Bigeye thresher shark Alopias superciliosus movements and post-release survivorship following capture on linked buoy gear
Off the US west coast, fishery development efforts have led to the recent recommendation for authorization of deep-set buoy gear (DSBG) and linked buoy gear (LBG), two commercial gear types for targeting swordfish at depth during the day. Because the new deep-set configurations interact with bigeye thresher sharks (Alopias superciliosus), a large pelagic species that is not typically retained for sale, this work has focused on documenting the fate of released catch. Bigeye thresher sharks (BETS) were captured and outfitted with pop-up satellite archival tags (Wildlife Computers sPATs and miniPATs) to assess movement trends and acute (30-d) post-release survival during exempted fishing trials authorized by National Marine Fisheries Service (NMFS) and the Pacific Fisheries Management Council (PFMC). Post-release survivorship was based on 14 BETS (∼157−230 cm LF) caught on LBG between August 2016 and December 2019 following fight times ranging from 17−300 min. Thirteen BETS survived the acute effects of capture, and one immediate mortality was documented following heavy entanglement in the monofilament leader. BETS were resilient to the acute effects of handling stress following capture on LBG, with an observed post-release survival rate of 93 %. Surviving BETS exhibited consistent diurnal dive patterns with sharks remaining well below the thermocline during the daytime (mean = 386 ± 62 m) and within the mixed layer at night (mean = 65 ± 25 m). With the exception of one tag that reported within close proximity of the SCB tagging location, BETS exhibited extensive southward movements (mean = 1235 ± 235 km) towards a potentially relevant migratory corridor for large pelagic sharks.