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C) c, ...... (0 t--"" 00 Another form of predation impact noted in the Colorado River Basin was channel catfish becoming lodged in the throats of Colorado pikeminnow. Sometimes, when Colorado pikeminnow eat channel catfish, the pectoral fin spines of the channel catfish catch in the throat of the Colorado pikeminnow, causing the Colorado pikeminnow to eventually die, Upper Basin residents' observations were reported by Vanicek and Kramer (1969), and some anglers thought this was a majorreason for Colorado pikeminnow decline. McAda (1983) reported an adult Colorado pikeminnow with a channel catfish lodged in its throat from the Green River. Pimental et al. (1985) conducted Colorado pikeminnow feeding experiments using despined and normal channel catfish, and rainbow trout, as prey. Channel catfish was not preferred by Colorado pikeminnow, compared with trout, and was only eaten after the Colorado pikeminnow starved for 5 days. Starved Colorado pikeminnow ate some of the despined catfish, and even fewer of the normal catfish, but they attempted to spit out any normal catfish they ingested, Pimental et ai, (1985) also reported on three other collections of channel catfish lodged in the throats of wild Colorado pikeminnow from the early 1980s. In October 1999, during SJRIP adult monitoring sampling, a large juvenile Colorado pikeminnow (330 mm TL, likely one of the stocked fish from 1996) was captured with ajuvenile channel catfish lodged in its throat. Although reports of channel catfish lodged in the throats of Colorado pikeminnow are rare, and this phenomenon is likely not a major limiting factor, channel catfish may also have a negative effect on adult Colorado pikeminnow. Because channel catfish are abundant in the San Juan River, they likely are responsible for the major predation occurring there, Common native fishes were the primary species eaten, which is reasonable since they were the most abundant fishes in the river, None of the common species populations appeared to be limited in the river, suggesting that predation was not a major limiting factor to native fishes. Roundtail chub numbers in the San Juan River may be low because of predation, but this has not been confirmed, Predation on larval razorback sucker and Colorado pikeminnow was not adequately assessed during the 7 -year research period because of the low numbers of the endangered species available. Red shiner exhibited predation on larval Colorado pikeminnow and razorback sucker in the laboratory (Ruppert et al. 1993), and predation in the wild was suggested as a major factor in determining year-class strength of Colorado pikeminnow in the Green River (Bestgen et al. 1998). Because red shiner are very abundant in the San Juan River (Propst and Hobbes 2000), they may be the most important predator on the two endangered fishes (Brandenburg and Gido 1999), Predation by nonnative fishes on larval endangered fishes may be a concern in the future as populations and reproduction of these two species increase, Competition For competition to occur, one or more of the individuals or populations involved in the interaction needs to have reduced fitness while one of them has a net gain (Wooten 1990). This typically happens in limited-resource situations. Douglas et al. (1994) suggested a "displacement hypothesis," where nonnative species displace native species without an actual shortage of habitat. They studied red shiner (nonnative) and spikedace (Meda JUIgida) interactions and noted that when the two species coexisted, spikedace used swifter habitats, suggesting red shiner displaced spikedace to lower-quality habitat. Competition can be difficult to demonstrate, and although widely cited as a major factor in declining native fish populations in the Colorado River Basin, it was established in very few cases (Douglas et al. 1994). Determining whether competition is a September 2000 3-46 Program Evaluation Report