Post-release survivorship and movements of bigeye thresher sharks, Alopias superciliosus, following capture on deep-set buoy gear
The bigeye thresher shark (Alopias superciliosus) is a large pelagic predator routinely caught in a new deep-set fishery off California. Deep-set buoy gear (DSBG) was recently developed to selectively target swordfish (Xiphias gladius) at depth during the daytime and is currently being trialed by a limited fleet of vessels operating under an exempted fishing permit (EFP) issued through the National Marine Fishery Service (NMFS) and Pacific Fisheries Management Council (PFMC). Although marketable, bigeye thresher sharks (BETS) are often released or discarded to conserve limited hold space for more valuable catch. This study outfitted BETS with pop-up satellite archival transmitters (PSATs) programmed for 30-day deployments to evaluate acute post-release survival in the DSBG fishery and begin to understand depth and temperature distribution for this poorly known species. Fourteen BETS (43–93 kg) were tagged from 2016 to 2018 with PSATs (Wildlife Computers sPATs and MiniPATs) following fight times that ranged from 15 to 171 min. Out of the 14 deployments, twelve individuals survived the acute effects of capture, one BETS died shortly after release and one individual was preyed upon 6 h after release. Given the lack of information surrounding the predation induced mortality, two survivorship estimates are provided, one that includes the event as a mortality resultant of the capture event (86% survivorship) and one that excludes the event from the analysis (92% survivorship). All surviving BETS exhibited consistent diurnal dive patterns in which the sharks remained below the thermocline during the day and within the mixed layer at night. Mean daytime depths ranged from 250 to 439 m, while mean nighttime depths ranged from 20 to 106 m. Daytime temperatures at depth ranged from 6 to 10 °C with the lowest temperature value (6.0 °C) recorded on the deepest dive (548 m). For all BETS, maximum night temperatures averaged from 22.5 to 25.1 °C. Two tagged individuals remained proximal to the study area (<150 km) with others moving up to >1300 km from the tagging site (mean = 1009 ± 455 km). Mean daily rates of horizontal movement ranged from 3.4 to 41.6 km day−1. The most common movement trend entailed BETS moving in a southerly direction towards a series of offshore seamounts along southern Baja California, Mexico (22°-26 °N and 114°-126 °W), a purported offshore corridor for other large pelagic sharks of the eastern Pacific.