Mean temperature (°C)

Mean temperature (°C)

FIGURE 5.16 Thermal niches of two stream fishes in Michigan, based on field distributions and thermal regimes defined by three categories of mean temperature (vertical lines) and three categories of temperature variation (horizontal lines). Brook trout (A), smallmouth bass (■). (Reproduced from Wehrly et al. 2003.)

some 300 Michigan sites (Figure 5.17), documenting the well-established higher diversity of warm-water streams over cold-water streams. This pattern has often been reported as a longitudinal gradient in which temperature increase is accompanied by an increase in river size and change in many other variables (e.g., Huet 1949), but because the Michigan study provided a wide range of temperature regimes within a relatively modest range of stream sizes, these results strongly implicate temperature as the causal variable.

Local-scale thermal heterogeneity may explain apparent exceptions when cold- and cool-water fishes occur in warm-water streams. In the Grande Ronde basin in northeastern Oregon, cold-water patches (at least 3°C colder than ambient) were found associated with side channels, lateral seeps, and floodplain spring-brooks, and riparian shading could effect further cooling, indicating that these could be thermal refuges for salmonids (Ebersole et al. 2003). Chinook salmon Oncorhynchus tshawytscha persist in streams in the John Day River basin in northeastern Oregon where water temperatures

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