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to them. Some nonnative fishes have been repeatedly stocked to offer recreational fishing opportunities by Federal and State agencies, commercial enterprises, and private citizens. The resiliency of nonnative fish species following removal from a gravel-pit pond along the Upper Colorado River was extremely rapid. Gardner Pond was drained, nonnative fish removed, and a channel excavated to connect the pond with the river. Within four months after the pond was reconnected with the river nonnative fishes invaded and recolonized the pond and five nonnative fish species successfully reproduced in eight months (Burdick et al. 1997). (B) Impacts of Nonnative Fish Introductions on Native Fishes. The impacts of nonnative fish species on native fishes depends upon (1) the aquatic habitats in a particular river reach, (2) the ___ composition of the fish community in that river reach, and (3) the life stages of threatened or endangered fishes 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 species that are better adapted to the physical features of existing aquatic habitats are likely to become more dominant. Many nonnative fishes 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 studies have demonstrated the negative impacts of nonnative fishes on native fish species in North America (Courtenay 1993; Dunsmoor 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). Predation of native fishes by nonnative fishes 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 fish 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 6