Large-scale, differential summer habitat use of three anadromous salmonidsin a large river basin in Oregon, USA

The summer distribution of three juvenile anadromous salmonid species was c ompared in the mainstem sections and nine tributaries of the Upper South Um pqua River Basin, Oregon, USA. Chinook salmon, Oncorhynchus tshawytscha (Wa lbaum), was found at highest densities in the mainstem and mid-elevation tr ibutaries. Coho salmon, O. kisutch (Walbaum), was essentially absent from t he mainstem, but was found at high densities in the low-elevation tributari es. Steelhead trout, O. mykiss (Walbaum), was found in the mainstem and all nine tributaries; the highest densities of this species were in the upper mainstem reaches and high-elevation tributaries. No significant correlation s were found between ranks of mainstem and tributary densities among the th ree species (Spearman's r(s); P > 0.05). Observed spatial segregation of ba sin habitat by these species during summer indicated that protection provid ed at the level of the river basin will be necessary to assure the continue d existence of all stocks.