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FIGURE 5.15 Relationship between the mean daily change in the total energy content of a 50 g brown trout and water temperature. The solid curve shows the values for fish on maximum rations and the dotted lines for proportions of the maximum energy intake. (Reproduced from Elliott and Hurley 2000.)

and black flies) dominate at the coldest temperatures, and the nymphs of a number of mayflies (Baetidae) and stoneflies (Nemouridae, Chloro-perlidae) are added to the assemblage as one proceeds downstream and waters warm above 4°C. At still warmer temperatures, caddisfly larvae and additional families of aquatic insects appear. Milner et al. (2001) used maximum summer water temperatures and channel stability to construct predictive models of macroinverte-brate diversity across seven glacial rivers in Europe. A combination of zonation and downstream addition of fish species was evident along an elevational gradient from 2,200 to 1,200 m in a Rocky Mountain stream (Rahel and Hubert 1991). Salmonids dominated headwater reaches but were replaced by a warmwater minnow-sucker (Cyprinidae-Catostomidae) assemblage below 2,000 m. Such longitudinal replacements may involve a combination of competitive effects and the influence of temperature on feeding and physiological performance. The longitudinal replacement of trout at high elevations in Rocky Mountain streams by the creek chub Semotilus atromaculatus at low elevations was attributed to both appetite loss and behavioral interactions and so was not solely a function of physiological performance (Taniguchi et al. 1998).

Fishery scientists have long been aware that fish assemblages vary along latitudinal gradients and can be classified according to their position and the maximum summer temperatures (Lyons 1996). In the heterogeneous glaciated landscape of Michigan, streams exhibit substantial regional variation in weekly mean temperature and in temperature fluctuation during warm seasons, allowing Wehrly et al. (2003) to determine the realized thermal niche of stream fishes based on three temperature categories (cold, <19°C; cool, 19-22°C; and warm, >22°C) and three temperature fluctuation categories (stable, <5°C; moderate, 5-10°C; and extreme, >10°C). The brook trout Salvelinus fontinalis and smallmouth bass Micropterus dololieu are good examples of cold-water and warm-water species, respectively (Figure 5.16). Overall fish diversity increased with mean water temperature across

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