クロマグロの定置網内における行動

概要

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Behaviors of bluefin tuna Thunnus thynnus once entrapped in the Sukizaki set net at the western part of Wakasa Bay were observed through the echo traces of the tele-fishfinder positioned in the second bag of the set net during the period from June to July in 1987. T. thynnus often entered into the set net during the times between after midnight and before sunrise, furthermore between around noon and before sunset. On the other hand, they rarely entered into during the times between after sunrise and before noon, furthermore between after sunset and after midnight. T. thynnus swam relatively at deeper layer during daytime, and shallower after midnight till before sunrise inside the set net. These facts suggest that T. thynnus has diurnal vertical movements. It was clarified that T. thynnus entered individually into the set net stayed for a shorter period inside, while the group for a longer period. Furthermore when an individual T. thynnus entered into a set net, simultaneously more the fish in group might be passing away the set net boundary, because the fish usually forms school. Therefore, generally much capture of T. thynnus in group might be one of the good barometers as good capturing im-plements and location of a set net.