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• behavior keep the spawning groups apart (Hubbs and Miller 1953). Although not well understood, the bonytail chub, humpback chub, Gila cypha, and the roundtail chub may be undergoing introgressive hybridization (Holden and Stalnaker, 1970). Several introduced diseases may also be taking their toll on native fishes, such as flukes, tapeworms, and protozoa. Seethaler (1976) indicated that the razorback suckers found in the Walter Walker Wildlife Area near Grand Junction, Colorado were parasitized by Lernea and blind eyes were common. Dotson et al (1960) thought a bacterial infection of fishes in the Green River was triggered by industrial and municipal pollution. The precise tolerances of razorback suc?;ers and bonytail chubs to water pollution have not been investigated, but wastewater loads have certainly increased in the Colorado and Gunnison Rivers in Colorado in years past, and may be a problem. CONCLUSIONS Bonytail chubs and razorback suckers are part of the western aquatic fauna which are difficult to observe and generally considered less worthy of management than organisms of value to sportsmen or industry. Both species are mainstream inhabitants of the extremely variable, yet harsh, upper Colorado River drainage in Colorado. Rapidly changing land and water use patterns have altered the natural habitat of these fish. The construction and operation of water impoundments, channelization of backwater habitat, and the release and proliferation of non-native fishes have adversely affected their habitat. Historical and recent stream inventories have demonstrated marked declines in both distribution and abundance of these fish in Colorado. Bonytail chubs have become extremely rare, if not extirpated, in Colorado. Only a few razor- back sucker adults can still be found in the Colorado and Gunnison Rivers in which they were formerly common. - 10 -