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    •  Chum salmon (Oncorhynchus keta) are hatched in fresh water from autumn to winter. After living in the river during the winter, they descend into the ocean in the spring. The juveniles that descend into the ocean move to the Sea of Okhotsk, where they feed and grow in the summer. They then further migrate to the Gulf of Alaska and the Bering Sea. Once they have sufficiently grown, sexual maturation starts, and they return to Japan. In the case of chum salmon, all juveniles descend to the ocean in the spring of the first year, but the returning age varies since the marine life varies at two, three, four, or more years. It is important to clarify factors influencing their life history patterns and the life history stage where depletion is significant.