Abstract
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.
