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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 level is necessary to maintain these habitats. Some efforts have been made to investigate the status of humpback chub in mainstream Green River (Tyus et a]. 1987; Rosenfeld and Wilkinson 1989), but 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 unaltered rivers {e.g., Yampa River, Little Colorado River) suggests that natural riverine environments-are important for recovery of the humpback chub. 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 lb 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 flam. 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 has been implicated in the decline of the bonytail chub in that .system, fish collections in DNM before and after the poisoning {Binns et al. 1963;. Vanicek and Kramer 1969; Vanicek et al. 1970) suggested that the downstream extent. of the poison was not a factor in the almost total extirpation of the species, from the Echo Park area. Flaming Gorge Dam operations could affect the future of bonytail chub re-introductions in the Green River system and needs to be- considered in reintroduction efforts. 22