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Uantitative Estimates of Post-Release Survival Rates of Sharks Captured In

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Uantitative Estimates of Post-Release Survival Rates of Sharks Captured In 2021 Quantitative estimates of post-release survival rates of sharks captured in Pacific tuna longline fisheries reveal handling and discard practices that improve survivorship Melanie Hutchinson1,2,3, Zach Siders4, Jennifer Stahl1,3 and Keith Bigelow3 1Joint Institute for Marine and Atmospheric Research, University of Hawaii, Honolulu, Hawaii USA 2Hawaii Institute of Marine Biology, University of Hawaii, Kaneohe, Hawaii USA 3National Oceanic and Atmospheric Administration, Pacific Islands Fisheries Science Center, Fisheries Research and Monitoring Division, Honolulu, Hawaii USA 4Fisheries and Aquatic Sciences, School of Forest Resources and Conservation, University of Florida, Gainesville, Florida USA PIFSC Data Report DR-21-001. Issued 10 March 2021. https://doi.org/10.25923/0m3c-2577 Contents Abstract ........................................................................................................................................................................ iv Key words: ............................................................................................................................................................. iv List of Acronyms ....................................................................................................................................................... v Introduction ................................................................................................................................................................ 1 Methods ........................................................................................................................................................................ 3 Fishing and Tagging ............................................................................................................................................ 3 Survival Analysis .................................................................................................................................................. 6 Covariates .......................................................................................................................................................... 6 Proportional hazard model ........................................................................................................................ 6 Survival Curves ................................................................................................................................................ 8 Survival Rate Projections to the Observer Data Set ............................................................................. 9 Focus on Oceanic Whitetip Mortality Mitigation ................................................................................ 10 Individual Post-interaction Survival ................................................................................................... 10 Hindcast Survival of Observed Interactions .................................................................................... 11 Results ........................................................................................................................................................................ 11 Fishery Characteristics, Observer Data Collections, and Tagging ............................................... 11 Survival Analysis ............................................................................................................................................... 14 Proportional hazard model fit ................................................................................................................ 14 Survival Curves ............................................................................................................................................. 14 Expected Survival Projections .................................................................................................................... 15 ESA Listed Oceanic Whitetip Shark Mortality Mitigation ............................................................... 16 Individual Post-interaction Survival ................................................................................................... 16 Hindcast Survival of All Observed Interactions .............................................................................. 16 Discussion ................................................................................................................................................................. 17 Species specific survival rates ..................................................................................................................... 18 Silky shark ....................................................................................................................................................... 18 Blue shark ....................................................................................................................................................... 19 Bigeye thresher shark ................................................................................................................................ 20 Shortfin mako shark ...................................................................................................................................... 21 Oceanic whitetip shark .............................................................................................................................. 22 Fishery and handling method effects on survival .............................................................................. 22 Delayed Mortality ............................................................................................................................................. 25 Conclusions, Recommendations, and Next Steps .................................................................................... 28 ii Acknowledgements .............................................................................................................................................. 29 References ................................................................................................................................................................ 30 Tables and Figures ................................................................................................................................................ 35 Appendix ................................................................................................................................................................... 47 Supplementary Material ..................................................................................................................................... 55 iii Quantitative estimates of post-release survival rates of sharks captured in Pacific tuna longline fisheries reveal handling and discard practices that improve survivorship Abstract Shark catch rates are higher in pelagic longline fisheries than in any other fishery, and sharks are typically discarded (bycatch) at sea. The post-release fate of discarded sharks is largely unobserved and could pose a significant source of unquantified mortality that may change stock assessment outcomes and prevent sound conservation and management advice. This study assessed post-release mortality rates of blue (Prionace glauca), bigeye thresher (Alopias superciliosus), oceanic whitetip (Carcharhinus longimanus), silky (C. falciformis) and shortfin mako (Isurus oxyrhincus) sharks discarded in the Hawaii deep-set and American Samoa longline fisheries targeting tuna in the central Pacific Ocean. The impacts on survival rates were examined considering species, fishery, fishing gear configuration, handling method, animal condition at capture and at release, and the amount of trailing fishing gear remaining on discarded sharks. Bayesian survival analysis showed that the condition at release (good vs. injured), branchline leader material, and the amount of trailing fishing gear left on the animals were among the factors that had the largest effect on post-release fate—animals captured on monofilament branchline leaders and released in good condition without trailing fishing gear had the highest rates of survival. This study shows that fisher behavior can have a significant impact on pelagic shark post-release mortality. Ensuring that sharks are handled carefully and released with minimal amounts of trailing fishing gear may reduce fishing mortality on shark populations. Key words: Bycatch, mortality, shortfin mako, bigeye thresher, silky, blue, oceanic whitetip, best handling practice iv List of Acronyms ASLL – American Samoa ASOP – American Samoa Observer Program BSH – Blue shark BTH – Bigeye thresher shark CITES – Convention on International Trade in Endangered Species CMM – Conservation and management measures CPUE – Catch per unit effort FAL – Silky shark HIDS – Hawaii deep-set fishery HISS – Hawaii shallow-set fishery MSA – Magnuson-Stevens Fishery Conservation and Management Act NMFS – National Marine Fisheries Service OCS – Oceanic whitetip shark PAT – Pop-off archival tag PIROP – Pacific Islands Region Observer Program RFMO – Regional Fisheries Management Organization SMA – Shortfin mako shark SPAT – Survivorship pop-off archival tag WCPFC – Western and Central Pacific Fisheries Commission WCPO – Western Central Pacific Ocean WPFMC – Western Pacific Fisheries Management Council v Introduction It is estimated that two-thirds of global elasmobranch species are threatened with extinction, with overfishing identified as a major contributor
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