Estimating post-release mortality of long-line caught tropical tunas in the Pacific Ocean

Post-release mortality experiments were undertaken on bigeye (Thunnus obesus) and yellowfin (T. albacares) tuna to assess potential post-release survival probabilities of conventional tagging on such individuals during commercial longline operations. Survival was estimated from the release of 32 tuna tagged with pop-up, satellite survivorship tags (sPATs) during a commercial fishing trip aboard a Hawaii based tuna longline vessel. The tagged fish ranged in size from 103 to 145 cm fork length in size. Of the 32 sPATs released, 27 reported earlier than the pre-programmed 60 days, and five never reported. Of the 27 tags that reported data one tag appeared to shed early while the fish was still alive and swimming 32 days post-release while the remaining 26 tags deployed early due to mortality or other events, as observed through either sinking, ingestion by predators, or detached tags. Most mortalities occurred within four days post-release. The overall survival probability was estimated at around 10%, with some evidence that reduced time on the hook may increase this probability up to 50%. These results suggest that bigeye and yellowfin tuna caught during typical longline operations are unlikely to be suitable for electronic or conventional tagging experiments. Our study also suggests all discarded tuna should be treated as mortalities (including those discarded alive) for stock assessment purposes.