Relationship between recruitment of the juvenile jack mackerel (Trachurus japonicus) and the warm water inflow in the Uwa Sea

Abstract

We investigated the relationship between the recruitment of the juvenile jack mackerel Trachurus japonicus (ranging from 30 to 80mm in fork length) to the coastal zone of Ehime Prefecture and the inflow of warm water. The fishing ground of the juveniles was formed in only the coastal zone of Uwajima Bay and northward. During the fishing season of the juveniles, the inflow of warm water to the fishing ground was observed when catch per purse seine fishing vessel, mainly operating in the ground, (CPUE) drastically increased. In addition, the CPUE after the inflow of warm water was significantly higher than before. From the above findings, it is likely that juvenile jack mackerel are recruited to the coastal zone of Uwajima Bay and northward with the inflow of warm water from the south. In 4 of 11 cases of the inflow of warm water observed at the time of the drastic increase in CPUE, the warm water flowed in from the middle of the Bungo Channel and was not accompanied by a rise in water temperature in the eastern side of the channel. Thus far, studies on variations in jack mackerel fishing conditions have focused on Kyucho, the northward advance of warm water of the Kuroshio system to the eastern side of the Bungo Channel, which causes a sudden rise in the water temperature of the coastal zone. However, our investigation indicates that the inflow of warm water from the middle of the Bungo Channel that is not accompanied by a rise in water temperature in the eastern side of the channel is also strongly related to the recruitment of the juveniles.