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Colorado squawfish (Tyus 1986; Tyus and Karp 1989). Minckley and Meffe (1987) reported that native fishes in the American Southwest are favored by flooding in streams. Periodic high flows in six unregulated Arizona streams reduced the number of sunfish and catfish, while native fishes were almost unaffected. The introduction of nonnative fishes also is implicated in the decline of the Colorado squawfish. The quiet waters of the first reservoirs constructed in the Lower Basin were inhabited initially by native fishes, including Colorado squawfish. Substantial catches of Colorado squawfish were made from Roosevelt Lake from 1913 through 1937 (Frazier in Miller 1961), and mainstem reservoirs on the lower Colorado River yielded Colorado squawfish of considerable size (Dill 1944; Wallis 1951) until the 1960's (Minckley and Deacon 1968). By the time lakes Roosevelt and Mead were filled, however, impounded waters became populated by a variety of introduced species (Minckley 1973) whose range expanded rapidly as additional reservoirs were built. In Arizona, about 28 fresh-water and 3 salt-water native species have been joined since the turn of the century by at least 60 introduced fishes (Minckley 1973). Of 55 fish species currently found in the Upper Basin, 42 were introduced (Tyus et al. 1982b). In the 1960's, programs were undertaken in various locations in the Colorado River to eradicate or reduce fish populations to improve opportunities for establishment of trout. During these eradication programs numerous Colorado squawfish along with other native and nonnative fish were killed (John Hamill, U.S. Fish and Wildlife Service, 1991, pers. comm.). Introduced fish may have subjected Colorado squawfish to biological interactions to which the latter were poorly adapted due to their previous isolation (Molles 1980). In this respect, the Colorado squawfish may be comparable to some geographically isolated island faunas that were quickly decimated by competition or predation with nonnative species (Molles 1980). Mortality as a result of Colorado squawfish choking, when preying on channel catfish, has been discussed by McAda (1983), Pimental et al. (1985), and others. The nature of interaction among native and introduced fish species is not well known. However, Karp and Tyus (1990) reported that age-0 Colorado squawfish may be negatively affected by small, nonnative fishes, particularly red shiner, flathead minnow, and green sunfish. Haines and Tyus (1990) found that age-0 Colorado squawfish were sympatric with 15 nonnative fishes, including red shiner and flathead minnow. They detected no segregation in habitat use between Colorado squawfish and nonnative fishes. Loss of fish habitat and habitat fragmentation due to stream blockage could pose a threat to the recovery effort and has been implicated in the systematic loss of the fish. Sheldon (1988) found that alterations of river drainages by fragmentation leads to a reduction of species diversity and species extinction. In the upper Colorado River, blockage of the Gunnison and upper mainstream by diversion dams have no doubt restricted access of the fish to presumed habitats (Valdez et al. 1982a). Construction of Flaming Gorge Dam inundated spawning habitats and blocked upstream passage. Construction of Taylor Draw Dam on the White River has blocked the movement of upstream, return migrants (Martinez 1986a) and resulted in a loss of about 80 km (50 miles) of adult habitat from which mature fish contributed to the spawning aggregations in both Gray and Yampa canyons (Tyus et al. 1987). Fragmentation of the rivers in the Lower Basin undoubtedly restricted movement over much of the historic range of the Colorado squawfish. 16 1 ~~~ ~~~