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. . fish~s that evolved in the Colorado River developed unique adaptations to the river's harsh environment of fluctuating flows, temperatures, and turbidities. upper Green River in Wyoming to the Gulf of California, including the Gila River system in Arizona. It was abundant over all of its range prior to the 1850' s [4]. The type specimen was obtained from the Colorado River in California [5], where it no longer exists. Because of these unique adaptations and long isolation from other surrounding river basins (Missouri Columbia , , etc.), the Colorado River, as a whole, has the highest percentage of endemic species of any river basin in North Ame:ica [1]. There are eight endemic fish found in the upper basln [2], two of which are recognized subspecies of more broadly distributed species. Of these, four are listed under the Endangered Species Act as endangered: Colorado squaw- fish (Ptychocheilus lucius), bony tail chub (Gila elegans) hum~b~ck chub (Gila ~), and Kendall Warm Springs dac~ (~hlnlchthys osculus yarrowi). Two others, the Colorado Rlver cutthroat trout (Salmo clarki pleuriticus) and the razorback sucker (Xryauchen texanus) are on state 1 ists of threatened and endangered species. The present range of natural populations of the Colorado' squawfish is restricted to the Upper Colorado River Basin. It is found in the Green River from the confluence of the Yampa River to its confluence with the Colorado River. It is also found in the Yampa, lower Duchesne, and White Rivers tributaries to the Green River. In the mainstem Colorado River it is found from Lake Powell to above Grand Junction and in the Gunnison and San Juan Rivers, tributaries to the mainstem Colorado. DISCUSSION OF THE ENDANGERED FISH OF THE UPPER COLORADO RIVER BASIN The Colorado squawfish occurs in a variety of habitats, especially in eddy and pool habitats protected from the main current. However, because of its feeding and spawning requirements, it can be found in various habitats throughout the river [4]. Movement of the Colorado squawfish appears to be related to flow, temperature, spawning, and feeding. Recent investigations by the U.S. Fish and Wildlife Service indicate the Colorado squawfish may require a relatively unrestricted movement to satisfy all of its life history requirements. Radio-tagged studies have shown that some individuals make long upstream and/or downstream migrations to specific areas [2]. Colorado squawfish have been tracked over 322 km (200 mi) in just a few months from the area in which they were first radio-tagged. Kendall Warm Springs Dace The Kendall Warm Springs dace, a subspecies of speckled dace, is found only in the heated outflow of Kendall Warm Springs, Wyoming. Its entire habitat consists of less than 305 m (1000 ft) of a small stream, which flows into the Green River in the most northern portion of the basin less than . , 50 km (31 ml) from the river's origin. Because this popula- t ion of speckled dace occupies a res tricted habitat in a r~stricted location, no further mention of this subspecies wLll be made. For a more detailed account of the Kendall Warm Springs dace, refer to Binns [3]. Humpback Chub Colorado Squawfish The humpback chub was also listed as endangered by the U.S. Fish and Wildlife Service in the Endangered Species List published in the Federal Register on 11 March 1967. It was described by R. R. Miller [6] from specimens collected in the lower basin of the Colorado River (Grand Canyon and another unknown location). The humpback chub is a medium- sized [less than 500 rom (20 inches) in total length] fresh- water fish of the minnow family. The greatest number of humpback chubs have been found in deep water canyon areas. They appear to be adapted to the turbulent waters in these canyons and are associated with the deep, swift, rocky areas, of the river [7]. The original distribution of the humpback chub is not fully known: it is assumed to be similar to that of the squawfish. But because of taxonomic confusion with related species and a paucity of earlier collections, an ac- curate assessment of its historic distribution and abundance is difficult [7]. The Colorado squawfish was listed as endangered by the U.S. Fish and Wildlife Service in the Endangered Species List published in the Federal Register on 11 March 1967. It is the largest species of the minnow fami ly (cyprinidae) native to North America. The largest known specimens seen in re~ent years have been about 0.9 m (3 ft) long and have welghed about 6.8 kg Os Ib) [1]. Maximum weight has been recorded as exceeding 36 kg (80 Ib) and lengths recorded of nearly 1.8 m (6 ft). Early records indicate that the Colorado squawfish was once found throughout the Colorado River system from the 538 539