ABSTRACT
As a result of its long isolation, the Colorado River basin (Fig. 1) supports some of the most distinctive ichthyofauna in North America. Several fish species have declined in the basin, which originally contained 32 native species of which 75% were endemic (Minckley, 1991). The first two listed as endangered were the humpback chub (Gila cypha) and the Colorado squawfish (Ptychocheilus lucius). Both were listed in 1967 (USDOl, 1967) as part of the Endangered Species Preservation Act of 1966, six years prior to the passage of the Endangered Species Act of 1973. Passage of this act relisted the humpback chub and Colorado squawfish (USFWS, 1974); later, in 1980, the bonytail (Gila elegans) was listed (USFWS, 1980), and in 1991 the razorback sucker (Xyrauchen texanus) (USFWS, 1991a) (Fig. 2). Several other fish in the Colorado River basin are now listed as species of concern (formerly category 2 species), including flannelmouth sucker (Catostomus latipinnis) and roundtail chub (Gila robusta) (USFWS, 1991b) (Fig. 2).
