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10 behavior between the two species could be one of the other reasons why even despined catfish were taken less frequently than trout. A final possibility relates to the second component of Ivlev's definition of prey vulnerability-- predator preference. Colorado squawfish may innately prefer other species over channel catfish, or they may have been conditioned to trout through years of feeding on them. There have been no studies on conditioning with squawfish; however, Mauck and Coble (1971) found prior conditioning had no effect on northern pike preference. In predator preference studies that in- volved ictalurids (Beyerle and Williams, 1968; Mauck and Coble, 1971), cat- fish were always the least preferred species but were still consumed to some degree. In our observations, it appeared that Colorado squawfish were just not interested in catfish as prey. Squawfish could be fed trout daily and would consume them immediately, yet they had to be starved for 5 days before they would even attempt to eat catfish. Perhaps the schooling behavior and bottom orientation of channel catfish made them uninteresting to squawfish. Nothing is known about how squawfish "hunt" their prey, but using Greenway's `1965) method of classifying predators by body type and mouth structure, Moyle (1976) classified Colorado squawfish as roving predators with lie-in- wait affinities. Thus, a single fish close to the surface may seem more attractive to a squawfish than a tightly schooled group of fish on the bot- tom. Only 8 attempts (6 observed, 2 unobserved but fish were eaten) were documented of squawfish eating CCSP. These squawfish were not injured in the attempts and the catfish were not lodged in the throat. Reist (1980) has shown that the size ratio between predator and prey is important in de- termining the effectiveness of spines in preventing predation. The Colorado squawfish reported by McAda (1983) was 550 mm long and the catfish was 120