The impacts of commercial purse seine fishing on the biology and ecology of the silky shark, (Carcharhinus falciformis): implications for science based management

Abstract

The silky shark (Carcharhinus falciformis) is a circum-tropical species found in all oceans between 20° North and South latitude. This distribution tends to overlap with the preferred habitat of three heavily exploited tropical tuna species (bigeye, Thunnus obesus, yellowfin T. albacares, and skipjack, Katsuwonus pelamis). The western and central Pacific Ocean (WCPO) is the most productive tuna fishery on the globe (2,613,528 mt in 2012) with the largest proportion of tuna landed via purse seine fishing operations (69%). In large part, this productivity is feasible because in the WCPO, the tuna purse seine fishery utilizes drifting fish aggregating devices (FADs) to attract and hold tuna and thereby reduce search times and cost. Juvenile silky sharks also aggregate in large numbers at FADs and, as a result, comprise the largest component of the incidental elasmobranch catch taken in the tropical tuna purse seine FAD fishery. A stock assessment revealed that silky shark spawning biomass, median size and CPUE have all declined and concluded that the WCPO stock(s) are overfished and that overfishing is occurring. Population analysis for this species has shown that high mortality during the juvenile life stages has the largest impact on population growth. Information on the basic biology, behavioral ecology and habitat use of this species is deficient. To fill these data gaps and better manage the impact that fishing has on this species I conducted an analysis of the survival rates and total mortality of sharks captured in purse seine gear. Survival was related to the stage during the fishing operation when sharks were released. This work was augmented by an investigation of the physiological disruptions that lead to mortality when sharks are captured in a purse seine. This was achieved by measuring stress hormones, blood gases and electrolyte concentrations in sharks sampled at various stages of the fishing process and by an analysis of the habitat use and movement behavior of juvenile silky sharks in the Pacific Ocean. Taken together, these data will be crucial to developing meaningful conservation tactics and future bycatch mitigation tools.