Assessing Bycatch Reduction Potential of Variable Strength Hooks for Pilot Whales in a Western North Atlantic Pelagic Longline Fishery

The pelagic longline fishery off Cape Hattaras in the western North Atlantic Ocean, which targets swordfish (Xiphias gladius, Linnaeus 1758) and tunas (Thunnus spp.), has a high frequency of interactions with marine mammals, particularly pilot whales (Globicephala spp.). The typical hooks used in the this fishery are size 16/0 "strong" hooks that straighten at 250 lb of pull (113 kg); some fishermen use alternative "weak" hooks that straighten at 150 lb of pull (68 kg). Other commonly used hooks in this fishery are size 18/0 strong hooks that straighten at 350 lb (159 kg) of pull, and the weak equivalent hooks that straighten at 225 lb (102 kg). Taking advantage of the size difference between large bycatch animals and relatively smaller target species, weak hooks could be implemented to reduce bycatch. Twenty-one pelagic longline sets were made targeting yellowfin tuna (T. albacares, Bonnaterre 1788) and bigeye tuna (T. obesus, Lowe 1839) using alternating 16/0 strong and weak hooks. Nine additional sets targeted swordfish with size 18/0 hooks and the same alternating hook methodology. No significant reduction in total tuna catch (α < 0.05) or of any target species, although weak hooks exhibited higher catch per unit effort (CPUE) for tuna and swordfish. The only species with a significant difference in total catch between strong and weak 16/0 hooks was the pelagic stingray (Pteroplatytrygon violacea, Bonaparte, 1832), with more individuals caught by the strong hook. The sets with 18/0 hooks had similar catches for all species, except the target species swordfish. Swordfish CPUE was higher with the strong hooks, and had significantly higher total catches. Seven weak hooks were retrieved straightened, and one of these hooks was observed being straightened by a pilot whale. While not conclusive, such results suggest further research into weak hooks for the reduction of large animal bycatch in the pelagic longline fishery.