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Family Cvprinidae Genus Gila ROUNDTAIL CHUB Gila robusta(Baird and Girard) A moderately streamlined minnow; caudal peduncle not pencil thin; length of head divided by depth of keel is greater than 3. Scales minute, may be absent locally; fins falcate; dorsal and anal fins normally have 9 anal rays. Adults silvery shading dorsally to dusky yellow or light green, spawning males have a pink cast to their fins. Adults can attain 18 inches in length and two pounds in weight. Range: The roundtail has historically been the most common member of the genus Gila in the Colorado River Basin extending up to the mountain foothills (Holden and Stalnaker 1975). In Colorado, the roundtail chub is found in the Col- orado River mainstem and larger tribu- taries (e.g., White, Yampa, Dolores, San Juan, and Gunnison rivers). Although abundant in most waters where it is found, the roundtail is declin- ing in the Gunnison River. Common in the late 1970's in the Gunnison River up- stream in the North Fork of the Gunnison, roundtail chub were not found in that river reach in either 1981 or 1982 (B. Nehring, personal communication). The species was found to be declining in numbers in the Gunnison River downstream of the North Fork to the town of Delta during the same time period. Habitat: A large river fish, roundtail occupy slow moving waters adjacent to areas of faster water. Groups of adult roundtail concentrate in quiet swirling water adjacent to fast moving water, swim- ming in small groups into the faster water "presumably to feed" (Minckley 1973). Young-of-the-year prefer shallow river runs while juvenile chubs concentrate in river eddies (Valdez, et. al. 1982) and irrigation ditches (Wiltzius 1978). Not a great deal is known about this species. Young feed on small insects, and algal films, while older chubs take both terrestrial and aquatic insects along with filamentous algae (Minckley 1973). Large roundtails are predators and will take small fish. Spawning habits are probably similar to other minnows and spawning takes place over a gravel substrate. Fertil- ized eggs are randomly scattered over the substrate with no parental care. Roundtail spawn in early summer in warmer water than the humpback chub, as spring runoff is subsiding (Valdez, et al. 1982). Young- of-the-year chubs (0.4 inches in length), indistinguishable as to roundtail, hump- back, or variant have been captured in Y - ? I I r1 l Distribution by County of the roundtail chub. early June from the Colorado River and in late July (Haynes and Muth 1982). Such a reproductive pattern could explain the decline of roundtail populations in the r i i I I N__v -? - ? 1 ti ?h? Gunnison River downstream of the Cure- canti Project because coldwater releases result in lower water temperatures all summer. Colorado's Little Fish 28 Adult male roundtail chub in breeding colors. Adult roundtail chub. Photo by Robin Knox.