Effects of logging on salmonids in small, steep eastern Cascade Range streams vary with stream gradient

Climate and land use each influence stream habitat features in ways important to fish populations, and land use effects on stream fishes and their habitat may vary with climate. To explore this hypothesis we examined the influence of two climatic types and two timber harvest histories on fish and aquatic habitat in 15 small, high-gradient headwater streams in the eastern Cascade Mountain Range, Washington. Fish abundance and mass and habitat characteristics were measured a total of three times per stream: twice in the summer of 2006 and once in summer 2007. Total mass of trout in study reaches was negatively related to logging in the dry ecoregion but not in the wet ecoregion. Data further suggest that in the high-energy environment of steep-gradient wet ecoregion streams, timber harvest effects on fish and fish habitat do not persist over time, while in lower-gradient dry ecoregion streams, these effects are still observable 25 years after logging. In addition to stream gradient, riparian canopy cover and invertebrate drift density were also important predictors in Random Forest (TM) nonlinear regression models of total fish mass. This study suggests that food abundance should be incorporated into habitat models predicting fish responses to stream habitat.