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-DRAFT August-October nursery periods. Streamflow modifications below major Federal reservoirs are currently being evaluated by the Bureau of Reclamation and the Service to determine the relationship between flows and survival of young Colorado squawfish (U.S. Fish and Wildlife Service 1987). Kaeding and Osmundson (1988) provided data indicating a relationship between slow growth of Colorado squawfish in the Colorado River above Lake Powell and the limited availability of warm water temperatures to support growth. They developed the hypothesis that the interaction between slow growth-and increased early life mortality due to changed habitat conditions was an important cause of the decline of the Colorado squawfish. Higher spring flows may be beneficial to Colorado squawfish spawning in the Yampa River and detrimental to introduced fishes (Wick et a1. 1983). Higher spring flows also may provide terrestrial food for Colorado squawfish (Tyus 1986). 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 sunfishes and catfishes, 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 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 freshwater and 3 saltwater. 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). Introduced fishes may have subjected Colorado squawfish to biological interactions to which the latter were poorly adapted due to their previous isolation (Moller 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 (Moller 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, fathead minnow, and green sunfish. 16