Abstract
Seasonal distribution and migration patterns of the steelhead trout and the Kamchatkan trout in the North Pacific were examined based on allelic variations of enzymes from specimens which were collected by Japanese research operations from 1976 through 1983.The western segment of the population mainly consisted of the Kamchatkan trout originating in the Kamchatka Peninsula in all months.As the fish extended to the west from spring through summer, the distribution of the steelhead trout extended to the western North Pacific.The westward extension of the steelhead was more evident in the immature population than in adults.As two major geographic units divided at the crest of Cascade Mountains, namely coastal and inland populations of the steelhead are defined in North America based on the differences of allelic frequencies, the immatures of the inland population exhibited a remarkable westward extension which reaches as far as 160°E longitude in summer.The distribution of the steelhead is distinguished by its extreme westward extension compared to the general distribution of Pacific salmon originating in North America.
