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

Eric B. Taylor
Journal of Animal Ecology
Vol. 59, No. 2 (Jun., 1990), pp. 455-468
DOI: 10.2307/4874
Stable URL: http://www.jstor.org/stable/4874
Page Count: 14
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Phenotypic Correlates of Life-History Variation in Juvenile Chinook Salmon, Oncorhynchus tshawytscha
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Abstract

(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. (5) Since these differences in phenotype were expressed in fish raised 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.

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