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.. ~, it '-1 1i ~! 82 The fish fauna of this basin is unique in that 74 percent of the data ~ native species are endemic only to the Colorado River system (Miller, nativE 1959). Four of these endemic fishes are known to be rare and declining countE in abundance in the river system: the Colorado squawfish (Ptychocheilus lucius), humpback chub (Gila cypha), bony tail chub (Q!!! elegans), and ened c humpback sucker (Xyrauchen texanus). The first two fish have been offi- the dl cially designated as "endangered" by the U,S, Department of the Interior, netic Fish and Wildlife Service (1976) and the other two were suggested for condil listing as "endangered" and "threatened," respectively, by the Desert Recenl Fishes Council in 1975, of pOI These endemic species are threatened by several factors including latiol the reservoirs and cold tailwaters resulting from the construction of endan high dams, and competition with introduced fish species, Dams have been constructed in canyon areas that were once the habitat for these fish Color and, in addition, have altered flow patterns, lowered mean water tempera- volum tures, and reduced turbidity. Although much remains to be learned about (>5,0 the habitat requirements of these endemic fish, the potential impacts of and m alterations to the river ecosystem must be evaluated if these unique tures species are to be protected from becoming extinct--as outlined by the chann Endangered Species Act (1973), PL 93-205. chub, In addition to the large numbers of introduced species competing with native fish, disease or parasitism may also be a factor in their decline. For example, an increased incidence of parasitism 1n the roundtail chub (Gila robusta) followed the establishment of various exotic fishes in origi the Moapa River, Nevada (Deason and Bradley, 1972), These investigators the :I: offered parasitism as a factor in the decline of this native chub, No carve -;:;;";}~~~';":~^fjfJf~ii~~~~ ':'< ' c'r.~, .;