Observing post-release mortality for dusky sharks, Carcharhinus obscurus, captured in the US pelagic longline fishery

摘要

The latest stock assessment for the dusky shark, Carcharhinus obscutus, in the western North Atlantic Ocean indicates the population is overfished and experiencing overfishing. As part of a rebuilding plan, the commercial and recreational retention of dusky sharks has been prohibited since 2000. Despite this prohibition, dusky sharks are bycatch in multiple fisheries, including the pelagic longline fishery; however, post-release mortality (PRM) rates have not been empirically determined for this gear. Herein we estimated PRM of dusky sharks captured by the US pelagic longline fleet in the western North Atlantic Ocean utilizing pop-up satellite archival transmitting (PSAT) tags. One hundred and twenty three dusky sharks were captured on commercial pelagic longline gear and time on the hook, based on hook timer data, ranged from 0.8 to 8.1 h (4.3 +/- 0.28 h). No at-vessel mortality (AVM) was observed for any dusky sharks in this study. Prior to release, 50 PSAT LIFE tags (Lotek Inc.) were attached to dusky sharks (females n = 12, 209 +/- 8 cm FL; males n = 4, 198 +/- 7 cm FL; unknown sex n = 34, 214 +/- 7 cm FL) to assess PRM rates in the pelagic longline fishery during a 30 day attachment period. Forty-three of the 50 deployed tags reported data with deployment times ranging from 1 to 28 days (11.2 +/- 9.8 days). Four dusky sharks were in poor condition at release and two individuals suffered PRM, which occurred within two hours after release. Total mortality rate (AVM + PRM) in the current study was 5.1%, far below estimates reported for bottom longline gear (97%), and reinforces the notion that PRM should be evaluated by species, season, and gear type.