First Descriptions of the Behaviour of Silky Sharks, Carcharhinus falciformis, Around Drifting Fish Aggregating Devices in the Indian Ocean

Silky sharks, Carcharhinus falciformis (Muller and Henle, 1839), are the primary elasmobranch bycatch species in tuna purse seine fisheries throughout the world's major oceans. Juveniles of this species commonly associate with drifting fish aggregating devices (FADs) deployed to enhance tuna catches in these fisheries. Here we present results from the first investigation into the behavior of juvenile silky sharks associated with drifting FADs in the western Indian Ocean. A total of 10 silky sharks were tagged with coded acoustic transmitters around drifting FADs equipped with acoustic receivers. Following release, all sharks undertook an excursion away from the FAD with which they were associated. Two individuals were subsequently never detected, while the initial absence period of the eight detected sharks ranged between 0.1 and 3.5 d. After returning, total association times averaged 5.19 d (sd=3.15 d) and related horizontal movement rates ranged from 8 to 50 km d-1. Short excursions away from the FAD were undertaken by some individuals, all of which lasted a few hours and were made at night. During periods of association, silky sharks typically occupied the upper 35 m of the water column for the majority of the observation period. These results provide new insights into our understanding of associative behaviors. Further studies are needed to improve assessment of the impacts of FADs on the ecology of this species, a major concern considering the large number of FADs deployed.