Catch Rates and Distribution Pattern of the Silky Shark, Carcharhinus falciformis, Caught by the Taiwanese Large-scale Longline Fishery in the Indian Ocean

Abstract

The silky shark, Carcharhinus falciformis, is widely distributed in tropical and temperate waters, and it is a common bycatch species for tuna longline fisheries. This study examined the distribution of and presents relative abundance indices of the silky shark in the Indian Ocean by using logbook and observer data from the Taiwanese large-scale tuna longline fishery between 2005 and 2019. Due to the high zero catch rate, a zero-inflated negative binomial (ZINB) model was used to standardize catch per unit effort. Due to a lack of detailed targeting information, the fishery strategy was identified by using cluster analysis based on catch composition and then incorporated as an explanatory variable related to the target species in the ZINB model. Size segregation was observed for males and females in the Indian Ocean. Juveniles were mostly concentrated between 10 degrees S and 10 degrees N. Cluster analysis results revealed five fishing clusters based on catch composition that explained the variance in the ZINB models. Our integrated approach improves the understanding of spatiotemporal silky shark dynamics in the Indian Ocean and can be used to derive relative abundance indices for stock assessment and management.