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(1) the aquatic habitats in a particular river reach, (2) the composition of the fish community in that river reach, and (3) the 1 i fe stages of threatened or endangered fi shes in that reach. Nonnative fish species adversely impact native fish species in several ways. Physical factors (i.e., habitat) regulate the capacity of a particular aquatic environment to support aquatic life. Because the amount of physical habitat available is finite, increases in the number of species present in a particular habitat usually equates to smaller populations of most species. Fish speci es that are better adapted to the phys i ca 1 features of exi st i ng aquat i c habi tats are 1 i ke 1 y to become more domi nant. Many nonnative fi shes are extremely adaptable and have become dominant in the Upper Colorado River Basin. (1) Impact of Predation on Native Fish Fauna. Minckley et al. (1991) strongly believe that predation by nonnative fishes was the most important factor in the lack of recruitment of the razorback sucker in Lake Mohave. Numerous studi es have demonstrated the negative impacts of nonnati ve fi shes on native fish species in North America (Courtenay 1993; Lassuy 1995; Li and Moyle 1993; Magnuson 1976; Marsh and Douglas 1997; Meffe 1985; Moyle et al. 1986; Ruppert et al. 1993; Scoppettone 1993; Taylor et al. 1984). Predat i on of native fi shes by nonnative fi shes is often difficult to detect because digestion of early life stages occurs rapidly, predators may regurgitate their stomach contents when captured, and the low probability of finding endangered fishes in the stomachs of predators due to their low availability (Moyle et al. 1986; Li and Moyle 1993). Although declines of native fishes in the Lower Colorado River Basin were attributed to nonnative fishes as early as 1944 (Dill 1944), the strongest evidence that predation is a major factor in endangered fi sh recruitment is documentation of survival and recruitment of razorback suckers in habitats where fish predators are excluded (Minckley et al. 1991). Good survival and growth of Colorado squawfish and razorback suckers were al so documented in ri vers i de ponds along the Upper Colorado River where predatory fishes were absent (Osmundson 1986; Osmundson and Kaeding 1989). These observat ions support the bel i ef that predation has contri buted. to the decline of some native Colorado River fishes. (2) Impact of Competition on Native Fish Fauna. Competition by two species occurs when food is limited, the food is shared, and one of the two species is adversely affected by sharing food (Moyle et al. 1986; Li and Moyle 1993). Low numbers of zooplankton occur in the main channel and backwaters of rivers (i.e., food is limited for early life stages of fish) in the Upper Colorado River Basin (Cooper and Severn 1994a, b, c, d; Grabowski and Hiebert 1989; Mabey and Schiozawa 1993). Di etary overl ap was reported between nonnative and native 6