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SY1V®PSIS INTRODUCTION . s The native chubs of the Colorado River system evolved in seasonally warm and turbid ~-~ water and appear to be highly adapted to the variable hydrologic conditions that occurred in the unregulated system. The roundtail chub (Gila ~-obusta) is thought to have been more suited to the tributary streams and upper portions of the mainstem Green and Colorado Rivers. The humpback chub (Gila cypha) has been characterized as the canyon-dweller and is currently found only in the canyon. portions of the Colorado River system. The bonytail (Gila elegans) was presumed to be the main channel, large river member of the genus and is functionally extirpated in the system. All congeners have been reported from the canyons of the Green River. ~; Humpback chub were first reported from the contiguous canyons of Desolation and Gray '~ (peso/Gray) on the Green River in 1975 (Holden and 5talnaker 1;975) as result of their investigations conducted in 1967-1971. The United States Fish and Wildlife Service included l.° ~' sampling in Desolation Canyon approximately ten years later as part of their Green River o ~ Investigations. They too documented the presence of all three native chubs (humpback, roundtail, and bonytail). The Utah Division of Wildlife has monitored the fish community in f---, Deso/Gray since 1989. The highest catch rates of native chub in the Green River sub-basin are found in Deso/Gray Canyons. However, Deso/Gray catch rates are at best 25% of those reported in Westwater Canyon and at Black Rocks on the Colorado River (Chart and Lentsch 1998b, McAda et al. 1994) ranking this population the third most abundant in the Upper Basin of the Colorado River. Today the endangered humpback chub, the roundtail chub, and unclassified chubs displaying characters of both species are the most prevalent members of the Gila genus found in Desolation and Gray Canyons. The humpback chub is an obligate riverine species, that requires temperatures in excess of 16°C to spawn successfully (Valdez and Ryel 1995). Hatching success under laboratory conditions was best at temperatures of 21-22°C (Hamman 1982). Spawning occurs on the descending limb of the hydrograph, and larvae remain in the vicinity of where they hatched. Recent research suggests that shoreline habitat is preferred over backwaters by YOY and age-1 humpback chub in Grand Canyon (Valdez and Ryel 1995, Converse et al. 1998). In these studies, sub-adult (< 200 mm TL) chubs frequented shoreline habitats in Grand Canyon. Chart and Lentsch (1998b) report similar findings ofnon-selective low velocity habitat use by YOY.chubs and no apparent preference for habitat by depth in Westwater Canyon of the Colorado River. During the five year study in Westwater Canyon (1992-1996), the greatest densities of YOY chub were found in 1996, the year that most closely approximated the historic average spring peak. Lower catch rates were recorded during years of both lower and higher flow. Colorado pikeminnow spawn in Deso/Gray Canyons; one of only two documented spawning locations in the Green River sub-basin. A portion of the adult pikeminnow population that resides throughout much of the year in the lower reaches of the Green River, in the Uintah Basin stretch of the Green River, and in the White River have been shown to migrate during the ~" spring and early summer months to Deso/Gray to spawn near Three Fords Rapid at RK 250. In addition, Deso/Gray supports a local population of year round residents comprised of young of t. ,i s the year, juveniles and adults. Colorado pikeminnow spawn on the descending limb of the spring ,e.. ii