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48 <br />The range of the humpback sucker has been markedly reduced in recent <br />years due to man's activities in the Colorado River system. Early observers <br />reported humpback suckers in the large numbers throughout the lower basin <br />(Evermann, 1916; Chamberlain, 1904). However, in the lower basin distribution <br />is now limited to several impoundments on the main Colorado and the species <br />is extinct below Davis Dam where they were once abundant (Minckley, 1973). <br />Humpback suckers are more widely distributed in the upper basin. Individuals <br />have been collected from the mouth of the Yampa River, in the Green from its <br />confluence with the Yampa River to its mouth, and in the Colorado River from <br />Grand Junction, Colorado to Lake Powell (McAda, 1977; Holden, 1973). <br />In this study a humpback x flannelmouth sucker hybrid was captured 51.8 <br />mi (96 km) above the confluence of the Green River on June 12, 1979. The <br />fish was caught behind a log where water depth was 2 ft (.66 m). Identifi- <br />cation was based on body shape, coloration, number of scales and the number <br />of dorsal and anal fin rays (Figure 23). This fish was a ripe male (465 mm, <br />125 gms) which had tubercles on the anal and caudal fins and released milt. <br />Humpback x flannelmouth hybrids were first reported in 1889 and more recently <br />by Hubbs and Miller (1953), Vanicek et al. (1970) and Holden and Stalnaker <br />(1975a). The incidence of hybridization between these two species appears <br />to be increasing. The situation is to be expected in an altered system where <br />one fish is considerably more abundant than another closely related fish and <br />both have similar reproductive requirements. <br />Other Endemic Fish Species <br />Roundtail Chub (Gila robusta) <br />The roundtail chub, commonly known as the Colorado chub, is native to <br />the Colorado River basin. It is decreasing in abundance as the Colorado