Turtle Bycatch Mitigation in the Hawaii Longline Fishery
Analyses of Observer Program Data for the Hawaii-Based Longline Swordfish Fishery for: Effects of sea turtle regulations on sea turtle interactions, catch rates of retained marketable species and catch rate of sharks; Economic viability and potential for temporal or spatial closures to reduce turtle capture rates; Comparison between 2005 and 2006 turtle catch rates and temporal distribution of effort to explain cause of loggerhead cap being reached in 2006 but not in 2005; and Hook position in a basket of caught turtles and retained fish.
Reducing sea turtle bycatch in pelagic longline fisheries may contribute to the recovery of sea turtle populations. The effectiveness and commercial viability of a turtle avoidance strategy, such as replacing J-shaped hooks with wider circle-shaped hooks, may be fishery-specific, depending on the size and species of turtles and target fish and other differences between fleets. Assessing the effects of turtle avoidance methods in individual fleets is therefore necessary.
Regulations based on research conducted in the U.S. North Atlantic longline swordfish fishery came into effect for the Hawaii-based pelagic longline swordfish fishery in May 2004. We conduct the first analysis of the U.S. National Marine Fisheries Service observer program database for the Hawaii-based pelagic longline swordfish fishery, in part, to infer possible effects of regulations on turtle interactions, catch rates of retained marketable species and shark capture. There were significant reductions in sea turtle and shark capture rates and reduced proportion of deeply hooked turtles, indicating increased post release survival prospects, without comprising target species catches. Results identify effective and commercially viable turtle avoidance methods that may be suitable for use in other longline fisheries worldwide, potentially resulting in substantial reductions in turtle bycatch in global pelagic longline fisheries.
The existence of confounding factors prevents definitive conclusions regarding single factor effects on turtle and fish interactions. Regulations designed to reduce turtle capture rates and proportion of deep-hookings, which came into effect in May 2004, changed the type and size of fishing hook and bait used by the Hawaii-based longline swordfish fleet (from using a 9/0 J hook with squid bait to a 10 degree offset 18/0 circle hook with fish bait).
Regulations designed to reduce seabird interactions, which came into effect for the Hawaii-based longline swordfish fishery in June 2001, include requirements for swordfish-targeting vessels to night set and dye bait blue, two changes that may affect sea turtle and fish capture rates. Prior to this rule coming into effect, swordfish vessels did not dye bait blue and initiated gear setting an average of 76 minutes earlier than after the requirement for night setting was instituted. Hawaii based swordfish vessels were subject to the night setting and blue bait requirements for the entire period after the sea turtle regulations came into effect and for the last eight months of the period before the sea turtle regulations came into effect.
Another confounding factor is variability in turtle abundance at fishing grounds. Analysis of the location of Hawaii-based longline swordfish effort by quarter for the periods before vs. after the sea turtle regulations came into effect indicate that there was generally no substantial differences in the temporal distribution of effort for these two periods.