Fishery bycatch research

To: Birding Aus <>
Subject: Fishery bycatch research
From: L&L Knight <>
Date: Sat, 23 Aug 2008 18:15:16 +1000
Pelagic and conservation minded birders may like to have a look at the following report ...

Understanding and mitigating vulnerable bycatch in southern African
trawl and longline fisheries
Wednesday,20 August 2008
WWF South Africa Report Series – 2008/Marine/002


Executive Summary____iv

Chapter 1: Seabird bycatch in the pelagic longline fishery off
Africa                                                                10
Chapter 2: Turtle bycatch in the pelagic longline fishery off southern Africa 38 Chapter 3: Shark bycatch in the pelagic longline fishery off southern Africa 59
Chapter 4: Seabird bycatch in the demersal longline fishery off
Africa                                                                80
Chapter 5: Chondricthyan bycatch in demersal longline and trawl
fisheries off South Africa 101
Chapter 6: Seabird bycatch in the demersal trawl fishery off South
Africa 126
Chapter 7: Albatross overlap with fisheries in the Benguela Upwelling System: implications for conservation and management 140
Chapter 8: Implications of night setting for seabirds and target
catches                                                            160
Chapter 9: Gear configurations, line sink rates and seabird bycatch in pelagic longline fisheries 166 Chapter 10: The use of circle hooks to reduce turtle bycatch and their effect on other vulnerable species 178
Chapter 11: Is a 5% fin to trunk ratio universally appropriate as a
disincentive to catch sharks? 185
Chapter 12: Line sink rates and implications for seabirds, target and non-target fish species in the South African demersal longline fishery

Executive Summary

Over the past decade there has been concern about the bycatch of
seabirds, turtles and sharks in fishing operations, in particular
longline and trawl fisheries. The incidental mortality of these
species has been widely held responsible for the declining populations and threatened conservation status of several species. These vulnerable, K-selected species are also top predators and as such play an important role in the functioning of marine ecosystems. They also
have an economic value both in terms of non-consumptive eco-tourism
activities and, at least in the case of sharks, a consumptive value.
Seabirds and turtles are also indicators of the health of the
ecosystem because they breed on land and their populations can
therefore be accurately monitored. This report addresses the real
world problem of understanding, managing and ultimately reducing the
impacts of longline (targeting tunas Thunnus spp., Swordfish Xiphias
gladius and Cape hakes Merluccius spp. and trawl (targeting Cape
hakes) fishing operations.
We estimate that 21 000 seabirds (0.44/1000 hooks) are caught
incidentally by South African longline and trawl fisheries, including eight endangered seabirds and one endemic species. A similar suite of seabirds is also caught in the Namibian hake longline fishery (crudely estimated to kill 20 000 birds per year). Key findings of this study confirmed that birds are most at risk in the southern region of South Africa’s west coast, in the vicinity of Cape Point and on the Agulhas Bank during the winter months June–October. Full moon and daytime sets were strong predictors of mortality in the longline fishery. Evaluation of observer data (1998–2005) confirm that Swordfish catch rates are greatest when setting takes place at dusk. Optimal line sink rates of 0.3 m.s-1 are a requirement of the South African longline
fishery yet gear configurations to achieve this sink rate have not
been established. Five gear configurations were investigated: the
American longline system using no weighted swivel, 60 g and 120 g
weighted swivels, the use of a wire trace and the Asian pelagic
longline system. The fastest line sink rates were achieved by the
addition of 120 g weighted swivel (0.35 m.s-1) however the relative
improvement from 60 g (0.24 m.s-1) to 120 g may not warrant the
additional cost and may further compromise crew safety. Despite
achieving the optimal rate of 0.3 m.s-1 on average, no weighting
regime achieved this rate consistently. Fishing effort could be
limited during full moon when catch rates are known to be
substantially higher. Although there is evidence elsewhere that
Swordfish catch rates increase over full moon, there was no
significant effect of moon phase on in catch rates in this fishery.
Mitigation measures to reduce the incidental mortality of seabirds in trawl fisheries include improved discard management to reduce the
attraction of seabirds to vessels. This report explored the
implications of reducing the availability of fisheries discards for
seabirds by understanding foraging patterns of Black-browed
Albatrosses Thalassarche melanophrys and White-capped Albatrosses T.
steadi through the use of satellite transmitters. Trawl activity on
the continental shelf break off South Africa provides large quantities of high quality and predictable food in the form of offal and discards for a range of species,
including non-breeding Black-browed Albatrosses and White-capped
Albatrosses. This study presents evidence that Black-browed
Albatrosses, in particular, forage to a large extent on natural prey, despite the high availability of discards from fishing vessels in the Benguela. Therefore, given the high incidence of albatross collisions with trawl cables, the benefit of a management decision to limit
discarding as a mitigation measure is likely to far outweigh the
disadvantage of reduced food in the form of fisheries waste.


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