Silky shark (Carcharhinus falciformis) bycatch in the French tuna purse-seine fishery of the Indian Ocean
Data collected through 20 observer fishing trips were used to quantify the number of silky sharks taken as bycatch by the French tuna purse seine fishery of the Western Indian Ocean. 1,385 immature silky sharks of which 85% was discarded at sea and 15% retained aboard, were observed as bycatch during 685 fishing sets observed from October 2005 to April 2008. Zero-inflated regression models fitted with Bayesian methods were used to explain silky shark bycatch as a function of fishing mode (free vs. fishing aggregating device-associated (FAD) schools), area, and season. Model results showed that silky sharks occurred in 24% of the fishing sets with an expected number of sharks per set estimated to be 2.02 ± 0.05. The 3 covariates were found to significantly explain both the presence and number of silky sharks caught by the French purse seiners. FAD was shown to have a strong positive effect on the number of silky sharks caught, an expected value of 4.3 sharks being taken in FAD associated schools versus 0.3 shark in free schools. There were significant differences in silky shark bycatch between seasons and areas with higher bycatch than average in July-September and in the South-East Seychelles area while fewer sharks were expected to be caught in the North Somali area. Results are discussed within the context of the ecosystem approach to fisheries for the analysis of ecosystem effects of fishing.