinney et al., 1967), before being released. Subsequent electroﬁshing in the following two summers indicated that the marked ﬁsh represented 30–40% of the stocked population. Some became smolts and migrated downstream over the falls. During autumn 1985, 17 mature returning sea trout (nine males and eight females) and two mature male, river-migrant, brown trout were recaptured in the lower part of the Burn. This was a minimum estimate of the number of returners, because not all ﬁsh had been marked and also because they were at liberty to enter and leave the Burn between the sampling visits. The second stocking of unfed fry was not followed through by a similar marking study and no further stocking has been carried out since 1983. In addition to the migrant trout, some individuals matured and remained in the Burn. These ﬁsh established a self-sustaining, resident population of brown trout, described below, isolated by the waterfalls from the returning migrants. Thus, the stocking of fry from pairs of sea trout provided clear evidence of their divergence into Burn-resident and river-migrant brown trout in addition to sea trout (Walker, 1990).

Results The present upper Allt Mor trout population During October 2002 and 2003, slightly more than 20 years after the initial stockings, the upper Allt Mor was revisited to determine the status of any remaining ﬁsh population. An area of about 1000 m2 , representing about 50% of the stocked section, was electroﬁshed by a rapid single-pass with a backpack shocker (Electrocatch), using smooth d.c. Capture efﬁciency was likely to have been ≥50%, except in two deeper pools, but on each occasion the catches of trout were very low (<0.05 ﬁsh per m2 ). The sampled ﬁsh were measured to the nearest mm (fork length), their state of maturity and sex were recorded and scales were taken for age determination and, in 2002, all were adipose-clipped for genetic studies.

392

Sea Trout Upper allt mor trout Oct 2002 and 2003 30

Numbers

25 Imm

20

Male

15

Female

10 5 0 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 Fork length (mm)

Fig. 27.2

The length composition of upper Allt Mor trout, in October 2002 and 2003.

Table 27.1 The mean lengths at ages of upper Allt Mor brown trout sampled in October 2002 and 2003 (sample sizes shown in brackets). Maturity

Mean fork length (mm ± st. dev.) at age (winter) 1st winter

2nd winter

3rd winter

4th winter

Total

Immature Mature males Mature females

(54)84 ± 9 (0) (0)

(14)136 ± 12 (16)143 ± 13 (0)

(1)160 (10)183 ± 15 (5)181 ± 6

(0) (1)230 (5)214 ± 9

69 27 10

Total

54

30

16

6

106

Figure 27.2 and Table 27.1 show the length composition and mean length at age of the combined samples. The trout ranged in length from 60 to 230 mm and in age from 0+ to 3+ winters. The fry were large by Scottish hill stream standards, some individuals exceeding 100 mm. However, the mature trout were small (<250 mm), typical of trout inhabiting hill Burns in Perthshire (Walker, 1994). Overall, there were 69 immature and 37 mature ﬁsh. The latter comprised 27 males and 10 females (a male/female sex ratio of 2.7 : 1.0). Bearing in mind the recent (20 years) origin of the Allt Mor population as progeny of sea trout, it is possible that a few of the juveniles continue to migrate from the upper Allt Mor, even though they cannot return. However, their numerical contribution to the main stem adult population must be insigniﬁcant. The two recent surveys of the upper Allt Mor indicate that the trout population there is sustained by perhaps 20–30, very small (170–220 mm), resident females, producing 1000–2000 ova (based on fecundity–length data on Scottish trout from Walker, 1994). Trout population structure in the main Findhu Glen Burn In the context of the establishment of the Allt Mor population, as in any stocking exercise, it is essential to consider the choice of the donor ﬁsh. Over a number of years, the spawning

Establishment of Resident Brown Trout

393

Table 27.2 The numbers and mean percentages (in brackets) of mature male and female sea trout and brown trout spawners sampled at the Findhu Glen Burn during 1980–2003. Year

Sea trout

Brown trout

Combined

Male

Female

Total

Male

Female

Total

Male

1980 1981 1982 1983 1984 1985 1986 Mean (%)

29 184 68 78 157 165 19 (39)

15 215 119 154 294 254 35 (61)

44 399 187 232 451 419 54 (100)

4 23 32 32 33 93 21 (98)

0 1 0 0 0 3 0 (2)

4 24 32 32 33 96 21 (100)

1990–2003 Mean (%)

99 (42)

137 (58)

236 (100)

63 (98)

1 (2)

64 (100)

Total (1980–2003)

799

1223

2022

301

5

306

Overall mean (%)

(40)

(60)

(100)

(98)

(2)

(100)

Female

Total

33 207 100 110 190 258 40 (46)

15 216 119 154 294 257 35 (54)

48 423 219 264 484 515 75 (100)

162 (54)

138 (46)

300 (100)

1100

1228

2328

(47)

(53)

(100)

stock of trout in the Findhu Glen Burn was sampled by repeated, often extensive, electroﬁshing visits, undertaken during autumn/early winter. The captured ﬁsh were measured, sexed, scale-sampled and marked and/or tagged before release. Quantitative estimates of juvenile densities were made at other times of the year. Most of the adult sampling took place during 1980–86, after which only sporadic visits were made, although larger annual samples were obtained during 1997 and 1998 (Table 27.2). The data are grouped for convenience into the two main periods, 1980–86 and 1990–2003. The combined catches of adults for the whole series comprised 2022 sea trout and 306 brown trout. Figure 27.3 indicates that the length composition of the sea trout and brown trout has remained stable, at least up to 1997/98, after which insufﬁcient sampling was carried out to detect change (Fig. 27.4).

Sea trout Typically of eastern Scotland (Nall, 1930; Shearer, 1990; Walker, 1990), most of the sampled sea trout were fairly short-lived. They had migrated to sea, some 60 km away by river course, as 2-year-old smolts (73%) and 3-year (26%), 4-year and 1year-old smolts being rarely found and 70% were returning to spawn as maiden ﬁsh aged 1+ sea winters (SW) (Table 27.3). Thereafter, there was a steep decrease in the representation of older sea-age classes. Mature ﬁnnock (0+ SW) were rare (4%) and previous spawners comprised 15–30% of the samples. Two ﬁsh were found spawning for the sixth time. Female sea trout consistently predominated over males (Table 27.4) and tended to live longer, except in the sampling period during 1990–2003, when fewer ﬁsh were examined. The overall sex ratio of sea trout examined was 1.0 : 1.58 males : females (P < 0.001).

394

Sea Trout (a)

1980–1986 500

Numbers

400 300

BT

200

ST

100 0 0

100

200

300 400 500 Fork length (mm)

700

800

1990–2003

(b)

Numbers

600

80 70 60 50 40 30 20 10 0

BT ST

0

100

200

300 400 500 Fork length (mm)

600

700

800

Fig. 27.3 The length composition of Findhu Glen sea trout and brown trout samples during (a) 1980–1986 and (b) 1990–2003.

1 000 000 ova from Sea trout (1981) 2 000 ova from brown trout

800 Sea trout (500 females) Parr Autumn 2.1+ (1985)

30 000 Autumn 0+ (1982)

190 BROWN TROUT (10 females) 9 000 Parr Autumn 1+ Parr (1983) 4 000 Smolts spring (1984)

Fig. 27.4 Generalised life cycle of Findhu Glen Burn sea trout. Note: Based on unpublished electro-ﬁshing survey data and frecundity estimates from Walker (1990). The different smolt and adult age groups have been amalgamated for simplicity. Mature male brown trout may be aged 1+ to 5+ years.

Establishment of Resident Brown Trout

395

Table 27.3 The sea-age composition of mature Findhu Glen sea trout sampled during 1980–2003. Year

Sea age (%) 0+

1+

2+

3+

4+

5+

1980 1981 1982 1983 1984 1985 1986 1990–2003

4 6 3 7 3 2 0 3

68 57 66 69 77 72 73 78

24 28 21 18 15 21 23 14

2 7 8 5 4 3 4 3

2 1 2 1 1 1 0 1

0 1 0 0 0 1 0 1

Mean

4

70

20

4

>1

<1

Table 27.4 The sex ratio of mature Findhu Glen sea trout sampled during 1981–2003 (annual sample sizes 187–451 ﬁsh). Year

1981 1982 1983 1984 1985 1990–2003

Maidens

Previous spawners

M

F

M

F

1 1 1 1 1 1

1.07 1.51 1.76 1.65 1.36 1.45

1 1 1 1 1 1

1.76 3.15 4.10 3.23 2.34 1.19

Brown trout The sampled mature brown trout were almost all male ﬁsh, mainly small, Burn residents, but including a small proportion of larger river migrants. The smallest Burn residents (<150 mm) may have been under-represented as they were occasionally observed to pass through broken meshes of hand nets. Only ﬁve mature female brown trout (<2%) were captured during the entire series. These ﬁsh ranged in length from 170 to 370 mm and were aged from 2+ to 5+ winters. Clearly, from the great scarcity and small size of the female brown trout, their contribution to the overall egg deposition within the Burn was negligible. However, males are likely to make a signiﬁcant genetic contribution to the overall stock by fertilising some of the sea trout ova, in the same way that diminutive, ripe male, salmon parr contrive to fertilise many ova from sea-run females (Jordan & Youngson, 1992). Population estimates While the length and age composition of the spawners appears to have remained relatively stable over the series, this does not necessarily imply stability in their numbers. Ideally, quantitative assessment would have been achieved through a ﬁxed trap, or a counter. As these

396

Sea Trout

facilities were lacking, spawner abundance was estimated indirectly by mark and recapture (Bailey-modiﬁed Petersen estimates) during the more intensive sampling period in the early 1980s. The results presented below are tentative as the methods used depend for their robustness upon various provisos that could not all be fully satisﬁed. The marked ﬁsh may not have been randomly dispersed within the overall population and the sampling for recaptures tended to concentrate upon key areas where the spawners congregated. Also, the population was open to emigration and immigration during the normal interval of a few days between marking and sampling. During 1985, in an attempt to counter the potential problem of lack of population closure, three successive estimates of the stock size were made, based on a series of sampling visits. A second method that was adopted applied the ratio of marked to unmarked ﬁsh among previous spawners in year N + 1 to the numbers of spawners in year N (i.e. 1 year earlier), when the marking was carried out. The inference is that the sea trout must home accurately and return to fresh water to spawn each year. Walker (1990) reported that of 1262 adult sea trout tagged in the Burn during the early 1980–86, 131 (10.4%) were recaptured in the Burn one or more years later. Anglers ﬁshing the River Earn reported 27 recaptures (2.1%), and a further 18 (1.4%) were caught in the local estuary nets, plus three in coastal bagnets, including one near Peterhead, Aberdeenshire and another at Amble, Northumberland. No recaptures were reported from areas, or at a time of year, that suggested that they might have strayed for spawning. In any case, some straying from the Burn would have been acceptable provided that it occurred equally among the marked and the unmarked ﬁsh, which seemed likely. On the other hand, unrecognised straying of sea trout from other populations into the Burn would have the effect of inﬂating the population estimates. The same inﬂating effect would occur through unidentiﬁed tag or mark loss, but the routine use of multiple tags or marks during the study reduced the risk of failure to identify recaptures. Similarly, a higher death rate of marked than unmarked ﬁsh would result in population overestimation, but no such evidence was found. With the above provisos, indications of the annual levels of sea trout abundance in the early 1980s are given in Table 27.5. No estimate is available for 1986 when the quantitative part of the study ended. The sexes are shown separately because, as mentioned earlier, the females had a higher survival rate than the males and tended to leave the Burn soon after spawning,

Table 27.5 Estimated numbers of Findhu Glen sea trout spawners during 1981–85. Year

Males

Females

Combined

Comments

1981 1982 1983 1984 1985 1985 1985

442 156 145 386 270 295 328

526 297 261 462 422 461 512

968 453 406 848 692 756 840

Year N Year N Year N Year N Year N Year N Year N

+1 +1 +1 +1 (22.10) (29.10) (6.11)

Establishment of Resident Brown Trout

397

while the males lingered to engage in multiple courtship and were potentially exposed to greater sampling effort. The estimates for years 1981–84 were based on the delayed mark and recapture method, that is, the proportion of marked ﬁsh among previous spawners (year N + 1). The estimates for 1985 were based on short-term mark and recapture data on three successive dates in the year of tagging (year N). In spite of the potential difﬁculties inherent in the methodology, described earlier, there is reasonably close agreement between years (range 406–968) and for the three dates in 1985 (692–840). The progressive increase in the estimates in that year may be explained by the arrival of later entrants to the Burn. The 1984 and 1985 estimates were based on 108 previous spawners sampled in 1985 of which 56 (52%) were recaptured ﬁsh. Ova deposition levels An estimate was made of the sea trout population fecundity during 1981–85, based on a regression formula for River Earn sea trout In F = 2.804 × ln L − 9.622

(Walker, 1994)

The data were weighted by the numbers of females within the length composition of the samples of the annual spawning stocks. Based on these calculations, it appears that more than 1 000 000 eggs are laid by sea trout in the Findhu Glen Burn in some years (Table 27.6). As the total area of the main stream and its tributaries accessible to sea trout is estimated at 45 000 m2 , the mean egg deposition amounts to some 22 eggs per m2 . This compares with the brown trout population resident in the upper area of the Allt Mor with an estimated 1 egg per m2 . Modelling of the Findhu Glen trout population A simple model providing an approximation of the population dynamics of the Findhu Glen Burn trout stock was derived from the above estimates of total ova deposition, the average Table 27.6 The estimated potential ova deposition in the Findhu Glen Burn by sea trout during 1981–85.a Years

Females

Total ova

1981 1982 1983 1984 1985a 1985b 1985c

526 297 261 462 422 461 512

1 043 584 589 248 517 824 916 608 837 248 914 624 1 015 808

a Based on a body length-weighted mean of 1984 ova per female sea trout and a regression from Walker (1994).

398

Sea Trout

juvenile densities from electro-ﬁshing surveys undertaken in the Burn, multiplied by the available area of stream, and from the estimated numbers of adults. The numbers of adult brown trout are derived from the estimated numbers of sea trout by using the proportion of brown trout to sea trout sampled in 1985. Because the ova contribution by female sea trout (99.8%) dwarfs that of the tiny numbers of female brown trout, almost all juvenile trout of the main Burn and accessible parts of its tributaries must be progeny of female sea trout.

Discussion The rapid establishment of a self-sustaining resident trout population in the isolated upper reaches of the Allt Mor stocked some 20 years ago with sea trout progeny demonstrates the continuing phenotypic and behavioural plasticity that allowed anadromous forms of S. trutta to recolonise fresh water habitats after the last Ice Age. In a modern simulation of such a natural recolonisation event, the progeny of sea trout obtained from a population in the Findhu Glen Burn with a strong history of anadromy, diverged into both resident and river-migratory brown trout and also sea trout. This outcome provided direct conﬁrmation of the division of a common spawning population into the separate life-history traits of the trout. It is consistent with the conclusion that the mixed trout stocks found spawning in the main Burn arise almost entirely from sea trout ova. Previously, Campbell (1977) concluded from the results of a trapping investigation in the Kirk Burn, supported by electro-ﬁshing surveys elsewhere in the catchment, that the brown trout of the River Tweed were largely or solely the progeny of female sea trout. Also, Skrochowska (1959, 1969) found that sea trout and brown trout phenotypes were produced from each of the batches of sea trout, brown trout and their crosses liberated into Polish rivers entering the Baltic Sea. In Norway, Nordeng (1983) showed similar divergence into anadromous and freshwater forms using progeny of migratory and freshwater-resident strains of Arctic charr (Salvelinus alpinus (L.)). Clearly, the knowledge that the two major trout phenotypes in eastern Scotland, sea trout and brown trout, derive from common breeding populations is fundamental for their better management and conservation. Current ﬁsheries legislation treats them like separate species. It is interesting that while only ﬁve mature female brown trout were sampled during the entire study in the main Burn, the stocking of a much smaller area of the upper Allt Mor with fry of main Burn sea trout origin resulted in a greater number of mature resident females. Proportionate to the numbers of brown trout adults sampled, 18 times as many mature females were sampled in the upper Allt Mor as were found in the Findhu Glen Burn. An increase in mature female brown trout was already apparent in the ﬁrst generation resulting from stocking, implying that this may have been partly because of acceleration of the onset of sexual maturity as a result of better environmental conditions in the ‘virgin’ upper reaches (Walker, 1990). A further swing towards female residence would be expected to occur progressively through intense natural selection, in that any ﬁsh that migrate will be unable to return to spawn. The low numbers of trout presently resident in the upper reaches of the Allt Mor (<0.05 ﬁsh per m2 ), can be explained by low egg deposition levels, perhaps compounded by continuing emigration. Whether or not that is the case, the resident female spawners are inadequate to fully populate the available habitat. At present, although

Establishment of Resident Brown Trout

399

the density of trout in the upper Allt Mor remains very low, the population is demonstrably sustainable. This rapid establishment of a self-maintaining brown trout population above impassable falls provides the opportunity to screen the ﬁsh for evidence of genetic selection. DNA from archived samples of scales from the donor sea trout and the general population at that time can be compared with samples from trout in the present Allt Mor and main Burn stocks. Further studies on the trout using such genetic techniques have now been initiated at FRS Freshwater Laboratory (J. Gilbey, pers. comm.). From the viewpoint of maximising ﬁshery potential for sea trout in the River Earn, the upper Allt Mor may be restocked annually with local sea trout ova or fry in order to make full use of the available nursery habitat, as the resident trout population appears to be incapable of maintaining that level of juvenile production. On the other hand, the use of above-falls areas of Scottish streams for the stocking of migratory ﬁsh is becoming increasingly of concern to those interested in conservation of trout biodiversity and the ecological integrity of fresh waters, although this practice has often taken place unchallenged in the past. Current scientiﬁc opinion strongly favours the conservation of identiﬁed remnant genetic strains of trout (Ferguson, 2004), while an argument can be made also to preserve ﬁshless habitat for other biota. On the other hand, above-falls areas that have already been stocked may cause less concern if they continue to be used as supportive nurseries for migratory trout ﬁsheries.

Acknowledgements The author wishes to thank The Baroness Willoughby de Eresby and her Ancaster Estates staff for their interest and cooperation in the Findhu Glen Burn studies. Colleagues R.B. Greer and A.E. Thorne provided considerable assistance with the original work, while John Young of the River Earn Angling Improvement Association has helped in the ﬁeldwork carried out in more recent years.

References Campbell, J.S. (1977). Spawning characteristics of brown trout and sea trout, Salmo trutta L., in Kirk Burn, River Tweed, Scotland. Journal of Fish Biology, 11, 217–30. Carlsson, J., Carlsson, J.E.L., Olsén, K.H., Hansen, M.M., Eriksson, T. & Nilsson, J. (2004). Kin-biased distribution in brown trout: an effect of redd location or kin recognition? Heredity, 94, 53–61. Ferguson, A. (2004). Gene pool. Salmo Trutta, 7, 48–51. Hindar, K., Jonsson, B., Ryman, N. & Stahl, G. (1991). Genetic relationships among landlocked, resident and anadromous brown trout. Heredity, 66, 83–91. Jonsson, B. (1985). Life history patterns of freshwater resident and sea-run migrant brown trout in Norway. Transactions of the American Fisheries Society, 114, 182–4. Jordan, W.C. & Youngson, A.F. (1992). The use of genetic marking to assess the reproductive success of mature male Atlantic salmon parr (Salmo salar L.) under natural spawning conditions. Journal of Fish Biology, 41, 613–18. Nall, G.H. (1930). The Life of the Sea Trout. Seeley Service, London, 335 pp. Nordeng, H. (1983). Solution to the ‘char’ problem based on Arctic char (Salvelinus alpinus) in Norway. Canadian Journal of Fisheries and Aquatic Science, 40, 137–87. Phinney, D.E., Miller, D.M. & Dahlberg, M.L. (1967). Mass marking of young salmonids with ﬂuorescent pigment. Transactions of the American Fisheries Society, 96, 157–62.

400

Sea Trout

Shearer, W.M. (1990). North Esk sea trout. In: The Sea Trout in Scotland (Picken, M.J. & Shearer, W.M., Eds). The Dunstaffnage Marine Research Laboratory, Oban, Scotland, pp. 35–45. Skrochowska, S. (1959). Migrations of sea trout, brown trout and their crosses tagged as smolts in the Vistula. Rapports et procès-verbaux des rèunions Conseil International pour l’explorations de la mer, 115, 5 pp. Skrochowska, S. (1969). Migration of the sea trout (Salmo trutta L.), brown trout (Salmo trutta m. fario L.) and their crosses. Polskie Archiwum Hydrobiology, 16(29), 125–92. Walker, A.F. (1990). Sea trout and brown trout of the river Tay System. In: The Sea Trout in Scotland (Picken, M.J. & Shearer, W.M., Eds). The Dunstaffnage Marine Research Laboratory, Oban, Scotland, pp. 5–12. Walker, A.F. (1994). Fecundity in relation to variation in life history of Salmo trutta L. in Scotland. PhD Thesis, University of Aberdeen, 150 pp. Walker, A.F., MacDonald, A.I. & Thorne, A.E. (1998). The sea trout and brown trout spawning stocks of the Findhu Glen Burn, river Earn system, eastern Scotland, in 1997. Freshwater Fisheries Laboratory Report, 6/98, 11 pp.

Chapter 28

Predicted Growth of Juvenile Trout and Salmon in Four Rivers in England and Wales Based on Past and Possible Future Temperature Regimes Linked to Climate Change I.C. Davidson, M.S. Hazlewood and R.J. Cove Environment Agency, Chester Road, Buckley, Flintshire, CH7 3AJ, UK

Abstract: Climate change scenarios modelled by the UK Climate Impacts Programme (UKCIP, 2002) indicate marked increases in temperature are likely across the British Isles in the next century. For example, by 2080, UK temperatures may be 2◦ C or 3.5◦ C higher than the 1961–90 annual average for low and high carbon dioxide emissions scenarios, respectively. Regional and seasonal increases may be even more marked – for example, by 2080, parts of the south-east may be up to 5◦ C warmer in summer for the high-emissions scenario. Temperature records, both globally and for the UK, suggest a warming tendency in recent decades. River temperature data presented in this chapter for the Thames, Wye, Dee and Lune demonstrate increasing and synchronous trends in annual mean temperatures at main river sites in the long (past 40 years on the Wye and Dee) and shorter term (past 20 years on all four rivers). This study utilises the growth models of Elliott et al. (1995) and Elliott & Hurley (1997) to predict size-at-age for juvenile trout and salmon on the Thames, Wye, Dee and Lune from (1) past and (2) possible future monthly mean water temperatures. The latter predictions were based on likely local increases in temperature up to 2100 derived from the low and high-emissions scenarios of the UKCIP (2002). Size-at-age predictions from past temperature data are compared with mean length estimates backcalculated from time-series of adult scales for sea trout (Dee) and salmon (Wye, Dee and Lune) and the degree of agreement between the results of these two approaches, and other evidence of growth changes (e.g. trends in smolt age), are discussed. Size-at-age predictions based on future temperature proﬁles are used to speculate about the possible impact of climate change on UK trout and salmon populations to the end of the century – exploring regional and species differences in the response to temperature change. Keywords: Salmo trutta L., Salmo salar L., climate change, modelling, predicted growth, parr.

Introduction In 2001, the Intergovernmental Panel on Climate Change (IPCC) noted that global average surface temperatures (air and sea combined) had increased by around 0.6◦ C during the twentieth century (IPCC, 2001). In the Northern Hemisphere, warming trends were even 401

402

Sea Trout

more signiﬁcant, with the decade of the 1990s being the warmest of the twentieth century and probably the warmest decade of the past millennium (IPCC, 2001). Computer modelling of past climate change by the UK Climate Impacts Programme (UKCIP) indicates that the increase in global average temperature seen in recent decades is unlikely to have been solely the result of natural processes (UKCIP, 2002). Indeed, a large part of the warming experienced in the past 50 years has been attributed to man’s inﬂuence through the production of ‘greenhouse gases’ – including carbon dioxide (CO2 ), methane and ozone. The rate at which ‘global warming’ occurs in the twenty-ﬁrst century is likely to depend on the volume of greenhouse gases produced in the coming decades. However, even if production of greenhouse gases is reduced, inertia in the climate system will mean that increases in temperature expected in the next 40–50 years may have already been determined by historic emissions of these gases (UKCIP, 2002). Global warming (as one aspect of climate change) is likely to have signiﬁcant consequences for many ﬁsh species. As ectotherms, ﬁsh normally have a body temperature near-identical to that of the surrounding water and therefore the rates of their biological functions are critically dependent on environmental temperature (Wood & McDonald, 1997). Extreme temperatures may be directly lethal to ﬁsh. In contrast the effects of sub-lethal temperatures – inﬂuencing processes such as growth and maturation – may be far more difﬁcult to predict. A further level of complexity arises when considering the effects of temperature on the ecosystem as a whole and the implications for ﬁsh populations. This study focuses on the relationship between temperature and salmonid growth. It utilises the growth models of Elliott et al. (1995) and Elliott & Hurley (1997) to predict size-at-age for juvenile trout and salmon on four rivers in England and Wales (Thames, Wye, Dee and Lune) from (1) past and (2) possible future temperature regimes (the latter based on the ‘low’ and ‘high’ carbon dioxide ‘emissions scenarios’ of the UKCIP, 2002) and compares predictions of past growth with ‘observed’ estimates to check the realism of model outputs. The implications for what future temperature changes might mean for both species of salmonid are discussed.

Materials and methods River temperature data Time series of river temperature data were compiled for the Thames, Wye, Dee and Lune (Fig. 28.1) from a combination of (1) in situ readings obtained from Environment Agency records and (2) predicted monthly average values based on regression relationships between in situ readings and local air temperature records (after the method of Crisp, 1992). All air temperature data were extracted from the Meteorological Ofﬁce website www.metofﬁce.com. As air temperature records were generally more extensive than those for river temperature, predictions based on the former were used to complete gaps in the data set where in situ readings were absent. On each river system, temperature data were collected from a single main river site located in the lower third of the catchment. Information on the sources

Predicted Growth of Juvenile Trout and Salmon

N

403

50 Kilometres

Lune

Dee

Severn

Wye Thames

Fig. 28.1 Location of study rivers (© Crown Copyright. All rights reserved. Environment Agency. 100026380 [2006]).

of temperature data and relationships used to convert from air to river temperature are summarised in Table 28.1. (Note: in some cases not all air or river temperature data were used to derive these relationships where monthly means were missing from annual data series.)

Predicted freshwater growth The growth models of Elliott et al. (1995) and Elliott & Hurley (1997) were used to predict size-at-age for 1 and 2-year-old trout and salmon from the Thames, Wye, Dee and Lune based on monthly mean river temperatures. (The latter derived from the data sets described above.) Wt = W0b + bc

(T − TLIM )t 100(TM − TLIM )

1/b

404

Sea Trout

Table 28.1 Temperature data for the Thames, Wye, Dee and Lune. River

Location (and grid reference)

Source and type of river temperature data Thames Teddington (TQ 1670 7150) Wye Redbrook (S0 5286 1108) Dee Manley Hall (SJ 3481 4146) Lune Lyon Bridge (SD 5815 6971) Source and type of air temperature data Thames St. Jame’s Park (TQ 2950 7970) Wye Malvern (SO 7830 4630) Dee N/A Lune Hazelrigg (SD 3740 8450)

Data type (and sampling method)

Period sampled

Single daily ‘midday’ (manual)

Jun 1986 to Dec 1999

Single daily 9.00 a.m. (automatic)

Jan 1996 to Dec 1999

Mean daily max/min (continuous chart) Variable – 1 day per 2 months to 4 days per month (manual)

Jan 1965 to Dec 1999

Mean daily max/min (manual)

Jan 1979 to Nov 1999

Mean daily max/min (manual)

Jan 1996 to Dec 1999

N/A Mean daily max/min (manual)

N/A Jan 1960 to Nov 1999

Monthly mean air (X) to monthly mean river temperature (Y) conversion P Regression relationship R2 Thames Wye Dee Lune

Y = −0.8537 + 1.0599X Y = 1.0347 + 0.8808X N/A Y = 2.0767 + 0.8332X

Feb 1980 to Dec 1999

0.972 0.965 N/A 0.838

≤0.05 ≤0.05 N/A ≤ 0.05

N 72 47 N/A 57

Jan 1987 to Dec 1997 Jan 1996 to Nov 1999 N/A Jan 1992 to Nov 1999

N/A = Not applicable.

where t is the time in days; W0 is initial ﬁsh mass; Wt is ﬁnal ﬁsh mass (after t days at T ◦ C); T is water temperature (◦ C); b is exponent for the power transformation of mass that produces linear growth with time = 0.308 for trout and 0.310 for salmon; c is growth rate of a 1 g ﬁsh at the optimum temperature = 2.803 for trout and 3.530 for salmon; TM is optimum temperature for growth = 13.11◦ C for trout and 15.94◦ C for salmon; TLIM is TL if T ≤ TM or TLIM = TU if T > TM ; TL is lower temperature at which growth ceases = 3.56◦ C for trout and 5.99◦ C for salmon and TU is the upper temperature at which growth ceases = 19.48◦ C for trout and 22.51◦ C for salmon. In the absence of information on emergence date, or appropriate ﬁeld length/weight measurements from any of the four rivers, a growth year of 1st April–31st March and an initial ﬁsh mass of 0.25 g for trout and 0.15 g for salmon on the 1st May was assumed in all cases. (The latter based on estimates given in Elliott et al., 1995; Elliott & Hurley, 1997.) In applying the model it was also assumed that zero growth occurred from 1st October to 1st April in the ﬁrst year for both trout and salmon. This followed comparisons of observed and expected trout and salmon growth rates by Elliott et al. (1995) and Elliott & Hurley (1997), who found that model predictions were improved when growth cessation was imposed from mid-September to mid-March in the ﬁrst year.

Predicted Growth of Juvenile Trout and Salmon

405

Mean weight predictions from these models were converted to length using the formula: (Ln Wt ) − a d where Ln is natural log and Lt is ﬁnal length in centimetre (at time t) and ‘a’ (= −4.43 for trout and −4.47 for salmon) and ‘d’ (=2.74 for trout and 3.00 for salmon) were taken from Elliott et al. (1995) and Elliott & Hurley (1997) and rearranged so that (Ln Lt ) =

(Ln Wt ) = a + d(Ln Lt ). Back-calculated freshwater growth Back-calculated lengths at each river annulus were derived from adult scales using the method described by Friedland et al. (2000). Scales were examined from .0+ and .1+ sea trout from the Dee (Davidson et al., 2001) and 1 and 2 sea winter (SW) salmon from the Wye, Dee and Lune (Davidson & Hazlewood, 2001). Measurements were obtained from cleaned scales viewed under magniﬁcation (×30 or ×50). For each ﬁsh sampled, radii distances were recorded from a single selected scale and measured to the nearest 0.5 mm.

Results Trends in river temperatures Annual mean river temperatures for the Thames, Wye, Dee and Lune (derived from the data sets described previously) are shown in Fig. 28.2. Over the common time period 1979–99, temperature records were highly correlated across all four rivers (r = 0.227– 0.569; P = 0.000–0.011) and in all cases an increasing trend in annual mean temperature 13 Thames Wye Dee Lune

Mean temperature (°C)

12

11

10

9

8

7 60

62

64

66

68

70

72

74

76

78

80

82

84

86

88

90

92

94

96

98

Year

Fig. 28.2 Year-to-year variation in observed and/or predicted annual mean river temperatures on the Thames, Wye, Dee and Lune, 1960–99.

406

Sea Trout Table 28.2 Linear regression relationships between year (X ) and annual mean river temperature (Y ) for: Thames, Wye, Dee and Lune, 1979–99 and Wye and Dee, 1965–99. R2

P

N

Period

Thames, Wye, Dee and Lune, 1979–99 Thames Log10 (Y ) = −5.986 + 0.0035405X Wye Log10 (Y ) = −3.796 + 0.0024182X Dee Log10 (Y ) = −2.463 + 0.0017447X Lune Log10 (Y ) = −0.171 + 0.0005830X

0.569 0.384 0.227 0.090

0.000 0.003 0.029 0.679

21 21 21 21

1979–99 1979–99 1979–99 1979–99

Wye and Dee, 1965–99 Wye Log10 (Y ) = −1.6436 + 0.001337X Dee Log10 (Y ) = −0.7096 + 0.000864X

0.409 0.183

0.000 0.010

40 35

1960–99 1965–99

River

Regression relationship

was apparent – signiﬁcantly so (P ≤ 0.05) except for the Lune (r = 0.090; P = 0.679). Log10 linear regression equations ﬁtted to each of the data sets (Table 28.2) indicated average increases in annual mean temperatures of 0.013–0.093◦ C per year with a latitudinal gradient in the rate of increase (i.e. the Lune, as the most northerly river, had the smallest rate of increase and the Thames, as the most southerly, the greatest rate). The more extensive temperature time series for the Wye and Dee, beginning in the 1960s, were also highly synchronous (Fig. 28.2) and showed signiﬁcant positive trends (P ≤ 0.05) in annual mean temperature equivalent to average yearly increases of 0.031◦ C and 0.020◦ C, respectively (Table 28.2b).

Past freshwater growth Temperature predicted mean lengths of 1 and 2-year-old salmon and trout – from the growth models of Elliott et al. (1995) and Elliott & Hurley (1997) – are shown in Figs 28.3 and 28.4 for the Thames, Wye, Dee and Lune over the past 20–40 years. In most cases predicted lengths for both species were relatively stable over the time series. The exceptions being for 2-year-old salmon on the Lune (r = −0.504; P = 0.028) and for 1-year-old trout on the Thames (r = −0.582; P = 0.007) and Wye (r = −0.364; P = 0.034) and 2-year-old trout on the Lune (r = −0.502; P = 0.029) where signiﬁcant negative correlations were evident between mean length and year class. (For the Wye this correlation was only signiﬁcant over the 1965–99 period and not the shorter 1979–99 time series common to all four rivers.) Mean lengths estimated from back-calculation are also shown in Figs 28.3 and 28.4 alongside those predicted by the growth models. For salmon, the former were based on scale measurements from both 1SW and 2SW adults. No back-calculation data are given for the Thames because salmon production on this river is considered to be maintained entirely by stocking. Back-calculation data for sea trout were only available for the Dee with mean lengths based on scale measurements from combined samples of .0+ and .1+ maiden adult ﬁsh (Fig. 28.4).

Predicted Growth of Juvenile Trout and Salmon Wye

Thames 250

250 Pred. RA1 Pred. RA2

200 Mean length (mm)

Mean length (mm)

200 150 100 50

Pred. RA1 BC RA1

150 100

0 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99

65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99

Year class

Year class Lune

Dee 250

250 Pred. RA1 BC RA1

Pred. RA2 BC RA2

200 Mean length (mm)

Mean length (mm)

Pred. RA2 BC RA2

50

0

200

407

150 100 50

Pred. RA1

Pred. RA2

BC RA1

BC RA2

150 100 50

0

0 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99

65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99

Year class

Year class

Fig. 28.3 Mean lengths of salmon at river ages (RA) 1 and 2 on the Thames, Wye, Dee and Lune: (1) predicted using the growth model of Elliott and Hurley (1997) and (2) estimated by back-calculation (BC) from adult scales.

Wye 250

200

200

Mean length (mm)

Mean length (mm)

Thames 250

150 100 50

150 100 50

Pred RA1 Pred RA2

0

Pred RA1 Pred RA2

0 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99

65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99

Year class

Year class Lune

250

250

200

200

Mean length (mm)

Mean length (mm)

Dee

150 100

Pred RA1 Pred RA2

50

150 100 50

BC RA1 BC RA2

Pred RA1 Pred RA2

0

0 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 Year class

65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 Year class

Fig. 28.4 Mean lengths of trout at river ages (RA) 1 and 2 on the Thames, Wye, Dee and Lune; (1) predicted using the growth model of Elliott et al. (1995) and (2) estimated by back-calculation (BC) from adult scales.

408

Sea Trout

Back-calculated lengths for both salmon and trout were usually (in 114 out of 121 cases) less than those predicted by the growth models – particularly so for salmon at river age (RA) 2. Temporal trends in back-calculated lengths were positive and signiﬁcant for RA1 and RA2 salmon on the Dee (r = 0.810; P = 0.000 and r = 0.721; P = 0.000, respectively) and for RA1 salmon on the Wye (r = 0.605; P = 0.010) and Lune (r = 0.554; P = 0.044). There was little evidence that annual variations in ‘observed’ lengths from backcalculation and lengths predicted by the growth models were synchronous; the exception being for 2-year-old salmon on the Dee where back-calculated and predicted lengths were signiﬁcantly correlated (r = 0.540; P = 0.017) (Fig. 28.3). Future freshwater growth The UKCIP (2002) has modelled four climate change scenarios for the UK corresponding to four global carbon dioxide emission ‘levels’. Namely: ‘low emissions’, ‘medium–low emissions’, ‘medium–high emissions’ and ‘high emissions’. Coupled to each scenario, average weather statistics have been predicted for three 30-year periods of 2011–40 (or the 2020s), 2041–70 (the 2050s) and 2071–100 (the 2080s). These statistics indicate change in relation to the average 1961–90 (or 1970s) climate (UKCIP, 2002). Average seasonal temperature changes for each scenario and in each 30-year period have been produced for a series of 50 km square covering the whole of the UK and Ireland and have been published on a series of colour-coded maps by UKCIP (2002). In each case, the seasons spring, summer, autumn and winter correspond to the monthly periods: March–May, June–August, September–November and December–February, respectively. Predicted increases in spring, summer, autumn and winter temperatures associated with the Thames, Wye, Dee and Lune catchments, were read directly from these maps for the low and high-emissions scenarios. (For each river this involved selecting a single 50 km square as representative of the catchment as a whole.) Monthly average temperatures on each river for the period 1961–90 were then adjusted upwards according to these seasonal temperature increases in order to simulate temperature proﬁles for the 2020s, 2050s and 2080s. The latter were used as inputs in the growth models of Elliott et al. (1995) and Elliott & Hurley (1997) to predict future size-at-age for salmon and trout on each river (Fig. 28.5). An example showing the calculation of future temperature proﬁles for the River Wye is given in Table 28.3 with the equivalent data for the Thames, Dee and Lune shown in Appendix 28.1. Note that, in the absence of temperature data for the Thames and Lune for the 1960s and 1970s (see Table 28.1), monthly average temperatures for these decades were derived from 1980s values adjusted according to mean temperature increases observed on the Wye and Dee. On the Wye and Dee, growth rates for 2-year old salmon under the low-emissions scenario improve up to the 2050s and then decline into the 2080s – although ﬁsh still attain mean lengths greater than those predicted for the period 1961–2000 (Fig. 28.5). Under the highemissions scenario, improved growth is also evident initially, but growth rates begin to decrease sooner (2050s) and more rapidly, such that predicted mean lengths in the 2080s are less than those of the past.

Predicted Growth of Juvenile Trout and Salmon

Length (mm)

Thames

100

50

50 2080s

2050s

2020s

1990s

1980s

1970s

1960s

Length (mm)

Dee

2080s

100

2050s

150

2020s

150

1990s

200

1980s

200

Wye

1970s

250

1960s

250

409

Lune

2080s

2050s

2020s

1990s

1980s

50

1970s

50

SL (L) SL (H) SL 1961–90 TR (L) TR (H) TR 1961–90

1960s

100

2080s

100

2050s

150

2020s

150

1990s

200

1980s

200

1970s

250

1960s

250

Fig. 28.5 Predicted mean lengths of salmon (SL) and trout (TR) at river age (RA) 2 based on observed (1960s–90s) and modelled (2020s–80s) river temperature proﬁles on the Thames, Wye, Dee and Lune for the high (H) and low (L) emissions scenarios of the UKCIP (2002).

Predicted growth patterns for salmon on the more northerly River Lune are similar to those for the Wye and Dee except that growth rates increase progressively from the 1960s and continue to do so almost to the end of the century – with a levelling off in the 2080s under the low-emissions scenario and a slight decrease in growth rate under the high-emissions scenario. In contrast to the above three rivers, on the Thames (as the most southerly river), predicted growth rates for salmon have been falling away gradually since the 1970s and continue on a steep downward trend under both low and high-emissions scenarios (Fig. 28.5). Growth predictions for trout on all four rivers are less favourable than for salmon with marked reductions in growth rate evident from the 2020s onwards – even under the lowemissions scenario (Fig. 28.5).

410

Mean river temperature (◦ C)

Period

Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Spring (MAM) Summer (JJA) Autumn (SON) Winter (DJF)

Mean temp increase (◦ C)

Low-emissions scenario

High-emissions scenario

Low-emissions scenario

High-emissions scenario

1961–90

2020s

2050s

2080s

2020s

2050s

2080s

2020s

2050s

2080s

2020s

2050s

2080s

6.5 8.6 11.5 14.2 16.0 15.7 13.6 10.7 7.2 5.5 4.4 4.6 8.9 15.3 10.5 4.9

7.2 9.6 12.8 15.1 17.1 16.7 14.9 11.8 7.9 6.7 5.4 5.5 9.9 16.3 11.5 5.9

7.6 10.1 13.4 16.0 18.1 17.8 16.2 12.8 8.6 7.3 5.8 6.0 10.4 17.3 12.5 6.4

8.0 10.5 14.1 16.5 18.6 18.3 16.9 13.3 8.9 7.3 5.8 6.0 10.9 17.8 13.0 6.4

7.2 9.6 12.8 15.6 17.6 17.2 15.6 12.3 8.2 6.7 5.4 5.5 9.9 16.8 12.0 5.9

8.0 10.5 14.1 17.0 19.1 18.8 17.5 13.8 9.3 7.8 6.3 6.5 10.9 18.3 13.5 6.9

9.1 12.0 16.0 18.8 21.2 20.8 19.4 15.3 10.3 9.0 7.2 7.4 12.4 20.3 15.0 7.9

1.0 1.0 1.0 1.0

1.5 2.0 2.0 1.5

2.0 2.5 2.5 1.5

1.0 1.5 1.5 1.0

2.0 3.0 3.0 2.0

3.5 5.0 4.5 3.0

Where, for example, the predicted March (Mar) mean temperature (T ) in the 2020s (MarT2020s ) is estimated as: MarT2020s = (MarT1970s /SpringT1970s ) × SpringT2020s .

Sea Trout

Table 28.3 Predicted monthly average river temperatures for the River Wye up to the 2080s, based on observed temperatures in the period 1961–90 and UKCIP (2002) modelled seasonal temperature increases for low and high-emissions scenarios.

Predicted Growth of Juvenile Trout and Salmon

411

Discussion Temperature records for the Thames, Wye, Dee and Lune examined here conﬁrm that warming has taken place in the past 20–40 years. Common patterns of temperature variation among these rivers suggest that broad-scale climatic processes have been inﬂuencing this change. At the same time, across-river differences in warming rates were also evident and appear to relate to geographical effects. For example, for the period 1979–99, the average rate of warming on the Thames, the most southerly of the rivers, was over 7× greater than the Lune, the most northerly river. Scenarios for future climate change (UKCIP, 2002) predict that warming trends are likely to be far more severe in the coming decades than those experienced in the past. Even under the least severe low-emissions scenario examined in this study, the future rate of global warming over the present century may be about four times that experienced during the twentieth century. For the high-emissions scenario, the future rate of warming may be about eight times that of the twentieth century (UKCIP, 2002). For the UK, average temperatures by the 2080s may be 2◦ C higher than the 1961–90 baseline for the low-emissions scenario and 3.5◦ C higher for the high-emissions scenario. The south and east are expected to experience greater warming than the north and west and warming in summer and autumn should be greater than in winter and spring. By the 2080s for the high-emissions scenario, parts of the south-east may be up to 5◦ C warmer in summer (UKCIP, 2002). These spatial differences in future warming patterns should be largely reﬂected in the temperature proﬁles predicted for the four rivers examined in this study. The location of these rivers is also broadly representative of the main salmon and sea trout producing areas in England and Wales – with, perhaps, the notable absence of the rivers, studied as examples, from the south-west peninsula and the north-east. Applying the models of Elliott et al. (1995) and Elliott & Hurley (1997) to these temperature proﬁles indicates that growth of salmon and trout in fresh water may alter markedly over the coming decades depending on the course of climate change, with strong regional and species differences apparent (Fig. 28.5). For example, for rivers in the south-west (Wye) and north (Dee and Lune), salmon growth rates in fresh water could generally improve under the low-emissions scenario but may decrease below current levels under the high-emissions scenario as temperatures exceed optimum levels in the latter half of the century. On rivers in the south-east (Thames) – where warming is expected to be greatest – declining growth rates could be more common, adversely affecting survival (e.g. because of starvation or greater susceptibility to predation) and, as a consequence, abundance (Elliott, 1994). For trout – under both low and high-emissions scenarios – predicted growth rates to the end of this century decrease progressively below current rates on all four rivers. Future warming trends are likely to have more severe consequences for trout than salmon because of their lower ‘thermal tolerance’ (Elliott, 1994). More speciﬁcally in the context of growth, trout have a narrower temperature range over which growth is positive (for ﬁsh on maximum rations); namely: 4–19◦ C (optimum 13◦ C) for trout compared with 6–23◦ C (optimum 16◦ C for salmon) (Elliott et al., 1995; Elliott & Hurley, 1997).

412

Sea Trout 2.5 Wye Severn

Mean smolt age (years)

Dee 2.0

1.5

1.0 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 00 02 Smolt year Fig. 28.6

Changes in MSA for salmon on the Wye, Severn and Dee, 1960–2002.

Elliott (1994) drew similar conclusions about the possible effects of global warming on growth and survival of juvenile trout, as did McCarthy & Houlihan (1997) who speculated about the positive and negative effects of warming on two contrasting salmon populations towards the extremes of the latitudinal range. On more southerly rivers (such as the Thames) salmon and trout may already be growing at a slower rate than they were 30 years ago, and in the case of trout, the size attained by a 2-year-old ﬁsh may be well below that of cooler rivers to the west and north (Fig. 28.5). Unlike future growth projections, past growth rates predicted for salmon on the Wye and Dee have been relatively stable over the past 30–40 years. This is in marked contrast to the decline in mean smolt age (MSA) apparent on both these rivers and (to a lesser extent) the Severn since the 1980s (Fig. 28.6), and suggests that factors other than (or in addition to) a general rise in temperature may be the main cause. Should growth rates for salmon improve into the middle part of this century on these rivers, then further declines in MSA may occur. Whether growth predictions for the future will prove accurate is uncertain. Part of this uncertainty is linked to the predictions for temperature change and their application (in this case) to riverine environments. For example, while mean air temperature is expected to increase over the next 100 years, increases in river temperature may be less marked – a difference which is likely to be greatest on aquifer-fed systems (such as the Thames and other ‘chalk’ rivers – many of which are located in southern England). Other sources of uncertainty stem from assumptions made in applying the growth models of Elliott et al. (1995) and Elliott & Hurley (1997). For example, size and time at emergence were held constant in the models (represented as a starting weight of 0.25 g on the 1st May

Predicted Growth of Juvenile Trout and Salmon

413

for trout and 0.15 g on the same date for salmon), although in sea trout both these variables are known to ﬂuctuate year-on-year and can signiﬁcantly inﬂuence subsequent growth and survival (Elliott, 1984; Elliott et al., 2000). Indeed emergence date has been shown to be inﬂuenced by temperature and climate change linked to the North Atlantic Oscillation (Elliott et al., 1999). Zero growth was also assumed in the ﬁrst year during the period 1st October–31st March. This followed comparisons of observed and predicted growth rates in trout and salmon by Elliott et al. (1995) and Elliott & Hurley (1997) who found predictions improved following a similar restriction. In the case of salmon this was based on observations on the Eden from the 1930s when the population comprised mainly ‘slow-growing’ 2-year-old smolts, but may be less apt today on rivers like the Wye and Dee which, over the past 20 years or so, appear to have been producing signiﬁcant numbers of ‘faster-growing’ 1-year-old smolts (Fig. 28.6). Despite these limitations, comparisons of (past) predicted and back-calculated lengths at RA 1 and 2 indicated reasonable agreement between the two estimates in terms of absolute size. For example, in the case of salmon (all rivers), 73% of BC lengths for age 1 and 2 ﬁsh were within 20% of the value predicted from the growth model, and 98% were within 30%. For trout (Dee only), all BC lengths were within 20% of the predicted values and 70% within 10%. This level of agreement was comparable with that found by Elliott and Hurley (1997) when they applied the same model to data from the Eden, and by Jensen et al. (2000) who applied the equivalent growth model for brown trout (Elliot et al., 1995) to data from populations around Europe. However, there were also differences between BC and predicted estimates. For example: (1) BC estimates were usually less than predicted estimates (as the growth model was based on ﬁsh fed on maximum rations; then constraints on food availability in the wild could help explain this difference); (2) temporal trends in BC and predicted estimates, although usually in the same direction, were often at markedly different levels of signiﬁcance and (3) there was little evidence of synchrony among BC and predicted estimates. Given that, for each river system, a single source of temperature data (from the lower main river) was used to predict the growth of the population as a whole, it is perhaps not surprising that these differences exist. Temperature data from one point on the catchment are likely to be only broadly indicative of the range of temperature regimes which operate in different sub-catchments (inﬂuenced by factors such as altitude, aspect, tree cover, etc.). They are also unlikely to account for the partitioning of total ﬁsh production among sub-catchments or the behavioural response of ﬁshes in avoiding adverse temperatures and seeking out preferred conditions. Whatever the consequences of climate change for freshwater populations of trout and salmon in England and Wales, these are likely to go beyond the effects of rising temperature on the growth and survival of ﬁsh. Physiological processes other than growth (e.g. smoltiﬁcation, maturation) may also be directly affected by temperature change (Wood & McDonald, 1997). Furthermore, warming is just one aspect of climate change expected in the future. Summers are likely not only to become hotter, but drier and winters milder but also wetter – with more frequent episodes of intense rainfall leading to increased ﬂooding

414

Sea Trout

(UKCIP, 2002). Increases in the frequency of droughts and ﬂoods may both adversely affect juvenile and adult populations of trout and salmon. The effects of global warming on the marine environment are also likely to be signiﬁcant (UKCIP, 2002) with important consequences for migratory salmonids in the sea (Hughes & Turrell, 2003). If, as climate models predict, we are now entering a period of rapid warming (UKCIP, 2002), then it may not be too long before speculation about the effects of changing weather patterns on our native trout and salmon populations becomes a detectable reality.

References Crisp, D.T. (1992). Measurement of stream water temperature and biological applications to salmonid ﬁshes, grayling and dace. Freshwater Biological Association. Occasional Publication 29, 1992. Davidson, I.C., Hazlewood, M.S., Cove, R.J. & McIlroy, J.T. (2001). Analysis of growth and survival of sea trout from the Welsh Dee. Atlantic Salmon Trust Project Ref 99/7. Environment Agency Wales (internal report). Davidson, I.C. & Hazlewood, M.S. (2005). Climate change and ﬁsheries: use of scale material to examine spatial and temporal trends in the freshwater and marine growth of Atlantic salmon in England and Wales. Environment Agency R&D report No. W2-0471SR. Elliott, J.M. (1994). Quantitative Ecology and the Brown Trout. Oxford Series in Ecology and Evolution, Oxford University Press, Oxford. Elliott, J.M. & Hurley, M.A. (1997). A functional model for maximum growth of Atlantic salmon parr, Salmo salar L., from two populations in northwest England. Functional Ecology, 11, 592–603. Elliott, J.M., Hurley, M.A. & Fryer, R.J. (1995). A new, improved growth model for brown trout, Salmo trutta. Functional Ecology, 9, 290–8. Elliott, J.M., Hurley, M.A. & Maberly, S.C. (2000). The emergence period of sea trout fry in a Lake District stream correlates with the North Atlantic oscillation. Journal of Fish Biology, 56, 208–10. Friedland, K.D., Hansen, L.P., Dunkley, D.A. & Maclean, J.C. (2000). Linkage between ocean climate, postsmolt growth and survival of Atlantic salmon (Salmo salar, L.) in the North Sea area. ICES Journal of Marine Research, 57, 419–29. Hughes, S. & Tarrell, W.R. (2003). Prospects for improved oceanic conditions. In: Salmon at the Edge (Mills, D., Ed.). Blackwell Science Ltd., Oxford, pp. 255–67. Intergovernmental Panel on Climate Change (2001). IPPC third assessment report: climate change 2001. In: Synthesis Report (Watson, R.T. and the Core Writing Team, Eds). IPPC, Geneva, Switzerland, 184 pp. Jensen, A.J., Forseth, T. & Johnsen, B.O. (2000). Latitudinal variation in growth of young brown trout (Salmo tratta). Joutnal of Animal Ecology, 69, 1010–20. McCarthy, I.D. & Houlihan, D.F. (1997). The effect of temperature on protein metabolism in ﬁsh: the possible consequences for wild Atlantic salmon (Salmo salar) stocks in Europe as a result of global warming. In: Global Warming: Implications for Fresh Water and Marine Fish (Wood, C.M. & McDonald, D.G., Eds). Society for Exapeimental biology, S. Seminar series 61, Cambridge University Press, pp. 51–77. UK Climate Impacts Programme (2002). Climate change scenarios for the UK: The UKCIPO2 Brieﬁng Report, April 2002. Wood, C.M. & McDonald, D.G. (Eds) (1997). Global Warming: Implications for Freshwater and Marine Fish. Society for Experimental Biology Seminar Series: 61. Cambridge University Press, Oxford.

Appendix 28.1 Predicted monthly average river temperatures for the Rivers Thames, Dee and Lune up to the 2080s, based on observed temperatures in the period 1960–90 and UKKCIP (2002) modelled seasonal temperature increases for low- and high-emissions scenarios. Period

Mean temperature increase (◦ C)

Low-emissions scenario

High-emissions scenario

Low-emissions scenario

High-emissions scenario

1961–90

2020s

2050s

2080s

2020s

2050s

2080s

2020s

2050s

2080s

2020s

2050s

2080s

6.6 9.1 12.8 16.5 18.4 18.2 15.7 12.1 7.9 5.4 4.3 4.1 9.5 17.7 11.9 4.6

7.3 10.1 14.2 17.9 19.9 19.7 17.0 13.1 8.6 6.6 5.2 5.0 10.5 19.2 12.9 5.6

7.6 10.5 14.8 18.8 20.9 20.7 18.3 14.1 9.2 7.2 5.7 5.5 11.0 20.2 13.9 6.1

8.0 11.0 15.5 19.3 21.5 21.3 19.0 14.6 9.6 7.7 6.1 5.9 11.5 20.7 14.4 6.6

7.3 10.1 14.2 17.9 19.9 19.7 17.7 13.6 8.9 6.6 5.2 5.0 10.5 19.2 13.4 5.6

8.0 11.0 15.5 19.8 22.0 21.8 19.6 15.1 9.9 7.7 6.1 5.9 11.5 21.2 14.9 6.6

9.0 12.5 17.5 21.2 23.5 23.3 22.3 17.1 11.2 9.5 7.5 7.3 13.0 22.7 16.9 8.1

1.0 1.5 1.0 1.0

1.5 2.5 2.0 1.5

2.0 3.0 2.5 2.0

1.0 1.5 1.5 1.0

2.0 3.5 3.0 2.0

3.5 5.0 5.0 3.5 Continued

Predicted Growth of Juvenile Trout and Salmon

a. Thames Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Spring (MAM) Summer (JJA) Autumn (SON) Winter (DJF)

Mean river temperature (◦ C)

415

Continued. Mean river temperature (◦ C)

Mean temperature increase (◦ C) High-emissions scenario

Low-emissions scenario

High-emissions scenario

1960–90

2020s

2050s

2080s

2020s

2050s

2080s

2020s

2050s

2080s

2020s

2050s

2080s

b. Dee Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Spring (MAM) Summer (JJA) Autumn (SON) Winter (DJF)

5.9 8.4 12.1 15.1 16.6 15.9 13.3 10.5 7.2 5.7 4.8 4.4 8.8 15.8 10.3 5.0

6.6 9.3 13.5 16.0 17.6 16.9 14.6 11.5 7.9 6.8 5.8 5.3 9.8 16.8 11.3 6.0

6.9 9.8 14.2 16.5 18.1 17.4 15.8 12.5 8.6 6.8 5.8 5.3 10.3 17.3 12.3 6.0

7.2 10.3 14.8 17.5 19.2 18.4 16.5 13.0 9.0 7.4 6.3 5.8 10.8 18.3 12.8 6.5

6.6 9.3 13.5 16.0 17.6 16.9 15.2 12.0 8.3 6.8 5.8 5.3 9.8 16.8 11.8 6.0

7.2 10.3 14.8 17.5 19.2 18.4 16.5 13.0 9.0 7.9 6.7 6.2 10.8 18.3 12.8 7.0

8.2 11.7 16.9 18.9 20.8 19.9 19.1 15.0 10.4 9.1 7.7 7.1 12.3 19.8 14.8 8.0

1.0 1.0 1.0 1.0

1.5 1.5 2.0 1.0

2.0 2.5 2.5 1.5

1.0 1.0 1.5 1.0

2.0 2.5 2.5 2.0

3.5 4.0 4.5 3.0

c. Lune Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Spring (MAM) Summer (JJA) Autumn (SON) Winter (DJF)

6.2 8.6 10.7 13.5 14.7 16.7 12.4 10.5 6.9 5.8 4.7 4.9 8.5 15.0 9.9 5.1

6.9 9.6 12.0 14.4 15.7 17.8 13.6 11.5 7.6 6.9 5.6 5.9 9.5 16.0 10.9 6.1

7.3 10.1 12.6 14.8 16.2 18.4 14.9 12.6 8.2 7.5 6.0 6.3 10.0 16.5 11.9 6.6

7.7 10.6 13.2 15.7 17.2 19.5 15.5 13.1 8.6 7.5 6.0 6.3 10.5 17.5 12.4 6.6

6.9 9.6 12.0 14.4 15.7 17.8 14.3 12.1 7.9 6.9 5.6 5.9 9.5 16.0 11.4 6.1

7.7 10.6 13.2 15.7 17.2 19.5 16.2 13.7 8.9 8.1 6.5 6.8 10.5 17.5 12.9 7.1

8.8 12.1 15.1 17.1 18.6 21.2 18.0 15.2 10.0 9.2 7.4 7.8 12.0 19.0 14.4 8.1

1.0 1.0 1.0 1.0

1.5 1.5 2.0 1.5

2.0 2.5 2.5 1.5

1.0 1.0 1.5 1.0

2.0 2.5 3.0 2.0

3.5 4.0 4.5 3.0

Sea Trout

Low-emissions scenario

416

Period

Chapter 29

Sea Trout (Salmo trutta L.) Exploitation in Five Rivers in England and Wales B.A. Shields1 , M.W. Aprahamian1 , B.D. Bayliss2 , I.C. Davidson3 , P. Elsmere4 and R. Evans5 1 Environment

Agency, Richard Fairclough House, Knutsford Road, Warrington, WA4 1HG, UK 2 Environment Agency, Ghyll Mount, Gillan Way, Penrith 40 Business Park, Penrith, Cumbria, CA11 9BP, UK 3 Environment Agency, Chester Road, Buckley, CH7 3AJ, UK 4 Environment Agency, Launceston Depot, Unit 19/26, Pennygillam Industrial Estate, Launceston, Cornwall, PL15 7ED, UK 5 Environment Agency, Cambria House, 29 Newport Road, Cardiff, CF24 0TP, UK

Abstract: Average rod exploitation rates (1993 to 2004), in terms of the proportions of the sea trout stocks that were caught, were highest on the Rivers Lune (20.5 ± 5.2%) and Fowey (17.9 ± 1.6%), intermediate on the River Kent (12.6 ± 2.7%) and lowest on the Rivers Tamar (4.6 ± 0.2%) and Dee (2.7 ± 1.1%). The average rod exploitation rates in terms of the proportions of stocks that were killed, were similarly highest on the Rivers Lune (9.2 ± 2.1%) and Fowey (10.1 ± 1.1%), intermediate on the River Kent (5.3 ± 1.5%) and lowest on the Rivers Tamar (2.9 ± 0.2%) and Dee (1.3 ± 0.8%). Rod catch was signiﬁcantly positively correlated with sea trout run size only on the Rivers Dee and Tamar. Rod exploitation rate was signiﬁcantly negatively correlated with run size on the Rivers Lune and Fowey. Average monthly rod exploitation rates peaked in June and July. Average annual exploitation rates by the net ﬁsheries were highest on the River Lune (5.68 ± 0.89%), but relatively low on the other rivers (<1%). Keywords: Sea trout, rod, net, ﬁshery, exploitation.

Introduction Sea trout (Salmo trutta L.) are widely distributed around England and Wales, with approximately 90 rivers supporting recreational rod ﬁsheries. Of these, 21 also support estuarine net ﬁsheries and there are a further 8 coastal net ﬁsheries. The average catch over the past ﬁve years (1999–2003) was 44 676 ﬁsh in the rod ﬁshery and 39 456 in the net ﬁshery, with a high proportion of the ﬁsh released in the rod ﬁshery (52%). Rod anglers and net ﬁshermen are licensed to ﬁsh for both salmon (Salmo salar L.) and sea trout. The average number of migratory salmonid rod licences issued annually between 1999 and 2003 was 28 593, corresponding to an average of 171 188 days/year. The average number of migratory 417

418

Sea Trout

salmonid net licences issued each year between 1999 and 2003 was 470 with netsmen ﬁshing an average of 10 904 net tides per year and 4689 net days per year (Anon., 2000, 2001, 2002, 2003, 2004a). Sea trout therefore provide a very signiﬁcant ﬁshery resource in England and Wales. Catches are often the only indicators of local stock abundance for many ﬁsheries, and the relationship between catch and the stock available to be caught is one of fundamental importance to ﬁsheries management. Substantial effort has been invested in examining this relationship for Atlantic salmon in the British Isles (Mills et al., 1986; Beaumont et al., 1991; Solomon & Potter, 1992; Crozier & Kennedy, 2001; Gargan et al., 2002; Whelan et al., 2002), while for sea trout, comparatively little has been reported (Mills et al., 1986; Solomon, 1995). The primary objective of the management of ﬁsheries for migratory salmonids in the UK is to ensure the well being and sustainable exploitation of both salmon and sea trout stocks. There is, therefore, a need to be able to predict how changes in ﬁshing effort – one of the principal methods of regulating ﬁsheries in the UK – affect the sea trout stocks and their associated ﬁsheries (Beach & Potter, 1987). Ideally, the total exploitation by both the rod and net ﬁsheries should ensure that sufﬁcient spawners escape the ﬁsheries to maintain recruitment (Solomon & Potter, 1992) and that any particular stock component is not signiﬁcantly impacted. The aim of this chapter is to describe the pattern and estimate levels of sea trout exploitation in both rod and net ﬁsheries in a selection of UK rivers, to evaluate its use in deriving indices of abundance and discuss the management implications for the species and the ﬁsheries it supports.

Methods Run size estimates The Environment Agency operates a network of resistivity ﬁsh counters and/or ﬁxed adult traps, providing estimates of sea trout (and salmon) run sizes that are independent of catch data. Resistivity counter data from the Rivers Kent, Lune, Fowey and Tamar and ﬁxed adult trap data from the River Dee were analysed in this study. For the Rivers Kent, Lune, Fowey and Tamar run estimates were based on the analysis of counter trace signals. Signiﬁcant overlaps in the size ranges of salmon and sea trout exist on the rivers Kent, Lune and Fowey. However, the counts could be accurately partitioned into sea trout and salmon based on video analysis on the River Kent and on trap catches on the Rivers Lune and Fowey. On the River Tamar the count was partitioned into sea trout and salmon on the basis of counter signal size as there was negligible overlap in the size ranges of the two species in this River (Nicholson et al., 1995; Aprahamian et al., 1997). Sea trout run estimates for the Dee are based on the mark-recapture of trap caught ﬁsh (for details see Davidson et al., 2006). For the Rivers Lune, Fowey and Tamar, the run size estimates represent the total estimate for all sea-age groups. For the rivers Dee and Kent, the run size estimates were partitioned into 0-sea winter (x.0+) and older (>x.0+) components.

Sea Trout Exploitation in England and Wales

419

Catch returns Anglers and net ﬁshermen in England and Wales who hold licences to catch migratory salmonids are required by law to submit full catch returns at the end of each ﬁshing season. Prior to 1993, anglers catch returns declared the number of sea trout (and salmon) caught per season, but did not segregate this catch into those ﬁsh that were killed and those that were returned alive to the river. Catch-and-release has become an increasingly signiﬁcant feature of recreational rod ﬁsheries, and since 1993 anglers have been required to record the number of ﬁsh returned alive. This allows rod exploitation rates to be expressed in terms of the total ﬁsh caught as well as the total ﬁsh killed. The latter is particularly important where catch and release rates are relatively high. Anglers are also asked to separate their catch into the x.0+ and x.>0+ components and provide details of the method used to catch the ﬁsh, the former enables exploitation rates to be determined for the different sea-age categories. Data examined in this study were based on the monthly declared sea trout catches from 1993 to 2004 inclusive (National Rivers Authority & Environment Agency Annual Fisheries Statistics). As a result of variations in the administration of the national rod licencing system, the return rate of rod catch returns has varied from 20–30% in 1993 to 71–76% for the period 1994 to 2004 inclusive (Milner et al., 2002). The percentage of the total salmon catch that has been declared in these returns has been estimated at 53% for 1993 and 91% for 1994 to 2004, using Small’s (1991) method of estimating the total rod catch from the declared returns. Speciﬁc declaration rates for sea trout catches have not been calculated, but for the purposes of this study are assumed to be the same as for salmon. Appropriate correction factors were therefore applied to the declared rod catch statistics, to account for these variations in under-reporting (declared catch x1.9 for 1993 and declared catch x1.1 for 1994–2004). Declared net catches were estimated to represent 92% of the total actual catch for all years in question (1993–2004 inclusive) (Anon., 2004b), and a correction factor (x1.09) was applied to the declared net catch statistics to account for this under-reporting. Exploitation The exploitation rate values for rods and nets in this study are the extant exploitation rates (Solomon & Potter, 1992), that is the annual catch divided by the total number of ﬁsh passing respective ﬁsheries during the whole year. However, two descriptions of the above have been used in this study in order to take account of the increasing prevalence of catch and release in the rod ﬁsheries. Extant exploitation rates are therefore presented on the basis of the total catch, and also on the basis of the total kill, as it could be reasonably argued that the high proportion of ﬁsh returned alive will not be lost to the spawning population. Radiotracking studies on salmon demonstrate around 80% of rod-caught salmon that are released, will survive to spawn (Walker & Walker, 1992; Webb, 1998). Note that exploitation rates calculated for ‘killed ﬁsh’ in this study do not take account of the mortality associated with catch and release angling, nor does it take account of the incidence of repeat captures of ﬁsh that have been released.

420

Sea Trout

Statistical analysis A General Linear Model was used to analyse for differences in the method by which the ﬁsh were caught among rivers for the period 2000–2004. The methods were separated into four categories; ﬂy, spinner, bait and not known, the latter accounting for between 1.6% and 3.2% of the catch.

Results Exploitation

Rod ﬁsheries Average annual rod exploitation in terms of the percentage of the stock caught was highest on the Lune with a mean (±95% CI) of 20.5% (±5.2), followed by the Fowey (17.9%±1.6), Kent (12.6% ± 2.7), Tamar (4.6% ± 0.2) and Dee (2.7% ± 1.1) (Table 29.1). In terms of the proportion of ﬁsh killed, the level of exploitation was similar on the Lune and Fowey at about 10%, and in the three other rivers, 5% or less of the stock was killed (Table 29.1). Signiﬁcant differences existed between the main method by which the majority of the ﬁsh were caught on the ﬁve rivers (P < 0.05), there was no signiﬁcant effect between years (P > 0.05). On the Lune the majority were caught on ﬂy (75.9%), as on the Tamar (64.5%), Dee (61.3%) and Kent (59.0%). However, on the Fowey the majority of the sea trout were caught using spinners (52.6%), ﬂy accounting for 29.4% of the catch. The proportion of bait caught ﬁsh was highest on the Kent at 24.7%, followed by the Fowey (15.7%), Dee (14.2%), Lune (6.8%) and Tamar (1.1%).

Table 29.1 The percentage of sea trout of all age groups caught and killed by the rod ﬁsheries on the Rivers Kent, Lune, Dee, Fowey and Tamar. Year

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Mean 95% CI

Kent

Lune

Dee

Fowey

Caught

Killed

Caught

Killed

Caught

Killed

Caught

18.0 18.7 10.5 19.0 12.7 10.5 8.8 11.2 4.9 10.6 13.7 13.1 12.6 2.7

9.5 7.9 3.6 9.3 4.6 4.8 4.4 4.5 1.7 4.6 4.3 4.6 5.3 1.5

23.4 14.0 No count 17.2 10.5 No count 19.4 28.0 12.9 25.1 21.1 33.9 20.5 5.2

12.3 8.5

1.7 7.6 1.7 2.7 2.2 2.8 2.1 2.0 1.8 3.2 2.2 N/A 2.7 1.1

0.8 5.0 0.8 1.1 0.7 1.2 0.9 1.0 0.7 1.0 1.1 N/A 1.3 0.8

No count No count 11.2 11.6 31.6 31.0 21.3 17.4 16.1 13.3 12.8 12.4 17.9 1.6

N/A – Data not available; see text for details.

9.6 5.1 9.4 11.3 3.3 11.3 9.0 12.2 9.2 2.1

Tamar

Killed

Caught

Killed

7.8 8.8 22.1 16.1 10.6 8.9 8.0 6.2 6.3 5.8 10.1 1.1

No count 3.6 6.2 4.7 4.3 4.7 4.8 5.4 3.2 4.4 4.3 5.3 4.6 0.2

2.8 4.2 3.6 3.3 2.3 3.0 3.5 1.6 1.7 2.6 3.4 2.9 0.2

Sea Trout Exploitation in England and Wales

421

In relation to various stock components, exploitation could only be examined on two rivers, the Dee and Kent. On the Dee the exploitation level in terms of the number of ﬁsh caught, was signiﬁcantly (P < 0.05) higher on the older ﬁsh x.>0+ (5.0% ± 0.3) than on the 0-sea winter (SW) component x.0+ (2.4%±0.5), as was the case in terms of the number killed, 3.1% (±0.2) compared with 0.9% (±0.4), respectively (Table 29.2). In contrast, the level of exploitation on the River Kent was higher on the x.0+ component (16.0% ± 2.1) than on the older ﬁsh (9.0% ± 1.4) (Table 29.2), though the difference was not signiﬁcant (P > 0.05). In terms of the percentage of ﬁsh killed, the exploitation rate was signiﬁcantly higher (P < 0.05) on the x.>0+ component (5.8% ± 1.0) than on the x.0+ age group (1.4% ± 0.2). On the Kent the exploitation level on the x.0+ component in terms of the numbers caught has increased steadily from approximately 9% in 2000 to approximately 25% in 2004, although no such trend was evident in terms of the proportion killed, nor for the older ﬁsh. The particularly low level of exploitation of x.>0+ sea trout in 2001 of 2.8% is likely to be related to reduced effort in the early part of the season as a result of access restrictions caused by an outbreak of foot and mouth disease. Monthly exploitation rates tended to be relatively high on each river in the early months of the season (Fig. 29.1), but these will be particularly uncertain as they are usually derived from small numbers of ﬁsh being caught from small runs. The peak runs of sea trout enter these ﬁve study rivers during June and July, with exploitation peaking in July (Fig. 29.1).

Net ﬁsheries Only the River Lune supports a substantial sea trout net ﬁshery (Table 29.3), with annual catches averaging 732 sea trout and exploitation rates ranging from 2.7% to 10.3% with Table 29.2 The percentage of 0-SW (x.0+) and older (x.>0+) sea trout caught and killed by the rod ﬁsheries on the Rivers Kent and Dee. Year

Kent x.0+ Caught

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Mean 95% CI

9.6 8.9 15.9 21.2 24.6 16.0 2.1

Dee

x.>0+ Killed

1.0 1.4 0.8 1.3 2.7 1.4 0.2

Caught

15.7 2.8 9.0 9.0 8.6 9.0 1.4

N/A – Data not available; see text for details.

x.0+

x.>0+

Killed

Caught

Killed

Caught

Killed

11.0 1.9 5.8 4.7 5.4 5.8 1.0

9.2 0.9 1.9 1.8 2.0 1.1 1.6 1.3 2.6 1.2 N/A 2.4 0.5

5.9 0.3 0.4 0.2 0.6 0.3 0.5 0.3 0.4 0.2 N/A 0.9 0.4

4.2 3.5 5.2 3.6 5.2 7.2 3.6 5.3 6.0 6.5 N/A 5.0 0.3

3.0 2.2 3.2 2.2 3.2 3.8 2.5 2.9 3.3 4.6 N/A 3.1 0.2

Ja nu Fe ary br ua r M y ar ch Ap ril M ay Ju ne Ju Au ly Se gus t pt em be O c r N tob ov er em D ec ber em be r

Exploitation

Fig. 29.1

10

20

Ja nu Fe ary br ua r M y ar ch Ap ril M ay Ju ne Ju Au ly Se gu pt st em O ber c N tob ov er e D mb ec e em r be r

pt

Exploitation

Kill

5

15

Dee Catch

Kill

5

0

Exploitation

Catch

Exploitation

Au g

ay ne uly J M Ju

us em t O ber c N tob ov er e D mb ec e em r be r

Se

Ja nu Fe ary br ua r M y ar ch Ap ril

10

Ja nu Fe ary br ua M ry ar ch Ap ril M ay Ju ne Ju ly Au gu Se st pt em O be ct r ob N ov er e D mb ec er em be r

20

Kent

0

20

15

10

5

Lune

15

10 5

Kill

0

Month Month

20 Fowey

15

10 Catch

5

Kill

0

Month Month

Tamar

Catch

Kill

0

Month

Mean monthly rod exploitation rates for all sea-age groups caught (95% CI) on the Rivers Kent, Lune, Dee, Fowey and Tamar (1993–2004).

Sea Trout

Exploitation

15

422

Ja nu Fe ary br ua r M y ar ch Ap r il M ay Ju ne Ju Au ly Se gu pt st em O ber ct N obe ov e r D mb ec e em r be r

20

Catch

Sea Trout Exploitation in England and Wales

423

Table 29.3 The percentage of sea trout of all age groups killed by the net ﬁsheries on the Rivers Kent, Lune, Dee, Fowey and Tamar. Year

Kent

Lune

Dee

Fowey

Tamar

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Mean 95% CI

0.07 0.03 0.03 0.00 0.04 0.02 0.15 0.00 0.54 0.00 0.09 0.00 0.08 0.09

7.07 7.76 No count 7.90 2.72 No count 7.02 3.09 4.17 3.13 3.58 10.31 5.68 1.89

0.40 1.52 0.37 1.81 1.20 0.35 0.38 0.21 0.17 0.81 0.43 N/A 0.70 0.37

No count No count 1.02 0.76 1.17 0.24 0.30 0.11 0.19 0.04 0.01 0.01 0.38 0.31

No count 1.19 1.25 1.74 0.46 0.35 0.57 0.57 0.71 0.46 0.36 0.00 0.70 0.34

N/A – Data not available; see text for details.

a mean of 5.7 ± 1.9% (Table 29.3). However since 1999, season restrictions and reductions in the numbers of licensed nets have generally – with the exception of 2004 – brought about a reduction in the level of exploitation (Table 29.3). Net exploitation on each of the other four rivers averaged <1% (Table 29.3). On the Kent, no sea trout were declared caught in the years 2000, 2002 and 2004 even though there were between six and eight netsmen licensed to ﬁsh in those years. Catch in relation to stock size

Rod ﬁsheries Total annual catch showed no signiﬁcant relationship with stock size on the rivers Kent, Lune and Fowey (Fig. 29.2). On the Tamar, catch increased linearly with stock size at a rate of 3.9% (P < 0.05). Although the Dee showed no relationship between catch and stock (Fig. 29.2), the relationship was heavily inﬂuenced by the 1994 data, which represented the highest catch for the period. If the 1994 data were excluded then catch showed a signiﬁcant positive linear relationship with stock size increasing at a rate of 2.3% (P < 0.05). Effort in terms of the number of licence days ﬁshed for migratory salmonids was available for all ﬁve rivers. Except for the Dee there was no relationship between catch per licence day (catch per unit effort [CPUE]) and stock size (N). For the Dee the relationship took the form: CPUE = 1.18E −6 N 1.071

r 2 = 0.46 P < 0.05

In relation to the two identiﬁable stock components, there was no signiﬁcant relationship between the catch and stock for either x.0+ or x.>0+ age groups on the Kent though this may in part relate to the small sample size (Fig. 29.3). On the Dee, older sea trout catch

424

Sea Trout Kent

1000 Rod catch

750 500 250 0 0

4000 Count

6000

8000

10 000 Count

15 000

20 000

Lune

4000 Rod catch

2000

3000 2000 1000 0 0

Dee

500 Rod catch

5000

1994

400 300 200

y = 0.022x + 3.089 R2 = 0.578

100 0

0

10 000 Count

15 000

20 000

Fowey

1500 Rod catch

5000

1000 500 0 0

2000

4000

6000

8000

10 000

Count Tamar

Rod catch

1000 750 500

y = 0.039x + 81.695 R 2 = 0.568

250 0 0

5000

10 000 Count

15 000

20 000

Fig. 29.2 The relationship between catch of all age groups and sea trout stock size. The trend line for the Dee does not include the data for 1994.

Sea Trout Exploitation in England and Wales Kent x.0+

400 Road catch

425

300 200 100 0 0

500

1000

1500

2000

2500

3000

3500

Count Kent x.>0+

Rod catch

400 300 200 100 0 0

500

1000

1500

2000

2500

3000

3500

Count Dee x.0+

Rod catch

400

1994

300 200 y = 0.016x − 3.337 R2 = 0.518

100 0 0

4000

8000 Count

12 000

16 000

Dee x.>0+

Rod catch

200

y = 0.048x + 3.850 R2 = 0.420

100

0 0

500

1000

1500

2000

2500

3000

Count

Fig. 29.3 The relationship between catch of x.0+ and x.>0+ sea trout and stock size. The trend line for the x.0+ ﬁsh from the Dee does not include the data for 1994.

426

Sea Trout

increased with stock size at a rate of 4.8% (P < 0.05). Whilst the relationship for the x.0+ was not signiﬁcant (P > 0.05), excluding the high x.0+ catch of 1994 indicated a signiﬁcant positive relationship between catch and stock (P < 0.05), with a gradient of 1.65%. The relationship between the exploitation rate (catch) and stock size for all sea-age groups of sea trout is shown in Fig. 29.4. On the Rivers Lune and Fowey the exploitation rate declined signiﬁcantly with increasing stock size (P < 0.05) and the relationship was well described by a power curve. The River Kent similarly showed a decline, though the relationship was not signiﬁcant (P > 0.05). On the Rivers Tamar and Dee the exploitation rate remained relatively stable at approximately 4% and 2% respectively, independent of stock size. Even when the data for 1994 is included for the River Dee, the relationship was Kent Rod exploitation

20

10

0 0

1000

2000

3000

4000

5000

6000

7000

Count

Lune

Rod exploitation

40

y = 131011.856x-0.942 R 2 = 0.622

30 20 10 0 0

5000

10 000 Count

20 000

Dee

10 Rod exploitation

15 000

5

0 0

Fig. 29.4

2000

4000

6000

8000 10 000 Count

12 000

14 000

16 000

18 000

The relationship between exploitation rate (catch) of all age groups and stock size.

Sea Trout Exploitation in England and Wales

427

Fowey Rod exploitation

40

y = 26580.365x –0.846 R 2 = 0.504

30 20 10 0 0

2000

4000

6000

8000

10 000

Count Tamar

Rod exploitation

10

5

0 0

Fig. 29.4

5000

10 000 Count

15 000

5000

10 000 Count

15 000

20 000

Continued.

Net exploitation

15

10

5

0 0

Fig. 29.5

20 000

Net exploitation of all age groups of sea trout from the River Lune in relation to run size.

not signiﬁcant (P > 0.05). There was no signiﬁcant relationship between the exploitation rate and stock size for either of the two stock components on the Rivers Kent and Dee.

Net ﬁsheries On all ﬁve rivers the relationship between the exploitation rate and run size was not signiﬁcant (P > 0.05), though on the Lune there is a suggestion that net exploitation declined with increasing run size (Fig. 29.5).

428

Sea Trout

Fish size distribution

Rod ﬁsheries The sea trout catch shows a negative binomial size distribution (Fig. 29.6). The majority of the catch in all rivers was of ﬁsh less than 0.45 kg (1 lb) and will consist mainly of x.0+ ﬁsh. Around 20% of the catch on the Rivers Lune and Kent consisted of ﬁsh between 0.9 and 1.4 kg (2–3 lb), and a similar size category represented approximately 10% of the catch on the Rivers Dee, Fowey and Tamar. On all ﬁve rivers there were very few ﬁsh (<3%) weighing more than 2.27 kg (5 lb) in the rod catch.

Net ﬁsheries The Lune net ﬁsheries catch very few ﬁsh of less than 0.9 kg (2 lb), the majority of the catch being in the 0.9–1.8 kg (2–4 lb) size category (Fig. 29.7). This indicates that the x.0+ component is virtually absent from the net catches (Harris, 2000), because of the selectivity of net mesh sizes (Evans et al., 1995).

Distribution of catches amongst anglers The distributions of catches and bag sizes (excluding x.0+) among anglers for the 2002 and 2003 seasons is presented in Table 29.4. On all ﬁve rivers the vast majority of anglers declared catching no x.>0+ sea trout. The proportions of anglers recording nil catches for the seasons on the Rivers Lune, Tamar and Fowey were similar (70.9–72.6%) whereas higher proportions of anglers failed to catch any sea trout on the Rivers Kent and Dee (80.1% and 91.5%, respectively). The distribution of bag sizes for the seasons, was described by a negative binomial distribution, with zero bag sizes being most common, and increasing bag sizes becoming increasingly less common. Bag sizes in excess of two sea trout per season were relatively common on the Rivers Lune, Tamar and Fowey, being accounted for by 14.4, 11.1 and 14.2% of anglers, respectively. However, they were relatively uncommon on the Rivers Kent and Dee, where they were accounted for by 7.6 and approximately 2.3% of anglers, respectively.

Discussion The assessment of sea trout stock in the Rivers Kent, Lune, Fowey and Tamar was derived from counter data. Though counters may underestimate the ﬁsh stock (Aprahamian et al., 1996), those that are properly designed and sited are very unlikely to overestimate the size of the ﬁsh population. The occurrence of non-migratory salmonids (resident brown trout or rainbow trout) has the potential to cause an overestimation of the migratory salmonid population; however, this was not a signiﬁcant issue on the ﬁve study rivers. The other main source of error is the accuracy of the catch returns. It has been estimated by Small (1991) that nationally, anglers declare 91% of their catch and though this under-declaration was accounted for in the analysis, the value for any speciﬁc river may differ from this national ﬁgure.

Sea Trout Exploitation in England and Wales Kent

60 Percentage

429

40 20 0 <1

1

2

3

4

8

9

10

>10

8

9

10

>10

9

10

>10

9

10

>10

10

>10

Lune

60 Percentage

5 6 7 Size category (lb)

40 20 0 <1

1

2

3

4 5 6 7 Size category (lb) Dee

Percentage

60 40 20 0 <1

1

2

3

4

5

6

7

8

Size category (lb) Fowey Percentage

80 60 40 20 0 <1

1

2

3

8

Tamar

80 Percentage

4 5 6 7 Size category (lb)

60 40 20 0 <1

1

2

3

4 5 6 7 Size category (lb)

8

9

Fig. 29.6 Size distribution of sea trout caught and killed by the rod ﬁsheries between 1994 and 2004 on the Rivers Kent, Lune, Dee, Fowey and Tamar, n = sample size.

430

Sea Trout Lune

Percentage

40

Haaf (n = 1218)

30

Drift (n = 143)

20 10 0 <1

1

2

3

4

5 6 7 Size category (lb)

8

9

10

>10

Fig. 29.7 Size distribution of sea trout from the net ﬁsheries on the River Lune between 2001 and 2004, n = sample size.

Table 29.4 Percentage of anglers catching various bag sizes of sea trout (excluding x.0+) in a season, 2002 and 2003 (N = total number of angler catch returns). River

Lune Kent Dee Tamar Fowey

Number of x.>0+ sea trout declared caught 0

1

2

3

4

5

6

7

8

9

10

>10

N

70.9 80.1 91.5 72.6 71.5

8.8 8.5 4.4 10.7 8.5

5.9 3.9 2.1 5.8 5.9

3.2 2.0 0.8 3.9 2.8

2.5 1.6 0.3 1.6 2.1

1.6 1.1 0.5 1.0 2.1

1.6 0.6 0.1 0.3 1.5

1.1 0.4 <0.1 0.7 1.3

0.8 0.1 0.1 0.7 0.7

0.9 0.3 <0.1 1.0 0.2

0.3 0.2 <0.1 0.3 0.9

2.4 1.3 0.2 1.6 2.6

2128 696 1523 575 684

The level of rod exploitation, in terms of the average total catch, varied from 2.8% on the Dee to 20.5% on the Lune. It was not considered to reﬂect differences in the methods used on the various rivers because ﬂy ﬁshing is the least restricted method from a legal perspective and on the Lune ﬂy ﬁshing caught the most ﬁsh. However, the Lune has the largest run of sea trout of these ﬁve rivers, and the high exploitation rate on the Lune is unlikely to be reﬂective of a small sea trout stock. It is likely that other human factors may be more signiﬁcant, particularly accessibility, angler opportunity or angling attitude to sea trout on a particular river. In terms of the proportion of ﬁsh actually killed, rod exploitation was less than about 10%, though this does not take into account any delayed mortality of the released ﬁsh. From studies on salmon, the mortality on released ﬁsh may be in the region of 15% (Walker & Walker, 1992; Webb, 1998; A. Gowans & C. Durie, pers. comm.), which would therefore produce a slightly higher effective rod exploitation rate than reported here. Monthly rod exploitation rates tended to be relatively high on all ﬁve rivers during the ﬁrst months of the season (April and May) possibly as a reﬂection of the relatively low runs during these months. Sea trout runs peak on these ﬁve rivers during June and July, which is also the time when rod exploitation reaches its highest levels, indicating both the vulnerability of these ﬁsh to capture at this time and also the targetting of these runs by anglers.

Sea Trout Exploitation in England and Wales

431

It was evident that on the Dee and Kent total exploitation in terms of proportion killed was higher on the older (x.>0+) sea trout when compared with the 0-SW component (x.0+). This may reﬂect the earlier average run timing of older ﬁsh, the effect of the size limit and also that anglers have a preference to take the larger sea trout to eat. The net ﬁshery also exploits sea trout in excess of 900 g (2 lbs), and this selective removal of the larger sea trout has a disproportionate effect on egg deposition (Walker, 1994a, b; Solomon & Czerwinksi, 2006). The majority of the anglers holding migratory salmonid licences catch no sea trout, and the proportions of anglers recording zero catches were similar on the Rivers Lune, Fowey and Tamar, at approximately 70%. On the Kent and Dee, the proportions of anglers recording zero catches were even higher at 80–90%, maybe because most anglers are targeting salmon on those rivers. Except for the Dee, there was no relationship between CPUE and stock size. This is likely to reﬂect the measure of effort used, the number of days ﬁshing for salmon and/or sea trout, and that most of the effort (on these rivers) is focused on salmon as opposed to sea trout. In a study carried out in 1992, it was estimated that the proportion of time ﬁshing for sea trout ranged from 4% on the Dee to 43% on the Fowey (Environment Agency unpublished data). The evidence for a relationship on the Dee suggests that the majority of the sea trout there, are caught by a few specialist sea trout anglers. Clearly, in order to fully understand the relationship between catch and stock, speciﬁc data on sea trout ﬁshing effort needs to be collected (Evans, 1996). In particular, the amount of time spent ﬁshing for sea trout at night, and during the day, and how much time is spent ﬁshing for both salmon and sea trout at the same time. At present, biological reference points (BRPs) have yet to be developed for sea trout, (Walker et al., 2006). BRPs aim to ensure a minimum spawning biomass is maintained such that the stock is conserved, whilst allowing exploitation on the excess (Potter et al., 2003). On rivers without operational counters, where it is necessary to use catch as some index of run size, it is essential to understand the relationship between catch and stock in order to be able to manage a sea trout ﬁshery in this way. However, this study illustrates that, based on the available data, there is no simple, consistent relationship between catch and stock, nor between CPUE and stock that could be used with conﬁdence to manage exploitation in ﬁsheries where there is no independent measure of stock size. The inconsistency in the relationship between CPUE and stock for the rivers studied here may be a consequence of the measure of effort in the rod ﬁshery not being sufﬁciently reﬁned. Improvements to the collection of CPUE data are being investigated (Evans & Greest, 2006).

Acknowledgements We thank N. Milner, M. Pawson, R. Sedgwick and R. Wyatt for their advice and comments.

References Anon. (2000). Fisheries Statistics – Salmonid and Freshwater Fisheries Statistics for England and Wales 1999. Environment Agency, Bristol, 46 pp.

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Anon. (2001). Fisheries Statistics – Salmonid and Freshwater Fisheries Statistics for England and Wales 2000. Environment Agency, Bristol, 46 pp. Anon. (2002). Fisheries Statistics – Salmonid and Freshwater Fisheries Statistics for England and Wales 2001. Environment Agency, Bristol, 48 pp. Anon. (2003). Fisheries Statistics – Salmonid and Freshwater Fisheries Statistics for England and Wales 2002. Environment Agency, Bristol, 48 pp. Anon. (2004a). Fisheries Statistics – Salmonid and Freshwater Fisheries Statistics for England and Wales 2003. Environment Agency, Bristol, 38 pp. Anon. (2004b). Annual Assessment of Salmon Stocks and Fisheries in England and Wales 2003. CEFAS and Environment Agency, Bristol, 79 pp. Aprahamian, M.W., Nicholson, S.A., McCubbing, D. & Davidson, I. (1996). The use of resistivity ﬁsh counters in ﬁsh stock assessment. In: Stock Assessment in Inland Fisheries (Cowx, I.G., Ed.). Fishing News Books, Oxford, pp. 27–43. Aprahamian, M.W., Nicholson, S.A., Best, P.M., Shaw, R.A. & Karr, E.T. (1997). Design and Use of Open Channel Resistivity Fish Counters. Fisheries Technical Manual No. 2. Environment Agency, Bristol, 112 pp. Beach, M.H. & Potter, E.C.E. (1987). MAFF Attitude to Fish Counters. Atlantic Salmon Trust, Counter Workshop, Montrose, Scotland, 15–16 September 1987. Beaumont, W.R.C., Welton, J.S. & Ladle, M. (1991). Comparison of rod catch data with known numbers of Atlantic salmon (Salmo salar) recorded by a resistivity ﬁsh counter in a southern chalk stream. In: Catch Effort Sampling Strategies: Their Application in Freshwater Fisheries Management (Cowx, I.G., Ed.). Fishing News Books, Oxford, pp. 49–60. Crozier, W.W. & Kennedy, G.J.A. (2001). Relationship between freshwater angling catch of Atlantic salmon and stock size in the River Bush, Northern Ireland. Journal of Fish Biology, 58, 240–7. Davidson, I.C., Cove, R.J. & Hazlewood, M.S. (2006). Annual variation in age composition, growth and abundance of adult sea trout returning to the River Dee at Chester, 1991–2003. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 76–87. Evans, D.M. (1996). Log books as a mechanism for assessing long-term trends in the salmonid ﬁsheries, with particular reference to the sea trout stocks of the River Tywi. In: Stock Assessment in Inland Fisheries (Cowx, I.G., Ed.). Fishing News Books, Oxford, pp. 110–25. Evans, D.M., Mee, D. & Clarke, D.R.K. (1995). Mesh selection in a sea trout, Salmo trutta L., commercial Seine net ﬁshery. Fisheries Management and Ecology, 2, 103–11. Evans, R. & Greest, V. (2006). The rod and net sea trout ﬁsheries of England and Wales. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 107–14. Gargan, P., Stafford, J. & Ó Maoiléidigh, N. (2002). The relationship between salmon rod catch, stock size, rod exploitation and rod effort on the Erriff ﬁshery, western Ireland. In: The Interpretation of Rod and Net Catch Data (Shelton, R.G.J., Ed.). Atlantic Salmon Trust, Moulin, Pitlochry, pp. 68–75. Harris, G.S. (2000). Sea trout stock descriptions: the structure and composition of adult sea trout stocks from 16 rivers in England and Wales. Environment Agency, R&D Technical Report W224, Environment Agency, Bristol, 92 pp. Mills, C.P.R., Mahon, G.A.T. & Piggins, D.J. (1986). Inﬂuence of stock levels, ﬁshing effort and environmental factors on anglers’ catches of Atlantic salmon, Salmo salar L., and sea trout, Salmo trutta L. Aquaculture and Fisheries Management, 17, 289–97. Milner, N.J., Davidson, I.C., Evans, R.E., Locke, V. & Wyatt, R.J. (2002). The use of rod catches to estimate salmon runs in England and Wales. In: The Interpretation of Rod and Net Catch Data (Shelton, R.G.J., Ed.). Atlantic Salmon Trust, Moulin, Pitlochry, pp. 46–67. Nicholson, S.A., Aprahamian, M.W., Best, P.M., Shaw, R.A. & Kaar, E.T. (1995). Design and Use of Fish Counters. R&D Note 382, National Rivers Authority, Bristol, 204 pp. Potter, E.C.E., MacLean, J.C., Wyatt, R.J. & Campbell, R.N.B. (2003). Managing the exploitation of migratory salmonids. Fisheries Research, 62, 127–42. Small, I. (1991). Exploring data provided by angling for salmonids in the British Isles. In: Catch Effort Sampling Strategies: Their Application in Freshwater Fisheries Management (Cowx, I.G., Ed.). Fishing News Books, Oxford, pp. 81–91.

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Solomon, D.J. (1995). Sea Trout Stocks in England and Wales. NRA R&D Report 25. National Rivers Authority, Bristol, 102 pp. Solomon, D.J. & Potter, E.C.E. (1992). The Measurement and Evaluation of the Exploitation of Atlantic Salmon. Atlantic Salmon Trust, Moulin, Pitlochry, 38 pp. Solomon, D.J. & Czerwinski, M. (2006). Catch and release, net ﬁshing and sea trout ﬁsheries management. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 434–40. Walker, A.F. (1994a). Sea trout and salmon stocks in the western Highlands. In: Problems with Sea Trout and Salmon in the Western Highlands. Atlantic Salmon Trust, Moulin, Pitlochry, pp. 6–18. Walker, A.F. (1994b). Fecundity in relation to variation in life history of Salmo trutta L. in Scotland. PhD Thesis, University of Aberdeen, 150 pp. Walker, A.F. & Walker, A.M. (1992). The Little Gruinard Atlantic salmon (Salmo salar L.) catch and release tracking study. In: Wildlife Telemetry: Remote Monitoring and Tracking of Animals (Priede, I.G. & Swift, S.M., Eds). Ellis Horwood, New York, pp. 434–40. Walker, A.M., Pawson, M.G. & Potter, E.C.E. (2006). Sea trout ﬁsheries management: should we follow the salmon? In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 466–79. Webb J. (1998). Catch and release: the survival and behaviour of Atlantic Salmon angled and returned to the Aberdeenshire Dee, in early spring and summer. Scottish Fisheries Research Report, No. 62, 16 pp. Whelan, K.F., Whelan, B.J. & Rogan, G. (2002). Catch as a predictor of salmon stock in the Burrishoole ﬁshery, Co. Mayo, western Ireland. In: The Interpretation of Rod and Net Catch Data (Shelton, R.G.J., Ed.). Atlantic Salmon Trust, Moulin, Pitlochry, pp. 76–84.

Chapter 30

Catch and Release, Net Fishing and Sea Trout Fisheries Management D.J. Solomon and M. Czerwinski Foundry Farm, Kiln Lane, Redlynch, Salisbury, Wiltshire SP5 2HT, UK

Abstract: A simple model is developed to consider the impact of various levels of exploitation of sea trout on the numbers of ﬁsh in a stock, the potential egg deposition and the numbers of larger ﬁsh of particular interest to anglers. Because sea trout are serial spawners the impact of exploitation tends to be cumulative on older and larger ﬁsh. Using the stock characteristics from a river in south-west England suggests that an exploitation rate of 30% results in a decrease of 17% in numbers of ﬁsh entering the ﬁshery, 47% reduction in egg deposition and 71% reduction in ﬁsh over 50 cm. It is proposed that control of exploitation can be a powerful ﬁshery management technique for increasing egg deposition and numbers of large ﬁsh, and examples are presented of situations where it appears to have been effective. Keywords: Salmo trutta L., exploitation, repeat spawners, lifetime egg deposition, ﬁshery regulations, conservation.

Introduction As the sea trout may return to the river to spawn several times in successive years if it lives to do so, ﬁshing mortality has the potential to play a major part in reducing the population and in making large ﬁsh scarce, through both growth overﬁshing and recruitment overﬁshing. Equally, reducing ﬁshing mortality by adopting catch and release for rod ﬁsheries and closing net ﬁsheries can remove such effects and allow a recovery. This chapter considers the theoretical background to this issue by the use of simple models of natural and ﬁshing mortality and age–fecundity relationships. It then discusses some situations where there is evidence that management of exploitation has apparently been effective.

Growth overﬁshing and recruitment overﬁshing With most ﬁsheries for Atlantic salmon being late on in their life, with little scope for growth after the period of exploitation, there is little risk of growth overﬁshing (i.e. cropping the stock too early so that overall yield is lower than if the ﬁsh had been left to grow before cropping). There is of course considerable scope for recruitment overﬁshing with salmon – that is, reducing the stock by exploitation to the extent that recruitment is affected. With sea trout being potential multiple spawners the situation is quite different, with scope for cropping of younger returnees to cause both growth and recruitment overﬁshing. In this 434

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435

chapter we explore the scope for both forms of overﬁshing and for control of exploitation to remedy the situation.

Modelling the effects of exploitation In order to model exploitation and its control we need to start with some assumptions regarding natural mortality across years. Based on collections of scales from a wide range of sources, Solomon (1995) noted survival across years of around 32% for virtually all age classes and for ﬁsh ﬁrst spawning at 0+, 1+ and 2+ sea winter (SW) for stocks from the south and west coasts of England and Wales (Fig. 30.1). For rivers on the east coast, exempliﬁed by the Tweed and Yorkshire Esk, survival between years is rather lower, ranging from 8% to 19% (Fig. 30.2). This different level of survival may be associated with marine conditions and the long-range migrations that these stocks make compared with those on the south and west coast; the level of exploitation in the north-east coast ﬁshery at the time the ﬁgures were collected may also be implicated. In any event, these survival ﬁgures reﬂect both natural and ﬁshing mortality and it is likely that natural survival is somewhat higher – perhaps around 40%. This is the ﬁgure we shall use here. We can now model the effects of various levels of exploitation on both survival to old age/large size and on potential egg deposition. But what levels of exploitation are realistic? Reliable ﬁgures for exploitation rates of sea trout are surprisingly elusive, and vary widely.

Number of fish in samples (log scale)

10 000 fish first spawned at .+ fish first spawned at .1+ fish first spawned at .2+ fish first spawned at .3+ 1000

100

10

1 0

1

2

3 4 Post-smolt age (years)

5

6

7

Fig. 30.1 Numbers of sets of scales collected from different age classes of sea trout from stocks on the south and west coasts of England and Wales as an indicator of survival between years. For ﬁsh spawned ﬁrst at .+ the line starts at 1+.

436

Sea Trout

Number of fish in samples (log scale)

10 000 fish first spawned at .+ fish first spawned at .1+ fish first spawned at .2+ fish first spawned at .3+ 1000

100

10

1 0

1

2

3 4 5 Post-smolt age (years)

6

7

8

Fig. 30.2 Numbers of sets of scales collected from different age classes of sea trout from stocks on the north-east coast of England as an indicator of survival between years. For ﬁsh ﬁrst spawned at .+ (0SW) the line starts at 1+.

Solomon (1995) presented some ﬁgures for rod exploitation rate from below 2% (Welsh Dee, where little sea trout ﬁshing is practised) through to around 30% (River Tawe). Figures for net ﬁsheries are even more scarce, with the complication that they are mostly size-selective for the larger ﬁsh. Perhaps the highest levels are likely to be some of the smaller rivers in south-western England, where sea trout appear to spend considerable periods in the estuary. Solomon (1995) noted an average annual catch of 202 ﬁsh with a mean length of around 500 mm on the estuary of the River Fowey; a concurrent trapping programme indicated that only about 164 ﬁsh over 500 mm were passing the tidal limit each year. The smallest ﬁsh in the net catch were around 400 mm, and it would appear realistic to suggest that the exploitation rate by the nets was of the order of 30% of ﬁsh over this size. It thus appears that exploitation rates can be up to around 30% each for both rods and nets. If both nets and rods were operating at this level on a single stock the overall level of exploitation would be 51%. We will therefore model 30% and 51% as examples of high levels of exploitation. Modelling was undertaken using a simple Excel spreadsheet. This allows different levels of natural survival and exploitation to be examined, using age–length data for any population of interest. As an example that part of the Fowey sea trout stock ﬁrst returning at a sea age of 1+ is used here. The output results are shown in Table 30.1. There are a number of interesting features of these results. Exploitation has only limited impact upon the numbers of ﬁsh entering the ﬁshery – a reduction of 17% in numbers by 30%

Net Fishing and Fisheries Management

437

Table 30.1 Output from spreadsheet model of effect of different levels of exploitation on numbers of ﬁsh and egg deposition. Length (mm)

1+ 1+SM+ 1+2SM+ 1+3SM+ 1+4SM+ 1+5SM+ 1+6SM+ Run size Millions of eggs Fish over 50 cm

406 461 499 579 632 630 630

Numbers of returnees with exploitation rate 0%

30%

51%

1000 400 160 64 25.6 10.2 4.1 1663.9 2.78 103.9

1000 280 78.4 22.0 6.1 1.7 0.5 1388.7 1.47 30.3

1000 196 38.4 7.5 1.5 0.3 0.1 1243.8 0.87 9.4

See text for details.

exploitation, and of 25% in numbers by 51% exploitation – because the run is dominated by ﬁrst-time returnees which have thus far been unaffected by the ﬁshery. The impact is much greater on the number and size of spawners; however, producing a marked reduction in egg deposition – a decrease of 47.1% at 30% exploitation and 68.7% at 51% exploitation. The greatest impact is on the older and larger ﬁsh, of particular interest to anglers. The number of ﬁsh over 50 cm entering the ﬁshery is reduced by 70.8% at 30% exploitation and by 91% at 51% exploitation. Even a modest exploitation rate of 10% gives a reduction in egg deposition of 18.7% and a reduction in numbers of ﬁsh over 50 cm of 31.1%.

Evidence of effective management It is remarkably difﬁcult to identify clear evidence of control of exploitation having led to an increase in the numbers of large ﬁsh and potential egg deposition, for a number of reasons. First, there are only a limited number of rivers where there has been a sudden reduction in a high level of exploitation. Second, the year by year variation in stock size, and the lack of monitoring facilities that can establish parameters of the returning stock, means that monitoring their effect is difﬁcult. In most cases we are dependent upon rod catches as the main monitoring tool. Rod catch is a far from perfect indicator of stock size, for a variety of reasons, and in many cases the size of individual ﬁsh is not recorded or at least not published. Finally, there may well be reasons other than exploitation and its control for changes in the fortune and age structure of stocks. For these reasons most of the available evidence of the beneﬁt of control of exploitation is inevitably circumstantial, but it is nonetheless worthy of examination. First, we consider the situation in the Rio Grande River in Tierra del Fuego, which rises in the Chilean part of the island but ﬂows to the sea through the Argentinean part, to the South Atlantic. The river is of medium size by UK standards, being about 120 km in length, and

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was stocked with what was believed to be a non-migratory strain of brown trout during the 1930s. These ﬁsh came from Puerto Montt in Chile, having originally been introduced from Europe (Leitch, 1991). The ﬁsh clearly thrived and for many years provided good ﬁshing for the estate owners and their guests. At some stage part of the stock successfully established a migratory life cycle, and large sea-run ﬁsh started to appear in the river. This attracted the attention of local net and rod ﬁshermen and a signiﬁcant and largely uncontrolled ﬁshery developed in the lowermost reaches of the river. There was a bag limit of ﬁve sea trout to be retained each day, but it was not strictly enforced. Organised angling tourism started on the river in 1986 with the opening of the Kau Tapen Lodge which allows ﬁshing about 25 km of river. From the start the management was enlightened, with ﬂy ﬁshing only and catch and release being the rule. As other estates followed suit in making ﬁshing available to visitors they too adopted the same philosophy; there are now seven ﬁshing operations on the Argentinean stretch of the river, with two more recently opened on the upstream Chilean stretch. Great strides have made to limit and control the ‘all-methods’ ﬁshery in the lower river, with the bag limit being reduced to three ﬁsh in the late 1980s, and subsequently to only one ﬁsh; this is now a possession limit, so it applies to the whole trip even if it covers more than 1 day. These regulations are now strictly applied, and all the lodge ﬁshing remains strictly catch and release. Coincident with this management regime has been a steady increase in numbers of ﬁsh caught, mean weight and numbers of very large ﬁsh. Some catch statistics from two Argentinean operations, Kau Tapen and Maria Behety, illustrate these trends (Table 30.2). In the ﬁrst 4 years at Kau Tapen in the 1980s the mean weight of the 1460 ﬁsh caught by ﬂy-ﬁshermen was 7.9 lb (3.6 kg), and the largest ﬁsh 20 lb (9.1 kg). The mean weight of the 1504 ﬁsh caught by ﬁshermen at Maria Behety (ﬁshing many of the same pools as Kau Tapen from the opposite bank) in 2002 was 11.4 lb (5.7 kg), with the largest ﬁsh just under 29 lb (13.2 kg) and 87 other ﬁsh over 20 lb (9.1 kg). Table 30.2

Details of catches on two estates on the Rio Grande.

Year

Kau Tapen No. of ﬁsh

1986 1987 1988 1989 1990 1991–1998 1999 2000 2001 2002 2003 2004

207 223 416 614 644

7492 4427 5910 2195 3745

Maria Behety

Mean wt. (pounds)

Largest (pounds)

No. of ﬁsh

11 8 7 7.6 9.7

20 20 19 20 25 No data available 985 28 1109 33 901 31 1504 26 558 29

Mean wt. (pounds)

Largest (pounds)

8.8 9.2 10.2 11.4 12.8

22.9 26.4 28.4 28.9 28.9

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Table 30.3 Scale-reading results from 18 sea trout caught by angling on the Rio Grande in January 2001. No.

L (cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

58 61 64 66 67 67 68 70 72 73 73 75 76 79 80 80 80 81

Wt. (lbs) 5.5 9 8 9 10 10 10.5 11 9 10 7.5 13 14 15 14 15 18 17.5

Pre-smolt age

Post-smolt age

2+ 2+ 2+ 2+ 2+ 2+ 3+ 2+ 2+ 2+ 2+ ? 2+ 2+ ? 3+ 2+ 2+

1+ 1+ 1+2SM+ 1+ 2+2SM+ 1+2SM+ 1+2SM+ 1+SM+ 1+SM+ 1+SM+ 1+4SM+ 1+2SM+ 1+SM+ 1+4SM+ 1+4SM+ 1+3SM+ 1+3SM+ 1+4SM+

This truly astounding quality of ﬁshing is founded on a stock of ﬁsh with a phenomenal growth rate in salt water (faster than anything recorded in Europe). However, careful management of exploitation also plays an important role, especially now that rod catches on the whole river have increased to the order of 10 000 ﬁsh per year. The ﬁrst-return maiden ﬁsh are for the most part just over a year at sea and are typically 55–65 cm in length and 4–9 lb (1.8–4.1 kg) in weight. Most of the rod catch comprises ﬁsh that have previously returned to the river and spawned one to ﬁve or more times. The life history of a sample of ﬁsh caught in January 2001 is indicated in Table 30.3. Of the 18 ﬁsh, only three were ‘ﬁrsttimers’, and one-third were returning for the fourth or ﬁfth time. This suggests a remarkably high survival rate, though the level of replacement scales among the samples is consistent with many of these ﬁsh having been caught, handled and released before. The mean weight of these ﬁsh at 11.4 lb (5.2 kg) suggests that they are reasonably representative of the catches in recent years. The development of lodge ﬂy ﬁshing on the Rio Gallegos, on the Argentinean mainland about 50 km north of the Rio Grande, is more recent but tells a similar story. The Bella Vista lodge was the ﬁrst to open in 1995, returning a catch of 50 ﬁsh with an average weight of 6 lb (2.7 kg); the largest that year was 12 lb (5.5 kg). In 2004 the catch was over 1000 ﬁsh with a mean weight of more than 10 lb (4.5 kg), and the largest ﬁsh 24 lb (10.9 kg). In 2001, a remarkable 30% of visiting anglers caught a ﬁsh of 20 lb (9 kg) or more during their week. Again, this appears to have been achieved by rigorous control of ‘all-methods’ ﬁshing, enforcement of the regulations and total catch and release by the lodge operation. As more lodge operations are introduced onto the river it is likely that they too will embrace

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the catch and release approach and ensure a healthy future for this river and its remarkable ﬁshery. There are also examples where control of exploitation has been associated with increased catches of large ﬁsh in the UK. Between 1990 and 1995 the net ﬁshery on the joint estuary of the rivers Taw and Torridge was closed as a conservation measure. In the 6 years before the closure the nets had reported a mean annual catch of just over 3000 sea trout with an average weight of 2.8 lb (1.27 kg). Although there was no signiﬁcant increase in the numerical reported rod catch (probably because of low reporting rate, normal ﬂuctuations in stocks and catches and the fact that the rod catch was dominated numerically by smaller ﬁsh that the net ﬁshery did not affect) there was an increase in numbers of large ﬁsh caught. In 1994 for example there were a number of 10 lb (4.5 kg) plus ﬁsh reported, including Taw ﬁsh of 13 lb 4 oz (6.0 kg) (the best on the river for many years), 12 lb 8 oz (5.7 kg) (a record for the hotel ﬁshery concerned) and a 10 lb 8 oz (4.8 kg) ﬁsh on the Torridge (the best on the river for many years).

Conclusion From a theoretical analysis it appears that while exploitation of sea trout stocks may only have a modest impact upon the overall numbers of ﬁsh entering the ﬁshery in the short term it can have a very much greater effect upon the egg deposition (and thus the numbers in future generations) and on the numbers of large sea trout of particular interest to anglers. Control of exploitation, including closing of net ﬁsheries and catch and release by anglers, is likely to play an important part in the management of sea trout sport ﬁsheries in the future.

Acknowledgements We are very grateful to Nick Zoll of Nervous Waters, who manage the Kau Tapen Lodge on the Rio Grande and Bella Vista on the Rio Gallegos, for the information on their operations.

References Leitch, W.C. (1991). Argentine Trout Fishing. Frank Amato Publications, Portland, OR, 192 pp. Solomon, D.J. (1995). Sea trout stocks in England and Wales. R & D Report 25, National Rivers Authority, Bristol, 102 pp.

Chapter 31

A Review of the Statutory Regulations to Conserve Sea Trout Stocks in England and Wales G. Harris Fishskill Consultancy and Resource Management Services, Greenacre, Cathedine Bwlch, Powys LD3 7PZ, Wales, UK

Abstract: The statutory regulations to control the exploitation of sea trout by rod and net ﬁsheries in England and Wales are reviewed in the context of a comprehensive strategy for the sustainable management of the resource. The adequacy of the regulations currently in force is considered and the requirement for new measures to protect and conserve both the quantitative and qualitative features of individual stocks is discussed. Conservation measures based on the introduction of a minimum size limit to protect immature 0SW whitling, a maximum size limit to protect both large multi-sea winter (MSW) and repeat spawning ﬁsh and catch limits to reduce the rate of total exploitation are outlined for the recreational ﬁshery. Keywords: Salmo trutta L., sea trout, conservation, ﬁshery regulations, size limits, catch limits.

Introduction The history of salmonid ﬁshery management in England and Wales has been largely dominated by the perceived needs of the more prestigious Atlantic salmon (Salmo salar L.) and the non-migratory ‘brown’ trout (Salmo trutta L.). Relatively little attention has been given to the special needs of the migratory ‘sea’ trout in its own right. Consequently, any measures to conserve sea trout stocks have been largely incidental to those introduced to protect salmon and brown trout. The general decline in the abundance and composition of salmon stocks throughout the British Isles since the early 1970s has coincided with an increase in the popularity of the sea trout as a distinct and separate component of the sportﬁshery. This, when coupled with concern about the recent collapse of sea trout stocks in parts of Ireland (Gargan et al., 2003) and Scotland (Anon., 2005a) and an increased awareness of the social and economic importance of the sea trout as the mainstay of the commercial (net) and recreational (rod) ﬁsheries on most rivers in England and Wales, has highlighted the need for a proactive approach to the sustainable management of the sea trout. It is perhaps indicative of the traditional lack of recognition of the status and importance of the sea trout that separate management strategies for both the Atlantic Salmon (National Rivers Authority, 1996) and for Trout and Grayling (Environment Agency, 2003) have been 441

442

Sea Trout

produced for England and Wales but nothing similar has been produced for sea trout. While both of these strategies contain elements that are either directly or indirectly relevant to sea trout, there are several notable omissions that highlight the general requirement for a separate and comprehensive overall strategy that addresses the special needs of the sea trout in terms of its variable life history, complex migratory behaviour and socio-economic importance. The broad thrust of the historical approach to the management of game ﬁsheries in England and Wales has been directed principally towards maintaining and increasing the quantity of ﬁsh available for capture. Very little thought was given to maintaining and improving the quality of the ﬁsheries. However, the recent adoption of the global concepts of ‘sustainability’ and ‘biodiversity’ by the UK Government has facilitated the introduction of a more comprehensive approach that combines both the qualitative and quantitative aspects of ﬁsheries management. This now places greater emphasis on the conservation of the natural diversity (including genetic diversity) and acknowledges that this may make an important contribution to the social and economic value of a ﬁshery. Harris (2000) proposed that one of the objectives of any management strategy for the conservation and qualitative improvement of sea trout stocks should be to strengthen the numbers of unspawned (maiden) and previously spawned ﬁsh represented in each sea-age group. This chapter develops that theme. It considers the statutory protection afforded to sea trout at critical stages and periods during their life history by the regulations currently in force throughout England and Wales and then discusses those adult stages where additional measures may be required to develop and then maintain a robust stock structure.

General background There are approximately 100 rivers in England and Wales that contain self-sustaining stocks of wild sea trout (Solomon, 1995). About 80 support signiﬁcant recreational (rod) ﬁsheries for both salmon and sea trout and commercial (net) ﬁsheries for both species continue to operate in the estuaries or near-coastal zones of some 40 of these rivers. Table 31.1 shows Table 31.1 Declared catches of salmon and sea trout by rod and net ﬁsheries in England and Wales in 2003. Sector

Rods Nets

No. of licences issued

Fishing effort (days)a

29 936 416

173 246 n/ab

Catch of sea trout

Catch of salmon

No. caught

Weight (tonnes)

No. released

No. caught

Weight (tonnes)

No. released

45 101 29 234

38.3 52.1

24 403 Nil

11 519 17 219

48.5 69.2

6425 13c

a Fishing licences cover both salmon and sea trout and effort cannot be broken down between species. b Information available in different formats (number of tides or days ﬁshed) that cannot be combined here. c Applies to only those salmon caught by netsmen before 1st June under the statutory provisions to conserve early running MSW ﬁsh. It does not apply to sea trout.

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Table 31.2 Declared catches of sea trout and salmon by rod and net ﬁsheries in England and Wales 1999–2003. Sector

Declared catch

1999

2000

2001

2002

2003

Rod ﬁshing

Sea trout catch Salmon catch Sea trout catch Salmon catch

46 786 12 508 40 068 34 146

41 322 17 596 46 022 50 998

40 374 14 383 44 926 43 243

49 796 15 282 37 018 38 279

45 101 11 519 29 234 17 219

Net ﬁshinga

a Care must be taken in interpreting the historical catch record from the commercial net ﬁshery because of

the steady and signiﬁcant decrease in the number of ﬁshing gears and the total ﬁshing effort over recent years.

the declared catch of sea trout and salmon by the rod and net ﬁsheries for the 2003 ﬁshing season and Table 31.2 compares the total declared catches over the past 5 years. The primary ﬁshery legislation for England and Wales is the Salmon and Freshwater Fisheries Act 1975. Among other things, this empowers the regulatory authority to introduce local ‘by-laws’ and ‘orders’ for any purpose speciﬁed within the Act under its secondary law-making provisions. The Environment Agency is now the regulatory authority appointed by the Government with the responsibility of implementing the 1975 Act. It has a statutory duty ‘to maintain, improve and develop salmon ﬁsheries, trout ﬁsheries, freshwater ﬁsheries and eel ﬁsheries’. Its powers with respect to migratory salmonids and eels extend out to the six-mile coastal limit, but they do not include the regulation of ﬁsheries for other species of marine or diadromous ﬁsh other than within the headland of certain deﬁned estuaries. It is important to note that the ﬁsheries legislation in England and Wales relating to salmon and trout has remained substantially unchanged for almost 150 years. The 1975 Act was merely a consolidation of earlier legislation going back to the Salmon and Freshwater Fisheries Act 1923 and this in turn was largely a consolidation of various statutes going back to the seminal Salmon Fisheries Acts of 1861 and 1865. Following the recent ‘Review of Salmon and Freshwater Fisheries’ in England and Wales (Anon., 2000), the Government has now accepted that the powers provided by the 1975 Act are outdated and inadequate for modern purposes and has undertaken to introduce new primary legislation to address its obvious deﬁciencies.

Existing regulations The principal statutory regulations currently in force to control the exploitation of sea trout and to protect different stock components can be categorised under four main headings.

Control of ﬁshing effort Every person who ﬁshes for salmon and sea trout must ﬁrst purchase a licence covering both species for the appropriate type of authorised ﬁshing gear.

444

Sea Trout

The historical public right of ﬁshing in tidal waters has been progressively derogated over the past century by the introduction of a series of local by-laws and net limitation orders that deﬁne the various types of commercial ﬁshing gear that may be used in a speciﬁed geographical area and ﬁx an upper limit on the number of such licences that are available for issue in any year. The number of licences issued for all types of commercial ﬁshing gear (nets and ﬁxed engines) has declined by about 60% over the past 20 years from 897 issued in 1985 to 354 issued in 2004 (Anon., 2005b). This reduction in the overall commercial ﬁshing effort has had four principal causes: (1) natural market forces linked to the low catch/proﬁtability of netting operations in some districts; (2) a reduction in the number of licences available within the provisions of a series of new net limitation orders associated with phasing out and the ultimate closure of known mixed-stock ﬁsheries for salmon; (3) introduction of new and more restrictive by-laws since 1999 to protect early running MSW salmon and (4) other measures to protect declining salmon stocks based on a combination of public and private initiatives that ﬁnancially compensate those licence holders who choose to relinquish their right of ﬁshing or to return the ﬁsh that they catch. In general terms, the remaining commercial ﬁsheries are now very highly regulated and controlled by a comprehensive range of local by-law restrictions covering (1) the type of ﬁshing gear (nets or ﬁxed engines) that may be used; (2) their precise area of operation; (3) the duration and timing of the ﬁshing season; (4) the weekly ‘close period’ when all forms of netting must cease; (5) their method of operation and (6) the dimensions, material and mesh size(s) used in the construction of nets. In contrast, there is no power in law for the regulatory authority to limit the number of licences available for rod-and-line ﬁshing. Every person who applies for a rod licence, and pays the appropriate duty, must be granted a licence. While the number of rod licences issued each year has increased several fold over the past 50 years, the total number issued since the introduction of a single ‘national’ rod licence structure for England and Wales in 1992 has remained relatively stable at around 30 000 each year. It is relevant to note (Tables 31.1 and 31.2) that the progressive decline in commercial ﬁshing effort has resulted in the total catch of sea trout taken by the rod ﬁshery exceeding that taken by the net ﬁshery in the past 2 years. Annual close seasons to protect spawning ﬁsh The fundamental importance of providing the fullest protection to salmon and sea trout as both gravid ﬁsh and as kelts during their spawning period has long been recognised within the provisions of the statutory legislation since the latter part of the nineteenth century. The main spawning period for sea trout in England and Wales is from early November until mid-December. This may vary slightly from year to year depending on water temperatures and river ﬂow and it may extend into January or even early February on those rivers in north-east England where a signiﬁcant proportion of the annual run enters the river in the late autumn and early winter period. Salmon and brown trout also spawn at about the same

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445

time as sea trout, but salmon usually start and ﬁnish spawning about 2 weeks later. Sea trout generally return to sea (as kelts) immediately after spawning and, unlike salmon, they tend not to remain in fresh water for more than a few weeks after spawning under normal conditions. This behaviour of sea trout kelts may partly explain their far higher incidence of survival and repeat spawning compared with salmon. A statutory close-season when no ﬁshing of any type may occur is in force all rivers to protect spawning ﬁsh and kelts. However, the duration of that period and the dates when it starts and ﬁnishes may be different for salmon and sea trout and across different rivers and regions. The start of the close-season for sea trout ranges from the end of September to the end of October and its ﬁnishing date ranges from the end of January to early May. However, within this local variability, the effective close-season on all rivers includes the months of November, December and January, and it therefore covers the critical periods for sea trout spawning and the downstream movement of kelts. The primary legislation also makes it an offence to take ‘unclean’ ﬁsh (i.e. ﬁsh that are ‘about to spawn’ or ‘kelts’ which have recently spawned). Minimum size limits to protect juveniles and smolts The downstream migration of smolts into the estuary is a crucial period in the life history of the sea trout. It represents the culmination of the period of juvenile production in fresh water and is the starting point that determines the initial strength of the subsequent adult runs likely to be derived from a single smolt year class. The typical age range of smolts in England and Wales is from 1 to 4 years, with the greatest proportion usually migrating at either 2 or 3 years of age (Solomon, 1995; Harris, 2000). The main period of smolt migration is typically between March and June (depending on local conditions of ﬂow and temperature), but there is increasing evidence (Elliott et al., 1992) that a signiﬁcant proportion of juveniles may migrate to sea as parr, silver parr or full smolts in autumn in some rivers. It is not yet known whether this autumn migration is an important and widespread occurrence. There are few direct observations on smolt length at the actual time of migration and these have been largely opportunistic and incidental to work on salmon smolts (Elliott et al., 1992; Solomon, 1995). Much more information has been derived indirectly by the back-calculation of length-for-age of maiden ﬁsh from scale-reading studies. While this may not present an accurate reﬂection of the situation at the actual time of migration because of the differential mortality among ﬁsh of different sizes, ages and periods of migration, it can be used in this study. Solomon (1995) reviewed the available data on the length of smolts in different age groups for 18 rivers in England and Wales. Mean lengths varied widely from 75 to 285 mm for the four age groups encountered. However, within the two dominant age groups the range was 140–183 mm for 2-year smolts and 177–216 mm for 3-year smolts. Local by-laws to protect juvenile parr and smolts by ﬁxing a minimum size limit below which all ﬁsh must be returned alive to the water immediately after capture have been in force for many years. Until recently, few administrative regions made any distinction

446

Sea Trout Table 31.3 Minimum statutory size limits for the retention of non-migratory (brown) trout and migratory (sea) trout currently in force (2004) for different administrative regions of the Environment Agency in England and Wales. Administrative regiona

North-west Welsh South-west

Sub-district

Gwynedd All other parts Cornwall Devon

Wessex Southern Yorkshire Northumbria

Minimum size limit Non-migratory (mm)

Migratory (mm)

200 210 230 180 200 250 230 230 180–200

300 210 230 180 250 350 380 230 250

a Excludes Thames and Midland regions where sea trout are uncommon.

between sea trout and brown trout when ﬁxing a minimum size limit and the regulations were traditionally based on what was deemed appropriate for brown trout. The current by-laws now make a legal distinction between the non-migratory and migratory forms of trout in most regions. The minimum size limits for non-migratory and migratory trout in force for the principal sea trout producing regions of England and Wales are shown in Table 31.3. These range from 180 to 250 mm for brown trout and from 180 to 380 mm for sea trout; with a common size limit (range 180–230 mm) applying to both sea trout and brown trout in only four of the nine areas. Notwithstanding certain limitations in the quality and scope of the available information, it would appear that, with one notable exception, the statutory minimum size limits now in force to protect sea trout smolts are generally adequate for that purpose. That exception is in Cornwall (south-west England). This geographical region contains several important sea trout rivers and is unusual in that the older (and therefore larger) 3- and 4-year-old smolt age groups are dominant. The minimum size limit of 180 mm in this district is less than the mean smolt sizes of 183 and 207 mm recorded respectively for these two dominant smolt age groups. Catch limits to reduce exploitation The introduction of statutory catch (or bag) limits is a recent innovation to reduce exploitation by the rod ﬁsheries on a few rivers in England and Wales. Catch limits do not apply to any net ﬁsheries at this time. The only administrative district in Wales with a statutory catch limit for sea trout is in the south-west area. This covers just six of the 35 principal ﬁsheries within the Welsh region and stipulates that anglers may not retain more than four adult sea trout (above the statutory

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447

minimum size limit of 23 cm) in any period of less than 24 h. There is, however, no further restriction on the number of ﬁsh that may be retained in any week or in any single season. For the 45 principal ﬁsheries in England, catch limits have only been introduced on two rivers in Devon. These specify that the maximum numbers of adult sea trout that may be retained by each angler in any day, week or season are 5/15/40 ﬁsh, respectively, on the Taw and 2/7/20, respectively, on the Torridge. The signiﬁcant decline in salmon stocks throughout England and Wales has resulted in the recent introduction of new regulations to protect salmon stocks in general and to conserve the multi-sea winter (MSW) component of all salmon stocks in particular. In addition to imposing a delayed start to the net ﬁshing season for salmon until after 1st June and the return of all rod-caught salmon caught before 16th June, these measures also include the promotion of catch-and-release for all salmon caught by anglers over the remainder of the season as a voluntary ‘code of conduct’. Although directly intended to increase the number of adult salmon that survive to spawn, the introduction of a delayed start to the commercial ﬁshing season will have provided some incidental protection for sea trout on those rivers where signiﬁcant runs of sea trout occur before June and where netsmen have not been given a special dispensation to continue ﬁshing for sea trout only (e.g. on the Tywi, Taf and Teiﬁ in south-west Wales). It is encouraging to note (Anon., 2004) that the adoption of catch-and-release for salmon has also been widely adopted as a conservation ethic by many sea trout anglers. This has resulted in the release of approximately 56% of all rod-caught salmon (6425 ﬁsh) and 54% of all rod-caught sea trout (24 403 ﬁsh) during the 2003 ﬁshing season (Anon., 2004). However, although Evans & Greest (2006) noted that the release rates of larger sea trout have increased in recent years, it is important to note that most of the sea trout returned by anglers over each season are likely to be the smallest and therefore less reproductively valuable of the ﬁsh caught.

Additional provisions While the existing regulations are adequate to protect spawning ﬁsh and downstream migrant kelts and seem broadly adequate to protect juvenile parr and smolts in virtually all rivers, they make no provision whatsoever to protect any qualitative features of adult stocks. This is a signiﬁcant omission that should be addressed: with particular attention given to the conservation of the following stock components. Whitling After passing through the estuary as smolts, the period of time that post-smolt sea trout spend feeding and growing in the sea before they return to the river to spawn for the ﬁrst time as ‘maiden’ ﬁsh may range from as little as a few weeks to as much as 3 years. Those ﬁsh that return to fresh water before their ﬁrst sea winter (as 0SW ﬁsh) are termed ‘whitling’ throughout much of England and Wales, but they are also known locally as ﬁnnock, herling or school-peal in other regions.

448

Sea Trout

The whitling stage of the sea trout represents a curious and seemingly contradictory phenomenon that is as yet little understood in terms of its evolutionary beneﬁt as a reproductive strategy. Its occurrence is even more confusing because there is some doubt about the general validity of the popular assumptions that all the immature whitling that return to a river are destined to spawn in the same year of entry into fresh water and that all the whitling that enter a river originate from within that same river system and are destined to spawn there (Nall, 1932; Elliott et al., 1992; Solomon, 1995). The annual importance of the whitling component to the spawning success of any single stock will depend on three considerations: (1) the number of immature maiden whitling present; (2) the number of repeat spawning whitling from earlier years that are also present in the same year and (3) the strength of the entire whitling component (maidens + previous spawners) relative to that of all older maiden sea-age groups and their categories of previous spawners. The proportion of immature (maiden) whitling in the 16 stocks studied by Harris (2000) ranged from 0.9% to 81%. Although very few whitling (<2%) were recorded for two rivers in north-east England, the proportions of maiden ﬁsh elsewhere were: >10% for 14 rivers, >20% for 12 rivers, >40% for 8 rivers and >60% for 4 rivers. For these 14 rivers, the proportions of ﬁsh within the whitling component that had spawned from 1 to 5 previous occasions were: >10% for 10 rivers, >20% for 5 rivers, >30% for 3 rivers and >40% for 1 river. For all 16 rivers, the entire whitling component (as a proportion of all seaage groups and categories of previously spawned ﬁsh encountered) represented: <5% for 2 rivers, >20% for 14 rivers, >40% for 11 rivers, >60% for 6 rivers and >80% for 5 rivers. Information on the mean length of whitling also showed a wide range in the sizes of the ﬁsh returning to different rivers of between 271 and 355 mm for the 31 rivers reviewed by Solomon (1995) and typically from 309 to 356 mm for those studied by Harris (2000). There are no minimum size limits currently in force to protect the whitling component of sea trout stocks other than those ﬁxed to protect juvenile parr and smolts (Table 31.3). At best, these provide token protection for only the very smallest ﬁsh within the annual whitling run. This is not seen as a problem in relation to the net ﬁshery as the statutory regulations on the size of mesh that may be used in the construction of nets are such that maiden whitling sea trout and the smaller sizes of repeat spawning whitling on their second return to fresh water (as 1SM ﬁsh) are not generally vulnerable to capture because they are theoretically able to pass through the mesh (Evans et al., 1995). However, the lack of any minimum size limit is very relevant to the extent of exploitation of whitling in the rod ﬁshery because they can be caught in very large numbers. It is apparent that the whitling component makes a signiﬁcant contribution to the numerical abundance and structure of most sea trout stocks in England and Wales and it may be of crucial importance to spawning success in terms of its contribution to total egg deposition in many rivers: although this has yet to be modelled. In addition, whitling may also be very important in maximising the use of the available wetted area within a catchment for spawning and juvenile trout production as they are better able to penetrate the smaller tributaries and they spawn in gravel of a smaller size than that utilised by larger ﬁsh.

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Multi-sea winter ﬁsh Those sea trout that return to fresh water to spawn for the ﬁrst time as maiden ﬁsh after spending at least two winters feeding and growing in the sea (i.e. as MSW ﬁsh) are normally larger than ﬁsh in the younger maiden sea-age groups. Most sea trout return to fresh water to spawn for the ﬁrst time either as 0SW or 1SW maiden ﬁsh, but the occurrence of variable proportions of older, MSW maiden ﬁsh has been observed. Solomon (1995) recorded that while the incidence of 3SW ﬁsh was rare and generally restricted to only the occasional ﬁsh in eight rivers in England and Wales, 2SW ﬁsh were more common and occurred in 23 rivers. The greatest proportion was in the Tweed (47%), but was less than 16% for all the remaining rivers. Harris (2000) reviewed the historical data on differences in the temporal stock composition of MSW ﬁsh for seven rivers where scale-reading studies had been undertaken over different periods of time. Only the Dyﬁ and Tywi had been studied previously on three separate occasions. In the Dyﬁ the proportion of 2SW sea trout had declined progressively from 11.1% in 1933 to 3.3% in 1970 and then to 1.2% in 2000. In the Tywi the decline was from 7.2% in 1970 to 5.6% in 1994 and then to 1.8% in 2000. While these results may not be strictly comparable because of selective sampling bias (Harris, 1995), they nevertheless indicate a signiﬁcant general decline in the incidence of MSW sea trout in these rivers. Robust regulations have recently been applied throughout England and Wales to the conservation of the MSW component of salmon stocks to maintain genetic diversity and to reinstate the early runs of salmon derived from this stock component that are important in increasing the duration of the salmon ﬁshing season. However, no similar measures are currently in force to conserve the MSW component of sea trout stocks; even though their importance in terms of genetic diversity and their greater contribution to spawning success as a consequence of their larger size and fecundity is precisely the same as that of MSW salmon. Previous spawners Unlike Atlantic salmon, which rarely survive to spawn more than once in the British Isles (Mills, 1989), adult sea trout exhibit the potential ability to live longer and to make multiple spawning visits to fresh water – if allowed to do so. The maximum number of spawning marks reliably recorded on the scales of any sea trout in the British Isles to date is x11 (Nall, 1930). Harris (2000) compared the relative proportions of maiden and previously spawned sea trout for 16 rivers and showed that ﬁsh which had spawned at least once represented a variable but often very signiﬁcant proportion of the annual stock. Values for all previous spawners as a proportion of all ﬁsh sampled (maidens and previous spawners) ranged from 6.5% to 51.5% and exceeded 20% for nine rivers. A further breakdown of the frequency of spawning within this stock component showed that the maximum number of spawning marks detected on the scales of any ﬁsh was: x2 in 16 rivers, x3 in 15 rivers, x4 in 14 rivers, x5 in 8 rivers and x6 in 3 rivers. The proportions of ﬁsh that were returning to spawn for at least the third time when captured ranged from 10.0% to 26.1% for all 16 rivers and for those returning to spawn for the fourth time it ranged from 2.5% to 12.7% in 15 rivers.

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There are two principal reasons for providing some appropriate measure of protection for the repeat spawning component of any stock of sea trout: 1. On returning to fresh water to spawn again after a period of further feeding and growth in the sea, each female ﬁsh will be larger and more fecund than on its previous spawning visit. Consequently, the cumulative contribution to spawning success derived from repeat spawning ﬁsh will be disproportionately greater than their numerical abundance within the total spawning population in any year. The largest sea trout recorded from the Dyﬁ in recent years weighed 10.9 kg (24 lb). It was a female ﬁsh that had spawned consecutively on x7 previous occasions (scale formula = 2.2+7SM+). By extrapolation from known data on possible size and fecundity for each year of return for Dyﬁ sea trout (Harris, 1970), it can be calculated that the cumulative contribution to spawning success over the lifetime of this one ﬁsh was roughly 100 000 eggs. This represents the egg deposition equivalent of roughly 130 whitling. 2. Multiple spawning sea trout, by deﬁnition, have proven their ability to survive in both the freshwater and marine environments and can be viewed as the ‘ﬁttest’ members within any stock. As such, they may have important genes as the expression of repeat spawning may be a heritable trait that represents an important part of the genetic diversity contained within each stock (Saunders & Bailey, 1980). There are currently no regulations to protect and conserve previously spawned sea trout after their migration to sea as kelts and during the ﬁshing season on their subsequent return to fresh water. Large ‘specimen’ sea trout Most rivers in England and Wales have produced sea trout that are ‘unusual’ or ‘exceptional’ for a particular river system because of their very large size at some time in the past. ‘Large’ is a relative term that may vary widely in different regions of the British Isles. Any sea trout in excess of 2.7 kg (6 lb) weight would be considered ‘large’ by Irish standards, whereas ﬁsh in excess of 4.6 kg (10 lb) or even 6.8 kg (15 lb) are not unusual in several rivers throughout much of Wales and in some English regions. Indeed a few rivers, such as the Dyﬁ and Tywi in Wales, regularly produce very large sea trout. Apart from any other consideration, these specimen sea trout have a very important ‘cachet’ value within the angling community that adds greatly to the attraction and economic value of a ﬁshery. A sea trout may become large by some combination of its longevity and the quality of its feeding in the sea (Harris, 2000). In north-east England sea trout are relatively short-lived and very few survive to spawn more than once or twice. However, they can attain a very large size because of rich feeding and very rapid growth in the sea. In contrast, sea trout in Wales and north-west England appear to experience poorer feeding conditions in the sea but can attain a very large size because they live longer and survive to make more spawning visits to fresh water. All maiden sea-age groups appear to have the potential ability to spawn repeatedly and eventually attain a larger size. Harris (1972) examined scales from 105 ‘specimen’ sea trout in excess of 10 lb from various regions of the British Isles. The proportions that had

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ﬁrst-returned to fresh water as maiden ﬁsh after each year of sea-absence were: 6.6% as 0SW whitling, 60.0% after 1SW, 28.6% after. 2SW and 3.8% after. 3SW. Of these, the largest ﬁsh weighed 11.2 kg (24.5 lb), the oldest ﬁsh was 11+ years of age and the maximum number of spawning marks was x8. There are currently no statutory regulations to protect very large ‘specimen’ sea trout.

Discussion The development of a long-term strategy for the conservation and sustainable management of our sea trout ﬁsheries will need to address the many unknowns and uncertainties that currently limit our ability to manage our sea trout stocks effectively and efﬁciently (Harris & Milner, 2006; Milner et al., 2006). This should be viewed as a stepwise process in its evolution and some of the more obvious requirements for better management information relating to the ways in which the ﬁsheries may need to be regulated can be brieﬂy listed as: (1) (2)

(3)

(4)

(5)

(6)

The determination of robust ‘biological reference points’ (BRPs) that deﬁne the status and well-being of individual trout stocks (Walker et al., 2006). The patterns of migration and feeding behaviour of adult ﬁsh in the sea (as both maiden ﬁsh and kelts) and the extent to which the apparent near-coastal feeding behaviour of most sea trout results in the occurrence of mixed-stock ﬁsheries. Reviews of the few studies undertaken in the British Isles (Elliott et al., 1992; Solomon, 1995) suggest a seemingly contradictory array of different behaviours that warrant further investigation. The nature and extent to which sea trout are subject to illegal or inadvertent capture in other ﬁsheries for marine ﬁsh species. We have no information other than a passing reference to the illegal landing of sea trout at seven locations by Pawson & Benson (1983). The extent of non-catch ﬁshing mortality on sea trout stocks from damage caused by the mesh size and materials used by commercial ﬁshing gears in estuarial and coastal waters. The only published information for sea trout in England and Wales (Evans et al., 1995; Solomon, 1995) relates to the Seine nets licensed for salmon and sea trout ﬁshing. Solomon (1995) suggests that large numbers of the smaller sea trout that can pass through the mesh may be damaged and we need much more information for other types of net, mesh sizes and materials used in ﬁshing for other species of ﬁsh in coastal and estuarine waters. The accuracy, reliability and interpretation of catch records from the rod and net ﬁsheries as true measures of the total catch and rate of exploitation in quantitative and, of equal importance, in qualitative terms. This core topic has recently been reviewed for salmon (Shelton, 2002), and there is a pressing need for an authoritative appraisal of the entire subject with respect to sea trout. The rates of exploitation by rod and net ﬁsheries on adult stocks and the selective nature of such exploitation on different stock components. Solomon (1995) and Shields et al. (2006) provide the only information available on exploitation rates in England and

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Sea Trout Wales. This relates to just eight rivers for the rods and only one river for the nets. It shows such a wide variability (range 0.5–30%) for the rods between different rivers that further studies, covering different ﬁsheries and a longer time series of observations on single ﬁsheries, are clearly indicated.

The history of ﬁsheries management has clearly established the practical merits of taking proactive steps to maintain a healthy ﬁshery before the symptoms of a stock collapse trigger reactive regulatory measures to address those symptoms – as has been necessary over the past decade for many salmon ﬁsheries throughout England and Wales. Although Evans & Greest (2006) have examined the long-term record of rod catches for 67 rivers in England and Wales over the 30-year period between 1974 and 2003 and concluded that the total national rod catch has generally remained stable over the period, they also noted that catches have declined on 31 of these rivers (signiﬁcantly so on 14 rivers) and it is evident that this decline has been largely offset by an increase in the catch from 19 other rivers (many of which are continuing to recover from the effects of historical pollution). Irrespective of any concerns about the accuracy of the catch data for the rod and net ﬁsheries and attempts to interpret that data into an estimate of stock abundance for any year (Shelton, 2002), it is a matter of fact that we are not yet in any position to judge if our sea trout stocks are healthy and performing at their fullest natural potential. More importantly still, in the absence of any BRPs (Walker et al., 2006), we do not know if the present rate of exploitation by the rod and net ﬁsheries is sustainable in the longer term. Nevertheless, within these unknowns and uncertainties, there are certain basic precautionary measures to conserve and improve existing stocks that can be implemented by the ﬁshing community as a matter of prudence and common sense. These are the introduction of: (1) a lower ‘minimum’ size limit to protect whitling; (2) an upper ‘maximum’ size limit to protect the larger, MSW and repeat spawning sea trout and (3) a catch limit deﬁning the maximum number of sea trout that may be retained in any period of ﬁshing by any one angler. Lower minimum size limit The present minimum size limit introduced to protect juvenile ﬁsh and smolts (Table 31.3) does nothing whatsoever to protect the whitling component that returns to fresh water during the same year that it migrated to sea as smolts. For many stocks in England and Wales, the runs of immature whitling should be viewed as the ‘seed-corn’ from which future stocks will be derived to a lesser or greater extent and which should be carefully protected and nurtured during their ﬁrst return as maiden ﬁsh. Thus, the minimum size limit should be increased to a length that affords whitling a signiﬁcant measure of protection. This would be approximately 36 cm (14 in.) for most rivers in England and Wales (Solomon, 1995; Harris, 2000). On returning to fresh water, sea trout lose their characteristic silver sheen. They become darker and their spotting becomes more obvious as they begin to become sexually mature and adapt their camouﬂage to their new freshwater environment. In some situations their external appearance can become virtually indistinguishable to that of certain types of brown

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trout found in the same river system. This can lead to enforcement difﬁculties with size limits and bag limits when sea trout are mistakenly (or intentionally) misidentiﬁed by their captors as ‘brown trout’ and killed. This problem would be avoided by the adoption for each river of the same minimum size limit for both the migratory and non-migratory forms of Salmo trutta in those rivers where whitling represent a major part of the total run of adult sea trout.This simple approach has ‘added-value’ as it would also provide important long-term beneﬁts in improving the future quality of the brown trout ﬁshery. Upper maximum size limit Speciﬁc measures to conserve MSW and multiple repeat spawning sea trout as discrete and separate components of the stock are impracticable because of the extensive overlap in the range of sizes encountered and the lack of any reliable diagnostic features based on their external appearance. However, the introduction of a maximum size limit above which all ﬁsh must be returned alive to the water immediately after capture could provide an important measure of protection for both stock components. There can be no generally acceptable ‘one-size-ﬁts-all’ criterion in this respect and so the actual maximum size limit selected for each river, which could also serve to protect large ‘specimen’ sea trout by default, would need to be carefully deﬁned in relation to the characteristics and structure of each stock. Thus, while the condition factor (K) expressing the length–weight relationship of sea trout can vary within and across stocks and is usually greater for maiden ﬁsh than for previous spawners (Nall, 1930), a length of 61 cm (24 in) equates very roughly to 2.7 kg (6 lb) for most Welsh stocks (Solomon, 1995; Harris, 2000). If, for example, this were to be adopted as the maximum size limit on the Dyﬁ and applied to known data on stock structure for that river (Harris, 1970), it would serve to protect: (1) all MSW maiden ﬁsh and all previous spawners within that group; (2) all 1SW group ﬁsh on their third return visit to fresh water and (3) all 0SW group ﬁsh on their fourth return visit to fresh water. This would then result in the release of roughly 15% of the rod catch to further supplement the spawning population. A similar size limit on the Coquet, where the major stock component consists of 1SW and 2SW maidens that rarely survive to spawn more than once, would protect 16% of the stock and embrace all 2SW ﬁsh and all 1SW ﬁsh on their second return visit. In contrast however, a maximum size limit of 61 cm would protect only 2.3% and 1.8% of the stock respectively for those rivers with slow growing ﬁsh and relatively few 1SW and no MSW ﬁsh such as the Tamar and Camel. Here, a lower maximum size limit would be required to protect a greater proportion of 1SW maidens and previous spawners. Catch limits With the notable exception of the nine rivers where a catch limit has already been applied, there are no statutory measures to restrict the total number of ﬁsh that may be retained by an individual angler at any time during the ﬁshing season. Every ﬁsh caught by lawful means may be retained by its captor regardless of the status and well-being of the ﬁshery. In the absence of any statutory power to restrict the total number of anglers and their total

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combined ﬁshing effort on any river system, this situation is potentially untenable in the absence of any reliable information on the status and well-being of individual stocks of sea trout. It is therefore appropriate that careful consideration should now be given to the introduction of a precautionary catch limit that ﬁxes the maximum number of sea trout that may be retained by any angler in order to restrict the rate of overall exploitation to within biologically ‘safe’ limits. This should be linked to the existing categories of ‘daily’, ‘weekly’ and ‘season’ rod licences issued by the Environment Agency and separate catch limit attached to each period. The actual limits applied on different rivers may vary in practice depending on an initial judgement of the intensity of the angling pressure and the status and well-being of each stock. However, for most ﬁsheries in England and Wales at this time, catch limits ﬁxing the maximum number of ﬁsh that may be retained by an individual angler at: (1) 4 ﬁsh any 1 day; (2) 10 ﬁsh in any 1 week and (3) 30 ﬁsh in any 1 ﬁshing season appear to represent a preliminary basis for further discussion at a local level on each river system until the range of catches by individual anglers has been modelled to determine more precisely the actual bag limits necessary to conserve existing stocks and to improve future stocks. Apart from any other practical consideration, it is to be noted that the very existence of a ‘catch limit’ serves as a constant reminder to each angler of the need to conserve stocks for future generations. The adoption of the proposals for upper and lower size limits would create a ‘slot’ deﬁning the range of sizes within which the catch limit would apply. While any one of the three elements outlined here could make a worthwhile contribution to future stock conservation on most rivers, due consideration should be given to their introduction as a basic package of measures that can be adopted and adapted to suit local circumstances. For most rivers in England and Wales where maiden and previously spawned whitling constitute a signiﬁcant or major part of the annual run of sea trout, priority should be given to the introduction of effective measures to protect whitling in the ﬁrst instance. It is not possible under the present ﬁsheries legislation in England and Wales to prohibit anglers from selling their catch: and it is inevitable that any proposals to introduce new catch limits and size limits will encounter opposition from certain sections within the angling community. However, the strength of that opposition will be weakened considerably when the respective Governments in England and in Wales act on their declared intention to accept the recommendation in the recent Review of Salmon and Freshwater Fisheries (Anon., 2000) to prohibit the sale of rod-caught ﬁsh as this will remove any incentive for anglers to catch and kill the greatest possible number of salmon or sea trout in order to proﬁt ﬁnancially from their ﬁshing. The existing statutory procedures for making new by-laws are complicated, onerous and protracted. They can also be very costly if, as may be necessary, a ‘Local Public Inquiry’ is required to resolve formal written objections to any new by-law proposals or if, as is also possible, compensation has to be paid to the owner of a ﬁshery for ﬁnancial loss occasioned by the introduction of any new regulations. Consequently, it is perhaps understandable that the Environment Agency gives careful consideration before promoting new statutory regulations including assessing that there is appropriate scientiﬁc evidence of the need to do

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so for reasons of stock conservation. There is therefore cause to see the private owners within each single river catchment become much more proactive in taking direct responsibility for protecting their ﬁsheries by the introduction of effective systems of voluntary rules that avoid the need to impose statutory regulations. It remains to be seen if this can be achieved within the framework of the often fragmented private ownership of freshwater ﬁsheries in England and Wales.

References Anon. (2000). Salmon and Freshwater Fisheries Review. Ministry of Agriculture Fisheries & Food, London, 199 pp. Anon. (2004). Fisheries Statistics 2003. Environment Agency, 28 pp. Anon. (2005a). Statistical Bulletin. Scottish Salmon and Sea Trout Catches, 2004. Fisheries Series No. Fish/2005/1. Anon. (2005b). An annual assessment of salmon stocks in England & Wales 2004. A preliminary assessment prepared for ICES, March 2005. Centre for Environment, Fisheries & Aquaculture Sciences and Environment Agency, 74 pp. Elliott, J.M., Crisp, D.T., Mann, R.H.K. et al. (1992). Sea trout literature review and bibliography. National Rivers Authority. Technical Report No. 3, 141 pp. Environment Agency (2003). National trout & grayling ﬁsheries strategy. Environment Agency, Bristol, 21 pp. Evans, R. & Greest, V. (2006). The rod and net sea trout ﬁsheries of England & Wales. In: Sea Trout: Biology, Conservation & Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff Wales, UK. Blackwell Publishing, Oxford, pp.107–14. Evans, D.M., Mee, D.M. & Clarke, D.R.K. (1995). Mesh selection in a sea trout, Salmo trutta L., commercial Seine net ﬁshery. Fisheries Management & Ecology, 2, 103–11. Gargan, P.G., Tully, O. & Poole, W.R. (2003). The relationship between sea lice infestation, sea lice production and sea trout survival in Ireland, 1992–2001. In: Salmon at the Edge (Mills, D., Ed.). Proceedings of the 6th International Atlantic Salmon Symposium, Edinburgh, UK. July 2002, pp 119–35. Harris, G.S. (1970). Some aspects of the biology of Welsh sea trout. PhD Thesis, University of Liverpool, 263 pp. Harris, G.S. (1972). Some specimen sea trout from Welsh, English & Scottish Waters. Salmon & Trout Magazine, 196, 223–34. Harris, G.S. (1995). The design of a sea trout stock description sampling programme. National Rivers Authority, R&D Note 418, 87 pp. Harris, G.S. (2000). Sea trout stock descriptions: the structure & composition of adult sea trout stocks from 16 rivers in England & Wales. Environment Agency R&D Technical Report W224, 93 pp. Harris, G.S. & Milner, N.J. (2006). Setting the scene: sea trout in England & Wales – a personal perspective. In: Sea Trout: Biology, Conservation & Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, Cardiff Wales, July 2004, pp. 1–10. Mills, D.M. (1989). The Ecology & Management of Atlantic Salmon. Chapman & Hall, London, 351 pp. Milner, N.J., Harris, G.S., Gargan, P. et al. (2006). Perspectives on sea trout science and management. In: Sea Trout: Biology, Conservation & Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff Wales, UK. Blackwell Publishing, Oxford, pp. 480–90. Nall, G.H. (1930). The Life of the Sea Trout. London, Seeley Service & Co., 335 pp. Nall, G.H. (1932). Sea Trout on the Solway Rivers. Fisheries Scotland, Salmon Fisheries, No. III, HMSO Edinburgh, 72 pp. National Rivers Authority. (1996). A strategy for the management of salmon in England & Wales. National Rivers Authority, Bristol, 36 pp. Pawson, M.G. & Benford, T.E. (1983). The Coastal Fisheries of England & Wales, Part 1: A review of their status in 1981. MAFF Directorate of Fisheries Research, Internal report No. 9, 54 pp. Saunders, R.L. & Bailey, J.K. (1980). The role of genetics in Atlantic salmon management. In: Atlantic Salmon: Its Future (Went, A.E.J., Ed.). Proceedings of the 2nd International Atlantic Symposium, Edinburgh. Fishing News Books Ltd. Farnham, pp. 182–200.

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Shelton, R. (Ed.) (2002). The Interpretation of Rod and Net Catch Statistics. Proceedings of a Workshop held at the Centre for Environment, Fisheries & Aquaculture Science, Lowestoft, England, 6–7 November 2001. Atlantic Salmon Trust ‘Blue Book’, 107 pp. Shields, B.A., Aprahamian, M.W., Bayliss, B.D., Davidson, I.C., Elsmere, P. & Evans, R. (2006). Sea trout (Salmo trutta L.) exploitation from ﬁve rivers in England & Wales. In: Sea Trout: Biology, Conservation & Management (Harris, G.S. & Milner, N.M., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 417–33. Solomon, D.J. (1995). Sea trout stocks in England & Wales. National Rivers Authority. R&D Report No. 25, 102 pp. Walker, A.M., Pawson, M.G. & Potter, E.C.E. (2006). Sea trout ﬁsheries management: should we follow the salmon? In: Sea Trout: Biology, Conservation & Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 466–79.

Chapter 32

An Appreciation of the Social and Economic Values of Sea Trout in England and Wales P. O’Reilly1 and G.W. Mawle2 1

Swyn Esgair, Drefach Felindre, Llandysul SA44 5XG, UK Environment Agency, Waterside Drive, Aztec West, Almondsbury, Bristol BS12 4UD, UK 2

Abstract: Most of the economic and social values associated with sea trout are poorly documented and difﬁcult to dissociate from those of salmon. The existence value of sea trout may be signiﬁcant and the presence of sea trout, especially leaping ﬁsh, enhances property values in urban areas. Most of the remaining net ﬁsheries for sea trout have little commercial value and, although there are probably exceptions, most netsmen probably ﬁsh for enjoyment as much as for proﬁt. Nationally, the sea trout net catch generates only about £160 000 gross income annually to netsmen. Some traditional net ﬁsheries may have a signiﬁcant heritage value; for example, the public in Wales is willing to pay £1.5 million to retain a minimum coracle ﬁshery. Rod ﬁsheries for migratory salmonids are worth over £100 million to ﬁshery owners across England and Wales, with sea trout probably now contributing as much to that value as salmon. Expenditure by salmon and sea trout anglers can contribute signiﬁcantly to local rural economies and constitutes an estimated £8 million annually to the Welsh economy; the greater part of this is probably now attributable to sea trout. All aspects of value can be changed and enhanced through effective management and marketing. A case study on the Teiﬁ suggests that sea trout rod ﬁsheries in Wales could generate around another 100 full-time equivalent jobs in the Principality compared with 1997 levels. Keywords: Sea trout, socio-economic value, angling, net ﬁshing, marketing, community beneﬁts.

Introduction Sea trout beneﬁt society in a range of different ways, their value to the individual depending on personal circumstances and preferences. In managing sea trout stocks and ﬁsheries for the beneﬁt of society, it is important to appreciate how these values are generated and to whom. This chapter summarises what is known about the current status of a range of social and economic values associated with sea trout in England and Wales, but values are not static. Their magnitude can change or be changed, not only with the status of sea trout stocks, and of similar resources such as salmon, but also with the preferences of society (such as anglers’ preference to ﬁsh for wild rather than stocked trout, Simpson & Mawle, 2001), the costs of ﬁshing (including travel) and the management and marketing of the associated ﬁsheries. 457

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Existence and associated values Fish are not only valued for themselves, similar to other fauna, but also because they are perceived as indicators of environmental quality (Department of the Environment et al., 1996). Their existence is valued. Salmonids, in particular, are widely recognised as requiring a good quality environment. As sea trout, similar to salmon, are frequently seen leaping at weirs when migrating upstream they are signiﬁcant indicators of the improving quality of urban rivers (Mawle & Milner, 2003). The better the environment, the more willing people are to live and work in a particular area, thereby generating local economic beneﬁts. In Cardiff, salmon and sea trout have been returning to the River Taff since the 1980s (Mawle et al., 1985). Apart from being a visible demonstration of improved river quality, they have also appeared as cultural symbols. Salmonids form part of a recent municipal sculpture in Cardiff and, more signiﬁcantly, the emblem of a ﬁsh is etched on the windows of the headquarters of the Cardiff Bay Development Corporation, the body responsible for the economic regeneration of the dockland area of the city. Increasing ﬁsh populations, including Salmo trutta L., in the Thames have also been used by London Docklands (1996) to indicate improved living conditions and thereby promote property. No estimates have been made for the existence value of sea trout but existence values for salmon can be substantial. In a study commissioned by the Environment Agency, Spurgeon et al. (2001) estimated that people resident in the Thames catchment would be willing to pay £12 million per year to have a breeding population of salmon in the river. However, it would be a mistake to assume that the existence value of salmon and sea trout is only an indicator of environmental quality. For example, it is evident that people derive pleasure, and presumably value, from watching salmon and sea trout. Well-known falls where these ﬁsh may be seen leaping are an attraction and visitors’ expenditure can be signiﬁcant for the local economy. O’Reilly (unpublished) has estimated that during the holiday season, expenditure by coach parties in the vicinity of Cenarth Falls on the River Teiﬁ is around £50 000 per year.

Fishery values Perhaps more obviously, sea trout have value because they can be ﬁshed for, usually in conjunction with salmon, providing income from the sale of the catch or recreation or both. The net ﬁsheries Net ﬁsheries for migratory salmonids have been in long-term decline. The number of licences issued to net for salmon and sea trout in England and Wales has decreased by about 60% since the 1980s to 372 in 2003. In part, this decline reﬂects the reduced value of the catch. The price paid for wild salmon has reduced since the 1980s with the increased availability and reduced price of farmed salmon. Prices recorded by the Fishmongers Company at Billingsgate, adjusted by the Retail Price Index, show that from 1979–2002, the price of wild salmon in August decreased from about £14/kg to £7/kg, while the price of farmed salmon decreased from £12/kg to £3/kg. Given the similarity of the two species

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the price paid for sea trout, usually less than that for salmon (e.g. Radford et al., 1991), has been similarly affected. In 2003, the declared catch of sea trout in England and Wales was just over 50 tonnes of which three-quarters were taken in the north-east coastal ﬁshery, mostly in T and J nets. Other signiﬁcant sea trout net ﬁsheries are: the coracle nets of the Tywi, Taf and Teiﬁ (2.3t; 17 licences); the Seine nets at the mouths of the Hampshire Avon and Stour (1t; 6 licences), Teign (0.6t; 6 licences) and Dart (1t; 12 licences); and the coastal ﬁsheries off East Anglia (3.6t; 45 licences).

Value to the netsmen So what is the catch worth to the netsmen? If the netting were being done on a purely commercial basis, the economic value would be the proﬁt generated from selling the catch. The sale price paid to netsmen is quite variable. For example, in 2003, the price paid to Haaf netsmen on the Solway estuary reduced as low as £1.50/kg though when ﬁsh are scarcer, as in June 2004, the price has been £4.50. Taking £3/kg as an average sales price, the income to netsmen from sea trout would have been about £160 000 in 2003. When combined with an estimated £280 000 from sales of salmon (69 t at £4/kg), the income to netsmen in 2003 from migratory salmonids would be about £440 000. In 1996, netsmen’s costs were estimated to be about 75% (MAFF, 1998) which suggests a proﬁt to netsmen, nationally, from both species of about £110 000 of which £40 000 is derived from sea trout. While this is a crude assessment, it does indicate that the level of proﬁt for most of the 372 licensed netsmen is small, and for many there may be none. It seems likely that, whether they sell their catch, many netsmen similar to most anglers are ﬁshing partly, if not largely, for enjoyment rather than for proﬁt. Indeed, some netsmen, such as on the Solway and River Teiﬁ, have indicated as much. However, for others, income from sea trout may be signiﬁcant. For example, in 2003 the average gross income from sea trout for netsmen on the north-east coast was about £2000 per licence.

Heritage value Net ﬁsheries may have other values than the proﬁt and recreational value to the netsmen. Some of the salmon and sea trout ﬁsheries use ﬁshing techniques with a long tradition. A prime example is coracle ﬁshing on the Tywi, Taf and Teiﬁ in West Wales, while the Solway Haafnetters Association claim that the Vikings started their ﬁshery. Are people aware of these traditional ﬁsheries and do they value them? A study commissioned by the Environment Agency (Environment Agency, 2004) indicates that such traditional ﬁsheries may indeed have what may be called a heritage value, though this value is not dependent on the catch or even the number of people ﬁshing. For example, the study estimated that the people of Wales were willing to pay, as a one-off payment, £1.5 million to maintain a minimum coracle ﬁshery in West Wales. The study also suggested that, through demonstrations and interpretative material, traditional ﬁsheries might contribute to their local economies through tourism.

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The rod ﬁsheries Nowadays, anglers catch more sea trout than netsmen do. In 2003, anglers took 60% of the declared catch, 45 101 sea trout, with only 29 248 reported by the netsmen. As with netting, angling for sea trout is often closely linked to salmon angling and evaluating sea trout alone is difﬁcult. Anglers ﬁshing for sea trout in England and Wales must hold an Environment Agency salmon rod licence. Each year, about 25 000 anglers currently buy such a salmon licence. The balance between these anglers’ interest in salmon and sea trout is unknown and will vary from river to river reﬂecting, in part, the local catches of salmon and sea trout and the timing of runs. Sea trout rod ﬁsheries are valuable in at least three different ways: (1) to the ﬁshery owner (all ﬁsheries are privately owned); (2) to the angler and (3) to the local economy. This chapter focuses on the values to the ﬁshery owner and to the local economy.

Value to ﬁshery owners Sea trout add value to the market value of ﬁshing rights, and so beneﬁt the ﬁshery owner, but how much they add is unclear. For salmon ﬁsheries there is a generally accepted rule-ofthumb whereby the market value of a ﬁshery is, on average, related to the size of the salmon catch, taking into account a number of other factors. One would expect the sea trout catch to be one such factor. Although Radford et al. (1991) did look for a relationship between ﬁshery value and the sea trout catch, as yet no such rule has been demonstrated empirically. As part of a national evaluation of ﬁsheries in England and Wales for the Environment Agency, Radford et al. (2001) used a per capita value of £8400 per salmon in the 5-year average annual catch to value ﬁshing rights for migratory salmonids at £128 million. The contribution of sea trout to the value of ﬁshing rights is subsumed within this and it should not be assumed that the sea trout catch is irrelevant. It is likely that the salmon catch works as a predictor of ﬁshery value not only because of the value anglers place on salmon but also because, on average, the larger the catch the bigger the ﬁshery. Given the similarity of the two species, one might expect that anglers might value catching salmon and sea trout of a given size equally. If they do, then a substantial component of the value of migratory salmonid ﬁsheries is attributable to sea trout which now represent about half of the declared rod catch by weight in England and Wales, and 80% by number (see Fig. 32.1).

Value to the local economy Although expenditure by anglers ﬁshing for salmon or sea trout may not be important for the economy of England and Wales as a whole, it may be signiﬁcant for the local economies of some rural areas. The Environment Agency has a duty under the Environment Act 1995 to maintain, develop and improve salmon, trout, freshwater ﬁsh and eel ﬁsheries in England and Wales. The latest statutory guidance from the Government (Defra, 2002; Welsh Assembly Government, 2002) on how to execute this duty emphasises that the Agency should ‘enhance

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100% 80% 60% 40%

salmon sea trout

20%

19 88 19 90 19 92 19 94 19 96 19 98 20 00 20 02

0%

Fig. 32.1 The relative proportions (%) of salmon and sea trout in the declared rod catch for England and Wales from 1988 to 2003.

the contribution salmon and freshwater ﬁsheries make to the economy, particularly in remote rural areas and in areas with low levels of income’. To date, the main focus on the economic contribution of sea trout ﬁsheries has been in Wales. An Environment Agency study (Spurgeon et al., 2001) estimated that salmon and sea trout angling contributed about £1 million in 1998 to the economy of the Teiﬁ catchment in West Wales. This value was an estimate of expenditure within the catchment using a multiplier of 1.1 but no account was taken of costs so it is a gross rather than net value. Based on the Teiﬁ study, Nautilus (2000), in a report for the Welsh Assembly Government, estimated that expenditure associated with salmon and sea trout angling contributed about £8 million to the Welsh economy. As with other values, it is difﬁcult to separate the relative contribution of salmon and sea trout though the contribution of sea trout is likely to have been signiﬁcant. At the time (1996–2000) anglers caught about ﬁve times as many sea trout, in Wales, as salmon. It is likely that the sea trout is more important than salmon to the Welsh economy.

A case study in supporting rural recovery via a sea trout ﬁshery In 2001, a sample survey of club membership and visitor permit sales indicated that since 1981 the employment generated via all recreational ﬁshing in Wales had decreased by at least 1000 full-time-equivalent (FTE) jobs – well over £30 million per year in angler expenditure. The Nautilus (2000) report estimated the employment based upon recreational ﬁsheries at around 1500 FTE jobs. Clubs dependent on river ﬁshing had suffered particularly; for example, over the 15-year period from 1985 to 2000, one South Wales club saw its membership decline from 250 to just 58. In West Wales, Llandysul Angling Association (AA) thrived in the days when brown trout were plentiful and salmon ran in the River Teiﬁ throughout the year, but as salmon and trout stocks declined so did club membership and visitor permit sales (see Fig. 32.2). The decline in salmon and some brown trout stocks was not helped by almost a decade of zero budget for capital improvement works by the ﬁsheries service in Wales. Throughout this period, investment in ﬁsheries marketing by the Wales Tourist Board consisted of just one image-building brochure written and donated by enthusiastic amateurs (O’Reilly, 1998).

462

Sea Trout 1000 900 800 700 600 500 400 300 200 100 0

Waiting list Day/week permits Full members

1980

1990

2000

Fig. 32.2 The number of full members and the number of short-term permits sold by Llandysul Angling Association in 1980, 1990 and 2000, showing the number of anglers on the waiting list for a full permit in 1980.

When it came to the selling – providing the detailed information and turning their interest into actual angling tourism holiday bookings – the ﬁsheries of Wales, for the most part angling clubs run by part-time amateurs, were unsupported by any state investment. The information on the website www.ﬁshing-in-wales.com – developed by Llandysul AA and its partners – attracted two million separate visits annually but there was no professional monitoring of its effectiveness to indicate what proportion of these site visits were being converted into angling holidays in Wales. In 1997, Llandysul AA set about arresting and reversing the decline and saw its 30 miles of sea trout ﬁshing as the key asset. The ﬁrst step was customer research. The club received 60% written responses to a reply-paid questionnaire asking members and visiting anglers what they felt was needed. Some respondents felt that the club could do more to help restore salmon and trout stocks; but many other factors scored much more highly. These were, in order of priority: • • • • •

more detailed information about each ﬁshing beat; better maps and signage for the club’s 22 ﬁshing beats; a source of up-to-date river reports; support for newcomers to river ﬁshing; improved access, especially for inﬁrm and disabled people.

By far the most repeated call was for high-quality, detailed ﬁshery information in a form that anglers can easily use. As well as the angling club, several community groups responded to these ﬁndings: the local canoe club, wildlife enthusiasts, accommodation providers and other tourism operators in the area were also actively involved. The recovery project was designed around the community’s aspirations and priorities. After struggling for many years to restore and protect ﬁsh stocks by improving river habitats – and making some modest gains, albeit on a small scale – the community’s efforts were given a major boost by the Welsh Assembly Government’s ‘Sustainable Fisheries’

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project and the Objective 1 programme. In 2002, community volunteers worked in partnership with the Environment Agency to open up several miles of a major tributary. That winter 68 salmonid spawning redds appeared there, and in 2003 the ﬁgure rose to 120 (Environment Agency, 2003). This pilot project also entailed major changes in marketing. On their own, the club’s image-building brochures and newsletters had not been enough to maintain visitor loyalty. A postal survey of 400 visitors to the club’s ﬁsheries had underlined the need to provide detailed information to overcome their doubts and fears when choosing a new holiday venue. Information about how, when and where to ﬁsh, clear maps, details of ﬂy hatches; tackle advice, where to stay in the valley, where to get food, ﬁshing tackle, etc. and things for nonﬁshing members of the group to do – all this went into a 150-page guidebook (O’Reilly, 1999). The ﬁrst 1000 copies of the book sold in 2 years, fully funding the rest of the publicity material and the club’s contribution to the habitat improvement programme. An interactive CD-ROM (O’Reilly, 2004) further cut the costs of publicity and raised the quality by including video and other multimedia material. The other vital ingredients for marketing success – the actual selling and making visitors welcome – were entirely dependent on the community. Anglers, canoeists, wildlife enthusiasts and accommodation providers worked together to put on Welcome Days throughout the tourist season. People were invited to the valley and given illustrated talks, casting lessons, ﬁshing advice and guided tours of various ﬁsheries; and they were introduced to accommodation providers. Each spring typically 50 people would attend, and of these about 40 became new members of the club, visiting the valley for 1 or 2 weeks per year. These newcomers to Wales made friends with local people. They had someone they could phone for advice on river conditions and ﬁshing news. Four years on, most of these newcomers are still returning to the Teiﬁ Valley for their ﬁshing holidays and occasional short breaks. The results speak for themselves. Full membership rose each year since the launch of the recovery initiative. Figure 32.3 shows that 2004 brought a further substantial increase in full members and short-term visitor numbers. To put it in economic terms: in 6 years Llandysul Angling Association has arrested the decline and reversed it, in so doing creating the equivalent of three FTE jobs dependent 900 800 700 600 500 1998

1999

2000

2001

2002

2003

2004

Fig. 32.3 Full membership and visitor permit sales, including juniors, during the ‘Recovery’ project (2004 results estimated based on data to end of July).

464

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on angling tourism. Provided that improvements to the ﬁshery can be maintained, the club has the potential to create another two FTE jobs, at which point it would be necessary to restart a waiting list for membership. Compared with the 1997 level, the potential increase in employment that could be generated is estimated to be ﬁve FTEs. It is instructive to view this from a national perspective. Llandysul AA generates about one-third of the sea trout rod catch from the Teiﬁ which itself provides 15% of Wales’s rod catch of sea trout. If the rest of Wales’s sea trout ﬁsheries could generate an increase in employment proportional to that at Llandysul, an additional 100 FTE jobs could be created across Wales, above the 1997 level. The potential, additional angler expenditure in Wales would be about £3 million a year, compared with the total contribution of salmon and sea trout angling of £8 million estimated by Nautilus (2000). Such an employment boost would be for the most part in rural areas where there are few other opportunities for economic development. To achieve this, Wales must invest effectively in its sea trout ﬁsheries and get the marketing and selling right.

Acknowledgements We are grateful to the ofﬁcials and members of Llandysul Angling Association for the use of their club records and to the Fishmongers Company for information on the prices of salmon at Billingsgate. We would also like to thank all the Environment Agency staff involved in the collation of the catch statistics and of course the ﬁshermen who provided them.

References Defra (2002). The Environment Agency’s objectives and contributions to sustainable development: statutory guidance. Department for Environment, Food and Rural Affairs. December 2002, 15 pp. Department of the Environment, Ministry of Agriculture, Fisheries & Food, and Welsh Ofﬁce (1996). The Environment Agency and sustainable development, 29 pp. Environment Agency (2003). Fish pass and river improvements spawn success. News release TC312/03MW. 1 December 2003. Environment Agency (2004). Study to develop and test a method for assessing the heritage value of net ﬁsheries. R&D Technical Report, 57 pp. London Docklands (1996). Advertisement. Daily Telegraph, 17 March, p. 35. MAFF (1998). Economic value of salmon net and rod ﬁsheries in England and Wales in 1996. Unpublished paper. Ref. HX 1183. MAFF, Economics (Resource Use) Division, 9 pp. Mawle, G.W. & Milner, N. (2003). The return of salmon to cleaner rivers – England and Wales. In: Salmon at the Edge (Derek, M., Ed.). Blackwell Science, Oxford, pp. 186–99. Mawle, G.W., Winstone, A.S. & Brooker, M.P. (1985). Salmon and sea trout in the Taff – past, present and future. Nature in Wales, New Series, 4 (1&2), 36–45. Nautilus (2000). Study into inland and sea ﬁsheries in Wales. Prepared for the National Assembly for Wales by Nautilus Consultants Limited, 120 pp. O’Reilly, P. (Ed.) (1998). Fishing in Wales. Wales Tourist Board, 58 pp. O’Reilly, P. (Ed.) (1999). Tribute to the Teiﬁ. Llandysul Angling Association, Llandysul, Wales, UK, 145 pp. O’Reilly, P. (2004). Multimedia guide to salmon, trout and sea trout ﬁshing and other outdoor activities in the Teiﬁ Valley. First Nature, Llandysul, Wales, UK, 120 pp. Radford, A.F., Hatcher, A.C. & Whitmarsh, D.J. (1991). An economic evaluation of salmon ﬁsheries in Great Britain. Volume I. Principles, Methodology and Results for England and Wales. A report prepared

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for the Ministry of Agriculture, Fisheries and Food. Centre for Marine Resource Economics, Portsmouth Polytechnic, 290 pp. Radford, A.F., Riddington, G. & Tingley, D. (2001). Economic evaluation of inland ﬁsheries. Environment Agency R&D Project W2-039/TR/1 (Module A). Simpson, D. & Mawle, G. (2001). Surveys of Rod Licence Holders. R&D Technical Report. Project W2-057. Environment Agency, Bristol, 100 pp. Spurgeon, J., Colarullo, G., Radford, A.F. & Tingley, D. (2001). Economic evaluation of inland ﬁsheries. Environment Agency R&D Project W2-039/PR/2 (Module B). Welsh Assembly Government (2002). The Environment Agency’s objectives and contributions to sustainable development in Wales: statutory guidance from the National Assembly for Wales, 15 pp.

Chapter 33

Sea Trout Fisheries Management: Should We Follow the Salmon? A.M. Walker, M.G. Pawson and E.C.E. Potter Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Pakeﬁeld Road, Lowestoft, Suffolk, NR33 0HT, UK

Abstract: The aim of this chapter is to evaluate whether tools used for managing Atlantic salmon (Salmo salar L.) ﬁsheries could be applied to sea trout (Salmo trutta L.). The scientiﬁc basis of management measures adopted for salmon ﬁsheries is reviewed and discussed, focusing in particular on setting appropriate targets (e.g. biological reference points, BRPs) and on the practicability of such measures and targets for management of sea trout ﬁsheries. We conclude that BRPs based on stock–recruitment relationships (as used for salmon) are not presently feasible for sea trout management, given the limited understanding of the complex array of life strategies demonstrated by S. trutta. We suggest, instead, that BRPs could be deﬁned in terms of juvenile abundance in relation to carrying capacity, bearing in mind the management requirement for conserving stock diversity both within and between anadromous and freshwater-resident components. Keywords: sea trout, management, salmon, biological reference points.

Introduction The sea trout (Salmo trutta L.) has an important ecological role in the majority of freshwater systems in the UK and Ireland, and the associated ﬁsheries have a considerable social, recreational and economic value (e.g. Mawle & O’Reilly, 2006). Despite this, efforts to conserve sea trout stocks and to manage their exploitation for recreational (anglers) or commercial purposes have often been reactive rather than proactive (Harris, 2006), and have been implemented principally as a by-product of salmon (Salmo salar L.) management. This weakness became particularly apparent during the late 1980s and early 1990s, when sea trout catches declined in many rivers throughout the UK and Ireland (Whelan, 1991; Anon., 1992; Walker, 1994a). Sea trout catches in rivers in England and Wales recovered quickly and stocks are now considered to be healthy: the declared rod catch in 2004 was 36 000 ﬁsh (Anon., 2005). In contrast, some sea trout stocks in rivers in the north and west of Scotland and Ireland that crashed during the same period have failed to recover (Gargan, 2000; Butler, 2002). While a number of possible causes have been investigated, it is generally agreed that the crashes were attributable primarily to reduced survival in the marine environment (Walker, 1994a; Poole et al., 1996). These events have highlighted the pressing need to address the speciﬁc management requirements of sea trout. 466

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Similar decline in the abundance of many salmon stocks throughout the species’ native range during the past two to three decades have forced scientists and managers to reconsider and improve their approach to management (e.g. Crozier et al., 2003). This has led to the adoption of a precautionary approach to salmon management and the use of biological reference points (BRPs) to provide the scientiﬁc basis for measures designed to bring about reductions in ﬁsheries exploitation and in reclamation and conservation of freshwater habitats. Within England and Wales, the Salmon and Freshwater Fisheries Review (Anon., 2000) recommended that sea trout ﬁsheries should be managed ‘to protect stocks from overexploitation’. The Review Group also suggested that the principles applied to the regulation and management of salmon ﬁsheries might be applied to sea trout. Given the broad similarities between the anadromous life history and coastal and freshwater ﬁsheries of salmon and sea trout, such an approach would appear to have merits, but sea trout have far more complex and varied life-history strategies than salmon. As a result, the pros and cons of adopting such an approach need to be explored in detail. In this chapter, we outline the management strategy and its scientiﬁc basis as presently applied to salmon stocks, focusing in particular on that applied in England and Wales. We then consider whether this approach might be suitable for the management of sea trout stocks, or whether alternative approaches might be more appropriate.

Salmon management The present approach to managing salmon stocks throughout the North Atlantic region follows the agreement by Parties to the North Atlantic Salmon Conservation Organization (NASCO) that salmon stocks should be conserved by ensuring that an adequate number of spawners enter each river to optimise annual production (NASCO, 1998). The derivation of an ‘adequate’ spawning stock size is based on the assumption that the number of ﬁsh produced in the next generation (recruitment) is related to the number of adult ﬁsh in the previous generation (stock). Salmonids are among the few ﬁsh species studied where this premise has been clearly demonstrated (Chaput & Prevost, 2001). Recruitment in anadromous salmonids is largely determined by density-dependent regulation in the early life stages because of limited resources (chieﬂy space and food) in fresh water (Gibson, 1993; Elliott, 1994). Though salmonid recruitment is strongly inﬂuenced both by intrinsic (genetic) and extrinsic (environmental) factors, long-term studies indicate that a density-dependent stock–recruitment (SR) model should generally apply (reviewed by Elliott, 2001). Several model types can be applied to explain the SR relationship (Elliott, 1994), each assuming that the proportional survival of offspring decreases as the stock size increases. The effect of this is that the number of recruits (or offspring that survive to adulthood) increases to a maximum as the spawning stock increases, and, in some modelling scenarios, may then decline again at high spawning stock levels (Fig. 33.1). Of course, the SR curve is simply a mathematical interpretation of the trend in recruitment at different stock levels, based on a scatter of observations from various years’ data. Not only should such curves be interpreted with care, but attention must be given to the variation in observations around the curve.

468

Sea Trout Replacement line

Adults produced (×1000)

6

SR curve

4

2

SMSY

0 0

2

4 Spawners (×1000)

Yield 6

Fig. 33.1 Ricker SR curve for hypothetical stock showing BRP, SMSY , adopted by NASCO as the CL for salmon stocks (Potter, 2001).

The prime objective of salmon ﬁsheries management is the conservation of the stock and the diversity within it. If this is achieved, we can then begin to determine sustainable levels of exploitation and consider optimum allocation of the exploitable surplus among ﬁsheries. The difference between the points on the SR curve and the replacement line (RL) (the number of ﬁsh that will give rise to an equal number of spawners in the next generation; RL in Fig. 33.1), at any given stock size, is the surplus that can be removed by the ﬁshery (yield), whilst still allowing sufﬁcient recruits to maintain that stock size. The exploitation rate associated with these variations in yield increases from zero, at the stock size where the RL crosses the SR curve, to a maximum where the stock size is reduced to almost zero. Thus, a wide range of exploitation rates may result in the stock and catches varying around what would appear from the SR curve to be stable, sustainable levels (Hilborn & Walters, 1992). However, the risk to sustainability varies considerably between different levels of spawning stock biomass, and there is a need to ensure reasonable protection against years in which recruitment is unexpectedly poor for other reasons. There is, therefore, a need to demarcate undesirable stock levels (and/or levels of ﬁshing activity), and the ultimate objective when managing stocks and regulating ﬁsheries will be to ensure that there is a high probability that these undesirable levels are avoided (Potter, 2001). This has been achieved by setting conservation limits (CLs) for stocks. A wide range of different BRPs have been proposed for managing ﬁsh stocks and ﬁsheries, and there is no ideal. In determining which point should be established as the CL for salmon stocks, consideration was given to the need to be able to set comparable reference points for the large number of different river stocks. One strong candidate is the stock size at which yield should be maximised in the long term (SMSY ). This point can be mathematically deﬁned for any density-dependent SR relationship, as the net gain curve (yield) is always

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dome-shaped (Fig. 33.1). ICES (1995) proposed that, by maintaining stocks at or close to SMSY , the maximum sustainable yield or catch should be generated in the long term (other factors being equal). However, the increasing slope of the SR curve below SMSY indicates that relative recruitment begins to fall rapidly as stock size diminishes and there is, consequently, an increasing risk that recruitment will be insufﬁcient to insure against the risk of stock collapse. NASCO (1998), therefore, adopted SMSY on an adult-to-adult Atlantic salmon SR relationship as a threshold or limit reference point, the CL. Stock levels below this are considered undesirable and are to be avoided. Clearly, the choice of SMSY as a limit means that the objective is not to maximise the catch (Potter et al., 2003) but to ensure stock/ﬁshery sustainability.

Salmon reference points for England and Wales In 1996, the National Rivers Authority (NRA, pre-Environment Agency, EA) in England and Wales launched their National Salmon Management Strategy (Anon., 1996), which included the requirement to develop river-speciﬁc CLs in terms of numbers of spawning salmon and egg deposition. Following examination of a number of approaches to BRPs, the use of SR curves and a CL approach was adopted (Milner et al., 2000). Estimates of stock and recruitment over a wide range of spawning escapements and a large number of years are required to derive a SR model (e.g. Buck & Hay, 1984; Elliott, 1993; Kennedy & Crozier, 1993), and such SR data are available for only about 30 of the 2000+ salmon stocks in the Atlantic region (Crozier et al., 2003, chapter 2). None was available from rivers of England and Wales in the mid-1990s. The NRA (Anon., 1998), therefore, chose to take as a base the salmon SR model for the River Bush in Northern Ireland (Kennedy & Crozier, 1993), a river similar in character to many in England and Wales and for which a long-term (1973 onwards) dataset of adult and smolt run size has been obtained using total-run trapping facilities. River-speciﬁc CLs for England and Wales are calculated based on the egg (adult escapement)-to-smolt (recruitment) SR curve for the River Bush, but adjusted for freshwater habitat availability in each river, and a replacement line (RL) derived from estimates of marine survival and other life-history parameters, also speciﬁc to each river (Wyatt & Barnard, 1997). River-speciﬁc habitat availability (wetted stream area) was quantiﬁed in terms of 22 categories of altitude and stream order (Strahler, 1952), assessed by GIS. The carrying capacities for 0+ and >0+ salmon parr for each category were derived from EA electro-ﬁshing surveys of pristine (i.e. not affected by adverse water quality or habitat) sites throughout England and Wales. Maximum smolt production for each river was predicted using category-speciﬁc Ricker-type egg-to-smolt curves from the Bush data, raised by the accessible wetted area of each habitat category in the catchment. The estimated number of eggs produced per smolt leaving the river (replacement) was calculated using river-speciﬁc sex and sea-age ratios for spawners, national estimates of marine survival for grilse (11%) and multi-sea winter salmon (MSW) (5%), based on data from the N. Esk River, Scotland (ICES, 2003), and a length–fecundity relationship derived for salmon from six Scottish rivers (Pope et al., 1961).

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Stock status in relation to the CL is assessed annually in terms of estimated egg deposition (e.g. Anon., 2005, Table 19), derived either from estimates of post-ﬁshery escapement based on the rod catch, raised to include an estimate of non-reported catch, and an estimate of exploitation rate or, in 13 rivers, from trap or counter data. The EA established management targets (MTs) for all 64 whole-river salmon stocks for which CLs had been set. The MT was deﬁned as the target stock size that would meet or exceed the CL in 4 years out of 5, based on variation in egg deposition estimates during the previous 11 years (Anon., 2004). Clearly, there are a number of areas of uncertainty associated with this method (Milner et al., 2000), not least the use of rod catches to estimate run size, and efforts continue to further develop the method and incorporate these uncertainties (Wyatt, 2002, 2003). Two other components of the salmon’s life history that could be monitored to assess the status of stocks are juveniles (fry and parr) and smolts, both representing freshwater production resulting from egg deposition. Juvenile abundance is surveyed on most salmonproducing catchments throughout England and Wales, and the smolt run is monitored on six rivers (Anon., 2004), but only estimated on two. At present, these data are not used as a formal BRP in the assessment of salmon stock status, but they are used as diagnostics to support interpretation of the CL compliance results.

Sea trout management Stock–recruitment model The extrapolation, or transport (Wyatt & Barnard, 1997), of a salmon SR relationship from one river to another, as from the River Bush to rivers of England and Wales, is based on the assumption that the population dynamics (eggs-to-smolts-to-eggs) of each stock are similar and that differences in river-speciﬁc production are primarily attributable to differences in habitat quantity and quality, that is carrying capacity. Establishing similar egg depositionbased BRPs to evaluate the status of sea trout stocks of England and Wales would, therefore, require SR data for at least one ‘typical’ sea trout river, and a method by which to ‘transport’ the relationship from this reference river to others, including knowledge of the numbers and sex composition of spawners, and a size–fecundity model. At present, published data for sea trout SR relationships are available for three rivers in north-western Europe: Black Brows Beck in England (Elliott, 1993); the Burrishoole system in Ireland (Poole et al., 1996) and the Bresle in France (Euzenat et al., 1999); data for all three are reported in this symposium (Elliott & Elliott, 2006; Euzenat et al., 2006; Poole et al., 2006). Black Brows Beck is a small, shallow tributary (length ∼500 m, mean width 0.8 m) of a spate river, the Cumbrian Leven, with a trout population supported entirely by sea-run ﬁsh. Almost all trout migrate to sea at age 2 and then return to spawn either the same year (males only) or the following year (both sexes); very few survive to spawn a second year (see Elliott, 1994). The Burrishoole system is a spate river catchment in western Ireland drained by approximately 45 km of shallow streams (16.7 km accessible to migratory salmonids), but

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characterised by two freshwater lakes (410 and 46 ha), where the majority of freshwater trout production occurs (Matthews et al., 1997). Before the stock collapse in the late 1980s, sea trout smolts were typically 2 (70%) or 3 (30%) years old and, based on catch returns for 1985 and 1986, the majority of sea trout returned as whitling (0 sea age: ∼56%) with the remainder being predominantly of sea ages 1 or 2 years (Poole et al., 1996). This system was stocked with almost 50 000 reared trout from 1993 to 1998 as part of a sea trout enhancement programme (Byrne et al., 2002). The Bresle is a chalk stream in the Upper Normandy-Picardy region of France, with a main channel of approximately 72 km in length, 40 km of which is accessible to sea trout and salmon. The majority (78%) of sea trout smolts go to sea at age 1, after which adults return annually to spawn, with the oldest ﬁsh having a sea age of 4 years (Euzenat et al., 1999). Thus, the three systems are very different in physical characteristics, as are the life histories of the sea trout they produce. Furthermore, the dynamics of sea trout stocks are, in many cases, far more complex and diverse than those of salmon, both between and even within river systems. Of key importance to diversity is the fact that, while some sea trout populations exhibit complete anadromy (Milner et al., 1993; Elliott, 1994), many are considered to be freely interbreeding fractions of a single trout population that includes both anadromous and freshwater-resident components. Progeny of anadromous and freshwater-resident trout have been shown to become both forms (Frost & Brown, 1967; Jonsson, 1985; Walker, 1990) and, while genetic differences have been reported between trout populations both within and between catchments, no study of neutral markers has provided conclusive evidence of genetic divergence between sympatric freshwater-resident and anadromous trout (Ferguson et al., 1995). Furthermore, the tendency to become anadromous often differs between the sexes, as evidenced by the female-biased sex ratios of sea trout during their spawning migrations (Le Cren, 1985), although the ratios may approach unity when maturing freshwater-resident brown trout are included in the calculations (Sambrook, cited in Solomon, 1995). As yet, the reproductive contribution of freshwater-resident trout to sea trout runs remains poorly understood, in part because of the technical difﬁculties in distinguishing between parr of resident, migrant or ‘mixed’ origins (but see Charles et al., 2004). Without this information, which is not accounted for in any measure of sea trout stock size based on monitoring spawning runs, it is impossible to transport a sea trout SR relationship between rivers. Even within the anadromous component, sea trout stocks demonstrate a considerable variety in life-history patterns. Within England and Wales, such variations include smolt age (1–4 years), sea age at ﬁrst spawning (0–2 years), spawning frequency (once to several), growth rate at sea, and pattern and timing of return to fresh water, as reviewed for 80 rivers (Solomon, 1995) and in greater detail for 16 rivers (Harris, 2002). These differences would each affect how representative a sea trout SR model from one river would be of any other. For example, as parr survival decreases with time, smolt production rates might tend to be negatively correlated with mean smolt age (MSA), as reported for both sea trout and salmon (Saltveit, 1990). Furthermore, Harris (2002) reported that the proportion of maturing whitling varied between 0% and >98% amongst the 16 stocks studied. This information is

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required for both the stock with a known SR relationship and those stocks to which this is transported in order to correctly derive effective spawning stock size from estimates of adult returns. Similarly, pattern and timing of return to fresh water will inﬂuence pre-spawning survival through natural and ﬁsheries-associated mortality rates, and hence will affect the relationship between returning stock and spawning stock. While fecundity is correlated with ﬁsh length, data from UK and Irish sea trout stocks (O’Farrell et al., 1989; Walker, 1994b; Elliott, 1995) suggest that length–fecundity relationships may differ between stocks, dependent on growth rates in fresh water (O’Farrell et al., 1989) or marine environments (Elliott, 1995), and that the use of a mean formula derived from these data to estimate egg deposition could lead to errors of up to 30% (Solomon, 1997). Given the variety and complexity of the population dynamics of S. trutta (resident and anadromous) stocks, it is highly unlikely that a SR model based on a single donor stock, especially one with such a limited dynamic as that of the Black Brows Beck population, or three stocks with such disparate population dynamics and habitats as those noted above, could be sufﬁciently representative of sea trout rivers throughout England and Wales. While some of these differences might be accounted for during the SR transport process, they would require a detailed knowledge of the population dynamics that is not available for most stocks. The resource (data and time) requirements to acquire this knowledge would be substantial and, though such an aspiration is laudable, there is a need for an alternative approach, at least in the short term.

Alternative BRPs for sea trout Catch-based indices The majority of salmon ﬁsheries now collate some form of catch data and, although hampered by the inﬂuence of variations in effort and catchability, catch-based indices can provide a guide to stock status. One alternative to a SR-based approach, therefore, might be to derive sea trout abundance indices from catch records and compare them against reference catch levels. This is part of the approach being developed for salmon stocks in Scotland (Potter et al., 2003), in response to concerns that whole-river CLs do not take sufﬁcient account of possible complex population structures within the rivers, and that the appropriate biological scale for setting CLs may be tributaries or smaller sections of the river (ICES, 1999). Rod catch records in Scotland have been collected in a systematic manner for many years, with catches of 1SW and MSW salmon recorded separately for each month of the ﬁshing season and on a sub-catchment scale. It is intended that management actions will be targeted at those sub-catchments where adult abundance indices and juvenile densities decrease below reference levels. In contrast, rod catches of sea trout in many rivers in Scotland, England and Wales may be poorly recorded, and there are additional problems with catch-based indices (for both sea trout and salmon) including the inﬂuence of non-stock related factors (e.g. national Spring Salmon by-laws since 1999; foot and mouth disease in 2001) on catch and effort. Furthermore, stock size may appear stable in the short term but be insufﬁcient to provide stable recruitment in the long term. It is difﬁcult, therefore,

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to envisage how such a catch-based approach could be applied across the variety of sea trout stocks in England and Wales. Moreover, sea trout catch records could miss a potentially signiﬁcant part of the spawning stock, the freshwater-resident trout. Therefore, sea trout catch data alone do not provide a sound basis for making decisions on management of sea trout ﬁsheries. Juvenile-based assessments Given the signiﬁcant difﬁculties of using adult-based assessments as a source of BRPs for sea trout (at least in the short–medium term), we should consider the other lifehistory components suggested earlier: juveniles and smolts. Salmon fry and parr abundance measures have been used to estimate spawner numbers in the previous winter (Kennedy & Crozier, 1993) or predict smolt output (Bagliniere et al., 1993). The EA routinely monitor the abundance of juvenile salmon and trout, particularly fry, throughout England and Wales. However, the parentage of juvenile trout can only be determined using destructive sampling methods, such as comparison of stable isotope ratios in recently emerged fry (Charles et al., 2004) or analysis of carotenoid pigment proﬁles (Youngson et al., 1997), neither of which could realistically be applied across large catchments. Whilst the adult salmon spawning stock of one small tributary has been genetically typed, allowing the parentage of juveniles to be assigned (Taggart et al., 2001), it is highly unlikely that resources would be available to apply this method to sea trout in large catchments. Similarly, the future form of juvenile trout cannot, at present, be predicted until a few days or weeks before their migration as smolts (Nielsen et al., 2004, but see Giger et al., 2006), because of the variable effects of environmental factors on life-history strategies. The present understanding is that anadromy within S. trutta is a threshold quantitative trait, that is, it is inﬂuenced by multiple genes and by the environment, but only expressed when the appropriate threshold combination, which presumably varies across and even within stocks, is reached (Ferguson, 2006). Thus, since the majority of sea trout stocks in rivers of England and Wales include both freshwater-resident and anadromous components, juvenile trout abundance data cannot, at present, be used to estimate sea trout spawning stock or to predict smolt output. Though smolt output clearly represents freshwater production and indicates the potential production of adults, our lack of understanding of the sea trout production dynamics means that we cannot transport a smolt output relationship from river to river. The monitoring programmes for salmon smolt runs on six rivers in England and Wales should certainly be adapted to provide sea trout data where possible. However, as there are at least 100 rivers producing sea trout in England and Wales (Solomon, 1995), this is not going to provide an index suitable for national sea trout management purposes.

Management of the Salmo trutta complex It is clear from the above that efforts to manage sea trout alone, and without considering the contribution of freshwater-resident trout, are severely hindered by their complex life history, diversity between stocks and our very limited knowledge of trout population dynamics.

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An alternative approach would be to consider the trout stock as a whole and to focus conservation efforts at that part of the stock that incorporates both freshwater-resident and anadromous components – the juveniles. A healthy juvenile trout population must provide the resource necessary for a sea trout population, though the environment and genes will determine what proportion adopt anadromy as a life strategy. For stock conservation purposes, the state of the juvenile population would have to be assessed in relation to potential freshwater production. This approach would be catchment-speciﬁc, and would explicitly focus management on both the ﬁsheries and on the environmental quality of juvenile rearing habitats in fresh water. There has been an extensive study of the freshwater habitat utilisation and requirements of juvenile salmonids (most recently reviewed by Armstrong et al., 2003; Klemetsen et al., 2003), and several attempts to model the abundance, density or biomass of juvenile S. trutta based on habitat characteristics within its natural (e.g. Belaud et al., 1989; Milner et al., 1993; Baran et al., 1996; Lek et al., 1996; Jutila et al., 1999; Maeki Petaeys et al., 1999) and introduced ranges (e.g. Lanka et al., 1987; Scarnecchia & Bergersen, 1987; Lambert & Hanson, 1989; Newman & Waters, 1989; Jowett, 1995; Van Winkle et al., 1998). These empirical models, for example, HABSCORE (Barnard et al., 1995; Milner et al., 1995), can only account for the spatial component of stream-dwelling trout abundance variance, but for trout in English and Welsh streams this can be up to 73% of the overall variance (including temporal, error and interaction), of which the (HABSCORE) models explained up to 63%. Moreover, the proportions of spatial and temporal variation vary substantially with the scale of analysis, from tributary to catchment level (Milner et al., 1995). However, they do offer a mechanism by which to predict the potential average maximum abundance, the carrying capacity, of juvenile trout for riverine parts of a catchment, against which to compare measured densities and set BRPs. Recently, the entire freshwater salmon habitat of Ireland has been quantiﬁed in terms of wetted surface area, channel gradient and water quality using GIS (McGinnity et al., 2003) and efforts continue to improve the river-reach classiﬁcation method for England and Wales (Wyatt & Barnard, 1997). However, it is not possible to predict the effects on adult sea trout numbers of either reducing or increasing juvenile abundance. Therefore, the a priori assumption of any juvenile-based BRP would have to be that, providing the juvenile abundance exceeded some predetermined level in relation to carrying capacity, the numbers of adult trout (anadromous and/or freshwater-resident) would be sufﬁcient to meet the needs of stock conservation and associated ﬁsheries. It is notable that recruitment at SMSY is generally between about 80% and 90% of maximum recruitment for typical salmonid SR curves (Healey, 1982; Potter et al., 2003). Thus a CL for juvenile trout production might be set at a similar proportion of the theoretical carrying capacity for S. trutta: this is analogous to the habitat utilisation index (HUI) of the HABSCORE models proposed by Barnard et al. (1995) as a benchmark for stream ﬁsheries potential. There would also be a need to take account of the variability of juvenile recruitment and uncertainty in assessment methods. This approach is based on the conservation of the whole stock in a catchment, and could be regarded as a reasonable ﬁrst step towards management for sustainable ﬁsheries. However, other stock characteristics, such as the size (length at age) and age structure of the stocks,

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relative and absolute abundance of repeat spawners and the run timing of different stock components, should also be considered. For many sea trout stocks, the longevity and multiple spawning of individuals, the contributions of resident and migratory ﬁsh, and the different run-timing patterns may buffer recruitment against the impacts of short-term environmental change to habitats. Clearly, such diversity should be conserved for the continued robustness of the stock and form the basis of diversity targets, as recommended for salmon (ICES, 2003). While population structure must be taken into account at all stages, it will only be practical to apply CLs on the same scale that they will be used in management. As with salmon (Potter, 2001), however, we need to know the extent to which a catchment-based reference level will ensure conservation at a ﬁner scale, and to what degree trout production and stock structure vary throughout a single catchment.

Conclusions Given the considerable problems with transport of any sea trout SR relationship to other rivers and the data requirements involved in the development of other, more representative, SR relationships, it appears that sea trout management cannot, in the immediate future, follow that of salmon. This is not to say that SR-based BRPs are unsuitable for sea trout stocks, but that this approach should only be considered as and when suitable data become available. Given the limited understanding of the dynamics of stocks that include freshwaterresident and anadromous components, we suggest focusing on trout as a whole, including both anadromous and non-anadromous individuals. BRPs should be deﬁned in terms of juvenile abundance in relation to carrying capacity, whilst considering the management requirement for conserving stock diversity both within and between anadromous and freshwater-resident components. Whatever methods are adopted, there is no doubt that considerable investment is required in order to improve our understanding of the population dynamics of trout stocks. Nevertheless, the social and economic value of sea trout ﬁsheries demands that stocks are conserved and associated ﬁsheries are run in a sustainable manner.

Acknowledgements The authors’ attendance at the 1st International Sea Trout Symposium and the preparation of this chapter were supported by the UK Department of Environment, Food and Rural Affairs.

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Matthews, M.A., Poole, W.R., Dillane, M.G. & Whelan, K.R. (1997). Juvenile recruitment and smolt output of brown trout (Salmo trutta L.) and Atlantic salmon (Salmo salar L.) from a lacustrine system in western Ireland. Fisheries Research, 31, 19–37. Mawle, G. & O’Reilly, P. (2006). An appreciation of the social and economic values of sea trout in England and Wales. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 457–65. McGinnity, P., Gargan, P., Roche, W., Mills, P. & McGarrigle, M. (2003). Quantiﬁcation of the freshwater salmon habitat asset in Ireland using data interpreted in a GIS platform, Dublin, Ireland. Central Fisheries Board Irish Freshwater Fisheries Ecology and Management Series No. 3, 132 pp. Milner, N.J., Wyatt, R.J. & Scott, M.D. (1993). Variability in the distribution and abundance of stream salmonids, and the associated use of habitat models. Journal of Fish Biology, 43A, 103–119. Milner, N.J., Wyatt, R.J., Barnard, S. & Scott, M.D. (1995). Variance structuring in stream salmonid populations, effects of geographical scale and the implications for habitat models. Bulletin Français de la Peche et de la Pisciculture, 337/338/339, 387–98. Milner, N.J., Davidson, I.C., Wyatt, R.J. & Aprahamian, M.A. (2000). The use of spawning targets for salmon ﬁshery management in England and Wales. In: Management and Ecology of River Fisheries (Cowx, I.G., Ed.). Fishing News Books, Oxford, pp. 361–72. NASCO (1998). Agreement on the adoption of a precautionary approach. Report of the ﬁfteenth annual meeting of the Council, Edinburgh. NASCO, pp. 167–72. Newman, R.M. & Waters, T.F. (1989). Differences in brown trout (Salmo trutta) production among contiguous sections of an entire stream. Canadian Journal of Fisheries and Aquatic Sciences, 46, 203–13. Nielsen, C., Aarestrup, K., Norum, U. & Madsen, S.S. (2004). Future migratory behaviour predicted from premigratory levels of gill Na+ /K+ -ATPase activity in individual wild brown trout (Salmo trutta). Journal of Experimental Biology, 207, 527–33. O’Farrell, M.M., Whelan, K.F. & Whelan, B.J. (1989). A preliminary appraisal of the fecundity of migratory trout (Salmo trutta) in the Erriff catchment, western Ireland. Polskie Archive Hydrobiologie, 36, 273–81. Pirhonen, J. & Forsman, L. (1998). Effect of prolonged feed restriction on size variation, feed consumption, body composition, growth and smelting of brown trout, Salmo trutta. Aquaculture, 162, 203–17. Poole, W.R., Whelan, K.W., Dillane, M.G., Cooke, D.J. & Matthews, M. (1996). The performance of sea trout (Salmo trutta L.) stocks from the Burrishoole system, 1970–1994. Fisheries Management and Ecology, 3, 73–92. Poole, W.R., Dillane, M., deEyto, E., Rogan, G. & Whelan, K. (2005). Characteristics of the Burrishoole sea trout population: census, marine survival and stock recruitment. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the Ist International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 279–306. Pope, J.A., Mills, D.H. & Shearer, W.M. (1961). The Fecundity of Atlantic Salmon (Salmo salar L.). Department of Agriculture and Fisheries for Scotland Freshwater Salmon Fisheries Research No. 26, 12 pp. Potter, E.C.E. (2001). Past and present use of reference points for Atlantic salmon. In: Stock, Recruitment and Reference Points: Assessment and Management of Atlantic Salmon (Prevost, E. & Chaput, G., Eds). INRA, Paris, pp. 195–223. Potter, E.C.E., MacLean, J., Wyatt, R.J. & Campbell, R.N.B. (2003). Managing the exploitation of migratory salmonids. Fisheries Research, 62, 127–142. Ricker, W.E. (1954). Stock and recruitment. Journal of the Fisheries Research Board of Canada, 11, 559–623. Saltveit, S.J. (1990). Effect of decreased temperature on growth and smoltiﬁcation of juvenile Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in a Norwegian regulated river. Regulated Rivers: Research & Management, 5, 295–303. Scarnecchia, D.L. & Bergersen, E.P. (1987). Trout production and standing crop in Colorado’s small streams, as related to environmental features. North American Journal of Fisheries Management, 7, 315–30. Solomon, D.J. (1995). Sea trout stocks in England and Wales. National Rivers Authority, R & D Report No. 25, Bristol, 102 pp. Solomon, D.J. (1997). Review of sea trout fecundity. Environment Agency, R&D Technical Report No. W60, Bristol, 22 pp. Strahler, A.N. (1952). Dynamic basis of geomorphology. Geological Society of America Bulletin, 63, 923–38.

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Symons, P.E.K. (1979). Estimated escapement of Atlantic salmon (Salmo salar) for maximum smolt production in rivers of different productivity. Journal of the Fisheries Research Board of Canada, 36, 132–40. Taggart, J.B., McLaren, I.S., Hay, D.W., Webb, J.S. & Youngson, A.F. (2001). Spawning success in Atlantic salmon (Salmo salar L.): a long-term DNA proﬁling-based study conducted in a natural stream. Molecular Ecology, 10, 1047–60. Van Winkle, W., Jager, H.I., Railsback, S.F., Holcomb, B.D., Studley, T.K. & Baldrige, J.E. (1998). Individual-based model of sympatric populations of brown and rainbow trout for instream ﬂow assessment: model description and calibration. Ecological Modelling, 110, 175–207. Walker, A.F. (1990). The sea trout and brown trout of the River Tay. In: The Sea Trout in Scotland (Picken, M.J. & Shearer, W.M., Eds). The Dunstaffnage Marine Research Laboratory, NERC, Oban, Scotland, pp. 5–12. Walker, A.F. (1994a). Sea trout and salmon stocks in the western Highlands. In: Problems with Sea Trout and Salmon in the Western Highlands. 24th November 1993, Atlantic Salmon Trust, Inverness, pp. 6–18. Walker, A.F. (1994b). Fecundity in relation to variation in life history of Salmo trutta L. PhD Thesis, University of Aberdeen, Aberdeen. Whelan, K.F. (1991). Disappearing sea trout: decline or collapse? Salmon Net, 23, 24–31. Wyatt, R.J. (2002). Estimating riverine ﬁsh population size from single- and multiple-pass removal sampling using a hierarchical model. Canadian Journal of Fisheries and Aquatic Sciences, 59, 695–706. Wyatt, R.J. (2003). Mapping the abundance of riverine ﬁsh populations: integrating hierarchical Bayesian models with a geographic information system (GIS). Canadian Journal of Fisheries and Aquatic Sciences, 60, 997–1006. Wyatt, R.J. & Barnard, S. (1997). The transportation of the maximum gain salmon spawning target from the river Bush (NI) to England and Wales, Environment Agency, R&D Technical Report No. W65, Swindon, 38 pp. Wyatt, R.J., Barnard, S. & Lacey, R.F. (1995). Salmonid modelling literature review and subsequent development of HABSCORE models. National Rivers Authority, R&D Project Record No. 338/20/W, 189 pp. Youngson, A.F., Mitchell, A.I., Noack, P.T. & Laird, L.M. (1997). Carotenoid pigment proﬁles distinguish anadromous and nonanadromous brown trout (Salmo trutta). Canadian Journal of Fisheries and Aquatic Sciences, 54, 1064–6. Zalewski, M., Frankiewicz, P. & Brewinska, B. (1985). The factors limiting growth and survival of brown trout, Salmo trutta m. fario L. introduced to different types of streams. Journal of Fish Biology, 27A, 59–73.

Chapter 34

Perspectives on Sea Trout Science and Management N.J. Milner1 , G.S. Harris2 , P. Gargan3 , M. Beveridge4 , M.G. Pawson5 , A. Walker6 and K. Whelan7 1

Address for correspondence: Environment Agency, C/O School of Biological Sciences, University of Bangor, Bangor LL57 2UW, Wales, UK 2 Fishskill, Greenacre, Bwlch, Brecon, Powys LD3 7PZ, Wales 3 Central Fisheries Board, Balnagowan, Mobhi Road, Dublin 9, Ireland 4 FRS Freshwater Laboratory, Faskally, Pitlochry, Scotland 5 CEFAS, Pakeﬁeld Road, Lowestoft, Suffolk, NR33 0HT, England 6 FRS Freshwater Laboratory, Faskally, Pitlochry, Scotland 7 Marine Institute, Furnace, Newport, Co Mayo, Ireland Abstract: The sea trout presents unique opportunities and problems for scientists and ﬁshery managers, because of its variety of life histories and occupancy of diverse marine, estuarine and freshwater habitats. This Symposium brought together much of the current knowledge and showed that sea trout stock assessment and ﬁshery management and underpinning science have advanced since the last large conference on this topic held 20 years ago. Sea trout have assumed greater signiﬁcance and economic value in recent years, but in parts of their natural range there are signiﬁcant concerns about the status of stocks that in some cases have suffered catastrophic or more gradual decline. Reasons for the decline are varied and include notably the impact of sea lice infection associated with marine salmon farming. Other problems are common to many anadromous ﬁsh, such as over ﬁshing, diffuse pollution, ﬂow regimes, access and ﬁsh passage, but complicated in sea trout by the possibility of response to environmental perturbation thorough shifts in life history strategy. Sea trout are the anadromous form of Salmo trutta, and the phenotypic plasticity in life history of the species is their dominant feature. The balance of genetic and environmental factors in determining the incidence of anadromy remains a central research question, but progress in this area is now being made as a result of improvements in genetic techniques and understanding. The Symposium made several recommendations for future management and research, particularly with respect to ecology and ﬁsheries in the sea (noting the opportunities for integration with freshwater studies), assessment methodologies and the basis of life history variation. Keywords: Sea trout, life-history variation, socio-economic importance, stock status, management developments, scientiﬁc advances, research needs.

Introduction Anadromous ﬁsh present particular challenges for scientists and managers. The sea trout exempliﬁes these perhaps more than any other ﬁsh species in the Northern Hemisphere

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because of the diversity in life-history strategy displayed across its extensive geographical range (see Ferguson, 2006; Jonsson & Jonsson, 2006). Fisheries protection, management and enhancement are difﬁcult in a species with such a plastic migratory habit without an adequate understanding of the factors affecting its life history. The sea trout’s occupation of freshwater, estuarine and marine habitats exposes it to environmental impacts and ﬁshing exploitation in all three. However, this characteristic of sea trout also brings opportunities. The successful completion of its life cycle requires good conditions in both freshwater and marine environments; thus the sea trout offers potential as a key sentinel ﬁsh species and as a link for integrating ecosystem studies across the freshwater–transitional–marine interfaces. This is in keeping with the trent of ﬁsheries and wider catchment management towards the ecosystem approach (Garcia et al., 2003, Water Framework Directive). A similar argument has been advanced for integrated studies on the scaenid spotted sea trout (Cynoscion nebulosus) in the eastern USA (Bortone, 2003). This variety of life histories also adds signiﬁcantly to the biodiversity of waters, including the many smaller coastal streams and tributaries which are home to juvenile sea trout, but which are often neglected by regulatory agencies. There are also ﬁsheries opportunities. As a natural resource, sea trout have socio-economic value that exceeds that of salmon in some areas. Moreover, in contrast to salmon, its main production takes place in local inshore waters where ﬁsheries and many environmental conditions have a reasonable prospect of being positively inﬂuenced by management actions. It is signiﬁcant that the Symposium attracted sponsorship and participatory support from a wide range of inﬂuential non-governmental bodies and government departments. Many groups including ﬁshermen, conservationists, ﬁsheries managers and scientists share the recognition of sea trout as the basis of valuable, sustainable ﬁsheries. It is essential to retain and develop this diversity of active interest. The determination of management goals and research priorities will be more effective if it is inclusive. All interested groups should be encouraged to participate and feel responsible for the results of managing stocks and ﬁsheries. This chapter summarises the principal messages from the Symposium and extracts some priorities for sea trout science and management, under three broad headings: (1) recognition of stock and ﬁshery status; (2) improved management of sea trout ﬁsheries and (3) future research to support better ﬁsheries.

Stock and ﬁshery status The status of sea trout stocks and ﬁsheries varies across its range according to the inﬂuence of local factors. In the western British Isles, where the coastal topography allows intensive marine salmon farming, sea trout stocks have collapsed or suffered dramatic declines because of the effect of sea lice infestations arising from their proximity to salmon farms (Butler & Walker 2006; McKibben et al., 2006; Poole et al., 2006). In the northern Baltic region over-exploitation in coastal ﬁsheries appears to have jeopardised stocks (Jutila et al., 2006) and in the Black Sea a combination of uncontrolled illegal ﬁshing in coastal waters and environmental problems in fresh water have brought stocks into the seriously ‘at risk’

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category (Okumus et al., 2006.). Elsewhere, sea trout stocks appear to be in a more healthy condition but, in common with Atlantic salmon, the potential for damaging effects of environmental factors is high. Many different types of sea trout ﬁsheries were described at the Symposium, but speakers from all countries reported the need for better data on catches, ﬁshing effort and exploitation rates determined at scales ranging from local to international. This was particularly so in the marine ﬁsheries where there are major gaps in knowledge, including the extent of illegal ﬁshing, whether targeted directly at sea trout or where they are taken as a by-catch in other types of ﬁshing. As so often in ﬁsheries, the regulation of sea trout catches and the underlying science are linked by the nature and quality of the monitoring programmes that service both routine stock assessment and the demand for data that is the raw material for much research. For sea trout in many countries, even the basic descriptions of population characteristics, stock structures and stability and ﬁshery trends are not as comprehensive as for salmon. While sea trout catch statistics are approaching adequacy in some parts, those for the freshwater brown trout are poor almost everywhere and if the two forms are to be managed as one, then signiﬁcant improvements in catch statistics for both are necessary. The Symposium showed that sea trout assessment is also constrained by limited availability of long time series of monitoring data, other than those provided by catches (e.g. electro-ﬁshing surveys, counters). The trade-offs between data costs and information value will be difﬁcult to negotiate, but if public stakeholders, policy makers and managers require robust scientiﬁc, risk-based decision-making they will have to make the appropriate investment.

Fisheries management for sea trout Considering ﬁsheries management, an obvious question is what is special about sea trout? Why should they be regarded and managed any differently from S. trutta as a whole? The taxonomist might say that they should not be, but that is to ignore some obvious and important practical differences that make sea trout very different from the freshwater trout for managers and ﬁshermen. From a biological perspective it is evident that anadromy is a threshold quantitative trait controlled by a combination of environment and genes (Ferguson, 2006). The sea-going habit confers large average size on returning spawners. Those trout populations dominated by the anadromous habit also have different freshwater age structures, within-catchment distributions and habitat linkages that differ from their freshwater relatives. Sympatry of the two forms is common, but most often it appears to be unbalanced through sex-selective migration favouring females. Genuine sympatry (as opposed to simply occurring in the same catchment) of abundant populations of both freshwater and anadromous form seems to be much less common, but we cannot be sure because systematic, extensive data are lacking. Fisheries for the migratory and resident forms often differ greatly in their locations and methods; consequently, legislation and regulatory controls are different. One could argue that such an approach is inconsistent with the taxonomy of S. trutta, and should be changed, but it does match many aspects of the biology, distribution and ﬁsheries of the

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two forms and so, pragmatically, the division is sensible. However, for the scientist and for science-based management the continuum is important and cannot be ignored. For example, to set biological reference points (BRPs), it will be necessary to know the contributions of both forms to total trout production, irrespective of whether the BRPs are based on juveniles (Walker et al., 2006), on adults as in salmon (e.g. Crozier et al., 2003) or on some combination of assessments. In the freshwater stage, the inability to distinguish anadromous from freshwater trout on morphological grounds means that carrying capacity, habitat models and effective population sizes will probably be based upon total S. trutta populations, perhaps incorporating age structure data. This may change with the development of tools to recognise the identities and location of anadromous and non-anadromous trout. But all this is speculative at this stage and is the subject of current research. However they are derived, BRPs and all sea trout stock assessments will require better consistency and standards of monitoring data for adults and juveniles. Habitat protection, restoration and enhancement in rivers have normally been applied in the interests of Atlantic salmon or non-migratory trout. Coincidentally, this may also meet sea trout requirements, because of their similarities in ecology, but this has not been tested. However, sea trout make greater use of small coastal streams and smaller tributaries than salmon for rearing (Baglinière & Maisse, 1999; Milner et al., 2006; Walker & Bayliss, 2006). Their size makes these waters often fragile and vulnerable to environmental pressures such as pollution of low ﬂows and they may require special designation and protection. Ferguson (2006) has outlined the implications of the genetic basis of anadromy noting that trout life-history strategy could shift with comparatively small environmental changes. The potential responsiveness of the migratory habit to freshwater environmental quality, operating through juvenile growth rate (Jonsson, 1985; Cucherousset et al., 2005; Jonsson & Jonsson, 2006) means that changes to productivity that may follow habitat alteration could affect sea trout in ways that are not yet understood or predictable. An undeveloped area of study that bears on this is the description and modelling of lifetime habitat usage by trout within catchments and how this might be inﬂuenced by the juxtaposition and connectivity of habitats at catchment scale. Similarly, the consequences of artiﬁcial stocking to restore or enhance sea trout stocks are not well understood. Stocking is an important ﬁshery management tool in some countries, especially those Baltic countries where loss of freshwater rearing areas makes it essential to support ﬁsheries artiﬁcially. However, its effectiveness in restoring natural populations has been questioned (e.g. Laikre, 1999; Jutila et al., 2006; Pedersen et al., 2006) and Lundqvist et al. (2006) have demonstrated some of the risks to wild stocks (see also Laikre, 1999; Ruzzante et al., 2004; Ferguson, 2006). In contrast, others have considered that the caution over stocking may have been over-played in some cases (e.g. Guyomard, 1999). Carefully planned and well-managed stocking has an essential role in stock restoration and conservation programmes, but in all cases it needs to be complemented by protection of resulting spawners through ﬁshery regulations (e.g. Jutila et al., 2006). The prior removal of limiting factors is also essential to success, for example the failure of sea trout stocking to restore stocks in some western Irish (Ferguson, 2006; Gargan et al., 2006) and Scottish rivers (Hay & McKibben, 2006) may have been because of the continuing problems of

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marine survival. The sea trout stocking debate brings ﬁsheries management hard up against the genetic/environment questions and, at the very least, a precautionary line needs to be adopted until the effectiveness and impacts of stocking are clearer. It is hoped that sound genetic principles will become incorporated into all future stocking programmes as has been the case in Denmark (Rasmussen, 2006). Signiﬁcant detrimental environmental factors reported by speakers from most countries were physical barriers to migration, reduced river ﬂows, siltation and nutrient enrichment through intensive agriculture, river habitat destruction and increased predation. Whilst the classical ﬁsh responses to environmental impacts (e.g. higher mortality, recruitment failure and contracted distribution) are to be expected in sea trout, the species is also capable of substantial changes in life-history patterns that could involve shifts between migratory and non-migratory behaviour, with enormous consequences for ﬁsheries. Most salmonid ﬁshery regulations in Europe have historically been implemented for the protection of salmon and non-migratory trout. But the case for a greater concern for sea trout has been recognised in some cases, and measures are in hand to reﬁne catch controls through various options such as bag limits, size and catch and release (Environment Agency, 2004; Butler, 2005; Harris, 2006). The use of catch and release or slot size limits to protect the larger, older multiple spawners seems to be of particular value for sea trout (Harris, 2006; Solomon & Czerwinski, 2006). The priorities for future sea trout management are the following: • •

• • • • •

protection of the smaller rivers, sub-catchments and streams generally favoured as spawning and nursery grounds by sea trout; better monitoring of catch and effort in all ﬁsheries in freshwater, estuarine and coastal zones; the inclusion of information on stock composition and ﬁshing effort is considered to be essential to complement and interpret basic catch data; better deﬁnition and implementation of regulations to prevent illegal ﬁshing or overﬁshing and to eliminate sea trout by-catch in coastal and estuarine ﬁsheries; modiﬁcation of catch regulations to afford better protection to the larger adult female sea trout; adoption of a precautionary approach for all trout stocking programmes; continuing effective control and management of sea lice on marine salmon farms to enable sea trout ﬁshery recovery in affected areas and provision of integrated scientiﬁc advice that takes account of other species of ﬁsh and a wide range of ecosystem components.

Progress in sea trout science In spite of decades of research on S. trutta, there are still many unresolved questions about the ecology, life-history strategies and population dynamics of the migratory form. The central role of anadromy and causes of its variation remain dominant issues and the determining inﬂuence of genetics has been well reviewed and discussed at the Symposium (Bruford, 2006; Ferguson, 2006). New methods and technologies are enabling better description of life-history patterns (e.g. Cucherousset et al., 2005), direct exploration of the role of the

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genome in phenotypic variation (Bruford, 2006; Ferguson, 2006; Giger et al., 2006) and in developing theories about the basis of anadromy. The population dynamics of S. trutta have been extensively studied in many countries and there are many key papers, reviews and books (e.g. Allen, 1951; Frost & Brown, 1957; Mills, 1971; Le Cren, 1973; Elliott, 1994; Baglinière & Maisse, 1999; Crisp, 1999). Perhaps the classic study has been that of Elliott & Elliot (2006), who demonstrated the great value of long-term data sets. Elliott’s seminal research was conducted on a very small stream with adult sea trout of limited sea-age diversity and few other competing species. While that does not detract from the insights it reveals into processes, there is much more to be discovered about population dynamics in different stream types where habitats will be more complex, the range of life-history strategies may be greater, ecosystem complexity is higher and multi-species interactions operate. Rasmussen (2006), Milner et al. (2006) and others have noted that, while electro-ﬁshing sampling (the commonest freshwater survey method) may be extensive, it is mostly conﬁned to smaller streams (e.g. <10 m width) and less is known about trout population dynamics in the larger channels. Whole catchment scale stock–recruitment data are still very sparse. At this Symposium progress in catchment-scale stock–recruitment studies was reported from two programmes, the Burrishoole in western Ireland (Poole et al., 2006), a lake–river system, and the Bresle in Normandy, France (Euzenat et al., 2006), a river only system. In both cases, the authors drew attention to the confounding effects of non-anadromous trout and interspeciﬁc interactions, principally involving salmon. The Burrishoole stock–recruitment relationship (Poole et al., 2006) suggested that the production of smolts, or juvenile recruits, was closely related to migratory trout egg deposition and largely independent of the freshwater trout production, supporting the hypothesis that in that catchment the propensity for marine migration is under strong genetic control. Elliott & Elliott (2006) drew attention to the risks in the use of stock–recruitment relationships for setting BRPs for management (see also Hilborn & Walters, 1992). As yet these case studies, from very different types of catchment, cannot be transposed to sea trout populations in general. BRP development, which is usually based on stock–recruitment relationships for salmon, will be constrained in the case of trout by this information gap and alternative or complementary methods will have to be explored (Walker et al., 2006). Sea trout growth in the sea is largely dependent upon water temperature and the abundance of food species. There is ample evidence of changes in coastal marine ecosystem composition that may be attributable to combinations of localised environmental pressures and wider climate change. The marine ecology of sea trout has still not received the attention it needs, but little that was new was revealed at the Symposium and this remains a priority topic. Sampling difﬁculties and costs are obvious constraints on such work, combined with the perceived lack of signiﬁcant mixed stock ﬁsheries that require international collaboration, as for salmon in recent years (Mills, 2003). There may be some exceptions to this, especially in the Baltic and possibly in the Irish Sea, but collaborative research is still unusual for sea trout even though it is the most practicable way to resource such studies. However, marine phase studies will be essential if science-based management is to be informed and thus effective.

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Climate change and freshwater productivity seem to be altering freshwater growth and smolt age (Davidson et al., 2006), which in turn may affect marine survival, growth and age at maturation and thus the age structure of the adult stock and its ﬁshery value. Gradually, knowledge of the distribution of spawning and rearing within rivers is accumulating (e.g. Walker & Bayliss, 2006), but there are still signiﬁcant gaps in understanding the functional links across population structure, habitat variation and connectivity. The use of genetic and isotopic methods to better describe the spawning distribution of anadromous trout and dispersal of their progeny in catchments offers great potential in this area. Spatial variation in environmental quality and accessibility to different catchment locations would be expected to inﬂuence the degree of anadromy shown by trout and temporal changes in such features may lead to changes in stock structure. In a European context, such understanding will be of value in interpreting monitoring data for purposes of the Water Framework Directive and in optimising programmes of measures.

Future research needs Considerable progress has been made in several scientiﬁc ﬁelds enabling advances in sea trout research that were previously impossible or impracticable. The Symposium presented exciting new developments in genetics, genomics, population dynamics, the statistical and ecological basis of BRPs and in Geographic Information Systems. These could provide a scientiﬁcally robust framework for practical assessment, environmental management (tuned to sea trout), modelling and decision making. The major strategic priorities for further research and investigation were identiﬁed as follows: •

• •

•

•

• • •

to explore how genetic and environmental factors interact in determining the incidence of the sea-going migratory habit – Ferguson (2006) discusses the genetic research needs in more detail; to develop an understanding of the stock–recruitment processes in trout, including interactions with other species, particularly salmon, in a wide range of stream types; to gain a better description and understanding of trout life tables, stock structures and life-history strategies in different geographical regions and to determine the temporal stability in stock structure and composition; to understand the genetic and ecological consequences of stocking on anadromy in trout and to develop and promote stocking practices that are protective of wild sea trout and are operationally effective; to improve the relationships among habitat availability, quality and trout production and life histories, which needs to consider catchment-scale connectivity of habitats, spawning distribution and population dispersal; to develop and implement BRP-based assessment and appropriate catchment management; to reﬁne estimates of the socio-economic value of sea trout and brown trout ﬁsheries and their potential for sustainable development; to investigate the distribution, movements and feeding migrations of post-smolts and adults in estuarine and coastal waters and to examine the inﬂuence of marine

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environmental factors on sea trout growth, maturation and survival in fresh water and the sea and to understand better the effects of ﬁshing on sea trout stocks, particularly the impacts of selective exploitation on the larger (mainly female) ﬁsh.

Much of this work will require the commissioning and support of long-term studies on trout population dynamics, genetics and ecology and this should be integrated with catchment-scale, multi-disciplinary ecosystem studies. Such large-scale research is necessarily collaborative. The Symposium noted with regret that similar recommendations from previous meetings in 1984 (Le Cren, 1985) and 1994 (ICES, 1994) had not been progressed. In a wider European context, it was noted that our limited understanding of fundamental sea trout biology is still too dependent on very few facilities where these longterm studies have taken place. When dealing with a species that exhibits such ﬂexible and variable life history and behaviour as the trout, the risks posed by extrapolating data among regions are even greater than that for the Atlantic salmon.

Concluding remarks This Symposium has brought together many of the current perspectives and much of the knowledge on sea trout across its geographical range. In the British Isles there has been signiﬁcant progress since previous meetings in terms of the sea trout’s higher proﬁle, biological understanding and the infrastructure of stock assessment and management. However, much of this has been on the back of progress for other species especially salmon, or as a result of reactive research following stock collapse and there is still much to do. However, the situation is less satisfactory in some other countries and it therefore behoves scientists, managers and stakeholders to work together to support sea trout management across its range. Overall, discussion at the Symposium conveyed the feeling of being at a watershed in scientiﬁc understanding and application to sea trout ﬁsheries management and in that sense much of the activity outlined in these proceedings represents work in progress. Investment permitting, the next few years should see major advances in biology, with genetics and genomics being key techniques, coupled with an extension into integrated ecological and life-history studies across freshwater and marine environments. This must lead to more informed, sustainable management and better protection of the species.

References Allen, K.R. (1951). The Horokiwi stream. Bulletin of the Marine Department of New Zealand Fisheries, 10, 1–231. Baglinière, J.-L. & Maisse, G. (Eds) (1999). Biology and Ecology of the Brown Trout and Sea Trout. Springer–Praxis Series in Aquaculture and Fisheries, 286 pp. Bortone, S.A. (Ed.) (2003). Biology of the Spotted Sea Trout. Marine Biology Series. CRC Press, Boca Raton, FL, 313 pp. Bruford, M.W. (2006). Applications and future prospects for genetic analysis in salmonid biology. In: Sea Trout: Biology, Conservation and Management. (Harris, G.S. & Milner, N.J., Eds). Proceedings of First International Sea Trout Symposium, July 2004, Cardiff, Blackwell Publishing, Oxford, pp. 248–56.

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ICES (1994). Review of the study group on anadromous trout. Trondheim, Norway, 29–31 August 1994. ICES C.M. 1994/M:4, 80 pp. Jonsson, B. & Jonsson, N. (2006). Life histories of sea trout. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 196–223. Jutila, E., Saura, A., Kallio-Nyberg, I., Huhmarniemi, A. & Romakkaniemi, A. (2006). The status of ﬁshing of sea trout on the Finnish coast of the Gulf of Bothnia in the Baltic sea. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 128–38. Laikre, L. (1999). Conservation and genetic management of brown trout (Salmo trutta) in Europe. Report of the concerted action on identiﬁcation, management and exploitation of genetic resources in the brown trout (Salmo trutta) (‘Trout Concert’ Eu Fair CT97-3882). Le Cren, E.D. (1973). The population dynamics of young trout (Salmo trutta) in relation to density and territorial behaviour. Rapport et Procés-Verbause des Réunions. Conseil International pour l’Exploration de la Mer, 164, 241–46. Le Cren, E.D. (1985). Sea Trout. Proceedings of the Atlantic Salmon Trust Workshop, Plas y Brenin, 1984. Lundqvist, H., McKinnell, S.M., Jonsson, S. & Östergren, J. (2006). Are reared anadromous brown trout compatible with the conservation of wild trout? In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 356–71. McKibben, M.A., Hay, D.W., Walker, A.F. & Northcott, S.J. (2006). Sea lice Lepeophtheirus salmonis infestations of post-smolts in Loch Shieldaig, Wester Ross: 1999–2003. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 372–76. Mills, D. (1971). Salmon and Trout: A Resource, Its Ecology, Conservation and Management. Oliver and Boyd, Edinburgh, 351 pp. Mills, D. (2003). The Ocean Life of the Salmon, Environmental and Biological Factors Inﬂuencing Survival. Atlantic Salmon Trust, Fishing News Books, Blackwell Science, Pitlochry, 228 pp. Milner, N.J., Karlsson, L., Degerman, E., Jholander, A., MacLean, J.C. & Hansen, L.-P. (2006). Sea trout (Salmo trutta L.) in Atlantic salmon (Salmo salar L.) rivers in Scandinavia and Europe. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 139–56. Okumus, I., Temel, S. & Atasaral, S. (2006). General overview of Turkish sea trout (Salmo trutta) populations. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 115–27. Pedersen, S., Christiansen, R. & Glüssing, H. (2006). Comparison of survival, migration and growth in wild, offspring from wild (F1) and domesticated sea run trout (Salmo trutta L.). In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 377–88. Poole, R., Dillane, M., deEyto, E., Rogan, G., McGinnity, P. & Whelan, K. (2006). Characteristics of the Burrishoole sea trout population: census, marine survival, enhancement and stock recruitment, 1971–2003. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 279–306. Rasmussen, G.H. (2006). Research activities and management of brown trout (Salmo trutta L.) in Denmark. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 342–48. Ruzzante, D.E., Hansen, M.M., Meldrup, D. & Ebert, K.M. (2004). Stocking impact and migration pattern in an anadromous brown trout (Salmo trutta) complex: where have all the stocked spawning sea trout gone? Molecular Ecology, 1113, 1433–55. Solomon, D. & Czerwinski, M. (2006). Catch and release, net ﬁshing and sea trout ﬁsheries management. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 434–40.

490

Sea Trout

Walker, A.M. & Bayliss, B.D. (2006). The spawning requirements of sea trout: a multiscale approach. In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 327–41. Walker, A.M., Pawson M.G. & Potter, E.C.E. (2006). Sea trout ﬁshery management: should we follow the salmon? In: Sea Trout: Biology, Conservation and Management (Harris, G.S. & Milner, N.J., Eds). Proceedings of the First International Sea Trout Symposium, July 2004, Cardiff, Wales, UK. Blackwell Publishing, Oxford, pp. 466–79.

Declaration 1. Sea trout stocks are apparently healthy in some regions, but in others there have been major collapses. The Symposium warned that continued neglect of the science and management of this species, coupled with increasing expansion of its ﬁsheries could threaten yet another valuable natural resource. 2. The Symposium concluded that governments, state agencies, ﬁshery managers and other stakeholders should no longer take this resource for granted and strongly recommended that immediate action be taken to protect and conserve sea trout stocks throughout their geographical range. 3. The Symposium noted the conservation value of sea trout. The exceptional variety of trout life histories and habitat use adds signiﬁcantly to the biodiversity of many types of waters. In the case of sea trout this includes rivers, lakes, estuaries, coastal waters and a huge network of otherwise neglected small coastal streams. Sea trout populations are thus particularly valuable in assessing ecosystem health in the context of the Water Framework Directive. 4. Sea trout ﬁsheries offer socio-economic beneﬁts that may be more widely distributed and in total may be greater than salmon, but studies and development of these have been largely ignored. It is crucial that research in this area is commissioned to encourage and inform sustainable development to maximise these beneﬁts. 5. Fishery management regulations for sea trout are poorly formulated and inadequately protective in some respects. There is a need to monitor more closely catches in all ﬁsheries. There is a need to better control ﬁshing in some licensed salmonid ﬁsheries, to eliminate illegal ﬁshing and sea trout by-catch in other coastal and estuarine ﬁsheries, and to more effectively control the genetic and ecological risks of stocking and in particular the impacts of parasitic infestation from marine aquaculture. 6. The Symposium noted the exciting new developments in sea trout related science. Genetics, population dynamics, the statistical and ecological basis of biological reference points and geographic information systems all offer potentially powerful and cost-effective management tools. But funding for research is quite inadequate. In particular, commitment is needed to support long-term, integrated ecosystembased freshwater and marine studies, incorporating ecology and genetics, to improve understanding of the sea-going migratory habit in trout. Nigel Milner & Graeme Harris Symposium Convenors 26th August 2004.

491

Index

Aberdeenshire Don R., x adaptive traits, 389, 398–9 adult life-tables, 95, 97–8 return rates, 315, 319–20, 353–4 size, 310, 353 survival, 435–6 age and growth, 76–87, 197–201, 280 age notation, 76, 91 anadromy ancestral state, 158 components of, 159–60 continuum of, 20, 159, 196, 224, 250 criteria for, 11 environmental control of, 163, 202–5, 229–30 genetic control of, 20, 72, 163–6, 196, 199, 202–5, 230–1, 248–9, 303, 485 threshold quantitative trait, 72, 157, 163, 170, 184, 230–1, 482 Angermasälven R., 361, 366 angling appeal, 2–3 associations, xiv code of conduct, xvi club membership, 461–3 expenditure, 464 guide, 463 literature, 1–3 aquaculture salmon farms, 26, 45–7, 54, 70, 73, 342, 300, 350, 372–5 sea trout ranching, 125 Arctic charr (Salvelinus alpinus), 160, 197, 200, 398 Argentina, 437 ascent to spawning grounds, 339 Atlantic salmon (Salmo salar), 13, 20, 139–51, 159, 200–1, 244, 248, 252, 258, 274, 280, 311, 318, 330, 343, 357, 401–16, 441, 443, 449, 458–9 autumn parr migration, 7, 290–1 Azov Sea, 115

bag limits see catch limits Baltic Sea, 128–37, 159, 356–69, 377, 382, 481, 483 Beaulieu R., 329 biodiversity, 399, 442, 480

Biological Reference Points (BRPs), xviii, 7, 113, 301, 320, 327–8, 431, 451, 466–75, 482 Black Brows Beck, 200, 257–76, 308 Black Sea, 115–26, 481 Black Sea trout (Salmo trutta, S. trutta labrax), 115–26 Border Esk R., 90, 92–100, 102–3 Bothnia, Gulf of, 128–37, 358–9 Bresle R., 307–21, 485 Britain & Ireland, 157–76 broodstock collection, 349–55, 358 brook charr (Salvelinus fontinalis), 160 brown (resident) trout, 1, 6, 26–7, 55–6, 119, 129, 300–1, 303–4, 312, 318, 327, 344, 346, 352, 389–99, 446, 481 contribution to sea trout recruitment, 71–2, 300, 303–4 replacing sea trout, 56 see also freshwater trout BRPs see Biological Reference Points Bulgaria, 116 Burrishoole System, 25–42, 227–8, 279–304, 308, 485 by-catch, 136 by-law restrictions, 444 Camel R., 90, 92–100, 453 capital value see owner value captive-release stocking, 355 carotenoid pigments, 199 carrying capacity, 272, 343, 378 Caspian Sea, 115 catch factors affecting, 54 improvement, 438, 440 limits, 438, 446–7, 452–4, 483 logbook schemes, 27, 37 net, xviii, 36–8, 107–13, 133–4, 417, 459 records, 7, 92, 129, 283, 418–19, 438, 440, 442–3, 451, 481 relationships, 298, 423–7, 431 return rates, 419 rod, xiii, 25–42, 47–9, 64, 90, 92, 107–13, 331, 285–8, 417–31, 434–40, 459–61 rod and net, xvii, 320 success, 428, 430 undeclared, 428

493

494

Index

catch-and-release, xvi, 101–2, 108, 139–44, 148, 283, 419, 431, 434–40, 447, 483–4 catchment-scale studies, 484 catch-per-unit-effort, 27, 36–7, 431 Caucasus, 116 channel size, relationship with catchment size, 147–9 Chile, 438 climate change, xx, 6, 54, 216, 346, 401–16, 485 close seasons, 444–5, 447 Clwyd R., 90, 92–100 collaborative studies, 320, 485 colonisation, 249, 389–99 post-glacial, 157–8, 197 commercial ﬁshing see net ﬁshing competition, 7, 258 interference, 150 inter-speciﬁc, 139–51, 320 intra-speciﬁc, 43, 139–40, 150–2, 303, 320 shadow, 258 condition factor (K), 243, 453 Connemara, 25–42, 60–73 conservation byelaw, 283, 445–51 Conservation Limits see Biological Reference Points conservation measures, 356–69, 399, 441–55 brown trout, 446 kelts, 444–5 large specimen ﬁsh, 450–1, 453 MSW maidens, 447, 453 multiple spawners, 449–50 parr and smolts, 445 whitling, 447–9, 453 Conwy R., 151 Coquet R., 90, 92–101, 453 coracle nets, 107, 458–9 CPUE see catch-per-unit-effort Crimea, 116 customer satisfaction, 461 cutthroat trout (Onchorhynchus Clarki), 11–21

Danish Strait, 382 Dee R., (Wales), 76–87, 91–7, 98–101, 104, 141, 145–6, 308, 402, 426 deﬁnition of ‘sea trout’, 5, 157–82 Delphi Fishery, 2, 25–42 Denmark 149, 166, 342–6, 377–87 density, 258, 262–3, 265–6, 272 density dependence, 151, 200, 211, 262–5, 268, 275–6, 280, 302, 319 disease, 2, 45 distribution of sea trout, 140, 196, 205

domestication, 171, 346 DNA analysis, 346 see also molecular markers droughts, 261, 266, 272 Dwyfor R., 90, 92–100, 104 Dyﬁ R., xiv, 90, 92–100, 107, 450 Earn R., 389–99 economic values, 108, 458, 460–1, 464, 480 ecosystem approach, 480 Eden R., 413 eel (Anguilla anguilla), 46, 343 effective population size, 164, 169, 250–1, 346 egg density, 26–64, 258, 271, 319 deposition, 52, 55, 71, 139–52, 280, 283–4, 293, 296, 312, 315, 397, 437, 440 deposition targets, xvi, 302 mean lifetime production (MLEP), 4, 227–9 size, 204, 206–7, 238 egg-smolt survival, 313, 315–16, 319 electro-ﬁshing, 129–31, 139–41, 344, 484 emergence time, 207, 258 energy dynamics, 200–7 England & Wales, xvii–xx, 1–7, 76–87, 88–105, 107–14, 141–8, 401–16, 417–31, 441–55, 457–64 enhancement programmes, 282–3, 300 Environment Agency, xvii–xx, 107, 443 environmental threats, xx, 26, 104, 123, 131, 342, 368–9, 377–8, 483 equilibrium density, 271–4 Erriff R., 25–42 estuary barrages, xx migration, 345 Europe, 139–52, 158 evolution, ix, 157 of migratory strategies, 161–3 Ewe R., 2, 45–56 existence value, 457–8 exploitation, 417–31 effects of stocking on, 369 impacts, 434–5 modelling, 435–7 rates, 420–3, 435–6 rods, 101 selective exploitation, 170–1, 384, 428–9, 430–7, 451, 486 Falklands, 231 fecundity, 226–7, 237–8, 283, 310, 450 feeding studies, 345, 451 Findhu Glen Burn, 225–6, 231, 389–99 Finland, 128–37

Index ﬁnnock, 15, 40–41, 45–56, 60, 70–1, 238, 240, 282–3, 285–6, 288, 292, 294, 310, 351–4, 393 see also whitling ﬁsh counters, 7, 331, 418, 428 passes, 136, 342, 345, 358, 364 ﬁshery closure, 122, 439 Fishery Research Services, 47, 350 ﬁshing effort, 443–4 licence sales, 444 mortality, 434–5 regulations, 123, 136–7, 359, 368, 438–40, 441–55, 444, 455, 483 Fishing Wales Programme, xv, 463 ﬁshless streams, 389–99 Fishmongers Company, 458 ﬁtness, 203, 205, 208, 212, 224, 350–5 ﬂesh value, 458–9 ﬂuorescent pigment, 391 Fowey R., 418 France, 307–21 freshwater trout, 158–9, 164 see also brown (resident) trout fry density, 258 fyke netting, 133–4

Galicia, 234–5 gene expression, 183–93, 252–3 ﬂow, 165–7, 249, 251 genetic control of migration, 161–4, 303, 320 biodiversity, 304, 357, 369, 442 differentiation, 164–6, 212, 249 heritability, 164, 170 heterogeneity, 166–8, 249 integrity, 345–6, 354, 358, 386 introgression, 123, 250, 357, 366 isolation, 25–7, 158, 163 markers see molecular markers races, 5, 7, 103 research, 175–6, 345–6 selection, 165 structuring, 118–19, 158 genomics, 183–93, 252 Georgia Fisheries Research Institute of, 116 gill nets, 133–4, 136 GIS (Geographic Information System), 332, 340 global warming see climate change grayling (Thymallus thymallus), 343 gravel size selection, 105, 202 growth, 203, 361–5, 380, 384, 386 back-calculated, 405–6 freshwater, 76, 78, 204, 207–8 marine, 47–51, 76, 84–7, 121–2, 485

495

models, 403–5, 411 prediction, 401–16 temperature effects on, 207–8 haaf nets, 459 habitat availability, 150–1, 311, 319 constraints, 211–12, 483 degradation see environmental threats improvement, 7 requirements, 134, 344, 485 restoration, 135, 482 Habitats Directive, 7 heritage value, 459 historical neglect, 1–7 homing, 345, 377, 383–5 hybridisation, 169–70 hydro-electric power schemes, 123, 342, 358 Ice Age, 25, 158 Iceland, 149 ICES, xi illegal & inadvertent capture, 122–4, 451, 481 Indalsälven R., 366 indigenous stocks, 346 interbreeding, 346 inter-generation ﬂuctuations, 272–6 Intergovernmental Panel on Climate Change, 401–2 introduced stocks, 438 Invermore R., 25–42, 60–73 Ireland, 25–43, 60–73, 141–8, 157–76, 279–306 Isojoki R., 129–30 isotopic analysis, 251 Karup R., 378 Kattegat, 345, 377 kelt conservation, 444–5, 450 migration, 243–4, 246, 345 Kent R., 90, 92–100, 328, 418 key factor analysis, 266 key feature analysis, 96, 99, 102 land-use regulations, 344 legislation, xiv, 389, 398 legislative procedures, 454–5 length see size length-at-age, 406–8 Lepeophtheirus salmonis see sea lice Lestijoki R., 129–30 licence sales, 417–18, 460 life cycle, ix–x, 4, 18, 119, 159, 184, 196–216, 224–32, 249, 259, 280, 310–11, 344, 394

496

Index

lifetime success, 227–9, 350 Llandysul Angling Association, 461–2 local community beneﬁts, 460 longevity, 210–11, 450 long-term studies, 76, 257–76, 279–304, 307–21, 344, 481–2, 486 Lune R., 90, 92–100, 141, 145–6, 402, 418

macrophytes, 377 maiden ﬁsh, 449–51 sea age, 94–5 management advantages, 4–5 aims, xvii–xx, 103–5, 280, 308, 418, 442, 481 priorities, 484 programmes, 123–6, 342–6, 357, 368–9 strategies, 124, 441–2, 451 Maree L. see Ewe R. marine feeding, marine, 54, 121, 200, 203, 450 movements, 5, 7, 245, 361, 365, 377–87 phase, 345, 485 protection zones, 369 survival, 41–3, 53, 55–6, 60–73, 285, 303, 377–87 return rate, 40–1 marketing, 461, 463 mark-recapture, 77, 378, 396 maturation, 82–6, 119–21, 209–10, 213–14 conﬂict with migration, 163–4, 202–3 genetic basis of, 163 maximum spawning marks, 449–50 Mean Smolt Age (MSA), 96, 245–6, 319 Melindwr R., 328 metapopulations, 366 microarrays, 183–93 microhabitat, 328–30, 332, 337 migration costs and beneﬁts, 203, 205, 212, 229–32, 249 distance, 121, 133, 239, 393 downstream, 38–9, 285–91 Onchorhynchus spp, 12–13 partial, 184, 196–7, 204 performance, 345 routes, 121, 381–2 strategy, 231–2, 275 upstream, 38–9, 240–2 migratory behaviour, 5, 7, 211–12, 377–87, 451 minor streams, 4, 103 misidentiﬁcation with brown trout, 452–3 mitigation schemes, 358 mixed stocks, xv, 7, 357, 369 model ﬁtting, 310 modelling errors, 338 molecular markers, 164–8, 183–93, 251

monitoring programmes, 124, 308, 320, 343, 437, 481 morphs, 56, 197–202, 250 mortality rate, 205, 214 multiple spawning see repeat spawning multi-scale models, 332, 340 National Assembly for Wales, xiii National Rivers Authority, 76 National Sea Trout R & D Programme, 88–9 nature-v-nurture, xviii, 5, 320 Nautilus Report, xiv, 108, 461 net buy-out, xv net ﬁshery value, 458 net ﬁshing, xiii, 133–4, 231, 417–31, 434–40 net limitation orders, 444 niche separation, 150–1 night ﬁshing, 2, 431 nomenclature, xi, 158–9 non-catch ﬁshing mortality, 451 North Atlantic Oscillation, 258 North Atlantic Salmon Conservation Organisation (NASCO), 369 North Esk R., 308 North Sea, 345 Norway, 141–8, 168 nutrient enrichment, 56 Objective 1 programme, 463 Onchorynchus spp., 11–21, 258 comparisons with Salmo trutta, 20–1 origin of broodstock, 350–5 osmoregulation, 54, 120–1 overﬁshing, 434 Owengowla R., 25–42, 60–73 owner value, 460 Paciﬁc salmon (see Onchorhynchus) Patagonia, 231 phenotypic plasticity, 20, 202, 250 polymorphism, 196–207 population dynamics, 274, 307–21, 343, 484 extinction 272–5 loss rates, 26–7 models, 397–8 size, 395–6 structure, 40, 285–6, 392–3 see also stock structure post-glacial colonisation, 103, 158 precautionary approach, 368–9, 452 predation, 274–5, 343, 345, 354, 367

Index previous spawners see repeat spawners private ownership, 455

quantitative trait variation, 168–9

races, 103 rainbow trout (Onchorhynchus mykiss), 18–20, 159, 165, 168 see also steelhead reaction norms, 203 redd, 327–8 counting errors, 338 counts, 258–9, 261 density relationship, 335–6 dimensions, 202, 328–30 habitat, 331–3, 335–7 over-cutting, 196, 202 substrate, 330 release location, 359–64 repeat spawning, 238, 382–3, 394, 434, 439 replacement scales, 439 reproductive characters, 205–7 research needs, 6–7, 175–6, 193, 215–16, 249, 301, 339, 346, 485–6 programmes, 342–6 resident trout see brown trout restoration stocking, 349–50 Ribble R., 90, 92–100 Rio Grande R., 437 river temperatures, 209, 215, 402–3 river trusts, xix Romania, 116 run estimates, 76–9, 309, 311–12, 314, 331, 418 timing, 51–3, 78–9, 240–2, 385 rural economies, 461–4

sale of rod caught ﬁsh, 454 SALMODEL, 140, 149 Salmon Advisory Committee, 350 Salmon & Freshwater Fisheries Act (1923), (1975), 443 Salmon & Freshwater Fisheries Review, xiv Salmon Research Agency, 282 Salmon Strategy for England & Wales, 441 Salvelinus spp, 17 sampling electro-ﬁshing, 259, 343, 350, 379, 391 fyke nets, 354 netting, 91, 259 rod catch, 91 scales, 47, 78, 89–91

497

tagging, 308–9, 314 trapping, 27–29, 38–40, 48, 61–8, 76–8, 91, 235–7, 259, 282, 308–9, 349, 379, 418, 436 scale reading, 47, 78, 88–92, 283, 310, 405, 435, 439, 450 Scandinavia, 140 Scotland, 45–57, 142–8, 167 sea-age structure, 93–4, 283, 295, 310, 312, 395 sea lice (Lepeophtheirus salmonis), 26, 45–56, 60–2, 73, 231, 237, 300, 354, 372–5 infection levels, 372–5 post-smolt infestation, 372–5 Slice treatment, 374 variation in resistance to, 199 sea spawning, 211 Sea Trout Task Force (Ireland), 26, 42 Sea Trout Working Group (Ireland), 26 secondary sexual characters, 197 sediment impacts see environmental threats selection pressures, 346 selective heritable trait, 303–4 sewin, xiii sex ratio, 237, 283, 310, 360, 364, 392–3, 395–6 sexual dimorphism, 199, 212–15, 226 Sheildaig L., 349–55, 372–5 size distribution, 40, 42, 49–50, 84–5, 121, 242, 245, 285, 288, 391–4 maximum limit, 452–3 minimum limit 445–6 slot-limit, 454 slob trout, 301 smelt (Osmerus eperlanus), ix smolt age groups, 64, 92–3, 209, 238–9, 244, 288–91, 310, 312, 445 density, 258 identiﬁcation, 310, 318 migration, 119, 239, 343, 344, 357 output, 38, 64–9, 134, 288–91, 295, 319, 351–2 recaptures location, 381, 383–5 release location, 381, 383–5 returns, 40–1, 372–5, 373 size, 64–6, 208–9, 238–9, 244, 310, 312, 319, 351–2, 359, 368, 384, 445–6 survival, 69–70, 286–8, 291–2, 303, 313, 316, 349–55, 368, 381–7 threshold size, 208 traps, 344 smoltiﬁcation, 345 genetic basis of, 183–93 physiological adaptation 160–1, 184–5, 198–201 socio-economic importance, xvii, 3 values, x, xiv–xv, 442, 457–64

498

Index

Spain, 149, 166, 234–46 spawning, 201–2, 283 altitude, 212 behaviour, 17–18, 119–20, 201–2, 214–15, 396–7 distribution, 331–2, 334–6 escapement, 283, 285, 293, 357 frequency, 95–6, 382–3, 449–50 gravel restoration, 340 habitat, 17–18, 134–5, 327–40 migration, 50, 54, 119–20, 357 post-spawning mortality in Onchorhynchus species, 18–19 post-spawning survival in sea trout, 80–2 runs, 119, 238, 310–11 segregation by size, 202 sites see redds targets see Biological Reference Points, 271–2 speciation, of trout, 157–8 specimen ﬁsh, 450–1 sportﬁshing, 441 steelhead see also rainbow trout, 12–20 stock census, 279–304 collapse, 3, 26, 42, 45–56, 60–73, 280, 298, 350, 441 descriptions, 88–105 groupings, 100–2 production, 280 stock status, xiv, 97–8, 104, 357, 452, 481 structure, 42, 76– 87, 88–105, 135, 439, 449 stocking, 6, 68, 125, 129, 132, 136, 157, 199, 203, 291–2, 300, 343, 345, 349–55, 356–69, 377–87, 389–99 comparisons, 351–4, 378, 381–7 genetic impacts of, 171–5, 249, 483 policies, 125, 343–4, 358, 378, 483 saltwater, 343 web site, 343 stock recruitment, 149, 292–7, 300–1, 308 stock-recruitment models, 257–76 assumptions, predictions, limitations, 152, 268–75 Beverton and Holt model, 268–70, 284, 295, 298–300, 302, 310, 315, 318 Cushing model, 268–70 habitat-based model, 275–6 Ricker model, 262, 268–70, 284, 295, 298–300, 302, 310, 315, 318 stock-recruitment relationship, 257–76, 297–8, 310, 315, 319, 485 straying, 212, 345, 361, 366, 396 strontium, 199 Sweden, 141–8, 166, 205, 356–69

Swedish Electro-ﬁshing Register, 141 survival rates, 280, 349–55 sustainability, xx, 442, 452 Sustainable Fisheries Programme, 462–3 sympatry, 140, 164–5, 199, 204–6, 251, 482 Taf R., 447, 459 Taff R., 458 tagging & marking acoustic, 344 Carlin tags, 129, 344, 358–9, 367, 384 data storage, 344 ﬁn clipping, 309, 350–1, 358 Floy tags, 239 ﬂuorescent spray branding, 389, 391 opercular marks, 309–10 PIT tags, 250, 351, 372 VI tags, 78, 351, 372 tag recovery rates, 134, 359–64, 367–8 Tamar R., 90, 92–100, 141, 145–6, 418, 453 Taw R., 90, 92–100, 102, 439 Tawnyard L., 25–42 Tay R., 225–6 taxonomy, 118 Teiﬁ R., 90, 92–100, 447, 459, 461 Teign, R., 90, 92–100 telemetry, 343–5, 357 temperature, 401–16 effect on growth (experimental), 208 effect on longevity, 210 trends, 405–6 tetraploidy, ix Thames R., 107, 402, 458 T & J nets, 459 Tierra del Fuego, 437 Tornionki, R., 129–30 Torridge R., 439 total allowable catch (TAC), 308 tourism, 26, 42, 438–9, 459, 461–4 tracking studies, 345 trap efﬁciency, 315 Trout & Grayling Strategy for England & Wales, 441 trout stocks, 282 Tummel R., 225–6 Turkey, 115–26 Tweed R., 102, 225–6, 435 Tyne R., 107 Tywi, R., xiv, 90, 92–100, 102, 107, 141, 145–6, 447, 450, 459 Ulcerative Dermal Necrosis (UDN), 2 Ulla, R., 149, 234–46 Umeälven R., 356–69

Index unfed fry, 391 UK Climate Impacts Programme (UKCIP), 401–2 upstream migration, 284–5 Vindelälven R., 356–69 voluntary rules & regulations, xv, 455 Wales, xiii–xvi Wales Tourist Board, 461 Water Framework Directive, xvii, 7, 485 Wear R., 90, 92–101, 107 weed clearance, 342

weight distribution see size distribution weight estimates, 360 Welsh Assembly Government, xiii–xv Wester Ross, 46, 372–5 wetland restoration, 345 whitling, 15, 26, 76–86, 447–9, 452 maturation of, 82–5 see also ﬁnnock wild-versus-stocked ﬁsh survival, 349–55 winter residence, 345 Wye R., 402

Yorkshire Esk R., 435

499