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INTRODUCTION The native fish fauna of the Colorado River Basin was labeled depauperate by early ichthyologists (Jordan 1891; Miller 1959; 1961). Although the term depauperate was originally used to indicate the low diversity in the basin, it probably also reflected the low value that early residents placed on native fishes. Native Americans apparently valued Colorado squawfish Ptychocheilus lucius, razorback sucker Xyrauchen texanus, and bonytaij Gila elegans as a food resource (Miller 1955; Gehlbach and Miller 1961). European settlers, however, saw no tangible value in the original fish assemblage, except in the late 18oos, when locally abundant Colorado squawfish and razorback sucker were captured for use as food or fertilizer (Behnke and Benson 1983). Most residents were nostalgic for the recreational and food gamefish they remembered catching in eastern states or ancestral Europe. In the early 1900s, they stocked gamefish into the Colorado River Basin to create these familiar resources. They attempted to add value to a fish fauna they viewed as basically worthless. Concurrent with these introductions were drastic changes in habitat and flow regimes caused by large dams constructed on the mainstream Colorado River. Decline and extirpation of native fishes quickly followed these introductions (Minckley and Deacon 1968); but it was uncertain if the native-fish fauna declined due to habitat changes, introductions of nonnative fishes, or both (Moyle 1986; Marsh and Langhorst 1988). In North America, extinctions within the past century were mostly attributed, in part, to either detrimental effects of introduced species (68%) or alteration of physical habitat (73%) (Miller et al. 1989). A total of 55 fish species exists in the upper basin of which 42 are not native to the basin (Tyus et al. 1982; Table 1). Nonnative species comprise 30 to 60% of the fish fauna west of the Rocky Mountains compared to fewer than 10% of the fish fauna east of the Rocky Mountains (Moyle 1986). Eastern fish faunas evolved in complex and shifting assemblages; with few changes to community structures or dynamics due to introductions (Grossman et al. 1982). Moyle (1986) noted that the west had seen wholesale replacement of its native fishes, mostly with eastern species. He identified habitat types in North America that favored establishment of introduced fishes. These habitats included reservoirs and other artificial non-flowing waters, coldwater lakes, coldwater streams, desert streams, isolated habitats, subtropical waters, and large rivers. Success of nonnative fishes also increased in "simple" communities with low native-species diversity (Minckley and Deacon 1968; Shafland 1979) and in areas subjected to physical and biological disturbances (Taylor et al. 1984). The Colorado River Basin has been susceptible to the establishment of nonnative fishes because of the low diversity of its native-fish fauna, the high degree of endemism of this fauna, and its highly altered physical habitat The Colorado River Basin has the highest level of endemism (74% of the native fishes are endemic species) of any major drainage in North America (Miller 1959), a result of geological isolation from other drainages for a long evolutionary time (Hubbard 1980). In other basins where the native fish fauna is less rich (e.g., the main Mississippi drainage), nonnative fish are also more abundant (Moyle 1986).