PARENT SESSION
Oral Session #43: Fish: Ecology and Conservation.
Presiding: B. Harvey
Tuesday, August 6. 1:00 PM to 4:45 PM. Cochise Meeting Room, TCC.
Heterogeneous thermal habitat for a stream fish assemblage.
Ebersole, Joseph*,1, Liss, William2, Frissell, Christopher3, 1 US EPA - Western Ecology Division, Corvallis, OR2 Oregon State University, Corvallis, OR3 Pacific Rivers Council, Polson, MT
ABSTRACT- Discrete coldwater patches associated with upwelling of hyporheic flow within warm streams provide potential thermal refuge for coldwater fishes during periods of heat stress. 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. Coldwater patches within the Grande Ronde River in northeastern Oregon generally occurred along the stream channel margin. 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 and exposure to solar radiation were greater in stream channels that were wider and shallower. Suitable refuge volumes for stream salmonids within coldwater patches may be restricted by strong vertical gradients in temperature associated with heating of surface layers superimposed upon vertical gradients in dissolved oxygen. Restoration of stream habitats for coldwater fishes in warm alluvial rivers should be based on actions that increase riparian vegetation, allow channels to return to natural width to depth ratios, and maintain and restore bed-forming processes that facilitate hyporheic flow exchange.
KEY WORDS: thermal refugia, hyporheic exchange, stream fish assemblages