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and humpback chub and channel catfish may be competing for food or microhabitat as suggested by capture of both species with baits in the same eddy habitats (Tyus and Minckley 1988; Karp and Tyus, in review). The high number of channel catfish in humpback chub spawning habitat, in addition to their potential ability to compete and prey on humpback chub (Kaeding and Zimmerman 1983; Karp and Tyus, in review; C. 0. Minckley, personal communication), suggests that this omnivorous introduced species may adversely affect survival of humpback chub in DNM. Channel catfish may be using mainstream habitats in the Green River for winter refugia as in some other river systems (Newcomb 1989). Flows or other conditions (e.g. temperature and fish movements) which may favor growth and reproduction of channel catfish in the Green River basin should be evaluated. Humpback chub in DNM spawn following highest spring flows at river temperatures about 20° C (Figure 7; Karp and Tyus, in review). This relationship between chub spawning and spring runoff has also been noted in the Blackrocks area of the upper Colorado River (Valdez and Clemmer 1982; Archer et al. 1985), and in the Little Colorado River, Ariz. (Kaeding and Zimmerman 1983; C.O. Minckley, pers. comm.). Spring runoff is important to reproductive success because spawning presumably occurs in shoreline eddies, during the period of declining flows and warming river temperatures following peak spring runoff. Availability of shoreline eddy habitat is greatest with spring flooding and decreases thereafter with decreasing summer flows. Loss or reduction of spring runoff could reduce availability of spawning habitat and, thus, adversely affect humpback chub reproduction. Habitat alteration may also promote hybridization with other species (Valdez and Clemmer 1982). Flow reductions and decreased temperatures have been implicated as factors curtailing successful spawn and increasing competition in the Colorado River (Kaeding and Zimmerman 1983). Although fall and winter habitat requirements of humpback chub are not well known, some observations in DNM indicated that the fish remain in pools and eddies of impounded water and rapids in low flow conditions (Karp and Tyus, in review). A minimum flow is necessary to maintain these habitats. Bonytail Chub Habitat requirements of the bonytail chub in the Green River basin are largely unknown. Fish collections in Echo Park (DNM) before and after closure of Flaming Gorge Dam indicated that the species was present in moderate numbers at the confluence of Yampa and Green rivers (Vanicek 1967). However, more recent investigations in that area have yielded few captures. Holden and Stalnaker (1975) reported the capture of 36 bonytail chubs in Yampa (lower 16 km) and upper Green rivers from 1968 to 1970. Holden and Crist (1981) collected one bonytail chub in the lower Yampa River in 1979, and USFWS biologists captured one suspected juvenile in 1987. Preliminary results of a radiotracking study of adult bonytail chub introduced into the upper Green River in 1988 and 1989 indicate that the fish exhibit crepuscular movements, and are relatively quiescent during the day and night (S. Cranney, Utah Division of Wildlife Resources, pers. comm.). Bonytail chub have apparently declined in the Echo Park area, possibly due to flow and temperature changes resulting from closure of Flaming Gorge Dam. A similar pattern has been noted in the Colorado River downstream from Glen Canyon Dam (Utah State Department Fish and Game 1964, 1969). Although the preimpoundment poisoning of riverine habitat in the upper Green River in 1962 22