Migration Dynamics of Released Hatchery Steelhead Smolts Influence Ecological and Genetic Risks

Hatchery steelhead migration behavior upon release can determine the type and magnitude of ecological and genetic risks posed to the receiving wild populations.  To evaluate the success or failure of conservation hatchery programs, we determined the relationship between migration behavior and ecological and genetic risks posed by an integrated steelhead hatchery on the lower Columbia River.  Over the course of seven years, an average of 62% (range 50-78%) of released hatchery steelhead smolts were estimated to have migrated rapidly downstream upon release.  Whereas, an average of 3% (range <1-7%) of released hatchery steelhead did not migrate and instead residualized over the summer.  Of the residualized fish, 72% were found in snorkel reaches just upstream and downstream of the release location.  This migration behavior combined with evidence of behavioral interactions, habitat use overlap and diet similarities between hatchery and wild juvenile steelhead suggest negative ecological effects may be present near the release site.  Additionally, evidence of successful reproduction by upstream residual migrants indicates negative genetic effects are likely.  We recommend that hatcheries producing steelhead should assess migration dynamics of released steelhead.  Risk reduction through the use of electrofishing to remove residuals, volitional release methods, and weirs to prevent upstream migration may be warranted.