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THE NATIVE FISHES OF THE UPPER COLORADO RIVER BASIN <br />Because of the long and effective isolation of <br />the Colorado River basin from invasion of fishes from <br />neighboring basins, only 13 species of fishes (Table <br />1) are native to the upper basin (that is, they oc- <br />curred in the basin naturally before man introduced <br />new species). Table 1 lists the common and scientific <br />names of the native fishes and their status on the <br />federal and Colorado state lists.1 <br />The seven species that occur in headwater streams <br />(cutthroat trout, mountain whitefish, the two sculpins, <br />the two mountain suckers, and the speckled dace) also <br />are native to other river basins such as the Columbia <br />and Missouri river basins and the Great Basin (several <br />separate basins where the streams never reach the <br />ocean but drain to internal sumps This distribution <br />indicates that these species have invaded the Colorado <br />River basin (or escaped from it) in relatively recent <br />geological times, and have not been isolated long <br />enough to evolve into different species. The remain- <br />ing six species -- squawfish, three chubs, and razor- <br />back and flannelmouth suckers -- are endemic species. <br />They have been isolated much longer, and have evolved <br />into species markedly different from their nearest <br />relatives in other river basins. Fossils of some <br />endemic species more than 3 million years old have <br />been found. All of the six endemic species also occur <br />(or did until recently) in the lower Colorado River <br />basin. Of the seven native but nonendemic species, <br />only the speckled dace and the bluehead mountain <br />sucker occur in the lower basin. <br />The native species have adaptive specializations <br />that enable them to live in different environments. <br />They are associated with specific types of habitats <br />and are not randomly distributed throughout the sys- <br />tem. For example, the cutthroat trout originally was <br />limited to clear, cold waters at high elevation before <br />it was replaced by non-native species of trout. The <br />six endemic species, with the exception of the round- <br />tail chub, were largely restricted to the large, main <br />river channels of the Colorado and Green rivers and <br />their major tributaries, such as the Yampa, Gunnison, <br />and San Juan rivers below the foothills, where the <br />water is warm in the summer. The roundtail chub's <br />optimum habitat seems to be the intermediate size <br />tributary streams. <br />------------ <br />1 The 14 fishes listed in Table 1 include 13 species <br />with 2 subspecies of dace. <br />Table 1. Common and scientific names of the native <br />fishes of the upper Colorado River basin, and status <br />of the species that are endangered or threatened. <br />Familv and common Scientific Statusa/ <br />Federal Colorado <br />Salmonidae: Trout, whitefish and grayling famil <br />Colorado River Salmo clarki <br />cutthroat trout pleuriticus T <br />Rocky Mountain Prosopium <br />whitefish williamsoni <br />Cyprinidae: Minnow family <br />Colorado River Ptychocheilus <br />Squawfish lucius E E <br />Humpback chub Gila cyphha E E <br />Bonytail chub Gila elegans E E <br />Roundtail chub Gila robusta <br />Speckled dace Rhinichthys <br /> osculus yarrowi <br />Kendall Warm Rhinichthys <br />Springs dace osculus thermalis E <br />Catostomidae: Sucker family <br />Razorback sucker Xyrauchen texanus E <br />Flannelmouth Catostomus <br />sucker latipinnis <br />Bluehead mountain Catostomus <br />sucker discobolus <br />Mountain sucker Catostomus <br /> platyrhynchus <br />Cottidae: Sculpin family <br />Mottled sculpin Cottus bairdi <br />Paiute sculpin Cottus beldingi <br />a/E: endangered <br />T: threatened <br />Great changes in the original river environments <br />of the Colorado River basin have favored the non- <br />native fishes. More than 30 species have been intro- <br />duced into the upper basin and now dominate most of <br />the basin's fish communities. All of the 13 native <br />fishes still occur in the upper basin but all have <br />been depleted in numbers. Five species have been <br />reduced sufficiently to be listed as endangered or <br />threatened in Colorado (Table 1). The squawfish, <br />bonytail chub, and humpback chub are also on the <br />federal list of endangered species. These five <br />species are discussed in detail in the following <br />sections. <br />9