Silky shark population trend in the Indian ocean derived from its associative behaviour with floating objects

Silky sharks (Carcharhinus falciformis) figure among the main pelagic shark species caught by the industrial tropical tuna purse-seine fisheries. However, this data was not used so far for estimating their population trends. In this study, using data from the European tropical tuna purse seine fishery, we provide an abundance trend for the silky shark, based on the associative behavior of this species with floating objects (FOBs). Two models were used, describing the dynamics of sharks associated to floating objects (FOBs) in a social and in a non-social case. The parameters estimates of the models were obtained by fitting the distribution of the number of sharks caught per set. The relative abundance indices were derived for the Seychelles area and the Mozambique Channel. For both areas, an upward trend was observed. In the Seychelles area, the abundance index increased by a factor of 3 from 2006 to 2018 and in the Mozambique Channel the increase reached a factor of 15. This modeling approach could be extended to other bycatch species to generate population trends and could be useful for future stock assessment analyses.