There is indication that the spawning ground of walleye pollock off western Hokkaido in the northern part of the Sea of Japan has disappeared. This is inferred from recent reduction in the fish catch by the coastal fisheries, which capture spawning adults. In the present research, the distribution of the walleye pollock eggs was investigated over the western Hokkaido, Sea of Japan in February 2006 and 2007. In Ishikari Bay, Stage1 eggs’ maximum density was actually small: 4 individuals/m², and the eggs were not collected in Soya and Rumoi region. However, in the southward from Shakotan Peninsula, Stage1 eggs’ maximum density was 19,350 individuals/m² in Iwanai Bay, and 1,574 individuals/m² in Hiyama region. These suggest that Iwanai Bay and Hiyama region (off Otobe) was the major spawning ground, and Ishikari Bay was a minor one. However, there was no spawning ground in Soya and Rumoi region recently. Ishikari Bay has many eggs transported but has few eggs produced. We assume that some eggs spawned off Otobe, Hiyama region, would be transported northward, and only a small amount southward, and that the location of the main spawning ground changed after 1980s by comparison with this result and past knowledge. As a result, the distance between main spawning grounds and nursery area became larger in recent years. Consequently, the eggs could encounter more often high water temperature, that is fatal to the development. It was thought that these didn’t allow successful transport of the eggs to the nursery area.

The spawning season and maturity of the marbled sole Pleuronectes yokohamae were examined from 1986 to 2001 in the waters from Harima Nada to Osaka Bay eastern part of the Seto Inland Sea, Japan. From the seasonal changes of gonadosomatic index (GSI) it was considered that the spawning season was from mid-December to mid-February and the peak period was from late-December to late-January. The age and size at maturity were determined by the GSI in December and January. Maturity rates of 1 year old female fish of 1 year 11 months to 2 year 2 months were 82.1% in 1986–1989 and 99.5% in 1998–2001 (1986–1989 vs. 1998–2001, Fisher’s exact probability test p<0.0001), and those of male fish were 94.1% and 99.6%, respectively (p=0.0003). Approximately 100% of 2 year old male and female fish were mature in both periods. The maturity rates of 1 year old females of 200–219 mm TL were 72.3% in 1986–1989 and 96.6% in 1998–2001 (p<0.004), and those of males of 180–199 mm TL were 90.5% and 100%, respectively (p=0.042). It was suggested that the age and size at maturity of marbled sole have decreased in recent years in this study area. The cause of these changes may be a density-dependent mechanism because of the remarkable decrease of catches in Harima Nada and Osaka Bay from 1986–1989 period to 1998–2001 period.