Should I Stay or Should I Go Now? The Clash Between Residual Hatchery and Wild Steelhead in the Hood River, Oregon

Impacts to wild fish related to residualism of hatchery origin smolts are highly variable and potentially negative.  Using existing PIT-tag data, our objectives for this study were to 1) quantify the potential minimum level of residualism and 2) assess the effects of residuals on wild steelhead within the Hood River, Oregon.  For outmigration years 2005 – 2013, we estimated the number of hatchery winter steelhead residuals by combining the marking rate during release year y with the recapture probability of PIT-tagged hatchery winter steelhead during year y+1 (CJS model, Program MARK).  The annual rate of residualism was subsequently integrated with additional variables in a multiple regression model (R version 3.1.2) to evaluate the effects of residualism on wild steelhead smolt productivity.  The estimated rate of residuals surviving to emigrate the following year was limited (mean = 1.75%, range [0.22% - 5.61%]).  Model selection processes (AIC) indicated that environmental conditions may affect both wild and hatchery juvenile steelhead similarly and that the estimated rate of hatchery residualism was positively correlated with wild smolt production (adjusted r² = 0.83, p ≤ 0.002).  Additional sampling should be conducted to determine the overwinter survival rate and to quantify the non-migratory component of residuals.