Phenotypic Correlates of Life-History Variation in Juvenile Chinook Salmon, Oncorhynchus tshawytscha

SUMMARY (1) Chinook salmon, Oncorhynchus tshawytscha (Walbaum), commonly follow two life-history patterns as juveniles. 'Stream-type' juveniles reside in streams for a year or more before abandoning territoriality and migrating to marine habitats. By contrast, 'ocean-type' juveniles migrate to sea sometime during their first year of life, often as newly emerged fry. (2) At age 1, stream-type chinook showed a distinct reduction in positive rheotaxis during both 'diurnal' and 'nocturnal' current response tests, consistent with their downstream migration to the sea at this age in nature. By contrast, ocean-type chinook showed no pronounced shifts in rheotaxis after age 2-3 months. (3) Ocean-type fish that migrate seaward after 2-3 months in freshwater were more aggressive than those that migrate as fry, but both groups were less aggressive than stream-type chinook. (4) Ocean-type chinook grew at a faster rate than did stream-type.