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Under warmer climatic conditions, the habitat avail- able to cool water species is expected to decrease and the distributions of these species will become more spatially fragmented. Rahel et al. (1996) investigated potential habitat loss in relation to climate warm- ing for salmonids occurring in streams of the North Platte River drainage in Wyoming. Under summer air temperature increases of 1.8 -9 °F, they estimated that 7 -76% of habitat would be lost, depending on the approach and amount of warming. In addition, population fragmentation was expected to occur as cold water populations were restricted to increasingly higher elevations. Eaton and Sheller (1996) looked at the effects of climate warming on 57 species of fishes in streams across the U.S., using temperature projections based on the Canadian Climate Center GCM (CCC GCM). They found a nearly 50% reduction in thermal habitat for cold and cool water species and a 14% decrease in habitat for warm water species. Overall, species with smaller ranges were projected to exhibit the largest habitat losses. Cold and cool water fish were least affected in locations that were higher in latitude or elevation. Additional studies on stream systems have confirmed significant effects on cold water fishes, but vary in their assessments of the impacts on cool and warm water fishes. For exam- ple, Mohseni et al. (2003) used a different approach to examine the impacts on climate change for the same set of 57 species used by Eaton and Sheller (1996). The results of their analysis projected a 36% decrease in cold water fish habitat and a northward shift in range. Changes in habitat for cool and warm water fishes was dependent on the assumptions for minimum temperature tolerance (32 °F vs. 35.6 °F ) and ranged between a 12 -15% decrease in habitat for cool water fishes and a 0 -31% increase in habitat for warm water fishes. Maximum temperature tolerance was not expected to have a significant effect on warm water habitat due to evaporative cooling of streams (Mohseni et al. 2003). Recognizing that both temperature and dissolved oxygen concentrations control the distribution of fish species in lakes, Stefan et al. (2001) simulated changes in both factors and examined impacts on fish habitat in North American lakes in response to projected climate warming. Using a doubled CO2 concentration scenario under the CCC GCM, their results suggest that climate warming will reduce the geographic area in which lakes have suitable cold and cool water habitat by 45% and 30% respectively. Suitable habitat for coldwater fish was likely to be restricted to deep lakes along the northern border of the U.S. In the south central and southeastern states, summer kill of cool water fish was expected to be more prevalent. However, warm water fish were expected to benefit in all lake types (Stefan et al. 2001). In rivers and streams with adequate dispersal corri- dors, species at the southern extent of their geograph- ical distribution may shift their distributions northward into cooler habitats (Allan et al. 2005). For lakes, differences in surface area, depth, lati- tude, and elevation are all factors that will influence response to climate change. Water levels are likely to be reduced in regions that experience increased evapotranspiration brought about by higher temper- atures and longer growing periods, unless offset by increased precipitation. As in streams, warmer air temperatures will raise water temperatures, especially in smaller and shallower lakes. In ponds and lakes deep enough to exhibit summer thermal stratification, warm water habitat will increase in depth, potentially forcing cool water organisms into deeper waters (Allan et al. 2005). At the same time, bottom waters may become depleted of oxygen due to higher decomposition of algae and organic matter settling out of warmer, more produc- tive surface waters. Cool water habitat may there- fore be constrained by increased warm water volumes above and oxygen depletion below (Figure 2 -3).