Undetected silky sharks (Carcharhinus falciformis) in the wells of the tropical tuna purse seine fleet in the Indian Ocean

The catches of the Spanish purse seine fleet targeting tropical tunas in the Indian Ocean have been systematically sampled in port and at sea by scientific research centres since the fleet began to operate in the 1980s. During these samplings, some silky shark (Carcharhinus falciformis) specimens were found in the wells of these vessels which had not previously been recorded by at-sea scientific observers. To quantify the occurrence of these undetected incidental catches of silky sharks, this study compared two sets of data: on-board data collected by scientific observers and port sampling data. The European Union's long-term data collection program (PNDB), coordinated by the Spanish Fisheries Secretariat, provides on-board data collected by scientific observers as well as port sampling data which is collected while a vessel arrives to the port to begin unloading. The sampling focused on target species up until January 2021, when sampling started including non-target species as well. The datasets examined in this study are from January 2021 to December 2022. The results show significant differences between silky shark sizes observed on-board and those measured in port. A logistic model indicates a significant probability of observing silky sharks in wells, with a strong goodness of fit and high discrimination capacity as a function of the total catch of the fishing operation. Further analysis reveals differences in average sizes and weights of silky sharks caught in free-schools compared with those caught with tuna schools aggregated beneath Fish Aggregating Devices or FADs. The presence of unnoticed silky sharks suggests unreported captures, indicating that rates of mortality of the species are underestimated. In conclusion, the research emphasizes the need to address undetected silky shark bycatch in the purse seine fishery in the Indian Ocean. Obtaining accurate data and understanding the magnitude of this bycatch are crucial for developing management strategies that mitigate the impact and promote the sustainability of silky shark populations in the region.