Benchmarks and Patterns of Abundance of Redband Trout in Oregon Streams: a Compilation of Studies

—This work summarizes pre-1998 studies of population abundance of stream resident redband trout Oncorhynchus mykiss ssp. in Oregon, and compares basin-level surveys of habitat and fish populations in streams of two ecoregions. Interquartile values of density and biomass were used to develop benchmarks of high, moderate, and low abundance. Comparison among Crooked River streams (Blue Mountain ecoregion) showed large differences in population abundance associated with watershed characteristics such as elevation, flow, temperature, land use, and disturbance history. Comparisons among Catlow Valley streams (High Desert ecoregion) found redband trout to be concentrated in discrete reaches associated with high spring flow or in narrow canyon reaches with riparian zones that were not intensively grazed by cattle. These reaches could possibly function as population refugia. Populations of redband trout in Catlow Valley streams were severely depressed during the time of sampling. Degraded stream habitat, drought conditions, and system connectivity appeared to be important factors associated with their status and recovery. * Corresponding author: [email protected]. 1 Present address: CSIRO Marine Research, GPO Box 1538, Hobart, Tasmania 7001, Australia. FIGURE 1.—Major drainages (shaded areas) with historic populations of redband trout and Oregon ecoregions. Ecoregions depicted here are the major ones designated by Clarke et al. (1991), except the High Cascades ecoregion, which is a subregion of the Western Cascades. Filled circles denote location of one or more sites from which abundance data were collected (Table 1). BM: Blue Mountains CP: Columbia Plateau CR: Coast Range EC: Eastern Cascades HC: High Cascades HD: High Desert KM: Klamath Mountains WC: Western Cascades WUV: Willamette KM CR WUV