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Higher water temperatures may have the most important implications for inland fisheries. Like plants and birds, most species of fish tolerate — and many require — winter cooling and summer warming by tens of degrees. Nevertheless, most fish have limits to how hot or cold the water can be before they must either find more hospitab le temperatures or die. Again like plants and birds, most freshwater fish are found throughout a fairly large region where the water is rarely too hot or too cold for them to live.
 As temperatures warm, many fish will have to retreat to cooler waters. In the Mississippi and Missouri River, colder water could generally be found by swimming upstream; and in the Great Lakes fish can find colder water at greater depths. But smaller rivers and lakes, as well as rivers that flow east or west generally do not cover as wide a range of temperatures. As a result, warmer temperatures could make these waters entirely uninhabitable for some of the fish that are found there today. Salmon and a few other fish that migrate between inland rivers and the ocean could move to more northern rivers; but most freshwater fish can not tolerate salt water and hence will never make this journey on their own. People will have to transport these fish to colder waters. (See abstract of the EPA report Ecological Impacts From Climate Change: An Economic Analysis of Freshwater Recreational Fishing in the Site's publications section)
Scientists are not yet certain whether the overall level of fishing will increase or decrease in these waters. Because warmer temperatures often promote biological activity, fisheries biologists generally believe that the increase in warm water species would more than offset the decline of cold water fish in the Great Lakes, and possibly in other freshwater fisheries as well. But the total amount of fish that can be caught is not the only important consideration.
For example, the results of a 1995 EPA study, Ecological Impacts From Climate Change: An Economic Analysis of Freshwater Recreational Fishing (see abstract), suggest that the overall diversity of fishing in U.S. rivers and streams is likely to decline. Of 14 cold- and cool-water species of fish that the study examined, all but one would have to retreat from at least one state where the fish is currently found if temperatures warm 2°F; and 7 of 10 warm-water species would also find at least one state too hot. Although the study did not attempt to quantify the advance of warm-water fish into cooler waters, data compiled by the Audubon Society and others indicate that all ten of those species are found all the way up to the Canadian border even today, suggesting that warmer temperatures are unlikely to cause a significant expansion of these species within the contiguous 48 states.
Other possible implications of climate change seem more likely to have adverse than beneficial impacts on fisheries. Perhaps most important, higher water temperatures lead to lower levels of dissolved oxygen. With the 7°F warming that could eventually occur, one study estimated that the majority of southeastern rivers will have oxygen concentrations below the level necessary to support most fish. Fish populations could also decline if global warming increases water pollution.
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