Elasmobranches bycatch in the French tropical purse-seine fishery of the eastern Atlantic Ocean: spatio-temporal distributions, life stages, sex-ratio and mortality rates

Marine megafauna, especially sharks and rays, are caught as bycatch by the tropical tuna purse-seine fishery. We studied their spatio-temporal distribution patterns by species and by the diversity of assemblages, as well as by differentiating juveniles and adults in the eastern Atlantic Ocean. We also investigated sex-ratios and mortality rates at release. Data were collected by scientific observers onboard French purse-seiners between 2005 and 2017. Among the 18 species of elasmobranches caught, 85.4% of the individuals were silky sharks. Distributions of catch per unit of effort (CPUE) by species, sex-ratios and diversity indices varied with life stages, areas, seasons and fishing modes (fish aggregating device vs. freeswimming tuna school sets). These differences appear to be linked to specific environmental conditions occurring in some areas and seasons. Higher elasmobranches catch rates in FAD sets (40%) compared to FSC sets (17%) were detected. Overall, this study highlights high elasmobranches bycatch rates, high mortality rates for most species (12.76–56.93%; average 45.8%), and high proportion of juveniles caught for the large majority of species (21.27–100%; average 87.4%).