Bigeye thresher shark Alopias superciliosus movements and post-release survivorship following capture on linked buoy gear
Introduction
Recent fishery development work off the U.S. West Coast has resulted in the recommendation for authorization of a deep-set fishery for swordfish that primarily operates within the Southern California Bight (SCB)1 . The soon-to-be-authorized fishery will allow for the use of two gear configurations, deep-set buoy gear (DSBG) and linked-buoy gear (LBG), designs that were developed to reduce spacial overlap with protected species by positioning up to 30 baited hooks below the thermocline during the daytime (Sepulveda et al., 2015; Sepulveda and Aalbers, 2018b).
Similar catch composition has been reported between the two configurations following both research and exempted trials, with catch predominantly consisting of swordfish (Xiphias gladius; >75 %) and bigeye thresher sharks (Alopias supercilliosus; <12 %). Although occasionally retained for sale, the vast majority (∼90 %) of bigeye thresher sharks (BETS) are released or discarded due to a limited market demand (Walsh et al., 2009; Sepulveda et al., 2019b). Given the need to better document the impact of a new deep-set fishery on the BETS resource, an initial study was conducted to quantify the post-release survival of BETS following capture on DSBG (Sepulveda et al., 2019b). Findings revealed a low post-release mortality rate, with ∼90 % of BETS surviving the acute effects of capture.
Despite many functional similarities in the design of LBG and DSBG (i.e. target depth, number of baited hooks, strike-indicator buoy system), LBG incorporates a heavy weighting system and extended horizontal mainline that may further restrict vertical and horizontal movement of catch while on the line (Fig. 1). Given that BETS are obligate ram ventilators and LBG hook depths occur near the oxygen minimum zone (OMZ; Levin, 2003), restricting movement has the potential to impact post-release survival. Further, because the horizontal footprint of LBG may be up to 5 nm, increased catch processing times have the potential to impact survival.
Considering the high vulnerability of BETS to over exploitation (Gruber and Compagno, 1981; Chen et al., 1997; Smith et al., 2008; Amorim et al., 2009) and the additional source of catch and effort likely to occur under the new California deep-set fishery, this work focused on assessing post-release fate of BETS following capture on LBG. Movement data for this poorly studied species were also used to characterize depth and temperature distribution and better assess how ongoing fishing operations may be tailored to further reduce future BETS interaction rates.
Section snippets
Location, permitting and data collection
Tag deployments were focused within the SCB from Santa Cruz Island (33.92, -119.63) to Oceanside, CA (33.24, -117.68) between August 2016 and December 2019 (Table 1; Fig. 2). Tagging activities were performed during LBG research trials conducted under a NOAA letter of authorization (LOA) as well as exempted fishing activities (EFP) sanctioned by the PFMC and NOAA West Coast Regional office (WCR). All cooperative fishers were trained on tagging and release protocols and had participated in
Survivorship
Survival rate was estimated from a total of 14 bigeye thresher sharks (157−230 cm LF) that were captured on 90 LBG sets (737 LBG sections; 2211 baited hooks) soaked over 646 h from August 25, 2016 through December 5, 2019. Fight times ranged from 17 to 300 min (mean = 64 + 77 min; Table 1) with haul-back times (included in overall fight time) ranging from 8 to 30 min (mean = 13.5 ± 4.5 min). Time depth recorders revealed a mean capture depth of 304 ± 20 m (range = 265−327 m) on LBG sets within
Discussion
This study offers insight into the post-release fate, movements and habitat utilization of a poorly known species that is routinely caught in a novel deep-set fishery for swordfish. The high rate of survivorship observed on both LBG and DSBG (Sepulveda et al., 2019b) suggests that BETS are remarkably resilient to capture stress and that overall fishing mortality rates in the new fishery may be considered low. Distinct vertical profiles were consistent with reports from previous bigeye thresher
CRediT authorship contribution statement
Scott A. Aalbers: Writing - original draft, Investigation, Formal analysis, Data curation. Michael Wang: Validation, Methodology, Writing - review & editing. Charles Villafana: Project administration, Resources. Chugey A. Sepulveda: Funding acquisition, Conceptualization, Visualization, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We are grateful for the continued support of the National Oceanic and Atmospheric Administration Bycatch Reduction and Engineering Program (BREP Award # NA18NMF4720288). Additional project support was leveraged from ongoing projects funded through the NOAA Saltonstall-Kennedy Grant Program along with contributions from the Pew Charitable Trusts, The Nature Conservancy, Santa Monica Seafoods, the George T. Pfleger Foundation, the Offield Family Foundation and the William H. and Mattie Wattis
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