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rare, juvenile Colorado squawfish may be abundant in the lower Green River (Tyus et al. 1987), and downstream drift of larvae suggests that a long- distance upstream movement by juveniles is needed to repopulate upstream areas. Such movement probably occurs during the late juvenile or early adult stage, because only large-sized fish are found in the upper Yampa River, and the highest concentration of juveniles is found in the lower Green river (Tyus 1986, 1990; Tyus et al. 1987). Evidence of predation by non-native fishes in both artificial and natural environments suggests that this factor may impact the survival of juvenile Colorado squawfish. Hendrickson and Brooks (1987) noted predation by yellow bullhead, Ameiurus natalis, and largemouth bass, Micropterus salmoides, on young Colorado squawfish stocked into the Verde River, Arizona. Osmundson (1987) noted predation by largemouth bass, green sunfish, black crappie, L. nigromaculatus, and black bullhead, A. melas, on young Colorado squawfish in gravel pits near the Colorado River and indicated that predation by channel catfish may have occurred. In addition, Coon (1965) reported channel catfish predation on Colorado squawfish in the Dolores River. Humpback Chub Humpback chub occur in several canyon-bound sections of the Green River basin, including the lower portion of the Little Snake River and Yampa, Whirlpool, Desolation and Gray canyons (Holden 1978; Tyus et al. 1982ab, 1987; Karp and Tyus 1990a; M. Moretti and E. Wick, pers. comm.). Survey studies conducted prior to and after closure of Flaming Gorge Dam indicated that all three forms of Colorado River chubs were locally common in the Green River; bonytail and roundtail chubs in upper Green River above Ouray, Utah; humpback chub in Desolation and Gray canyons and historically in Flaming Gorge basin of upper Green River (Smith 1960; Holden 1978; Vanicek 1967; Seethaler et al. 1979). Although intensive netting and electrofishing efforts in the Green River mainstream by USFWS 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 in part, to confusion with specific identification of some individuals (Tyus et al. 1987; Rosenfeld and Wilkinson 1989; M. Moretti, pers. comm.). More intensive studies are needed to further elucidate the systematics of the 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 and Clemmer 1982). Thus, the presence of intermediate forms in altered systems (e.g., Green River, Colorado River) and the paucity of such forms in some unaltered rivers (e.g., Yampa River, 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 DNM, particularly Yampa Canyon (see Tyus and Minckley 1988; Tyus and Karp 1989; Karp and Tyus 1990a) because this information is largely unknown far the mainstream Green River. We presume habitat use and needs are similar throughout the Green River basin. 22