Beverly et al 2004 Trial setting of deep longline
In general, longline fisheries for pelagic species within the WCPO deploy either “shallow” or “deep” set gear. So called “regular” longline gear was estimated to hang at a depth of 50 to 120 m while deep longline gear covered a wider range from 50 to 300 m –– deploying 4––6 hooks between floats (per basket) for shallow sets and an average of 13 hooks per basket for deep sets (Suzuki and Warashina 1977). Deep longlining was introduced to the WCPO in the 1970s and is widely practiced by the major fleets to target deep-swimming bigeye and albacore tuna (Sakagawa et al. 1987). Modern tuna longline vessels may deploy more than 30 hooks per basket and utilize a “line shooter” to set additional mainline between floats to sink the line even deeper. In contrast, typical swordfish style longline sets are very shallow with only four or five hooks per basket and no use of a line shooter. For a detailed description of pelagic longline gear see Beverly et al. (2003) and Swenarton and Beverly (2004). One problem with shallow-set longline gear is that it places the hooks within the upper mixed layer of the ocean, bringing the gear into conflict and potential interaction with surface fisheries; i.e. subsistence, recreational and small-scale handline and troll fisheries, as well as large-scale purse seine fisheries. Shallow-set gear also produces significantly higher interaction rates with protected or ecologically sensitive bycatch species that are easily overharvested, such as marine turtles, seabirds, marine mammals, oceanic sharks, manta rays and whale sharks. Shallow-set gear also competes with important sport and recreational species prized by surface fisheries such as the marlins, spearfish, sailfish, wahoo, and dolphinfish. In response, deep-set longline gear has been actively promoted as one means to improve targeting while decreasing the likelihood of interactions with protected species. The importance of reducing interaction rates with protected species cannot be overemphasized. In recent years, longline fisheries have been significantly curtailed and even closed in attempts to mitigate interaction rates with marine turtles and marine mammals.