Bycatch survival of shortfin mako sharks (Isurus oxyrinchus) in the U.S. Atlantic pelagic longline fishery.

Severe population declines of shortfin mako sharks (Isurus oxyrinchus) in the Atlantic Ocean have led to the implementation of conservation measures, notably fishing retention bans and live-release regulations, aimed at substantially reducing fishing mortality to allow stock recovery. While retention bans can eliminate harvest mortality, their effectiveness can be reduced if survival of sharks encountered as bycatch and not retained is low. We quantified at-vessel survival (AVS) and post-release survival (PRS) and estimated overall bycatch survival probability of mako sharks for the U.S. Atlantic pelagic longline fishery. Based on fisheries observer records (n = 7821) between 2000-2020, we found AVS varied regionally from 0.77 (95% CI: 0.74-0.80) in the northmost observation region to 0.65 (95% CI: 0.61-0.69) in the Gulf of Mexico (GOM). We found significant negative correlations between AVS and soak time, surface temperature, mainline length, and shark size. Based on pop-up archival satellite tags (n = 27) deployed from pelagic longline vessels in the WNA during 2022-2024, PRS was 0.87 (95% CI: 0.74-0.93). Overall mean bycatch survival probability varied regionally from 0.64 (95% CI: 0.51-0.68) in the northmost observation region to 0.59 (95% CI: 0.49-0.64) in the GOM, which given the low productivity rates of mako sharks may be low enough to hinder recovery efforts if mako sharks are encountered as bycatch in significant numbers. Pairing retention bans with actions that reduce incidence of bycatch would likely provide the greatest benefit to population recovery. Our research highlights the importance of quantifying survival regionally and between fleets, as variability in fishing practices and environmental conditions can result in different bycatch survival outcomes, which can be important considerations in stock assessment.