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, If! .; . ~ Present distribution of the humpback chub as indicated by r~cent collecti?ns include the followin~ upper basin locat~ons: Desolat~on, Gray, and Labyrinth Canyons in the Green River; Dinosaur National Monument in the Green and Yampa Rivers; Cross Mountain Canyon on the Yampa River; and Black Rocks, Westwater, and Cataract Canyons on the mainstem Colorado River. In the lower basin, recent collections include the following: Marble and Grand Canyons in the Colorado River and the lower 23 km (14 mi) of the Little Colorado River. Gray, Westwater, and Black Rocks Canyons appear to support the only major concentrations of humpback chub in the Upper Colorado River Basin [6]. Based on tagged and radio transmitter studies the humpback chub, unlike the Colorado squawfish, do not ty;ical- ly migrate over large stretches of river. They seldom leave their canyon habitats. Bony tail Chub . The bony tail chub was listed as endangered by the U.S. F~sh and Wildlife .S:rvice in the. Federal Register on 23 April 1980. It was ong~nally descnbed by Baird and Girard [8] f:om collections from the Zuni River, a tributary of the Llttle Colorado River. Bony tails commonly reach 300-350 mm (11-14 inches) in total length in the upper basin. The roundtail chub (Gila robusta) and humpback chub are closely related to the bony tail chub, and the three were often con- fused in the early literature. Many early references to the bony tail chub refer, in fact, to the roundtail chub [1]. As a result of this confusion, documentation of historic distribution and abundance, as with the other chubs is difficult [7]. The bony tail chub is believed to origin~lly have ranged throughout the Colorado River system in the main channels and larger tributaries from Mexico to Wyoming. Presently, small concentrations of bony tail chub are thought to exist in GJay Canyon in the upper basin, and In Lake Mohave [7] and Lake Havasu [1] in the lower basin No bony tail reproductive success has been identified in a~y of these areas. The bony tail chub's abundance has declined steadily until it is now the most rare endemic fish in the entire upper basin [2]. Little habitat information is available for the bony tail chub, except that they appear to oCfuPY deep, swift semi- rocky areas in main channels [7]. o,M) --- CAUSES OF DECLINE OF THE ENDANGERED FISHES Several papers have recently discussed the causes of decline of the endangered Colorado River fish in the upper basin ([1,4,7], draft bony tail chub recovery plan, draft revised recovery plans for the Colorado squaw fish and hump- back chub). The major impacts, according to these papers, are 1) the result of dams and reservoirs, 2) the removal of water from the system, and 3) the introduction of competing non-native fish to the system. Each of these factors has significantly altered the aquatic habitat and community structure. Dams and Reservoirs The single most important factor identified by most sources as causing the decline of the endangered Colorado River fishes in the upper basin has been the construction and operation of dams and reservoirs (primary mainstem). Hundreds of miles of flowing river habitat have been converted into great impoundments. Riverine habitat downstream from the dams has been drastically altered in flow, temperature, chemistry, and biota. Migration routes for the larger ranging fish, such as the Colorado squawfish, have been blocked. Over 20 mainstem and tributary dams (at least half of these in the upper basin) have been constructed on the Colorado River since the first major dam, Hoover Dam, was built in 1935. Over 20 more dams and reservoirs are either authorized, part ially completed or are contemplated for the mainstem Colorado and Green Rivers in the upper basin alone. Dams and reservoirs have affected the endangered fishes of the Colorado River in several ways. They have altered natural flow and temperature regimes and water qualities, reduced total annual discharges through evaporation and diversion (dewatering), converted lotic habitat to lentic, and blocked migration routes. The total effect of these impacts has resulted in a 37% reduction in suitable habitat for the endangered fishes in the upper basin. This does not include the subt Ie biot ic and abiot ic impact s wh ich have reduced the suitability and extent of preferred niches in the remaining 63% of the inhabitable river reaches of the upper basin. Through construction of impoundments across the river, the squawfish's ability for long-range migrations is impeded. 541