Post-release mortality of oceanic whitetip sharks caught by purse seiners – POREMO project

Oceanic whitetip shark (Carcharhinus longimanus – OCS) is a sensitive species present in the Indian Ocean that is classified as globally “Critically Endangered”. OCS are occasionally bycaught by tuna purse seine fleets operating in the Indian Ocean. The retention of OCS is prohibited and despite the efforts made to handle and release those sharks, post-release mortality (PRM) of OCS needs to be assessed. In the frame of the POREMO project that is dedicated to this particular task, onboard observers deployed electronic tags (16 sPATs and 3 miniPATs) on OCS released by French, Spanish and Mauritian purse seiners in the western Indian Ocean between 2018 and 2023. Based on the 16 tags that reported data, we determined the overall PRM rate and explored the effect of fish condition upon release, individuals’ size, sex, and time spent on deck before release on the PRM. We found that the overall PRM for OCS bycaught by purse seiners was 18.75 %. Our results show that PRM is dependent on fish condition upon release, with full survival for lively individuals (100 %) and poorer survival chances for the injured and moribund ones (20 and 0 %). The time that individuals spend on deck directly affects their condition at the time of release, subsequently influencing their likelihood of survival. Furthermore, the size of individuals size appears to enhance the survival prospects of injured and moribund individuals. Those results suggest that handling carefully and releasing promptly bycaught OCS would likely improve their PRM.