FISH ASSEMBLAGES AND ENVIRONMENTAL CORRELATES IN LEAST-DISTURBED STREAMS OF THE UPPER SNAKE RIVER BASIN

Fish assemblages and environmental variables were evaluated from 37 le ast-disturbed, 1st- through 6th-order streams and springs in the upper Snake River basin, western USA. Data were collected as part of the ef forts by the U.S. Geological Survey National Water Quality Assessment Program and the Idaho State University Stream Ecology Center to charac terize aquatic biota and associated habitats in least-disturbed coldwa ter streams. Geographically, the basin comprises four ecoregions. Envi ronmental variables constituting various spatial scales, from watershe d characteristics to instream habitat measures, were used to examine d istribution patterns in fish assemblages. Nineteen fish species in the families Salmonidae, Cottidae, Cyprinidae, and Catostomidae were coll ected. Multivariate analyses showed high overlap in stream fish assemb lages among the ecoregions. Major environmental factors determining sp ecies distributions in the basin were stream gradient, watershed size, conductivity, and percentage of the watershed covered by forest. Lowl and streams (below 1,600 m in elevation), located mostly in the Snake River Basin/High Desert ecoregion, displayed different fish assemblage s than upland streams (above 2,000 m elevation) in the Northern Rockie s, Middle Rockies, and Northern Basin and Range ecoregions. For exampl e, cottids were not found in streams above 2,000 m in elevation. In ad dition, distinct fish assemblages were found in tributaries upstream a nd downstream from the large waterfall, Shoshone Falls, on the Snake R iver. Fish metrics explaining most of the variation among sites includ ed the total number of species, number of native species, number of sa lmonid species, percent introduced species, percent cottids, and perce nt salmonids. Springs also exhibited different habitat conditions and fish assemblages than streams. The data suggest that the evolutionary consequences of geographic features and fish species introductions tra nscend the importance of ecoregion boundaries on fish distributions in the upper Snake River basin.