2017 Volume 54 Issue 3 Pages 185-195
A midwater float system with long （100m） float lines for deep setting of pelagic longline was developed to reduce sea turtle bycatch and improve fishing efficiency, focusing on the difference in swimming depth between sea turtles and tuna species. Longline setting with midwater floats which are small floats attached to the mainline allows all the hooks of one basket to be set within narrower depth range than the conventional setting. Long float lines could set the main line with all hooks in the deeper range for tuna habitat to avoid the hooks entering the sea turtle habitat. Longline operations were carried out in the Indian Ocean. One midwater float with a buoyancy of 2300gf or 2000gf was attached to each center of five baskets on the mainline, both ends of which were hung with 100m long float lines. The conventional setting with 40m float lines without any midwater float was also conducted as a control. No bycatch of sea turtles occurred in the midwater float setting while one olive ridley sea turtle was caught in the conventional setting. All hooks of the midwater float settings were set below 150m depth, while the hooks in the conventional setting were set in the wide depth range from 109m to 230m. The hook position in a basket catching bigeye tuna was recorded to estimate the depth of catching. The CPUE of bigeye tuna was higher in the hooks below 150m. The estimated catch number of bigeye tuna in longlines with midwater float setting was nearly 30% more than using the conventional setting.