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on natives. Such indirect evidence may include inferences from field data or results of laboratory studies. Direct evidence of predation includes native fishes obtained from stomach contents of the nonnative fishes and by visual observation of predation. Indirect evidence strongly suggests a link between the decline of native fishes to the proliferation of nonnative fishes has been given by many workers (Dill 1944, Wallis 1951, Jonez and Sumner 1954, Miller 1961, Vanicek 1967, Rinne 1971, Vanicek and Kramer 1969, Baxter and Simon 1970, Moyle 1976, Holden 1977, Joseph et al. 1977, Allan and Roden 1978, Deacon 1978, Miller et al. 1982 and references therein, Kaeding and Zimmerman 1983, Hinckley 1983, Wick et al. 1983, Bestgen and Propst 1989, Marsh and Hinckley 1989, Tyus and Karp 1989, Tyus and Beard 1990, Tyus and Nikirk 1990, Valdez et al. 1990, Hinckley and Deacon 1991 and references therein, Propst and Bestgen 1991, Rinne 1991, Rinne and Hinckley 1991, Scoppertone 1993, Ruppert et al. 1993, Trammell et al. 1993, and Valdez and Ryel 1995). Other workers have ~ studied dietary overlap and postulated that competition for food and/or space was occurring (Jacobi and Jacobi 1982, McAda and Tyus 1984, Grabowski and Hiebert 1989, Muth and Snyder 1995, Valdez and Ryel 1995). Laboratory studies have documented agonistic behavior, resource sharing, and vulnerability to predation (Papoulias and Hinckley 1990, Karp and Tyus 1990, Johnson et al. 1993, Beyers et al. ~ 1994). Direct observations, including stomach content analyses, of predation by nonnative fishes have been reported for many species native to the Colorado River basin, including Colorado pikeminnow, razorback sucker, and humpback chub (Table 1). The ~ list is extensive and should leave no doubt that predation by nonnatives is a powerful force. The number of predator species is great, especially for the early life history stages of the razorback sucker. However, it has been difficult to document predation on larvae in nature. Part of the difficulty in documenting predation in early studies is that the rapid digestion of some of the centrarchid fishes was not appreciated. ~ Langhorst and Marsh (1986) found that razorback sucker larvae were only distinguishable in stomachs of green sunfish (Lepomis cyanel/us) for about 30 minutes. After that time the larvae essentially were dissolved. The table is supplemented by reports of humpback chub with characteristic bite marks that have been attributed. to channel catfish. These marks could not have been made by native cyprinids or catostomids because they lack jaw teeth (Kaeding and Zimmerman 1983, Karp and ~ Tyus 1990). The nonnative fishes can be divided roughly into three assemblages based on the threat posed to recovery of the endangered fishes. The first is comprised of small cyprinid species (e.g., red shiner, sand shiner, and fathead minnow) that are abundant ! mainly in backwater habitats of the warmer, low-gradient river reaches. Although these cyprinids are small, they are very aggressive and will prey on larvae in the backwaters that serve as nursery habitat for the Colorado pikeminnow (Dunsmore 1993 and 1996, Muth and Snyder 1995). The second group consists of centrarchid fishes (e.g., largemouth bass, green sunfish) that occupy deeper and more permanent pools that ! 27