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15 and Jacobi 1981; McAda and Tyus 1984). One or more of the following species; redside shiner, common carp, white sucker, or fathead minnow; were identified in different studies as prey consumed by yearling, juvenile, or adult Colorado squawfish (Vanicek 1967; Grabowski and Hiebert 1989; Osmundson and Kaeding 1989b). Introduced fishes used as prey may pose a threat if they have anti-predatory mechanisms that are deadly to their predators, especially if native predators are naive to this defense. Several authors have noted the occurrence of channel catfish lodged in the throats of Colorado squawfish. Apparently, pectoral fins of catfish lock out and become lodged in the esophagus of squawfish (Vanicek 1967). McAda (1983) collected a Colorado squawfish (550 mm TL) with a 120 mm TL catfish in this condition in 1974. He believed it was a rare occurrence based on the large number of Colorado squawfish collected without this problem and considered this to have had little influence on the decline of Colorado squawfish. Wick et al. (1985) also collected a Colorado squawfisIl (591 mm TL) with a 174 mm TL channel catfish lodged headfirst in its throat Pimentel et al. (1985) reported three occurrences of Colorado squawfish choking on channel catfish in the Green River in 1982 and 1983. They also performed laboratory studies of adult Colorado squawfish (mean TL = 444 mm) and small channel catfish (mean TL = 77 mm). Colorado squawfish that did consume channel catfish were not injured and catfish did not lodge in their throats. A possible explanation for the lack of lodging was that Colorado squawfish in this study were considerably larger than the prey. Colorado squawfish did not prefer channel catfish and fed on other species when possible. Removal of spines did not enhance predation on channel catfish and Colorado squawfish fed on them only after being starved for over 5 days. Pimentel et al. (1985) suggested that environmental conditions, such as cold water, might make channel catfish sluggish and more vulnerable to predation. Spatial interactions Any desirable space in limited supply can be contested, creating competition between nonnative and native species. There can be competition for habitat, feeding station, or spawning space. Using a preferred space and protecting it from others might benefit a fish that is resting, feeding, spawning or performing any necessary activity. Benefit may be obtained from an area that provides better water quality, depth, velocity, substrate, or some other parameter. Spatial competition is often tested by determining overlap of microhabitat use between two species. A high degree of overlap may indicate competition for a defined space, but for competition to occur microhabitat must be limited Habitat use of some introduced fishes in the upper basin was summarized by Miller and Hubert (1990) and Valdez (1990) summarized additional information on nonnative distribution and life history. Holden (1977) suggested possible competition based on similar microhabitat use by rare and introduced species. His collections of YOY squawfish and humpback chub on the Green River were dominated by young nonnative species. McAda and Tyus (1984)