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I I I I I I I I I I I I I I I I I I I In the Colorado River, humpback chub are locally abundant in Westwater Canyon and Black Rocks, and young and juveniles are present. An occasional adult has been found outside these preferred areas. Adult humpback chub prefer deep, swift runs and eddies with a rocky substrate (Valdez and Cl enrner 1982). Young prefer backwaters or other available slack water habitats near the deep, swift runs. Adults apparently remain fairly stationary in these favored habitats with few movements or migrations to other areas. little is known concerning spawning of humpback chub. Spawning occurred at temperatures of about 160C during spring runoff in May and June in B1ac1< Rocks in 1980 and 1981 (Valdez and Clerrvner 1982). Spawn ing probably occurs over gravel-cobble bars associated with the favored deep canyon habitats. Radiotel emetry in 1983 indicated that spawning probably occurred in the Black Rocks area and that little move- ment outside that area occurs (Miller et al 1984, Archer et al 1984). The major loss, or decline, of humpback chub in the past has been due to habitat alterations by large dams, such as in the Grand Canyon or the F1 aming Gorge area. Most present popu1 ations in the upper basin appear to be suffering from hybridization with roundtail chub (Valdez and Clerruner 1982), a less obvious but very effective method of el imi- nating a species. The major reason for this hybridization appears to be changes in flow. A popu1 ation near De Beque is now apparently all hybrids with the roundtail chub (CRFRT 1982a; Valdez et a1 1982), and populations in Desolation Canyon of the Green River a1 so appear to be primarily hybrids (Tyus et a1 1982). Reduced flows have tended to shift the distribution of roundtail chubs downstream from that observed historically (Jordan 1891) and; therefore, they are probably more abundant in humpback chub habitat than they were historically. Also, - 26 -