Post-release mortality of hammerhead sharks Sphyrna mokarran and Sphyrna lewini after discard from longline fishing gear

Great hammerheads Sphyrna mokarran and scalloped hammerheads Sphyrna lewini are globally listed as Critically Endangered because of population declines largely driven by overfishing. As a result, these species are protected with no-take or limited quota regulations in several countries. However, both are commonly caught as bycatch in commercial fisheries and post-release mortality (PRM) remains poorly understood, resulting in uncertainty in what type of management measures may be most effective for limiting fishing mortality in these species. We used acceleration data-loggers and pop-up satellite archival tags to quantify PRM of great and scalloped hammerheads caught on bottom longline fishing gear in waters off Florida, Louisiana, and Alabama, USA, and assessed whether soak time limitations could be an effective method to reduce overall mortality. Under relatively short soak times (approximately 1–3 h), at-vessel mortality (AVM) in great hammerheads (AVM 19%, n = 77) and scalloped hammerheads (AVM 13%, n = 76) was not significantly predicted by sex, size, or water temperature, but was correlated with hooking duration. PRM was 43% (of n = 21) for great hammerheads and 12% (of n = 25) for scalloped hammerheads following hooking durations ranging from 6 min to 3.3 h. In great hammerheads, PRM was associated with decreasing blood pH and chloride concentrations, increasing lactate levels, and worsening observer assigned release conditions, whereas no at-vessel metrics were significantly associated with PRM in scalloped hammerheads, although the PRM sample size in this species was low. Collectively, these results increase our understanding of the effectiveness of soak time limitations and other management initiatives for reducing bycatch mortality in hammerhead sharks, while allowing more accurate estimates of total fishing mortality to be incorporated into assessment of stock status for these threatened species.