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1, 46 DISCUSSION AND SUMMARY Literature Review Increases of number of nonnative fish have been documented in the Colorado River (Osmundson and Kaeding 1989a), Green River (Behnke 1980; Holden 1980; McAda et at. 1980), Yampa River (Joseph et at. 1977; Miller et al. 1982a), and White River (Crosby 1975; Lanigan and Berry 1979, 1981; Miller et at. 1982b). The abundance of nonnative fishes has also been shown to be related to abiotic factors such as habitat alteration and flow events (Cooper 1983; Meffe 1984; Castleberry and Cech 1986; Minckley and Meffe 1987; Valdez 1990; Osmundson and Kaeding 1991). In the upper basin, predation has been documented on wild Colorado squawfish by channel catfish (Coon 1965) and black bullhead (Taba et a1. 1965). In the lower basin, channel catfish consumed humpback chub (W. L Minckley, personal communication in Karp and Tyus 1990b) and there is evidence of other predation attempts by channel catfish on humpback chub (Kaeding and Zimmerman 1983; Karp and Tyus 1990b). Razorback sucker eggs have been found in channel catfish stomachs in Lake Mohave (Bozek et at. 1984). Stomach contents from one or more of the following species; channel catfish, common carp, northern pike, rainbow and cutthroat trout" green sunfish, black bullhead, largemouth bass, striped bass, or walleye; were examined by several researchers who failed to find eggs or remains of rare fish (person and Bulkley 1982; Bozek et al. 1984; Grabowski and Hiebert 1989; Tyus and Karp 1990). Inability to detect predation through stomach content analysis does not discount predation of endangered fishes. Inability to detect predation may be due to small sample size of predator stomachs, rarity of endangered fish, rapid digestion, mastication, or regurgitation, of prey (Bozek et at. 1984; Tyus and Beard 1990). Even if predation is documented, its affect on the prey population may be difficult to predict, especially in wild, natural situations. Several studies have shown the negative affect that predation can have on fish numbers. Because of the difficulty of documenting predation on wild, rare species, nonnative fish predation effects were documented most frequently on native fishes reared in large numbers, in confined areas, or after stocking (Hendrickson and Brooks 1987; Osmundson 1987; Marsh and Langhorst 1988; Marsh and Brooks 1989). Predation was confirmed as a significant source of mortality for artificially propagated YOY and yearling Colorado squawfish raised along the Colorado River in riverside ponds (Osmundson 1987). Predation was also confirmed as a significant source of early life mortality on razorback sucker reared in an isolated backwater in Lake Mohave (Marsh and Langhorst 1988). The backwater was invaded by exotic predators that eliminated the razorback sucker population. Exotic fishes identified as major predators in the two studies were largemouth bass, green sunfish, black crappie, black bullhead, channel