Geographic Variation in the Occurrence of Red- and White-Fleshed Chinook Salmon (Oncorhynchus tshawytscha) in Western North America
Flesh color, as reported by fishery processors, was surveyed in coded-wire tagged chinook salmon (Oncorhynchus tshawytscha) sampled from the 1983–85 Southeast Alaska commercial troll fisheries. Flesh color data were collected from 11 373 fish originating from 95 locations in western North America between southcentral Alaska and central California. White-fleshed chinook salmon were most common in stocks from coastal British Columbia, Southeast Alaska, and the tributaries of the Fraser River, British Columbia. Heterogeneity between neighboring locations was also greatest in this region, the most recently glaciated area of western North America. Differences in flesh color proportions are consistent with the hypothesis that chinook from the Pacific refuge colonized rivers in British Columbia and Southeast Alaska after the recession of the last Pleistocene glaciation. Genetic drift of populations founded by small numbers of parents is one explanation of the pattern of occurrence of white-fleshed chinook salmon. An alternate explanation is that geographic variation in the occurrence of red- and white-fleshed chinook salmon reflects evolutionary adaptations of these fish during early life history to local freshwater environments.