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W.. Evidence To Support The Belief That Introduced Fishes Have Negatively Affected la. Iq? s The Endangered Colorado River Fishes Decline of Colorado squawfish (Ptychocheilus lucius), humpback chub (Gila cypha), bonytail (Gila elegans) and razorback sucker (Xyrauchen texanus) populations in the Colorado River basin has occurred since the coming of technologic man. This decline is most evident in the lower basin where many rivers formerly habitat for native fishes have been dammed or entirely dewatered as the result of water-development projects. These and other lower basin rivers have also been extensively colonized by introduced, non-native fishes. Because much of this environmental alteration occurred well before field investigations of fish populations were performed, it's not possible to definitively state which particular alteration was the most important cause of the decline. However, in some instances the effect of some types of alteration is so profound that it alone was sufficient to account for elimination of native fishes. Such is the case for complete dewatering, as well as for the severe reduction of seasonal-high water temperatures in river reaches downstream from dams with hypolimnetic discharges. Although these particular perturbations clearly have demonstrable negative effects, the effect of introduced fishes on native fishes is not so clearly evident. ''Nonetheless, there is evidence that strongly suggests such effect exists and could be important. We consider this evidence to be of three types: observational, experimental and deductive. Observational Evidence Observational evidence is actual documentation of introduced fishes preying upon or competing with wild, native fishes in nature. Although such evidence would perhaps be among the most useful to support the argument, these data are among the most difficult to obtain. We have found only one record (Coon, 1965, cited in Seethaler 1978) of endangered fishes being found among the stomach contents of introduced predator species. In that case, Coon reported two, presumably small, Colorado squawfish among the stomach contents of 58 channel catfish (Ictalurus punctatus) that he had collected from the Dolores River in September, 1963. Seethaler (1978) suggested this observation was especially significant because squawfish were rare in, the Dolores at the time of Coon's study. Few additional observational data are available. McAda (1983) and Pimentel et al. (1985) reported collecting adult-size Colorado squawfish from the Green River that had channel catfish firmly lodged in their throats. These Colorado squawfish evidently prey on the catfish, whereupon the catfish erected their spiny fins and firmly lodged themselves in the squawfish throat.