A cross‐taxa assessment of pelagic longline by‐catch mitigation measures: conflicts and mutual benefits to elasmobranchs

Abstract

Elasmobranch mortality in pelagic longline fisheries poses a risk to some populations, alters the distribution of abundance between sympatric competitors, changing ecosystem structure, processes and stability. Individual and synergistic effects on elasmobranch catch and survival from pelagic longline gear factors, including methods prescribed to mitigate bycatch of other vulnerable taxa, were determined. Overall relative risk of higher circle vs. J-shaped hook shark catch rates conditioned on potentially informative moderators, from 30 studies, was estimated using an inverse-precision weighted mixed-effects meta-regression modelling approach. Sharks had a 1.20 times (95% CI: 1.03-1.39) significantly higher pooled relative risk of capture on circle hooks, with two significant moderators. The pooled relative risk estimate of ray circle hook catch from 15 studies was not significant (RR = 1.22, 95% CI: 0.89-1.66) with no significant moderators. From a literature review, wire leaders had higher shark catch and haulback mortality than monofilament. Interacting effects of hook, bait and leader affect shark catch rates: hook shape and width and bait type determine hooking position and ability to sever monofilament leaders. Circle hooks increased elasmobranch catch, but reduced haulback mortality and deep hooking relative to J-shaped hooks of the same or narrower width. Using fish vs. squid for bait increased shark catch and deep hooking. Pelagic stingray (Pteroplatytrygon violacea) catch and mortality were lower on wider hooks. Using circle instead of J-shaped hooks and fish instead of squid for bait, while benefitting sea turtles, odontocetes and possibly seabirds, exacerbates elasmobranch catch and injury, therefore warranting fishery-specific assessments to determine relative risks.

Sea Turtle Bycatch Mitigation in U.S. Longline Fisheries

“…However, regulations requiring use of fish bait to reduce sea turtle bycatch must be balanced against the potential target species catch loss, a concern that has been previously evaluated (Watson et al, 2005;Yokota et al, 2009;Curran and Bigelow, 2011;Coelho et al, 2012). Further, bait choice can also potentially increase bycatch of certain sharks or other vulnerable species (Foster et al, 2012;Santos et al, 2012;Gilman et al, 2016b). As with other bycatch reduction techniques, success in adopting these measures may be fishery dependent.…”

“…Several studies and symposiums have evaluated the effectiveness of circle hooks across and found that reductions are not necessarily achieved for all non-target bycatch species taxa (Kerstetter and Graves, 2006;Serafy et al, 2012;Huang, 2015). In some cases, circle hooks may increase bycatch of sharks (see Gilman et al, 2016b). There is a clear need for further investigation of cross-taxa bycatch solutions in pelagic longline fisheries.…”

Using logbook data to determine the immediate mortality of blue sharks (Prionace glauca) and tiger sharks (Galeocerdo cuvier) caught in the commercial U.S. pelagic longline fishery

“…The immediate mortality of incidentally caught sharks can be substantial (Dapp et al, 2016a), and incidental capture has been identified as a major driver of declines in shark populations (Dulvy et al, 2014). Immediate mortality rates of sharks during longline capture can be influenced by a multitude of factors, such as, but not limited to, capture duration, hook type, hook size, species-specific biology, and crew-specific handling practices (Campana et al, 2009;Morgan and Carlson, 2010;Dapp et al, 2016aDapp et al, , 2016bGilman et al, 2016). Factors contributing to the immediate mortality of sharks caught by the U.S. Atlantic pelagic longline fishery have previously been examined by using the data set of the National Marine Fisheries Service (NMFS) Pelagic Observer Program (POP) (Serafy et al, 2012;Gallagher et al, 2014).…”

“…Although this rule was implemented to reduce bycatch mortality of species of turtles (NMFS 2 ), circle hooks are less likely than traditionally used Jhooks to be swallowed by some species of shark. The use of these hooks, therefore, reduces occurrences of internal injuries and immediate mortality rates in some species (Kerstetter and Graves, 2006;Campana et al, 2009;Serafy et al, 2012;Gilman et al, 2016;but see Yokota et al, 2006). In accordance with the methods of Serafy et al (2012), we considered only specific regulatory periods in our analysis.…”

“…Several studies have reported that the use of circle hooks instead of J-style hooks can reduce immediate mortality of blue sharks (Campana et al, 2009;Carruthers et al, 2009;Afonso et al, 2011;Afonso et al, 2012;Epperly et al, 2012;Godin et al, 2012;Gilman et al, 2016; but see Kerstetter and Graves, 2006;Yokota et al, 2006;Curran and Bigelow, 2011;Serafy et al, 2012) because such hooks decrease the likelihood of hooking in the gut (gut-hooking) in this species (Watson et al, 2005;Afonso et al, 2011). We cannot determine the specific postrelease survival rate for released animals, but delayed death due to gut-hooking may have been a further significant cause of mortality that Table 3 The effect of sea surface temperature (SST), regulatory period, and geographic zone on the immediate mortality of blue and tiger sharks in the U.S. Atlantic pelagic longline fishery during the period 1992-2008.…”