Present condition of walleye pollock spawning ground formation in the Sea of Japan off western Hokkaido, viewed from the recent condition of the egg distributions

Abstract

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.