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Galeorhinus Galeus) in South Africa View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by South East Academic Libraries System (SEALS) ASPECTS OF THE ECOLOGY AND MANAGEMENT OF THE SOUPFIN SHARK (GALEORHINUS GALEUS) IN SOUTH AFRICA A thesis submitted in fulfillment of the requirements for the degree of MASTER OF SCIENCE of RHODES UNIVERSITY by MEAGHEN ERICA MCCORD February 2005 ABSTRACT Global trends in teleost fisheries indicate significant population declines. Thus, alternative fisheries are being developed to meet the growing economic and nutritional demands of the expanding human population. Recently, it has been established that elasmobranch fisheries may fulfill these demands. As many elasmobranchs possess life- history characteristics that make them particularly vulnerable to overfishing, it is imperative to develop management strategies prior to the inception of these fisheries to ensure sustainable resource utilisation. In South Africa, elasmobranchs have been commercially exploited since the 1930s. Although generally considered an under-exploited resource, the potential for growth within these fisheries has been recognized. In 2005, the commercial shark fishery will undergo a transition from medium to long-term rights allocations. This represents an ideal opportunity for scientists and managers to develop precautionary species-specific management plans for commercially exploitable elasmobranch species. The soupfin shark (Galeorhinus galeus) is one of the principal target species in South Africa’s shark fisheries. Given its inherent susceptibility to overexploitation, G. galeus was selected as a management priority by South Africa’s regional fisheries organisation. The purpose of this study was to examine and describe the stock status of G. galeus in South Africa, and to develop a precautionary fishery management plan to ensure the sustainability of this resource. Age, growth, and mortality calculations for G. galeus were made from research survey data collected between 1996 and 1999. A small sample size precluded independent analyses of females. The maximum recorded age for G. galeus was 33 years. Estimated von Bertalanffy growth parameters from observed length-at-age for males and combined -1 -1 sexes were: L∞ 1542.8 mm TL, K 0.21 year , t0 -2.79 year and L∞ 1560.3 mm TL, -1 -1 K 0.19 year , t0 -3.03 year , respectively. The age-at-50% maturity was determined to be 6 years, corresponding to 1011 mm TL for males and 1100 mm TL for combined sexes. ii Natural mortality was calculated as 0.126 yr-1. The rate of instantaneous total mortality was calculated as 0.27 yr-1. Catch trend analysis showed that catches and CPUE of G. galeus are increasing in the demersal longline fishery, and decreasing in the handline fishery. Decreasing catches and CPUE were observed in fishery-independent research survey data. The status of the soupfin shark stock was modelled using per-recruit analysis. The SB/R model indicated the soupfin shark is being optimally exploited and spawner biomass is at 43% of pre- -1 exploitation levels. Current fishing levels ( F = 0.14 yr ) approximate the FSB40 level ( F = 0.17 yr-1); thus, an increase in fishing pressure may lead to stock collapse. It was determined that the current age-at-capture (7.9 years) should be increased to 10 years, or 1420 mm TL, to maximize yield and minimize the possibility of recruitment failure. The results of this study indicate a need for immediate scientific and management intervention in South Africa’s soupfin shark fishery. An assessment report and fishery management plan for G. galeus was compiled, and several management options were proposed. These include the implementation of licence and size restrictions, as well as seasonal/area closures. The potential for an experimental gillnet fishery should be investigated. iii TABLE OF CONTENTS ABSTRACT........................................................................................................ii DEDICATION...................................................................................................vi ACKNOWLEDGMENTS ..............................................................................vii CHAPTER 1: GENERAL INTRODUCTION 1.1 Fisheries management ..................................................................................1 1.2 Taxonomic and biological characteristics of chondrichthyans ...................5 1.3 Management of elasmobranch fisheries in South Africa...........................11 1.4 An introduction to the soupfin shark (Galeorhinus galeus) ......................13 1.5 The soupfin shark fishery of South Africa.................................................14 CHAPTER 2: STUDY AREA & GENERAL SAMPLING METHODS 2.1 Study area....................................................................................................21 2.2 Sources of data............................................................................................21 2.3 Data analysis ...............................................................................................22 CHAPTER 3: CATCH TRENDS 3.1 Introduction.................................................................................................24 3.2 Materials and Methods ...............................................................................25 3.3 Results .........................................................................................................26 3.4 Discussion ...................................................................................................31 CHAPTER 4: BIOLOGY, AGE, GROWTH, AND MORTALITY OF THE SOUPFIN SHARK (GALEORHINUS GALEUS) 4.1 Introduction.................................................................................................35 4.2 Materials and Methods ...............................................................................37 4.3 Results .........................................................................................................45 4.4 Discussion ...................................................................................................55 CHAPTER 5: YIELD-PER-RECRUIT ANALYSIS OF THE SOUPFIN SHARK (GALEORHINUS GALEUS) POPULATION OF SOUTH AFRICA 5.1 Introduction.................................................................................................61 5.2 Materials and Methods ...............................................................................66 5.3 Results .........................................................................................................69 5.4 Discussion ...................................................................................................76 CHAPTER 6: GENERAL DISCUSSION 6.1 Introduction.................................................................................................81 6.2 Framework ..................................................................................................87 iv CHAPTER 7: ASSESSMENT REPORT AND PROPOSED FISHERY MANAGEMENT PLAN FOR THE SOUPFIN SHARK (GALEORHINUS GALEUS) IN SOUTH AFRICA 7.1 Introduction.................................................................................................91 7.2 Biological synopsis .....................................................................................93 7.3 Overview of the fishery ..............................................................................94 7.4 Long-term objectives for the fishery..........................................................99 7.5 Specific management objectives ................................................................99 7.6 Consultative process .................................................................................101 7.7 Stock status................................................................................................103 7.8 Current management issues......................................................................109 7.9 Management strategy................................................................................109 7.10 Enforcement measures............................................................................115 7.11 Financial responsibilities ........................................................................117 7.12 Performance review ................................................................................117 7.13 Synthesis .................................................................................................118 APPENDIX 1..................................................................................................121 REFERENCES...............................................................................................123 v DEDICATION This thesis is dedicated to my parents – thank you for your infinite support throughout my university career. vi ACKNOWLEDGMENTS I would like to express my sincere gratitude to the following individuals for their support and contribution for the duration of this degree: • Professor Warwick Sauer of the Department of Ichthyology and Fisheries Science (DIFS), Rhodes University for providing me with the opportunity to travel to and work in such a beautiful country. Also, thank-you for providing me with insight and support over the last two years. • Dr. Olaf Weyl of the DIFS for granting me sanity when I hit a modelling wall – you cannot imagine how much I have appreciated your assistance. • Dr. Tony Booth of the DIFS for his mathematical expertise and critical abilities. • Dr. Malcolm Smale of the Port Elizabeth Museum and Mr. Marcel Kroese of Marine and Coastal Management
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