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

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Abstract
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. 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. 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. Ocean-type chinook grew at a faster rate than did stream-type. Since these differences in phenotype were expressed in fish in a common laboratory environment they are concluded to have a genetic basis. Furthermore, the differences in behaviour and growth rate were appropriate for different durations of freshwater residence and, hence, probably reflect adaptive divergence within O. tshawytscha.