Survival rate of silky sharks ( Carcharhinus falciformis) caught incidentally onboard French tropical purse seiners

Currently, French purse seiners in the Indian Ocean release all sharks and rays that are caught in order to reduce the fishery induced mortality of elasmobranchs. Through participation in two commercial fishing trips, we first recorded the number of sharks (primarily silky sharks, Carcharhinus falciformis) that were alive or dead, once they had been sorted by the crew on the upper and lower decks. More sharks were observed in the lower deck (73%) than in the upper deck. The silky sharks observed on the upper deck were significantly larger than the ones found in the lower deck. The immediate mortality (sharks that were dead at the time of observation) rates appeared to be linked with the location of the individual, as more sharks were found dead on the lower deck than the upper deck. The immediate mortality rates also increased with the set size (tonnage). In total, 20 silky sharks (125.3 plus or minus 33.8 cm total length) were tagged with MiniPATs (Wildlife Computers, Redmond, WA, USA) to study their survival after release. Six tags clearly showed mortality directly after release, while data from three tags suggested delayed mortality after 2.5, 14 and 15 days. Nine tags showed that the sharks survived. Two tags failed to report data and one was incorrectly initiated. Following these findings a 'best practices' manual for fishers will be prepared to increase rates of survival of sharks caught by purse seine vessels. However, other methods prior to the sharks being brought onboard must also be investigated.