Increased straying by adult steelhead trout,Salmo gairdneri,following the 1980 eruption of Mount St. Helens

Synopsis This paper describes the magnitude and geographic dispersion of adult steelhead trout, Salmo gairdneri, diverted from Columbia River tributaries which were impacted by the 1980 volcanic eruption of Mount St. Helens. Compelling circumstantial evidence suggests that for 1–3 yr after the eruption, large numbers of adult steelhead migrating toward affected tributaries entered two non-affected tributaries flowing into the Columbia River upstream of natal streams. Streams downstream of affected tributaries did not appear to receive substantial numbers of displaced fish. I estimated that the percentage of non-natal steelhead migrating into the lower portions of the nearest upstream river increased from 16% pre-eruption to 45% post-eruption. Winter-run steelhead strays probably originated from both the Cowlitz and Toutle rivers, whereas summer-run strays originated predominantly from the Toutle River. Increased natural production in tributaries resulting from the infusion of large numbers of strays was of insufficient magnitude to be detected by the methods of this study.