The Post-release Condition of FAD Associated Silky Sharks (Carharhinus falciformis) Caught in Tuna Purse Seine Gear

Silky sharks (Carcharhinus falciformis) are frequently caught in tuna purse seines set around fish aggregating devices (FADs). During a recent Bycatch Mitigation Research Cruise in the western central Pacific Ocean (WCPO) sponsored by the International Seafood Sustainability Foundation (ISSF) we were able to characterize the frequency of capture and size frequency characteristics of silky sharks caught on drifting FADs and conduct experiments to assess the post-release survival of sharks released at various stages of the fishing operation. We used a combination of Pop-off satellite archival tags (PSATs) and acoustic and conventional I.D. tags to assess behaviour and post-release survival. In addition, we took blood samples for analysis of various biochemical stress indicators that can be used to aid in interpretation of the post-release survival results. To establish baseline blood gas, metabolite and stress hormone levels, some sharks were caught, sampled and tagged prior to the onset of purse seine fishing operations or while still free swimming inside the net. These baseline levels can now be compared with the blood chemistry of sharks caught and released at different stages of the fishing operation. During the 31 sets of the cruise, we interacted with a total of 295 silky sharks, deployed 28 satellite tags and took blood samples from 86 sharks caught during various stages of the fishing operation. Here, we present the preliminary findings of the survival rates of a sub-set of silky sharks released with "survival Pop-off archival tags" (SPATs). Initial results indicate that sharks released in excellent condition and those that are landed early in the fishing operations have increased survival rates post-release.