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PROLOGUE Gila Complex of the Colorado River Three fish species of the genus Gila inhabit the mainstem Colorado River, including the humpback chub (Y. ~ Miller, 1945), bonytail (Y. elegans Baird and Girard, 1853), and roundtail chub (Y. robusta Baird and Girard, 1853). These species are considered part of a morphologically diverse group or complex of western minnows that includes several congeneric species outside of the Colorado River Basin, with a pervasive influence of hybridization throughout their evolutionary histories ~Dowling . and DeMarais 1993). This apparent mtrogresslve hybridization has resulted in high phenotypic plasticity with morphologic integrades present in all sympatric populations of Colorado River Gila (Holden and Stalnaker 1970, Valdez and Clemmer 1982, Kaeding et al. 1990). Gila ~ and G. elee:ans appear to be specialized derivatives of the G. robusta complex, and may have arisen in response to special conditions in large erosive C~lorado River habitats (Smith et al. 1979, Minckley et al. 1989), an hypothesis that is being supported by recent allozyme and mitochondrial DNA analyses (Dowling and DeMarais 1993, Starnes 1995). These three chub species belong to the Class Osteichthyes (bony fishes), Order Cypriniformes, ~ Family Cyprinidae (carps and minnows), which ~ the most diverse and widespread family of fishes 10 N~ America with over 240 recognized species (Amencan Fisheries Society 1991). These chub species are part of the Gila complex and represent half of six recognized species or subspecies inhabiting the Colorado River basin, including the humpback chub (Y. ~, bonytail (Y. elegans), roundtail chub (Y. robusta), Virgin River chub (Y. robusta seminuda), Pahranagat roundtail chub ill..!. iordanD, and Gila chub (Y. intermedia). The .other three taxa, Virgin River chub, Pahranagut roundtail chub, and Gila chub, are isolates and primarily tributary inhabitants, although historic hybridization with other forms of Gila is evident. Humpback chub (Gila cYDha) The humpback chub was described in 1945 by R.R. Miller (1946) from specimens taken in Grand Canyon. It. was included in the first List of Endangered Species issued by the Office of Endangered Species on March 11, 1967 (32 FR 4001). The humpback chub was classified as "endangered" because of declines in distribution and abundance throughout its range. It was afforded full protection under the Endangered Species Act of 1973, as amended. It is surmised that the humpback chub speciated from a Q. elegans-like form in canyons of North em Arizona (i.e., Grand Canyon) about 3-5 million years ago (Miller 1946, Holden 1968, Minck1ey et al. 1986), during the mid-Pliocene arid early Pleistocene epochs. During this time, the Colorado River was cutting through the Kaibab upwarp of the Colorado Plateau to join the ancient upper basin with the lower Hualapai Drainage System (McKee Humpback chub