At-vessel and post-release mortality of the dusky (Carcharhinus obscurus) and sandbar (C. plumbeus) sharks after longline capture

Abstract

Since the mid 1990’s, stock assessments of shark populations in the northwest Atlantic by the National Marine Fisheries Service (NMFS) have indicated that sandbar (Carcharhinus plumbeus) and dusky (C. obscurus) populations are overfished. In response to these assessments, the dusky shark bottom longline fishery was closed in 2000, followed by the sandbar fishery in 2005. A common management strategy is to mandate the release of prohibited species if caught as bycatch. However, a major assumption is that most released sharks survive, which may not be the case. Longline operations were conducted during 2011 and 2012 to capture and tag sandbar and dusky sharks, in order to assess (1) at-vessel mortality, (2) post-release mortality, and (3) investigate the effects of soak-time on each of these variables for each species. Our findings show that dusky sharks experience high at-vessel and post-release mortality, with mortality occurring more frequently after 3–5h on the line. While time-on-the-line (TOL) increased dusky shark at-vessel (16% when less than 3h TOL; 27.5% when more than 3h TOL) and post-release mortality (11% when less than 3h TOL; 42% when more than 3h TOL), sandbar shark at-vessel mortality was low (0% when less than 3h TOL). Nonetheless, the apparently hardier sandbar sharks did exhibit 29% post-release mortality after 3h TOL. The total mortality (at-vessel+post-release, including moribund individuals) for dusky sharks was 97% after being captured on a demersal longline when TOL surpassed 3h. These data clearly elucidate an interspecific difference in vulnerability to longline gear between these species.