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Pathogens have successfully reduced fish abundance. The <br />channel catfish virus disease (CCVD) impacts only channel <br />catfish. Young are particularly vulnerable to this highly <br />communicable, selective disease, which can be fatal 50-95% of the <br />time (Plumb et al. 1989). <br />Physicochemical Control.-Control of undesirable fishes also <br />can be achieved with physicochemical approaches, including the <br />manipulation of water levels, temperature, flow, and turbidity. <br />Many authors have suggested that nonnative fish species became <br />established in the western United States at least partly because <br />historic flow regimes were modified to resemble flows that favor <br />these species. A return to larger, historic amplitudes in <br />seasonal discharge may allow native fishes to better compete with <br />nonnative fishes (McAda and Kaeding 1989a). <br />Meffe (1984) reported that the Sonoran topminnow <br />Poeciliopsis occidentalis, a native to the arid southwest, was <br />most rapidly replaced by mosquitofish in areas that rarely <br />flooded, and long-term coexistence of the two species may occur <br />in areas that frequently flood. Minckley and Meffe (1987) found <br />that nonnative fishes (i.e., common carp, red shiner, fathead <br />minnow, ictalurids, mosquitofish, largemouth and smallmouth bass, <br />green sunfish, and bluegill) were reduced in abundance or <br />completely eliminated after major flooding in unregulated Arizona <br />streams, whereas abundance of native fishes was rarely affected. <br />29