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.
In order to determine the factors affecting the stock fluctuation of asari clam Ruditapes philippinarum in Ise Bay, Japan, we examined the high-density depth zones (i.e., centers of distribution) of R. philippinarum and standing stocks at three depth zones (depth zone I: shallower than water depth 1 m; depth zone II: water depth 1–5 m; depth zone III: water depth 5–10 m) in the four regions (the Suzuka region including the Wakamatsu and Shiroko subregions, the Karasu region, the Matsusaka region including the Miwatarigawa and Matsunase subregions, and the Ise region) of Ise Bay, Japan, from 2012 to 2014. Based on the comparison of clam densities among depth zones in each region, the depth zones I were the centers of distribution in the Karasu, Matsusaka, and Ise regions from 2012 to 2014, while the depth zone II was considered as the center of distribution in the Shiroko subregion of the Suzuka region in November 2013 and May 2014. In May 2014, the standing stock of asari clam in the Suzuka region was 4,246 metric tons, which was the largest among the four regions from 2012 to 2014. The standing stock at depth zone III with a large area accounted for 70% of the stock in the Suzuka region in May 2014. A relationship between the standing stock and dissolved oxygen concentration (DO) suggested that the appearance/disappearance of hypoxia water masses (DO≤3 mg・L-1) affected large fluctuation in standing stocks.