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Aspects of the Biology of Three Exploited Deepwater Sharks

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Aspects of the Biology of Three Exploited Deepwater Sharks Aspects of the biology of three exploited deepwater sharks Centrophorus squamosus, Centroscymnus coelolepis and Deania calceus (Elasmobranchii: Squalidae) from the continental slopes of the Rockall Trough and Porcupine Bank A thesis presented to the National University of Ireland in fulfilment of the thesis requirement for the degree of Doctor of Philosophy in Zoology By Maurice Clarke B.A. (Mod.) Head of Department: Department of Zoology, Professor Eamon J. Duke University College, Belfield, Dublin 4 Supervisors: Professor John J. Bracken Department of Zoology, University College, Belfield, Dublin 4. Dr. Paul L. Connolly Marine Institute, Divisional Director Marine Fisheries Services Division, Abbotstown, Dublin 15. October 2000 This work is dedicated to my parents Declaration I hereby declare that I am the sole author of this work, now submitted in fulfilment of the thesis requirement for the degree of Doctor of Philosophy of the National University of Ireland. I also certify that the material in this thesis has not already been accepted for any other degree and is not being submitted for any other degree. Signed ___________________ Maurice Clarke Permission is hereby given to the Library of the National University of Ireland to reproduce this thesis, in whole or in part, at the request of other institutions or individuals. This permission covers single copies only, made for study purposes and is subject to the usual conditions of acknowledgement. Signed____________________ Maurice Clarke Contents Abstract……………………………………………………………………... vi Declaration………………………………………………………………….. vii List of Tables……………………………………………………………….. ix List of Figures………………………………………………………………. xiii List of Plates………………………………………………………………… xviii 1 1. Introduction…………..………………………. 1.1 Exploitation of deepwater sharks…………………………... 2 1.2 The deepwater environment of the continental slopes……. 6 1.3 The study area……………………………………………….. 12 1.3.1 Geology……………………………………………………….. 12 1.3.2 Hydrography………………………………………………….. 14 1.3.3 Biology……………………………………………………….. 15 1.4 Previous studies……………………………………………… 17 1.4.1 General biology and population structure…………………….. 17 1.4.2 Reproduction………………………………………………….. 20 1.4.2.1 Maturity……………………………………………………….. 24 1.4.2.2 Fecundity……………………………………………………… 25 1.4.2.3 Cycles…………………………………………………………. 26 1.4.3 Age and Growth………………………………………………. 28 1.4.4 The effects of fishing…………………………………………. 32 1.4.5 The species under study………………………………………. 36 1.4.5.1 Centroscymnus coelolepis…………………………………….. 36 1.4.5.2 Centrophorus squamosus……………………………………... 39 1.4.5.3 Deania calceus………………………………………………... 42 1.5 The scope and purpose of this study…………………….…. 45 ii 2. Materials and Methods……..……………………. 49 2.1 General procedures………………………………………….. 49 2.1.1 Trawl surveys…………………………………………………. 52 2.1.2 Long-line surveys……………………………………………... 53 2.1.3 Port Sampling………………………………………………… 53 2.1.4 Data storage and data management…………………………… 54 2.1.5 Catch per unit effort……………………………………..……. 55 2.1.6 Discard rate…………………………………………………… 55 2.1.7 Length frequency distributions……………………………….. 57 2.1.8 Sex ratios……………………………………………………… 57 2.2 Age estimation procedures………………………………….. 59 2.2.1 Measurement of spines……………………………………….. 59 2.2.2 Treatment of spines of Centrophorus squamosus……………. 63 2.2.3 Treatment of spines of Centroscymnus coelolepis…………… 63 2.2.4 Treatment of spines of Deania calceus………………………. 64 2.2.5 Histological preparation of spines……………………………. 64 2.2.6 Readability of spine cross-sections…………………………… 64 2.2.7 Growth curves………………………………………………… 65 2.3 Reproduction ………………………………………………... 66 2.3.1 Maturity………………………………………………………. 66 2.3.2 Gonadosomatic index………………………………………… 68 2.3.3 Hepatosomatic index…………………………………………. 68 2.3.4 Treatment of gonads………………………………………….. 69 2.3.5 Interpretation of gonad slides………………………………… 69 2.3.6 Fecundity……………………………………………………… 70 2.4 Mortality…………………………………………………….. 70 iii 3. Results……………...………………………….. 75 3.1 Population structure……………..…………………………… 75 3.1.1 Distribution of Centroscymnus coelolepis………..……………. 75 3.1.2 Length frequencies of Centroscymnus coelolepis……..……….. 84 3.1.3 Distribution of Deania calceus………………………..……….. 88 3.1.4 Length frequencies of Deania calceus…………………….….. 95 3.1.5 Distribution of Centrophorus squamosus…………………….. 98 3.1.6 Length frequencies of Centrophorus squamosus……………... 105 3.1.7 Discarding…………………………………………………….. 108 3.2 Reproduction…………………………………………………. 113 3.2.1 Reproductive cycle and maturity of Centroscymnus coelolepis. 113 3.2.2 Fecundity of Centroscymnus coelolepis………………………. 120 3.2.3 Spermatogenesis in Centroscymnus coelolepis……………….. 121 3.2.4 Gonadosomatic and Hepatosomatic index of Centroscymnus 139 coelolepis and Centrophorus squamosus……………………... 3.2.5 Reproductive cycle and maturity of Centrophorus squamosus…………………………………………………….. 139 3.2.6 Fecundity of Centrophorus squamosus……………………….. 147 3.2.7 Spermatogenesis in Centrophorus squamosus………………... 147 3.2.8 Reproductive cycle and maturity of Deania calceus………….. 151 3.2.9 Fecundity of Deania calceus………………………………….. 152 3.2.10 Spermatogenesis in Deania calceus…………………………... 152 3.2.11 Testicular structure and spermatogenesis……………………... 157 iv 3.3 Age estimation……………………………………………….. 159 3.3.1 External structure of the spines……………………………….. 159 3.3.2 Internal structure of the spines………………………………... 159 3.3.3 Spine growth………………………………………………….. 161 3.3.4 Obtaining age estimates………………………………………. 162 3.3.5 Readability and precision……………………………………... 165 3.3.6 Empirical growth……………………………………………… 170 3.4 Mortality……………………………………………………… 182 4. Discussion………...………………………………… 206 4.1 Missing size ranges…………………………………………... 206 4.2 Distribution and Abundance…………………………….…... 221 4.3 Size selectivity of fishing gear…………………………….….. 214 4.4 Discarding………………………………………………….…. 216 4.5 Changes in GSI and HSI with maturation……………….…. 219 4.6 Assessing maturity……………………………………….…… 221 4.7 Age estimation……………...……………………………….…. 230 4.8 Growth and mortality……………………………………….... 233 4.9 The impacts of fishing………………………………………… 236 4.10 Management of the fishery…………………………………... 238 4.11 Conclusions……………………………………………………. 240 Acknowledgements…………………………………………………………… 242 References……………………………………………………………………. 244 Appendices……………………………………………………………………. 271 v Abstract Aspects of the biology of three exploited sharks (Elasmobranchii: Squalidae) from the continental slopes of the Rockall Trough and Porcupine Bank (NE Atlantic) were examined using specimens taken during trawl and long-line surveys (December 1995 to December 1999) and commercial samples obtained from trawlers (March 1997 to June 1998). The species under study were Centroscymnus coelolepis, Centrophorus squamosus, and Deania calceus. The entire depth range (300 m – 1,800 m) of each species was sampled. C. squamosus and D. calceus were abundant between 700 m and 900 m. C. coelolepis was more abundant deeper (1,300 m) but gravid females were more abundant in shallower waters. Small specimens of each species were absent from the study area. Trawls and long-lines selected for different size ranges of C. coelolepis and D. calceus, though not C. squamosus. Deepwater bobbin and French high headline trawls did not select for different size ranges of C. coelolepis. Unlike the other species D. calceus is not landed. Discard rates from long-liners exceeded 40 % of total catch depending on depth fished. While percentage discard rates from trawlers were lower, an estimated 745 t of this species were discarded in 1996 alone. Sexual maturity was achieved at large size, more than 75 % of maximum length in each species. No evidence of seasonal cycles in reproduction was found from spermatogenic analysis or the occurrence of maturity stages. Mean ovarian fecundity was low; C. squamosus, 8, C. coelolepis and D. calceus 13. Dorsal spine sections were used for age estimation. Significant positive regressions of spine width and total length were found and the growth rates of first and second spines were not significantly different. Estimates of 21-70 years (C. squamosus) and 11-35 years (D. calceus) were obtained. Agreement within 1 year was found for more than 93 % of D. calceus and 88 % of C. squamosus first and second spines. Empirical and von Bertalanffy growth data suggest that growth had slowed down or ceased before the onset of maturity. Estimates of natural mortality (M) for C. squamosus (0.066 – 0.087) and D. calceus (0.116-0.177) were low. Fishing mortality (F) for D. calceus was estimated to have reached the level of M for females but below M for males. This study suggests that these species have conservative life-history strategies. No management plan currently exists for the multi-species deepwater fishery in the northeast Atlantic. In the absence of suitable abundance indices, stock assessment should be based on known life history parameters of target and by-catch species. Such an approach will facilitate the rational management of the fishery. vi Aspects de la biologie de trois requins profonds, Centrophorus squamosus, Centroscymnus coelolepis et Deania calceus (Elasmobranchii: Squalidae) du talus continental du Fossé de Rockall et du Banc de Porcupine Une thèse présentée devant L’Université Nationale d’Irlande Pour obtenir le titre de Docteur ès Lettres par Maurice Clarke Soutenue le 8 décembre 2000 devant le Jury: Professor John Bracken Président Professor Kazunari Yano Rapporteur Dr. Tom Hayden Rapporteur Dr. Paul Connolly Examinateur Correspondance à: Maurice Clarke, Marine Institute, Abbotstown, Dublin 15, République d’Irlande. Tel: 00353 1 8228202,
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