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. , 12 this reach during the spawning season: (c) the occurrence of larvae in and downstream of the reach: and (d) the availability of suitable physical habitat conditions in the reach. At certain flow regimes, the IS-mile reach above the Gunnison confluence appears to have all the necessary physical components to support spawning (i.e., cobble substrate and riffles adjacent to deep pools>. Field studies of the confirmed spawning sites in the Yampa and Green Rivers indicate that Colorado sQuawfish use this type of habitat for adult staging and deposition of eggs. In addition, water telllleratures in the 15-mi 1 e reach do reach temperatures conducive to Co 1 orado sQuawfi sh s~awni ng. The IS-mile reach is actively used by Colorado sQuawfi sh throughout the year for a variety of purposes (spawning, wintering, general adult use, etc.). The Service believes that these data point to the need to prevent further degradation of adult Colorado sQuawfish habitat in the 15-mile reach. HUMPBACK CHUB Two major populations of humpback. chub (.!il.l.g ~) occur in the upper Colorado River at the Black Rocks area of Ruby Canyon (river miles 135 to 137) and in Westwater Canyon (river mil es 116 to 124). Because these reaches are deep and narrow, the Service believes that major flow changes would be necessary to significantly reduce the availabil ity of deepwater habitat used by adult fish. It is expected that the conservation measures included in this opinion to protect the Colorado sQUawfi sh a 150 wi 11 minimi ze adverse impacts to the hlJlT1)back. chub. For this reason, the Service has determined that the proposed action is not likely to jeopardize the continued existence of this species. BONYTAll CHUB little is ~ about the biological requirements of the bony tail chub, as the species greatly declined in numbers in the upper basin shortly after 1960. Until recently, the Service considered the species extirpated from the upper basin. However, a ~imen collected at Black Rock.s in 1985 which exhibits many bony tail characteristics could indicate a small, extant population (Kaeding et al. 1986). Also, during recent studies, several suspected bonytai 1 chub have been captured in Cataract Canyon (Valdez 1988). It is assumed that if this species persists in the Colorado River, its preferred habitat would be extensive reaches of the river downstream from the area of direct influence of this project. The conservation measures included in this opinion for Colorado sQuawfish will help to preserve the natural conditions of the Colorado River and should preserve any nseining habitats of the bony tail chub. Therefore, the Service has detel1lined that the effects of water withdrawals due to this Federal action are not likely to further jeopardize the continued exi stence of the bonytai 1 clUl.