Capture-Related Mortality and Post-Release Survival of Pelagic Sharks Interacting with Tuna Purse Seines in the Eastern Pacific Ocean

Pelagic fish are known to aggregate with floating objects in large numbers and this behavior is frequently exploited by purse seine fisheries across the globe to increase the landings of tuna. Non-target species are incidentally caught in large numbers as well, but as this bycatch may be less commercially valuable, it is typically discarded. Recently, the survival of many bycatch species has been questioned and mounting evidence suggests capture-related stress may often lead to post-release mortality. To investigate the total mortality of the shark bycatch in the purse seine under drifting floating objects (i.e., fish aggregating devices) in the Eastern Pacific Ocean, pop-up satellite tags were deployed on sharks to monitor post-release behavior and survival. Blood samples were analyzed to investigate the physiological stress response associated with capture. Results indicate a large proportion of sharks did not survive (79% of silky sharks and 50% of scalloped hammerheads), and the blood data suggest that oxygen deprivation during the capture event may cause these high mortality rates. Taken together, these findings emphasize the need to develop methods that minimize their capture and stress in order to maximize their survival.