U.S. Geological Survey

Fish communities in the Willamette Valley, Oregon, and their relationships to stream habitat and water quality: A multivariate statistical approach

Fish communities from 40 stream reaches in the Willamette Basin, Oregon, were evaluated during 1993-95 following National Water-Quality Assessment (NAWQA) protocols. Physical habitat and water-quality were measured to determine relationships between fish communities and their environments. Stream reaches in areas of agricultural (n=17), urban (n=6), forested (n=9), and mixed (n=8) land use were sampled. Classification and ordination analyses were used to examine patterns in environmental data (PCA), fish relative abundance (TWINSPAN), and environmental and fish data combined (CCA). TWINSPAN and CCA analyses discerned four mutually exclusive groups based on physicochemical variables, and fish species and health: 1) cold-water mountainous reference sites, 2) small agricultural and urban sites, 3) large river sites with mixed land use, and 4) sites dominated by introduced species and high percentages of fish abnormalities. Physical habitat (percent riffles and riparian quality) were the primary variables describing fish communities among all sites. Minimum dissolved oxygen concentration (DO) and maximum water temperature were also important. When reference and large river sites were excluded, water-quality variables (minimum DO, pesticide concentration, total phosphorus concentration, and maximum water temperature) became the primary descriptors of fish communities among small agricultural and urban sites.