Differential Reproductive Success of Hatchery and Wild Summer-Run Steelhead under Natural Conditions

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Abstract
We compared the relative reproductive success of naturally spawning, summer-run hatchery and wild steelhead trout Salmo gairdneri by electrophoretic examination of juveniles for a specific genetic marker. The success of hatchery fish in producing smolt offspring was only 28% of that for wild fish. We also found that 62% of the naturally produced summer-run smolts were offspring of hatchery spawners. Their dominance occurred because hatchery spawners, within the watershed we examined, effectively outnumbered wild spawners by at least 4.5 to 1. We suggest that, under such conditions, the genetic integrity of wild populations may be threatened. We also suggest that hatchery fish may be an important component of the spawner-to-smolt recruitment relationship for summer-run steelhead.