Fine-scale 3-dimensional movement behaviour of silky sharks Carcharhinus falciformis associated with fish aggregating devices (FADs)

The silky shark Carcharhinus falciformis is the primary elasmobranch bycatch in the global tuna purse seine fishery using fish aggregating devices (FADs). Information on the associative behaviour of this species with floating objects remains limited. Here the use of various electronic tags provided important new insight into this behaviour. Thirty-eight juvenile silky sharks (69 to 116 cm total length; TL) were tagged with acoustic tags at 9 drifting FADs equipped with satellite-linked acoustic receivers in the western Indian Ocean (total monitoring = 154 d). Presence/absence and swimming depth data were transmitted from the receivers. A subset of 17 individuals was also fitted with pop-up satellite archival tags (PSATs; n = 13), or internal archival tags (n = 4). Behavioural data were successfully collected from 20 of the tagged sharks, covering a total of 300 d. Fine-scale movements of one individual were observed by active tracking, lasting 2 h 46 min. Sharks remained associated with the FAD where they were tagged for extended periods (2.84 to 30.60 d, mean = 15.69 d). Strong diel changes were observed in both FAD association and swimming depth. Typically, individuals moved away from FADs after sunset and returned later that night, then remained closely associated until the following evening. Vertical behaviour also changed around sunset, with sharks using fairly constant depths, > 25 m, during the day and switching to rapid vertical movements during the night, with descents > 250 m recorded. The actively tracked individual returned to a FAD from >1.2 km away. Long residence times and close association highlight the vulnerability of silky sharks to incidental capture in FAD fisheries.