Multiscale thermal refugia and stream habitat associations of chinook salmon in northeastern Oregon

We quantified distribution and behavior of adult spring chinook salmon (Onc orhynchus tshawytscha) related to patterns of stream temperature and physic al habitat at channel-unit, reach-, and section-level spatial scales in a w ilderness stream and a disturbed stream in the John Day River basin in nort heastern Oregon. We investigated the effectiveness of thermal remote sensin g for analyzing spatial patterns of stream temperature and assessed habitat selection by spring chinook salmon, evaluating whether thermal refugia mig ht be responsible for the persistence of these stocks in rivers where water temperatures frequently exceed their upper tolerance levels (25 degrees C) during spawning migration. By presenting stream temperature and the ecolog y of chinook salmon in a historical context, we could evaluate how changes in riverine habitat and thermal spatial structure, which can be caused by l anduse practices, may influence distributional patterns of chinook salmon. Thermal remote sensing provided spatially continuous maps of stream tempera ture for reaches used by chinook salmon in the upper subbasins of the Middl e Fork and North Fork John Day River. Electivity analysis and logistic regr ession were used to test for associations between the longitudinal distribu tion of salmon and cool-water areas and stream habitat characteristics. Chi nook salmon were distributed nonuniformly in reaches throughout each stream . Salmon distribution and cool water temperature patterns were most strongl y related at reach-level spatial scales in the warm stream, the Middle Fork (maximum likelihood ratio: P < 0.01), and most weakly related in the cold stream, the North Fork (P > 0.30). Pools were preferred by adult chinook sa lmon in both subbasins (Bonferroni confidence interval: P less than or equa l to 0.05); however, riffles were used proportionately more frequently in t he North Fork than in the Middle Fork. Our observations of thermal refugia and their use by chinook salmon at multiple spatial scales reveal that, alt hough heterogeneity in the longitudinal stream temperature profile may be v iewed as an ecological warning sign, thermal patchiness in streams also sho uld be recognized for its biological potential to provide habitat for speci es existing at the margin of their environmental tolerances.