Deep setting of tuna longline hooks using mid-water float system with long float lines to avoid sea turtle bycatch.

Abstract

Sea turtle bycatch issues are of special concern in tuna longline fisheries. To avoid sea turtle bycatch, we developed a mid-water float system, which is a method of setting the longline hooks at almost the same depth. When enough long float line was deployed with the mid-water float system (the mid-F system), all hooks could be set in water deeper than where sea turtle predominantly forage and in the depth of tuna habitat. Sea trials of full scale longline gear with mid-water float and long float line (long FL) were carried out in the Indian Ocean in December 2005. One mid-water float (buoyancy of 2200gf or 2560gf) was attached to the center of one-basket mainline, both ends of which were hung with two long float lines (100m). The conventional longline setting (the length of float lines were 40m) without any mid-water float was also conducted as a control. The mainline with long float lines were set at a depth from 125m to 175m. The depth range of hooks with mid-F system was at most 50m, while that of the conventional longline setting was over 125.5m. The mid-F & long FL system allowed all hooks to be set at sufficient depth for capture of tuna, with an associated avoidance of sea turtle bycatch as well as more effective tuna catch.