Abstract
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 associa- tive behaviour of this species with floating objects remains limited. Here the use of various elec- tronic 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). Pres- ence/absence and swimming depth data were transmitted from the receivers. A subset of 17 indi- viduals 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.
Highlights
Tuna purse seine vessels targeting tropical tuna species deploy large numbers of drifting fish aggregating devices (FADs) throughout the world’s tropical and subtropical waters
While the possibility that they died cannot be excluded, data received from the pop-up satellite archival tags (PSATs) fitted to 8 of these individuals indicated that they survived the tagging process
Data were analysed from 20 sharks tagged with coded acoustic tags, of which 5 had a PSAT and 1 was recaptured with an internal archival tag, which was recovered after 141 d at liberty
Summary
Tuna purse seine vessels targeting tropical tuna species deploy large numbers of drifting fish aggregating devices (FADs) throughout the world’s tropical and subtropical waters. Over time these FADs aggregate huge numbers of tunas as well as several non-tuna pelagic species. The principal species targeted through this practice are skipjack tuna Katsuwonus pelamis, yellowfin tuna Thunnus albacares and bigeye tuna T. obesus. Mar Ecol Prog Ser 539: 207–223, 2015 oceanic triggerfish Canthidermis maculata and wahoo
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.