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17 Holden 1978; Vanicek 1967, Seethaler et al. 1979). Although intensive netting and electrofishing efforts in the Green River mainstream by Service and Utah Division of Wildlife Resources biologists yielded over 500 chubs (Miller et al. 1982c; Tyus et al. 1987; M. Moretti, pers., comm.), status of humpback chub in that system is not well understood, due j,n part to confusion with specific identification of some fish (Tyus et al. 1987;~Rosenfeld and Wilkinson 1989; M. Moretti, pers. comm.). More intensive studies are needed to further evaluate the systematics of Colorado River chubs and to determine their habitat needs in the Green River basin. The morphological variation that occurs in some areas of sympatry may be induced by recent habitat change (Valdez ahd Clemmer 1982). If preferred spawning conditions and/or habitats are unavailable for one or more species, spawning might occur during other times and/or places that promote hybridization. Thus, the presence of intermediate forms in altered systems (e.g., Green River and Colorado River) and the paucity of such forms in some unaltered rivers (e.g., Yampa River and Little Colorado River) suggests that natural riverine environments are important for recovery of the humpback chub. The following paragraphs summarize information on habitat use of humpback chub in Dinosaur, particularly Yampa Canyon (Tyus and Minckley 1988; Tyus and Karp 1989; Karp and Tyus 1990a) because little of this information is available for the mainstream Green River basin. During spring and early summer, humpback chub in the Green River basin are most prevalent in high-gradient, whitewater reaches dominated by rocky runs, riffles, and rapids. Adult fish are most often collected in seasonally flooded shoreline eddies that are downstream of large boulders and upstream of rapids. Juveniles appear more common in sma]ler eddies in rocky shoreline runs. Humpback chub remain rare in Dinosaur, and only 133 juvenile and adult fish were captured during spring sampling efforts from 1986-1989 (Karp and Tyus 1990a). Feeding habits of humpback chub are relatively unknown in the Green River basin, but stomachs of a few fish contained hymenopterans and plant debris. Humpback chub also feed on Mormon crickets and presumably other foods at various levels within the water column (Karp and Tyus 1990a). Although fall and winter habitat requirements of humpback chub are not well known, some observations in Dinosaur suggest that the fish remain in pools and eddies of impounded water and rapids in low flow conditions (Karp and Tyus 1990a). Minimum flows are thus required to maintain such habitats from midsummer through late winter, but specific flow leve]s for the Green River are not determined. Saawning Humpback chub generally spawn between temperatures of 16-20 °C but may also spawn in temperatures as low as 11.5 °C (Valdez and Clemmer 1982) and as high as 20.5 °C (Kaeding and Zimmerman 1983). Eggs are adhesive, but little is known about preferred substrate for egg deposition. Hatchery success diminishes as temperatures vary from their optimum of 20 °C (Hamman 1982a; Marsh 1985). The eggs generally hatch in 5-1 days at 19-20 °C (Hamman 1982a).