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Deductive Evidence Perhaps the most compelling, albeit circumstantial, evidence supporting the argument that introduced fishes have negatively affected populations of endangered Colorado River fishes is obtained through a deductive assessment based on our general knowledge of fishery ecology, the Colorado River ecosystem and the changes that have occurred therein. It is generally agreed that the overall decline of native fishes of the Colorado River basin is the result of stream dewatering, altered flow and temperature regions downstream from dams and water diversions, poor land-use practices that have affected water quality and quantity, "pollution," the negative effects of introduced fishes and perhaps other factors. However, the relative importance of each of these is not always understood. Indeed, aside from the obvious detrimental effect of dewatering, only that of reduced temperatures below dams has been clearly demonstrated. However, there are river reaches in the upper basin (e.g. lower Colorado and Green rivers) that have not had their flow or temperature regimes or water quality meaningfully altered but nonetheless their populations of endangered fishes have significantly declined. Because these reaches have been very successfully colonized by-several introduced species (Tyus et al. 1982), one cannot escape the seemingly well-founded speculation that introduced fishes played an important role in this decline. Kaeding and Osmundson (MS) present this argument in more detail and also point out that unusually slow growth of Colorado squawfish in the upper basin (the upstream limit of historic range) makes their population especially vulnerable to the negative effects of increased early-life mortality that would result from competition with or predation by introduced fishes. Additional information indicating potential competition between Colorado squawfish and introduced fishes was provided by McAda and Tyus (1984). They analyzed diets and habitat use of age-0 Colorado squawfish and those of sympatric species. McAda and Tyus found high dietary overlap among squawfish, channel catfish and red shiner (both introduced species) and- high spatial overlap among squawfish, red shiner and fathead minnow (also an introduced species). However, the authors cautioned that such overlap is perhaps not important if dietary and spatial resources are not limited. SUMMARY Although definitive data are lacking, available information strongly suggests introduced fishes have negative effects on populations of endangered fishes in some river reaches of the Colorado River basin. Like Colorado squawfish, these introduced forms are commonly considered warm- water species. Coldwater fishes, such as salmonids, do not persist in river reaches occupied by Colorado squawfish and therefore do not have important direct negative effects on this or other endangered Colorado River fishes. References Brooks, J.E. No date. Annual reintroduction and monitoring report for razorback sucker, Xyrauchen texanus, in the Gila River basin, Arizona, 1985. Arizona Game and Fish Department, unpublished mimeographed report.