A midwater float system for pelagic longlines was developed to reduce sea turtle incidental catch and to improve tuna catch efficiency. This paper presents changes in the three-dimensional shape of the pelagic longline with midwater float by water flow. The 3-dimensional underwater shape of the pelagic longline with a mainline of 180 m length in one basket was measured, using an ultrasonic positioning system and GPS buoy, and simultaneously the water currents at 11 depth levels (from 10 In to 60 In at 5-m intervals) were measured by an acoustic Doppler current profiler. The whole longline gear drifted in the direction of the current at the same depth where the mainline was located, because the mainline was most influenced by the drag of the water flow. Similarly, the hook of the branch line was shoaled in the current direction. Due to the current component parallel to the mainline, the mainline was skewed downstream, and the slack of the downstream part in the mainline became large while the upstream part of the mainline was shoaled. Especially, when a large difference was found in the current direction and velocity between the vertical depths, complicated 3-dimensional deformation occurred on the mainline.

A midwater float is a small float attached to the mainline of pelagic longlines to standardize the hook depth. In this study theoretical equations are presented for estimating the buoyancy of the midwater float required to lift the joints of the midwater float line on the mainline to the target depth. Sea trials using full scale tuna longline gear with midwater floats were carried out in the Indian Ocean in December 2004 and 2005, in order to examine the validity of the theoretical equations. In the sea trials, two types of midwater float settings, single midwater float setting and double midwater float setting, were tested and compared with the conventional setting. As a result, the joints of the midwater float line on the mainline were successfully lifted to the target depth as expected, demonstrating the validity of the theoretical equation. The range of hock depths in the midwater float setting was less spread over depths than in the conventional setting, and therefore, use of long float lines (100 m) with the midwater float setting allows all hooks to avoid entering the sea turtle habitat of shallower than 100 m depth. Factors affecting shoaling of the longline with the midwater float are also discussed.

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.