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South African Demersal Trawl Fishery

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South African Demersal Trawl Fishery View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by South East Academic Libraries System (SEALS) THE ASSESSMENT AND MANAGEMENT OF BYCATCH AND DISCARDS IN THE SOUTH AFRICAN DEMERSAL TRAWL FISHERY. A thesis submitted in fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY of RHODES UNIVERSITY by SARAH ANN WALMSLEY June 2004 Abstract Over the past few decades it has become recognised that an ecosystem approach is required to manage world fisheries. Management strategies must ensure that non-target (bycatch) as well as target catches are sustainable. To achieve this, detailed commercial catch and biological information is required. The composition of catches made by trawlers operating off the south and west coasts of South Africa was investigated. Distinct fishing areas were identified on each coast, based on target species and fishing depth. Catch composition differed markedly among the areas defined. Although hake Merluccius sp. dominated South Coast catches, a large proportion of the catch was composed of bycatch. On the West Coast, hake dominated catches and this domination increased with increasing depth. On both coasts approximately 90% of the observed nominal catch was processed and landed. Estimates of annual discards suggested that the fishery discarded 38 thousand tons of fish per annum (16% of the nominal trawl catch). The data also indicated that hake discarding, the capture of linefish and the increased targeting of high value species might be cause for concern. Spatial analysis indicated that a variety of factors such as trawling position, catch size and catch composition affects bycatch dynamics. The monkfish Lophius vomerinus is a common bycatch species that has been increasingly targeted by demersal trawlers. This study showed that L. vomerinus is a slow-growing, long-lived species (West Coast males L∞ = i -1 68.50cm TL, to = -1.69yr, K = 0.10yr ; West Coast females L∞ = 110.23cm TL, -1 to = -1.54yr, K = 0.05yr ; South Coast sexes combined L∞ = 70.12cm TL, to = - 0.80yr, K = 0.11yr-1), that matures at approximately 6 years of age. These traits could have serious management implications for the species. Per-recruit analysis suggested that the stock might be overexploited, although further investigation is required to confirm this. Solutions were suggested for each of the concerns raised, taking cognisance of the differences observed between the South and West Coasts and the economic dependence of South Coast companies on bycatch. The needs of future research were considered. ii Contents Abstract.............................................................................................................i Contents ......................................................................................................... iii List of figures ...................................................................................................v List of tables...................................................................................................viii Acknowledgements........................................................................................ xii Chapter 1 - Introduction. ..................................................................................1 The bycatch issue.....................................................................................1 Bycatch assessment and management ....................................................4 Bycatch in trawl fisheries ..........................................................................5 The South African demersal trawl fishery .................................................9 The Cape south coast.............................................................................17 The West Coast ......................................................................................19 The bycatch and management of the South African trawl fishery...........22 Thesis aims and structure.......................................................................29 Chapter 2 - Bycatch and discarding in the South African demersal trawl fishery: the Cape south coast. .......................................................................33 Introduction ................................................................................................33 Material and methods.................................................................................37 Data collection ........................................................................................37 Data analysis ..........................................................................................39 Results .......................................................................................................42 Discussion..................................................................................................55 Chapter 3 - Bycatch and discarding in the South African demersal trawl fishery: the West Coast..................................................................................62 Introduction ................................................................................................62 Material and methods.................................................................................63 Data collection ........................................................................................63 Data analysis ..........................................................................................65 Results .......................................................................................................67 Discussion..................................................................................................78 Chapter 4 - The spatial distribution of bycatch and discards..........................87 Introduction ................................................................................................87 Material and methods.................................................................................90 The spatial distribution of bycatch and discards .....................................92 The spatial distribution of hake discards.................................................92 Identifying factors influencing bycatch and discards using GAM ............93 Results .......................................................................................................98 The spatial distribution of bycatch and discards .....................................98 The spatial distribution of hake discards.................................................99 Identifying factors influencing bycatch and discards using GAM ..........113 Discussion................................................................................................124 iii Chapter 5 - The biology, distribution and preliminary stock assessment of the monkfish Lophius vomerinus in South Africa. ..............................................135 Introduction ..............................................................................................135 Material and methods...............................................................................137 Length-weight regressions....................................................................142 Age and growth.....................................................................................143 Reproduction ........................................................................................146 Diet .......................................................................................................147 Distribution............................................................................................148 Per-recruit analysis ...............................................................................148 Results .....................................................................................................152 Length-weight regressions....................................................................152 Age and growth.....................................................................................152 Reproduction ........................................................................................153 Diet .......................................................................................................154 Distribution............................................................................................155 Per-recruit analysis ...............................................................................155 Discussion................................................................................................174 Chapter 6 - Managing South Africa’s trawl bycatch .....................................184 Introduction...........................................................................................184 Background...........................................................................................186 Discussion ............................................................................................193 Conclusion............................................................................................227 Appendices ..................................................................................................229 Literature cited .............................................................................................242 iv List of figures Fig. 1.1: Graphic illustration of the components of the catch as defined in the text. .....................................................................................................3 Fig. 1.2:
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