Ecological and societal context of catch and discards: identifying opportunities for bycatch mitigation in swordfish and tuna pelagic longline fisheries

Carruthers, Erin H. (2012) Ecological and societal context of catch and discards: identifying opportunities for bycatch mitigation in swordfish and tuna pelagic longline fisheries. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Bycatch, defined here as catch discarded for regulatory, economic or personal reasons, from pelagic longline fisheries has contributed to wide spread population declines of sharks and sea turtles. Opportunities to reduce impacts in these fisheries occur throughout the fishing process and depend upon the fishing practices within fleets, and upon the behaviour of target and bycatch species. The overall objective of this thesis was to identify bycatch mitigation opportunities within the Canadian Atlantic pelagic longline fishery, which targets swordfish (Xiphias gladius), warm-water tunas (bigeye, Thunnus obesus; yellowfin T. albacares; and albacore, T. alalunga) and mahi-mahi (Coryphaena hippurus). Bycatch includes common sharks and rays (blue shark, Prionace glauca; pelagic stingray, Pteroplatytrygon violacea), and endangered sea turtles (leatherback Dermochelys coriacea; loggerhead, Caretta caretta). Bycatch mitigation approaches such as shifting to circle hooks, increased the likelihood that shark bycatch would be released alive and with less severe hooking injuries. Shorter longline soak times also increased hooking survival among most of the common bycatch species. Shorter soak times would not decrease catch of the most common landed species (swordfish), but this shift in fishing practices could negatively impact fisher safety. Interviews with active longline captains revealed operational difficulties and unintended ecological impacts with proposed bycatch mitigation approaches. Longline captains also reported innovative uses of bycatch mitigation tools that could increase post-release survival of common bycatch species in this and other pelagic longline fleets. Finally, the combined analysis of fisheries observer data, qualitative data from fishers' knowledge interviews, and concurrent environmental data suggested that high blue shark catch rates were related to local oceanography - and did not reflect behavioural differences between blue shark and swordfish. Clearly, there are opportunities for bycatch mitigation within the Canadian pelagic longline fishery for swordfish and tunas. However, the process of interviewing pelagic longline captains revealed both interest in reducing bycatch, but also suspicion of research efforts. Such trust issues will need to be addressed in subsequent research as the combined use of fishery assessments, detailed oceanographic data, practical fishing knowledge, and on-the-water observations will be needed to decrease the amount of and harm to discarded bycatch.

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