Shark bycatch and mortality and hook bite-offs in pelagic longlines: Interactions between hook types and leader materials
This study addressed the influence of hook type (circle vs J-hook) and leader material (nylon vs wire) on longline catch and mortality rates of target and bycatch species in a pelagic longline fishery targeting swordfish, Xiphias gladius, and tunas. A total of 603 individuals (53% classified as bycatch) were caught on 17,000 hooks. Sharks constituted 45% of the bycatch. Bite-offs (i.e. missing hooks) corresponded to ∼33% of the shark catch and occurred mostly on nylon leaders (97%). Hook type had no significant effect on catchability or mortality of any species or groups. However, nylon leaders caught more bigeye tuna, Thunnus obesus and all target species combined, while wire leaders caught more blue shark, Prionace glauca and all sharks combined. If bite-offs were assumed to be undetected sharks, differences in shark catchability between leader types disappear. Moreover, significant differences in blue shark catch rate between leader types was found in J-hook treatments only. Higher proportions of live sharks were found on wire leaders. The catch and mortality rates of sharks in longline fisheries may be underestimated when monofilament leaders are used. This study highlights the need for understanding the role of every longline component in gear performance analysis.