STATUS AND CONDITION OF FISH ASSEMBLAGES IN STREAMS OF THE TUALATIN RIVER BASIN, OREGON

We conducted fish inventories at 38 sites on fifteen streams of the lo wer Tualatin River Basin as part of a study to document fish species a nd assess the impacts of urbanization on native fish assemblages. We u sed three-pass electrofishing techniques to survey each site in spring , summer, autumn, and winter. We collected 25 species of fish from ten families. twelve species from five families (6.3% of the total catch) were exotic to Oregon. Reticulate sculpin Cottus perplexus, a native fish tolerant of habitat degradation, comprised 68.4% of the catch. Nu mber of species per stream ranged from 5 to 15; number of native speci es ranged from 4 to 10. Sites in the upper sections of streams contain ed the lar esl number of trout, native minnows, and sculpins, whereas lower sites contained more diverse species assemblages and a larger nu mber of introduced fish. We found a significant difference in the numb er of native species among sites of different stream order, bur no sig nificant difference when all species were considered. Our catch also v aried seasonally, likely due to species motility, gear selection, and variation in life history stages. Native species intolerant to habitat disturbances (torrent sculpin Cottus rhotheus and Oncorhynchus spp.) comprised only 1.7% of the total catch, and 2.0% of the total catch ex hibited parasites or physical anomalies. The relatively low number of intolerant species, high proportion of fish with parasites or anomalie s at some sites, introduction of exotic species, and reported habitat deficiencies suggest that native fish assemblages in the basin are at least moderately unhealthy.