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r Whether these squawfish would have succumb as a result is unknown, as is the importance of such possible mortality to the overall mortality of the Colorado squawfish population. However, there are undocumented reports of Colorado squawfish found dead with channel catfish lodged in their throats (Seethaler 1978). Circumstantial evidence of probable predation by introduced fishes or endangered Colorado River fishes was provided by Kaeding and Zimmerman (1983). They reported humpback chub and channel catfish were often seen in close association with one another in the Little Colorado River. The fact that many humpback chub captured by Kaeding and Zimmerman showed apparent catfish bite marks lead the authors to conclude that predation by catfish might be an important source of mortality for humpback chub in the Little Colorado. Channel catfish is widespread and abundant in the upper Colorado River basin (Tyus et al. 1982). Experimental Evidence Two experimental investigations have provided evidence that introduced fishes may have important negative effects on endangered Colorado River fishes. Osmundson (1986) performed studies of predation on fingerling - Colorado squawfish by largemouth bass (Micropterus salmoides) in ponds as well as in the laboratory. He found that bass preferred squawfish as food over, :fathead minnow (Pimephales promelas) and red shiner (Notropis lutrensis), prey species common in the river. Osmundson's work may provide important insight into predation by bass on Colorado squawfish in river environments. Although his bass were larger than those normally found in the adjacent Colorado River, the Colorado has small bass (35-100 mm TL) in many of its backwaters. It's not known whether these young bass are the result of reproduction in the river or in adjacent riverside ponds. However, if the preference for squawfish reported by Osmundson also exists between small bass and larval-size squawfish, seemingly small numbers of bass could have important negative effects on young squawfish in backwaters--the habitat from which these size classes of both species are most often collected. Brooks (no date) used razorback sucker in an experiment similar to that of Osmundson. Fingerling-size razorbacks were stocked into the Gila River, Arizona, in September, 1985. Post-stocking analyses of channel catfish and flathead catfish (Pylodictis olivaris) stomacher revealed 46 and 79%, respectively, contained stocked razorback sucker. Based on the high density of these catfishes in rivers considered candidates for razorback reintroduction efforts, Brooks concluded potential losses of stocked razorback would be very large. Moreover, he indicated presence of such predators is the most important factor affecting success of recovery efforts for razorbacks and that predators must be removed, or at least greatly reduced in numbers, before such recovery can be expected.