Heterogeneous thermal habitat for northeast Oregon stream fishes Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/zg64tq270

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  • Discrete coldwater patches within warm streams provide potential thermal refuge for coldwater fishes during periods of heat stress. This analysis focused on heterogeneity in stream water temperatures as influenced by local influx of cooler subsurface waters. Using field thermal probes and recording thermistors, we identified and characterized coldwater patches potentially serving as thermal refugia for coldwater fishes. Coldwater patches within the surface waters of summer-warm streams generally occurred along the stream channel margin and were classified into four morphological types. Isolation, measured as distance of the coldwater patch from the main channel, was related to the composition of fish assemblages occurring within coldwater patches. Assemblages dominated by catostomids used coldwater patches furthest from the main channel, while salmonids utilized coldwater patches closer to the main channel. Increased isolation of coldwater patches from the main channel was hypothesized to incur greater risks of predation and greater energetic demands on fishes moving between coldwater patches and the main channel. Coldwater patch isolation distance increased in stream channels that were wider and shallower, and riparian canopy density over coldwater patches decreased with increasing channel width:depth. Strong vertical gradients in temperature associated with heating of surface layers of coldwater patches exposed to solar radiation superimposed upon vertical gradients in dissolved oxygen may restrict suitable refuge volumes for stream salmonids within coldwater patches. These relationships suggest that stream channel and riparian characteristics may influence connectivity and suitability of coldwater patches for stream fishes. Coldwater patch availability (measured as frequency and relative area) was positively associated with higher abundances of rainbow trout within stream reaches, after accounting for other habitat characteristics influencing abundance. Chinook salmon abundance was positively associated with coldwater patch frequency, but was not associated with coldwater patch area within streams. These results suggest that the physiognomy, distribution and connectivity of coldwater patches, important attributes determining the effectiveness of these habitats as thermal refugia for stream fishes, are associated with reach-level channel bedform and riparian features. We propose that patterns of thermal heterogeneity associated with channel morphology may provide an important aspect of habitat structure for stream fishes.
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