We investigated vertical distributions of the short-necked clam larvae and dissolved oxygen (DO) concentrations in Mikawa Bay, Japan, from spring 2010 to autumn 2012. Larvae of the short-necked clam were usually found in the bottom layer or almost uniformly distributed in the water column. However, in the summer, hypoxic water mass formed at the bottom layer, and the larval distribution was restricted to the surface or middle water layers. The occurrence of D-shaped larvae was rare in layers with DO below 4 mg・l-1, while that of umbo and full grown larvae was limited inlayers with DO below 2mg・l-1. Diurnal vertical distribution of the larvae was observed when no hypoxic water massformed at the bottom layer: larvae moved from the bottom at daytime to the surface layers at nighttime. Interestingly, they remained at the surface or middle layers for the whole day when the bottom layer of the water mass became hypoxic. These phenomena suggest that short-necked clam larvae avoid hypoxic water masses. Larval occurrence peaks (mainly D-shaped larvae) were observed during May to June (before hypoxic water mass formation) and in October (when the hypoxic water mass disappeared). These occurrence peaks were accompanied with spawning peaks. The survival rates of larvae in spring, however, seemed to be lower than those in autumn, suggesting that hypoxic watermass formation in spring may lead to high larval mortality.

The body sizes of Japanese common squid Todarodes pacificus, which migrated to waters off the Pacific coast of Hokkaido, exhibited long-term changes. It was hypothesized that the long-term body size changes corresponded to population dynamics and regime shift. In addition, the body-size changes might be attributed to changes in the hatching period. In the present study, we examined a long-term trend in body-size using the specimens that were collected off the Pacific coast of Hokkaido from 1965 to 2008. In addition, the relationship of body sizes to hatching dates was examined for the periods from 1999 to 2008. The annual mean body size tended to be larger during 1974–1984 than duringthe earlier (1965–1973) and later (1991–2008) periods. Such a decadal trend of body sizes seemed to correspond tothe long-term trends in catch per unit effort and the regime shift. The hatching dates during 1999–2008 ranged fromNovember to April, with a main season extending from February to March. Moreover, a negative relationship was found between the hatching dates and body sizes, suggesting the influence of the hatching period on changes in body-size.

The water qualities observed in Mikawa Bay, such as water temperature, salinity, dissolved oxygen (DO), chemical oxygen demand (COD), chlorophyll a, particulate organic nitrogen (PON), total nitrogen (TN), total phosphorus (TP), and dissolved inorganic nitrogen (DIN), and dissolved inorganic phosphorus (DIP) were analyzed using the monitoring data since the 1970s. As a result, it was found that the water-quality properties of the horizontal distributiondiffer locally, and that there is a long-term trend in the variations of water properties. The transparency was low in the water column in the first half of the 1990s when TN, and TP concentrations were higher and red tide had occurred frequently in the same time. After that, TN and TP concentration decreased due to the reduction of nitrogen and phosphorus from the land. However, the hypoxic water area tended to expand in contrast with the decrease in TN, TP, total number of red tide days, and the increase in transparency. The results of multiple regression analysis, from 1980 to 1997, fishery catches of short-necked clam (Rudittapes philippinarum) had greatly contributed most to the suppression of hypoxic water. On the other hand, the hypoxic water area tended to expand regardless of fishery catches of short-necked clam that had been increasing since the second half of the 1990s. The water temperature rapidly increased between 1997–1998, and the cause of the expansion of the hypoxic water area was thought to be environmental change of Mikawa Bay. However, since 2011, the hypoxic water area has been shrinking. This was caused by the weakening of the stratification by a decrease in the water temperature, an increase in short-necked clam resources, and construction of artificial tidal flats.