A Genetic Comparison of Sympatric Populations of Summer and Winter Steelheads

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Abstract
The 1973 and 1974 year classes of adult, wild summer and winter steelheads (Salmo gairdneri) were genetically compared at five polymorphic enzyme loci: alpha‐glycerophosphate dehydrogenase‐1; lactate dehydrogenase‐4; malate dehydrogenase‐3,4; superoxide dimutase‐1; and phosphohexose isomerase‐3. Overall genetic differences between summer and winter steelhead groups were not significantly greater than genetic differences observed for 1973 and 1974 year classes compared within each group. These findings, in conjunction with field observations of known summer‐winter steelhead spawning pairs, suggests that summer and winter steelheads in the Kalama River are not at present reproductively isolated.