Silky shark post release survival evaluation in tropical tuna purse seiners using hoppers with ramps

Silky sharks are the principal elasmobranch bycatch species encountered in tropical tuna purse seine sets. In this work we present results from research cruises on purse seiners equipped with hoppers with ramps on their working deck. The hopper’s tray improves the chances of spotting sharks occurring within the tuna catch, and the door can greatly prevent sharks from accidentally going down to the lower deck, from where release times are delayed. The ramp adds a safety element, as less shark handling is required by crew, and helps speed up release. Release times for sharks on these vessels were mostly less than two minutes. Our results show that shark survival can be significantly enhanced using hoppers with ramps, doubling and even tripling shark survival rates reported in previous purse seine studies. However, there can be post-release survival (PRS) variability, which can be attributed to different factors. Particularly, time in the sack seems to affect the state in which sharks arrive on deck, presumably due to an inability to respire for obligate ram ventilators whilst confined in the sack. Therefore, hoppers with ramps have the capacity to maximize the survival of sharks arriving onboard in good condition, but research should also focus on actions prior to and during the set, that ensure sharks are released before sacking or at least arrive on deck in the best condition possible.