Spawning Characteristics of Sympatric Populations of Steelhead Trout (Salmo gairdneri): Evidence for Partial Reproductive Isolation

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Abstract
We differentially tagged upstream migrant adult summer and winter steelhead trout (Salmo gairdneri) of hatchery and wild origin in the Kalama River, Washington, and subsequently observed them spawning under natural conditions in a tributary. We found that summer steelhead spawned within the study stream on a spatially random basis. The steelhead spawning interval was 6 mo long, during which hatchery summer steelhead spawned first, followed by wild summer, hatchery winter, and wild winter steelhead. Significant temporal spawning differences were found between hatchery summer and wild winter fish, and between wild summer and wild winter steelhead. We developed estimates of potential reproductive overlap between spawner groups that indicated that most reproductive activity involved members of the same spawner group. Estimated genetic exchange among groups was primarily between hatchery and wild fish within races. Of all possible spawner group comparisons, the smallest potential reproductive overlap estimated was between wild summer and wild winter steelhead. We concluded that, although wild summer and wild winter steelhead were not completely reproductively isolated, a substantial degree of successful gene flow restriction occurred, thereby providing a mechanism for the maintenance of racial identity. Additionally, spatial and temporal spawner segregation was of insufficient magnitude to prevent the potential infusion of hatchery steelhead genes into the wild summer steelhead population.

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