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HUMPBACK CHUB Gila cypha (Miller) A streamlined minnow; concave skull; a prominent nuchal hump at occiput (back end of head marked by a line separating scaleless and scaled portions of epidermis); caudal peduncle thin but not long; snout overhangs upper lip; scales often minute or absent on keel; fins falcate with 9 dorsal fin rays and 10 or more anal fin rays; in young less than 5.9 inches eye diameter less than 2/3 caudal peduncle depth. Adults are dark on top and light below. Fins rarely have yellow-orange pigment near base. Adults are usually 12-16 inches long and weigh from 3/4-2 pounds. Range: Humpback chub historically ranged in the mainstem Colorado River downstream to below the Hoover Dam site (Miller, 1955). Present populations are restricted to areas in, and upstream, of the Grand Canyon. In Colorado, the hump- back chub has been found in the Yampa, Gunnison, Green, and Colorado rivers. The greatest numbers of humpbacks in Colorado are taken at the Black Rocks area of the Colorado River downstream of Grand Junction. The humpback chub is a federal and state endangered species. Habitat: This big river fish lives where water depth, velocity, and turbidity make direct observation difficult. The Black Rocks area is a river reach of deep, fast water with a sand, gravel, boulder bedrock stream bed. The fish are not found in areas of swift current, but prefer slower eddies and pools (T. Lytle, personal com- munication). Humpback chubs are found to be concentrated in such river areas, and not found in other habitat types. Con- versely, exotic and other native species are not normally found in humpback habitats, (Valdez et al. 1982). Tagging studies show that humpback chubs in the Black Rocks area rarely move more than 0.5 miles from the spot they were first collected and marked (Miller et al. 1982). Both young and adult chubs are taken in the Black Rocks area, indicating that the fish may reproduce at this site. Spawn- ing takes place when water temperatures are 58-62°F (Valdez et al. 1982) and Ham- man (1982) noted that 2% of humpback eggs are hatched when incubated at 54-56°F while 79-84% hatch in 66-68°F water. One possible explanation of this fish's disappearance in some areas would be coldwater releases downstream of man- made impoundments. Valdez et al. (1982) Family Cyp4Wue Genus Gila suggests that reduced river flows allow the roundtail chub, a fish of "shallower, less swift water than the humpback;' to suc- cessfully inhabit some deepwater areas during low water periods where hump- back chubs were previously isolated resulting in competition and hybridiza- tion. For proof, Valdez notes the presence of a tentatively identified humpback x roundtail in the area of the Colorado River near the town of DeBeque. 29 Colorado's Little Fish Adult humpback chub. Note the tag at the base of the dorsal fin. Distribution by County of the humpback chub.