Environmental, operational, and physiological causes of elasmobranch mortality during fisheries capture

Commercial and recreational fisheries around the world typically target specific catch species, but often capture other species incidentally, some of which are retained (byproduct) and others are discarded (bycatch). This incidental fisheries capture is a major threat to global elasmobranch (shark and ray) populations because bycaught animals can die during capture or post-release. Furthermore, their life history traits of slow growth, late maturity, and low fecundity put elasmobranchs at generally greater ecological risk of overexploitation than bony fish species. Although the potentially deleterious effects of incidental capture are raising conservation and sustainability concerns, factors influencing the immediate and post-release mortality rates of elasmobranch discards are not fully understood. Accordingly, a greater understanding of the factors affecting these mortality rates is necessary to develop fisheries management strategies that reduce the detrimental effects of fisheries capture on elasmobranch bycatch. This study aimed to address these knowledge gaps by investigating environmental, operational, and physiological factors contributing to immediate and post-release mortality in elasmobranchs. This was achieved by a combination of meta-analysis, field sampling, laboratory analysis, and modelling of capture data. Species-specific analyses were conducted on log-book and empirical data and revealed that several factors influenced immediate mortality rates in longline-caught sharks, including capture duration (bronze whalers), sea surface temperature (blue sharks), fishing location(blue sharks, tiger sharks), regulatory period (blue sharks, tiger sharks), target catch (blue sharks, tiger sharks), and an interaction effect between regulatory period and fishing location(blue sharks). Following gillnet capture, total length was the only factor examined that affected immediate mortality rates (blacktip reef sharks). To determine how contrasting respiratory strategies and fishing gear types can influence mortality, I conducted a multi-species meta-analysis on published elasmobranch immediate and post-release mortality rates. This meta-analysis revealed that elasmobranch immediate and post-release mortality are more likely during trawl and gillnet capture than longline capture, and that obligate ram-ventilating species are particularly susceptible to mortality. Hematological analyses showed that high concentrations of plasma lactate and potassium were correlated with poor capture condition in bronze whalers. As high plasma lactate and potassium concentrations are associated with struggling and insufficient oxygen intake during capture, this result further demonstrated the vulnerability of obligate ram-ventilating sharks to immediate mortality caused by fisheries capture. My results suggest that fisheries management strategies such as reductions in allowable soak times and the mandatory use of circle hooks can effectively reduce immediate mortality rates in several shark species, such as bronze whalers and blue sharks. Following a reduction in immediate mortality rates, fisheries management strategies that promote the rapid release of bycatch can be effective. However, several elasmobranch species and groups have inherently high immediate mortality rates following capture (e.g. nervous sharks and juvenile blacktip reef sharks). Given the high immediate mortality rates of these species, catch and release is not a viable conservation strategy. Fisheries regulations designed to protect species with high immediate and post-release mortality rates should instead minimize the likelihood of interactions with fishing gear (e.g. gear modifications such as changing minimum allowable gillnet mesh size or spatio-temporal closures). The strategies recommended can be implemented to reduce the pressures global fisheries exert on elasmobranch bycatch populations.