However, the clams in the study area spawn only during the summer because of the slow progress of sexual maturation at low temperatures, which are controlled by the cold Oyashio current
In particular, the waters off Fukushima are squeezed between the Kuroshio, which flows northeastward near Chiba Prefecture and then turns eastward, and the colder and fresher Oyashio Current
, which flows south from the Arctic Ocean.
A: Roughly speaking, the sub-arctic Oyashio current
, which flows southward along the Pacific coasts of Hokkaido and the Tohoku region, and the Kuroshio current, which flows northward from the Kyushu and Shikoku regions, collide off Choshi, Chiba Prefecture during this period of the season.
Hashimoto and Ishito (1991) suggested that eggs are transported from Funka Bay southward to the Tohoku area by the coastal branch of the Oyashio Current, but there has been no study to verify this hypothesis.
Inada and Murakami (1993) suggested that the pollock catch and distribution in the Tohoku area are strongly affected by the coastal branch of the Oyashio Current but little information is available on the relationship between oceanographic conditions and juvenile distribution and abundance in this area.
In Funka Bay, the Oyashio Current
flows from spring to summer, and the Tsugaru Warm Current flows from autumn to winter (Ohtani & Akiba 1970, Ohtani 1971, 1981).
The low SST in the waters bounded by 38-40[degrees]N and 141-143[degrees]E is indicative of a large inflow of Oyashio Current
waters (Hirai and Yasuda, 1988), which is a cold water current and has high productivity (Odate, 1994), into the Kuroshio-Oyashio transition zone, where is one of the main feeding grounds of mackerels (Odate, 1961; Watanabe, 1970; Watanabe and Nishida, 2002; Fig.
The localities between the eastern Tsugaru Strait and Todohokke in the Pacific Ocean where the sea urchin was not found are affected by low water temperature, caused by the Oyashio Current