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I I I I I I I I I I I I I I I I I I I reported by several investigators (Hagan and Banks 1963, Vanicek 1967, Flagg 1982). This . s believed to be alien to the Colorado River Basin, and transferred from other river ia non-native fishes. 3.3.2 Nonnative Fishes At least 67 species of nonnative fishes have been introduced into the Colorado River Basin during the last 100 years (Minckley 1982, Tyus et al. 1982, Carlson and Muth 1989, Minckley and Deacon 1991, Maddux et al. 1993, Tyus and Saunders 1996). Tyus et al. (1982) reported 42 nonna . ecies established in the upper basin, and Minckley (1982) reported 37 in the lower basin. M ... f these species were introduced actively as game and forage fishes, while others were incidenr~llY mtroduced with game fishes or passively as bait fish. Native and nonnative fishes that occupy the lower Colorado River basin overlap broadly in their physical habitat and resource uses, such that spatio-temporal overlap is evident at all life stages (i.e., larval, juvenile, adult; Pacey and Marsh 1998). The assemblage of nonnative fishes now present in the lower basin has feeding, behavioral, and reproductive attributes that allow it to displace, replace, or exclude native fi ecies, supporting the hypothesis that the presence of nonnative fishes alone precludes suce . e-cycle completion of components of the native fauna. Pacey and Marsh (1998) conclude that no attribute of physical habitat or resource use can be identified that markedly or marginally favors one group offish over another, and that habitat manipulation is not an option. Segregated management is recommended in which native fish management focuses on providing aquatic habitats free of and protected from nonnative fishes, and populated with native populations or assemblages deriv appropriate genetically defined stocks. These recommendations are being implemen repatriated riverside habitats (e.g., oxbows, flooded bottomlands) in which hatchery reare onytail are being released. At least three year classes of bonytail have been reported from one of these ponds near the Cibola National Wildlife Refuge ((Mueller 2000; Personal communication, e.O. Minckley 2000). In the upper basin, the extent of effects from nonnative fishes is not fully known, but there is compelling evidence that predation and competition from introduced species has contributed to the decline of all of the Colorado River native fishes (McAda and 1980, Minckley 1982, Tyus and Saunders 1996). Predation on native fishes has been doc 15