Estimating at-vessel mortality rates of shortfin mako sharks caught in the US pelagic longline fishery and examining environmental drivers of their depth use in the North Atlantic Ocean
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Shortfin mako sharks, Isurus oxyrinchus, have historically been a small part of recreational and commercial fisheries in the United States (U.S.). Recent stock assessments have identified declines in mako shark stocks that have caused widespread concern among researchers and managers. The National Marine Fisheries Service (NMFS), in accordance with recommendations from the International Conservation Committee for Atlantic Tunas (ICCAT), have introduced live release regulations for all commercial and recreational fishers that catch mako sharks. However, if mako sharks are caught as bycatch in commercial fisheries experience high at-vessel mortality rates (AVM) than these conservation measures may do little to help rebuild the stock. I examined AVM of mako sharks in two regions of the US Atlantic pelagic longline fishery, the Gulf of Mexico (GOM) and the Western North Atlantic (WNA), and AVM was found to be higher in the GOM (32.4 percent, 95 percent CI: 29.5 - 35.4 percent) and 25.7 percent (95 percent CI: 24.6 percent - 26.8 percent) in the WNA. At-vessel mortality was evaluated in relation to environmental, biological, and fishery characteristics. The variables influencing AVM in these two regions differed where sea surface temperature (SST), hook depth, and shark size were important in the GOM, whereas soak time, shark size, and SST were important in the WNA. In both regions, AVM increased in warmer waters. The relationship between shark size and AVM differed between regions where larger sharks in the GOM were more susceptible to AVM and intermediately sized sharks (approximately 80 - 250 cm) had higher AVM in the WNA. Lastly, AVM decreased when hooks were set deeper in the water column in the GOM, and AVM increased with longer soak times in WNA. Combining the estimates of AVM from this study and a PRM rate of 0.358 (sd = 0.06; Bowlby et al. 2021), the probability of survival of a shark hooked on a longline in the GOM was estimated at 42.6 percent (95 percent CI: 33.7 - 52.1 percent) and 47.4 percent (95 percent CI: 38.6 - 56.9 percent) in the WNA. Given the high AVM rates in both regions and the relatively low change of survival, the most effect management strategies to help rebuild mako shark stocks should focus on bycatch reduction. Further, I examined the mako shark vertical habitat use patterns in relation to environmental drivers in a dynamic and diverse marine ecosystem of the North Atlantic Ocean. Overall, mako sharks' vertical habitat use was related to temperature and mixed layer depth. Increased vertical habitat was correlated with the mixed layer depth where sharks dove to greater depths more frequently as the mixed layer deepened. Although encountering a broad range of temperatures, mako sharks tended to inhabit mean temperature ranges from 17 - 20 ºC. In this study, it appeared that mako sharks altered their vertical habitat to find waters that were within their optimal temperature range.
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M.S.