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SUMMARY OF KEY FINDINGS Native and non-natives fishes that occupy the lower Colorado River overlap broadly in their physical habitat and resource uses. Spatio-temporal overlap was evident at all life stages (larval, juvenile, and adult). Common resource uses involved critical life-history functions such as feeding (subsuming maintenance and growth); however, a substantially greater number and proportion of non-native fishes than native species are piscivorous predators. Spatial, temporal and thermal aspects of spawning were comparable among native and non-native fishes, notwithstanding requirements of cold- vs warm-water taxa. Faunas were distinctive in mode of reproduction: native species all are broadcast spawners without parental care, while non-natives are predominated by nest-building and some level of parental protection of eggs and young. No attribute of physical habitat or resource use can be identified that markedly or marginally favors one group of fishes over another, and we cannot envision habitat manipulations or features that could be made to accomplish such a goal. Rather, the evidence supports an hypothesis that presence of non-native fishes alone precludes successful life-cycle completion by components of the native fauna. This array of non- native fishes now present has feeding, behavioral, and reproductive attributes that allow it to displace, replace, or exclude native kinds. The lower Colorado River is physically altered and hydrologically regulated. New habitats created by dams include impoundments that represent large lakes, and thermally depressed, downstream tailwaters. These are dramatically different from the lower Colorado River before dams. Nonetheless, there is ample evidence to support an argument that these habitats would be occupied by native fish communities, and these native fishes would find resources necessary to life-cycle completion and population maintenance, if there were no non-native fishes. It is absurd to suggest that the Colorado River would be devoid of fishes if alien species had not been introduced to the system.