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I I I I I I I I I I I I I I I I I I I Although water quality has changed in many regions of the Colorado River Basin, there is no evidence that any single water chemical parameter, or any combination of parameters, ificant factor in the decline and status of the endangered fishes. Although increased salinity (as total dissolved solids, TDS) is reported a common effect of flow depletion, laboratory studies by Bulkley et al. (1982) showed that Colorado pikeminnow, humpback chub, and bonytail are tolerant to much higher TDS levels than ever observed in the basin. Although heavy metals, such as mercury, zinc, cadmium, copper, and lead, have been identified as potential contaminants of t w gered fish, there is little compelling evidence to suggest that any of I . these elements conti .Je significantly to the declined status of these species. I ".... ....&. ... 3.5.2 Hybridization The humpback chub and bonytail are part of a morphologically diverse group of western cyprinids that includes several congeneric species. This Gila complex consists of six forms that inhabit the Colorado River Basin, including the humpback chub (G. cypha), bonytail (G. elegans), roundtail chub (G. robusta), Virgin River chub (G. robusta seminuda), Pahranagat roundtail chub (G. r. jordani), and Gila (G. intermedia). The humpback chub, bonytail, and roundtail chub are mainstem sympatri s with substantial evidence of introgressive hybridization (Dowling and DeMarais 1993), while the Virgin River chub, Pahranagat roundtail chub, and Gila chub are isolates and primarily tributary inhabitants, although historic hybridization with other forms of Gila is evident. Humpback chub and bonytail appear to be specialized derivatives of the roundtail chub complex, and may have arisen in response to special conditions in large erosive.habitats (Smith et al. 1979, Minc al. 1989); a hypothesis that is supported by recent allozyme and mitochondrial DNA analys owling and DeMarais 1993). Intraspecific and interspecific morphological variation IS extensive where humpback chub, roundtail chub, and bonytail occur sympatrically. This apparent introgressive hybridization has resulted in high phenotypic plasticity with morphologic intergrades present in all sympatric populations of Colorado River Gila (Holden and Stalnaker 1970, Smith et al. 1979, Valdez and Clemmer 1982, McElroy and Douglas 1995, Douglas et al. 1989, 1998, Kaeding et al. 1990). These intergrades suggest, to some, extensive hybridization with possible concomi genetic diversity and evolutionary adaptive traits (Valdez and Clemmer 1982, Rosenfi 19