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CHAPTER III AFFECTED ENVIRONMENT water temperature. Because young humpback chub are difficult to distinguish from young roundtail and bonytail chub, the habitat and distribution of young humpback chubs are unknown. Bonvtail Chub This species is extremely rare; only incidental collections have been made in recent years. Very low numbers may occur in Gray Canyon of the Green River and Black Rocks (Figure III-B-3). The last sightings of fish identified as bonytail chub was a fish captured at Black Rocks on the Colorado River in 1984, a specimen captured from Lake Powell near Page, Arizona, in 1985, and a few observations from the Green River. The habitat requirements of this species are not well known. Razorback Sucker Adult razorback suckers are found in a variety of habitats including quiet eddies, pools, and mid-channel runs. They are usually found over a sand or silt substrate but occur over gravel and cobble bars in the spring during spawning. The largest population of razorbacks is in the upper Green River between the confluence of the Yampa River and the confluence of the Duchesne River (Figure III-B-4). Adults also occur in the Colorado River near Grand Junction, Colorado, although numbers there are very low. Spawning is known to occur over sand and gravel bars in the Green River near Dinosaur National Monument and at the mouth of Ashley Creek near Jensen, Utah. Recent collections of several ripe adults from large flooded bottoms in the Green River near Jensen and Ouray, Utah, suggest that fish are either using these sites as staging areas prior to spawning or that they may spawn at these sites. Survival of the young razorback suckers is low. Recruitment is apparently unsuccessful since no young or juveniles have been collected with intensive sampling on the Green and Colorado Rivers since 1979. Several possible explanations for the lack of reports on young razorback suckers include: (1) reproductive failure, (2) predation of eggs and young by nonnative fishes, and (3) competition with nonnative fishes for food and habitat. C Native Fishes (Other than the Rare and Endangered Fishes) Seven species of fish native to the Colorado River are still found in the Upper Basin and are not rare (Table III-B-1). The roundtaii chub, flannelmouth sucker, and speckled dace are considered common to abundant. The roundtail chub, present in low numbers in the Green and San Juan Rivers, is present in the Green River above Flaming Gorge Reservoir and the flannelmouth sucker occurs in the reservoir. The mountain whitefish is common in the colder headwater areas of the Green and Colorado Rivers and their major tributaries, and the bluehead sucker is common in the high-gradient warmwater areas. The mottled sculpin and mountain sucker are found in low numbers in most reaches of the Upper Basin, but the sculpin is common only in the Gunnison River. See Tyus et al.., (1982: 12-70) for a more thorough coverage of distribution and abundance of native fish. III-12 1