LEPIDORHOMBUS WHIFFIAGONIS (Walbaum, 1792) from OFF the WEST COAST of IRELAND
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AGE, GROWTH, REPRODUCTIVE BIOLOGY AND POPULATION DYNAMICS OF THE COMMON MEGRIM LEPIDORHOMBUS WHIFFIAGONIS (Walbaum, 1792) FROM OFF THE WEST COAST OF IRELAND By Stephen M. Robson B.Sc. (Hons.) Degree of Doctor of Philosophy in Fisheries Biology Galway-Mayo Institute of Technology Supervisor of Research Dr. Pauline A. King Submitted to the Higher Education and Training Awards Council, June 2004 This work is dedicated to my friends and family Declaration I hereby declare that I am the sole author of this thesis and that the material in this thesis has not nor is being currently submitted for any other degree. I authorise the Higher Educational and Training Awards Council to lend this thesis to other institutions or individuals for the purpose of scholarly research. Signed _ £ i M. Robson. Permission to Copy I authorise the Higher Educational and Training Awards Council to reproduce this thesis by photocopying or by other means, in total or part, at the request of other institutions or individuals. This permission covers single copies only, made for study purposes and is subject to the normal conditions o f acknowledgement. Age, growth, reproductive biology and population dynamics of the common megrim Lepidorhombus whiffiagottis (Walbaum, 1792) from off the west coast of Ireland Stephen M . Robson ABSTRACT A total of 4,964 common megrim Lepidorhombus whiffiagonis, were examined from commercial catches in Divisions V II b^, o ff the west coast o f Ireland, between April 1997 and February 2001. A sex ratio o f 31.8:1 females to males was recorded, with a higher percentage (15%) of fish being male at lengths <25cm TL, while no males were recorded >45cm TL. The largest and oldest fish recorded was a 51.3cm TL female, aged 16 years and weighing 1012.61g. The smallest was a 15.1cm TL female, weighing just 18.24g. Females ranged in age from 2-16 years old, while the male age range was considerably smaller, that o f 3-8 years old. Age at recruitment (tr) was determined to be at age 6 years o f age fo r both female and male fish. Values o f 0.91 and 1.43 were recorded for total mortality (Z) for female and male respectively. Three different growth models were used to describe the most accurate growth rates for the fish. Average values of 50.72cm for Loo, 0.097 fo r K and -4.78 for to were recorded for female fish, while Leo = 41.59cm, K = 0.138 and to = -3.062 were determined for males. Yield per recruit curves revealed that optimum yield occurred at a fishing mortality (F) o f 1.2 and 1.4 fo r female and male megrim respectively. Five maturity stages were recorded for the species. Fecundity ranged from 26,522-640,523 oocytes. Megrim were determined to be total spawners, spawning once annually between January and early March. Estimates o f 24.0cm TL and 25.0cm TL for Lso»/» were calculated for female and male histologically determined maturity at length. Maturity at age estimates for A50«/o o f 2 years and 2.5 years were recorded for female male fish respectively. An overall mean prevalence o f atresia (Pa) o f 63.5%, and relative intensity o f atresia (la) o f 24.2% was determined. Table of Contents Dedication................................................................................................... i Declaration .................................................................................... ii A b stra ct............................................................................................ i i i Table of Contents ......................................................................... iv -v Acknowledgements.......................................................................... v i-v ii List of Tables, Figures & Plates........................................................ v iii- x ii Chapter One: Introduction .............................................................. 1-12 Chanter One : P a rt 1. 1.0 The common megrim Lepidorhombus whiffiagonis.................... 3 1.1 Importance o f megrim in Irish demersal fisheries....................... 4 1.2 ICES Division Sub Area V II megrim stock................................ 4 Chapter One: Part 2. 1.3 Previous megrim studies.......................................................... 10 Chapter One: Part 3. 1.4 Purpose o f the present study..................................................... 11 Chapter Two : Materials & Methods.................................................. 13-44 Chapter Two : P a rt 1. 2.0 General introduction to sampling and laboratory methods............ 15 Chapter Two: Part 2. 2.1 General introduction to age and growth methods........................ 19 Chapter Two: Part 3. 2.2 General introduction to reproductive methods............................ 27 2.3 Histology..................................................................................31 2.4 Gonadosomatic Index............................................................... 41 2.5 Condition Factor...................................................................... 41 2.6 Fecundity................................................................................ 41 Chapter Two: Part 4. 2.7 General introduction to statistical methods................................ 44 iv Chapter Three : Results............................................................................... 45-178 Chapter Three : Part 1. 3.0 General introduction to sampling, length and weight results 47 Chapter Three: Part 2. 3.1 General introduction to age and growth results........................... 77 Chapter Three : Part 3. 3.2 General introduction to reproductive results................................ 124 Chapter Four : Discussion................................................................................. 179-220 Chapter Four : Part 1. 4.0 Sampling, lengths & weights discussion..................................... 181 Chapter Four : Part 2. 4.1 Age and growth discussion........................................................ 184 Chapter Four ; Part 3. 4.2 Reproduction discussion........................................................... 195 Chapter Four : Part 4. 4.3 Future studies...........................................................................219 Bibliography........................................................................................................ 221-231 Appendix I............................................................................................................ xiii Appendix H ...........................................................................................................xiv-xxiv Appendix HI........................................................................................................ {143-148) Appendix IV........................................................................................................ xxv-xli v ACKNOWLEDGEMENTS Firstly, I would like to thank ray project supervisor, Dr. Pauline King, for her guidance, dedication and patience. This project could not have been carried out or completed without her expertise and advice. I would also like to thank Dr. David McGrath for his critical advice and encouragement. Many thanks to Mr. Brian Ottway, Dr. Pat Dineen and especially Dr. Colin Pybus fo r his assistance w ith statistical analyses. Also, other staff o f the Biology department of GMIT for their advice throughout this study. I would like to acknowledge Dr. Malachy Thompson and Dr. Gerry Quinn, former Heads o f School and Life Science Department respectively, for their continuous support. I would also like to acknowledge the current Heads of School, Dr. Des Foley and Dr. Seamus Lennon. Much of this project could not have been carried out without the assistance of the Biology technicians, Mr. Stephen Barrett and Ms. Mary Veldon. I would especially like to acknowledge Stephen for his regular and invaluable help in collecting fish samples from the auction hall at Rossaveal. I would like to thank the library staff at GMIT, especially Ms. Mary O’Rourke, for their invaluable assistance in obtaining the relevant literature and for always being readily available to help. Many thanks are due to the Science computing staff especially Celia, Pat and Liam for their assistance with all those annoying computer problems. Thanks must be given to the Industrial Liasion Manager Mr. Tom Conlon, and to Ms. Ann Murphy and Ms. Patricia Bergin. I would like to thank the School of Science secretaries for all their help in providing stationary, photocopying facilities and other assistance, as well as Mr. Brendan Forde in the Science Stores for providing the necessary chemicals as required. Much of this project could not have been carried out without the w illing co operation o f the Martin Ryan Marine Institute o f NUIG. Large parts o f the histology was carried out there, and I would like to thank the Institute and the late Professor Brendan Keegan for facilitating the work, and the technicians Mr. Albert Lawless and Mr. John Galvin. 1 would also like to thank the Martin Ryan Institute caretaker, Mr. Keith McCauldron for his obliging and helpful manner. Many thanks must be given to the Galway and Aran Fishermen’s Co-operative in Rossaveal for providing the monthly samples o f fish, especially Mr. Paul Connery (Export manager) for his assistance in organising and securing the samples. Thanks are due also to Galway Bay Seafoods fo r providing additional samples o f fish. I am grateful to the ageing technicians of the Marine Institute, Galway, especially Helen McCormick and Fiona Woods, who validated ageing techniques. I would also like to acknowledge some