Technical mitigation measures for sharks and rays in fisheries for tuna and tuna-like species: turning possibility into reality

Abstract

Tuna fisheries have been identified as one of the major threats to populations of other marine vertebrates, including sea turtles, sharks, seabirds and marine mammals. The development of technical mitigation measures (MM) in fisheries is part of the code of conduct for responsible fisheries. An in-depth analysis of the available literature regarding bycatch mitigation in tuna fisheries with special reference to elasmobranchs was undertaken. Studies highlighting promising MMs were reviewed for four tuna fisheries (longline, purse seine, driftnets and gillnet, and rod and line – including recreational fisheries). The advantages and disadvantages of different MMs are discussed and assessed based on current scientific knowledge. Current management measures for sharks and rays in tuna Regional Fishery Management Organizations (t-RFMOs) are presented. A review of relevant studies examining at-vessel and postrelease mortality of elasmobranch bycatch is provided. This review aims to help fisheries managers identify pragmatic solutions to reduce mortality on pelagic elasmobranchs (and other higher vertebrates) whilst minimizing impacts on catches of target tuna species. Recent research efforts have identified several effective MMs that, if endorsed by t-RFMOs, could reduce elasmobranchs mortality rate in international tropical purse seine tuna fisheries. In the case of longline fisheries, the number of operational effective MMs is very limited. Fisheries deploying driftnets in pelagic ecosystems are suspected to have a high elasmobranchs bycatch and their discard survival is uncertain, but no effective MMs have been field validated for these fisheries. The precautionary bans of such gear by the EU and by some t-RFMOs seem therefore appropriate. Recreational tuna fisheries should be accompanied by science-based support to reduce potential negative impacts on shark populations. Priorities for research and management are identified and discussed.

Meta‐analysis of post‐release fishing mortality in apex predatory pelagic sharks and white marlin

“…Moreover, Campana, Joyce and Manning () and Musyl and Gilman () used similar survival models and prognostic variables and independently reached the same conclusion: the only significant variable to explain survival outcomes was health condition code at retrieval and tagging. However, in terms of practical and clinical significance and “Best Handling Practices” to promote survival, it is clear that eliminating severely injurious handling practices could lower mortality rates (Favaro & Côté, ; Molina & Cooke, ; Poisson, Filmalter et al., ; Poisson et al., , ; Poisson, Séret et al., ).…”

“…In a recent evaluation against the IUCN Red List criteria, about one‐quarter of elasmobranch species were listed as threatened due to overfishing (Dulvy, Fowler, Musick, Cavanagh & Kyne, ) and reducing fishing mortality is a global priority to rebuild stocks (Camhi, ; Dulvy, Baum, Clarke, Compagno & Cortés, ). Due to declining populations, more species are being protected through retention bans (IATTC, ; ICCAT, , , , ; IOTC, , ; Poisson, Crespo, Ellis, Chavance & Pascal, ; WCPFC, , ) with implicit or explicit aims of reducing fishing mortality, which will result in higher discard (i.e. released after capture) rates.…”

“…For fisheries management and conservation, having precise survival rates of discards and at‐vessel mortality rates are necessary to advance and justify non‐retention, catch‐and‐release policies (Ellis, Phillips & Poisson, ; Graves, Marcek & Goldsmith, ; Moyes et al., ; Pine, Martell, Jensen, Walters & Kitchell, ) and other by‐catch mitigation strategies to rebuild stocks (Carruthers, Schneider & Neilson, ; Dapp, Huveneers, Walker, Drew & Reina, ; Favaro & Côté, ; Gilman et al., ; Poisson et al., ; Uhlmann et al., ). In fisheries population biology, it is necessary to account for the two main sources of fishing mortality: at‐vessel or catch (i.e.…”

Fisheries Bycatch of Chondrichthyes

“…For instance, the United States National Marine Fisheries Service (NMFS) in USA and the Department of Fisheries and Oceans (DFO) in Canada have conducted observer programs to monitor commercial fishing vessels targeting tuna, swordfish and sharks. In addition, other measures including the reduction of fishing effort targeting sharks, time/area closure in certain seasons and gear restrictions have been adopted in the North Atlantic [116,[118][119][120]. Notably, many large pelagic sharks are overfished or in overexploitation and are often removed as bycatch in longline fisheries targeting tuna and swordfish in the North Atlantic as a whole [11,54,117].…”

“…Recently, specific technical mitigation measures have been proposed and implemented in order to reduce bycatch mortality. Some authors suggested the use of circle hooks rather than J-hooks to increase post-release survival of large pelagic sharks in longline fisheries [71,72,120].…”