Assessing the Impacts of Commercial Tuna Purse Seine Fishing On Silky Shark Populations

The silky shark, Carcharhinus falciformis is taken in both target shark fisheries and as by-catch in commercial longline and purse seine fisheries worldwide, making this species one of the ocean's most commercially exploited elasmobranchs. Commercial landings are estimated at over 309,000 individuals caught annually in the western and central Pacific Ocean (WCPO) alone. A recent stock assessment in the WCPO concluded the stock is overfished at levels far in excess of the maximum sustainable yield. Due to concerns about the status of the population this study was initiated to quantitatively assess the impact the commercial purse seine fishery has on silky sharks and to identify the point in the fishing operation when sharks sustain injuries that result in mortality. During a chartered research cruise aboard a purse seiner in the WCPO, blood was drawn from candidate sharks captured during different stages of the fishing operations to identify key physiological parameters associated with capture stress and mortality. Survival and mortality of these sharks was validated using the satellite transmissions of pop-off archival tags (PAT) from 14 silky sharks that were both tagged and blood sampled. Post release mortality was predicted using pH and lactate levels (n = 87) and was found to increase once the sharks had been confined in the sack.  Any efforts to reduce the mortality of sharks captured in purse seine gear need to focus on releasing sharks before this stage, while they are still free swimming.