Reducing shark bycatch in tuna fisheries: adaptive spatio-temporal management options for the eastern Pacific Ocean

Purse-seine tropical tuna fishing in the eastern tropical Pacific Ocean (EPO) results in the bycatch of several sensitive species groups, including elasmobranchs. Effective management of ecosystems balances conservation and resource use, but requires actionable knowledge that accounts for both trade-offs and synergies. Seasonal and adaptive spatial management measures can be effective to reduce the impact of fisheries on non-target species while preserving, or even increasing, target species catch. Exploring the potential distribution and impact of fisheries closures in the open ocean, where highly dynamic environmental conditions drive distributional changes in biological communities throughout the year, requires the identification of persistently high-risk areas, where the likelihood of encountering and catching unwanted bycatch species, relative to the target species, is high. We used fisheries observer data from 1995–2021 to explore the spatio-temporal persistence of areas of high bycatch risk for two species of oceanic sharks, silky shark (Carcharhinus falciformis) and oceanic whitetip shark (Carcharhinus longimanus), and low tuna catch rate areas—defined as areas of high fishing inefficiency (i.e., poor fishing areas). We found that if areas of high fishing inefficiency were closed throughout the study period, and effort reallocated proportionally to reflect historical effort patterns, yearly tuna catch may have increased by 1–11% while the bycatch of silky and oceanic whitetip sharks could have decreased by 10-19% and 9%, respectively. Prior to fishing effort redistribution, bycatch reductions would have accrued to 21–41% and 14% for silky and oceanic whitetip sharks, respectively. Our analysis builds on past evidence and demonstrates the high potential for reducing elasmobranch bycatch in the EPO, while not compromising the catch rates of target tuna species. It also highlights the need to consider new dynamic and adaptive management measures to more efficiently fulfill conservation and sustainability objectives for exploited resources in the EPO.