Rates of at-vessel mortality and post-release survival of pelagic sharks captured with tuna purse seines around drifting fish aggregating devices (FADs) in the equatorial eastern Pacific Ocean

Abstract

Pelagic fishes are well known to aggregate in large numbers under floating objects and this behavior is frequently exploited by purse seine fisheries targeting tunas. Non-target species (e.g., sharks) are often caught as well, but they are typically discarded as they do not have sufficient commercial value. To investigate the total mortality of pelagic sharks in the equatorial Eastern Pacific Ocean associated with the tuna purse seine fishery deploying drifting fish aggregating devices (FADs), we measured rates of at-vessel mortality and deployed pop-up satellite archival tags (PSATs) to monitor post-release survival and behavior. Between 2011 and 2012, at-vessel mortality rate ranged from 15% to 70%, and total mortality rate (i.e. the combination of at-vessel and post-release mortalities) ranged from 80% to 95%. Taken together, our findings document the high mortality of sharks incidentally captured in the tuna purse seine fishery that employs drifting FADs, indicate a correlation to set size, and suggest the need to develop methods that minimize shark bycatch in this fishery.