Fishery Study of Tuna in the Western Tropical Pacific I

The study for relation between the tuna fishing condition and the oceanographic environment are discussed on the basis of data obtained from tuna fishing experiments in the western Tropical Pacific Ocean in June, 1990. The hook depths of longline gear were measured with the depth meter and estimated by Yoshihara's method. The greater part of measured value of hook's depths were shallower than estimated one. The sinking rates of longline changed with respect to the time and stations. Though the actual depth of hooks varied due to many changing factors (e.g. an ocean current) & etc, the depths of capture of tuna were determined from estimated value of hook's depth. It was found that a lot of yellowfin tuna was captured in the upper layer of the thermocline and bigeye tuna near the center of the thermocline