Blue shark and swordfish catch rates in Hawaii’s shallow-set longline fishery: before and after regulations aimed to reduce sea turtle bycatch (DRAFT)
To reduce capture and mortality of endangered and threatened sea turtles, United States longline vessels targeting swordfish in the Pacific Ocean have operated under extensive fisheries regulations since 2004. We analyzed longline observer data from the Pacific Ocean to assess the impact of these regulations on targeted swordfish (Xiphias gladius) and bycatch of blue sharks (Prionace glauca). Using generalized additive mixed models (GAMMs), we investigated relationships between the nominal catch-per unit effort (CPUE) of blue sharks and swordfish and using operational components such as fishing location, hook type, bait type, hooks between floats, use of light sticks, and sea surface temperature. For blue sharks, GAMMs identified a significantly higher catch on J hooks with squid or fish bait relative to circle hooks with fish bait. For swordfish, J hooks with squid bait caught significantly more relative to circle hooks with fish bait, however there was no significant difference of catch when comparing J hooks with fish bait to circle hooks with fish bait. Confounding variables such as year and terminal gear components (hook type, bait) are discussed. Single factor analysis identified that catch rates of blue sharks and swordfish were significantly lower after the regulations, which were lower by over 2.4 times for blue sharks, yet by only ~8% for swordfish. These results indicate that the use of mitigation measures to reduce sea turtle bycatch, specifically large circle hooks and fish bait, can provide a significant conservation value by reducing blue shark bycatch, yet may also result in a slight reduction in targeted swordfish catch rates.