Behavioral Interactions among Hatchery-Reared Steelhead Smoltsand Wild Oncorhynchus mykiss in Natural Streams

Abstract
The potential for hatchery fish to negatively impact wild fish has been identified as a concern for dwindling stocks of naturally produced anadromous salmonids in the Pacific Northwest. Using a control–treatment approach, we performed a multiscale examination of potential behavioral impacts of releases of hatchery-produced steelhead Oncorhynchus mykiss (anadromous rainbow trout) on preexisting wild populations of O. mykiss (anadromous and potamodromous) over a 4-year period. We released approximately 33,000 conventionally reared hatchery steelhead smolts (treatment) into an upper Yakima River tributary in 1991, 1992, 1993, and 1994 and investigated behavioral interactions and small-scale displacement (0.2–5.0 m). Snorkelers conducted behavioral observations and observed small-scale displacements in treatment and control streams for approximately 1 month following releases. Hatchery steelhead were generally larger than wild O. mykiss and dominated most (68%) contests. The types of behavioral inter...