Drivers of at-haulback mortality of sharks caught during pelagic longline fishing experiments

Elasmobranchs (sharks and rays) are a critical part of the bycatch in tropical pelagic fisheries (longline, purse seine, gillnet). The induced mortality can be a major threat to populations especially for vulnerable or endangered pelagic elasmobranchs. Even though retention bans are enforced for some species, it is crucial to reduce the mortality before individuals are released (at-haulback mortality) in longline fisheries. So far, little is known about the drivers of this at-vessel mortality for elasmobranchs. We used data collected during longline fishing experiments (ECOTAP program) in French Polynesia (Central South Pacific Ocean) between 1993 and 1997. Multivariate logistic regression models were used to assess the influence of factors on the survival of the blue shark, oceanic whitetip shark, shortfin mako, and silky shark. Factors related to fishing techniques, biological and environmental conditions have an impact on at-vessel mortality of several shark species. Epipelagic species (oceanic whitetip and silky shark) are more likely to die when caught at depths deeper than their usual mixed surface layer habitat, and the odds of survival increase with individual's size. Mesopelagic species (blue shark and shortfin mako) survival significantly decreases with time spent being hooked. In regards to the impact of these factors on the at-haulbacvk mortality, mitigation measures aiming to survival at release are discussed.