Habitat Use by Fishes and Pacific Giant Salamanders in Small Western Oregon and Washington Streams

Abstract

The habitat use patterns of juvenile salmonid fishes Oncorhynchus spp., Pacific giant salamanders Dicamptodon spp., torrent sculpins Cottus rhotheus, reticulate sculpins C. perplexus, and larval lampreys Entosphenus tridentatus and Lampetra spp. were examined in 30 small streams in western Oregon and Washington. Fish and salamander densities and sizes were compared between different habitat types (pools and riffles) in summer and winter. Species density, length, and diversity (richness and dominance) were also correlated with physical variables measured at the reach and watershed scales. In the summer, densities of coho salmon O. kisutch, cutthroat trout O. clarki, and larval lampreys were significantly higher in pools than in riffles (P < 0.01), whereas densities of age-0 torrent sculpins were higher in riffles than in pools (P < 0.01). In winter, densities of coho salmon, cutthroat trout, steelhead O. mykiss, and young-of-the-year trout fry were higher in pools than in riffles (P < 0.01). Cutthroat trout, steelhead, Pacific giant salamanders, and torrent sculpins found in pools were larger than those found in riffles during summer (P < 0.01). Steelhead and young-of-the-year trout fry were heavier for a given length in pools than in riffles, whereas both reticulate and torrent sculpins were lighter (P < 0.01). These morphological differences within species are likely the result of differences in food availability between habitats or differences in morphology between conspecifics inhabiting riffles and pools. Reach- and watershed-scale physical variables (e.g., stream gradient, percent pool area elevation, drainage area, and precipitation) explained from 22% to 63% of the variation in species density among streams. This, combined with the results of previous studies, suggests that large-scale habitat variables may be better predictors of fish densities among streams and pools than variables measured within individual habitat units.