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back chub habitat, as ~tailvaters of dams may be adverssly affecting h~P i• suspected for other rare fish (Colosado squawfish, bonytail chub; Joseph et al. 1977). Such reductione~may have altered rciallhythatlneeded perfa=mance to a point where humpback chub habitat, cape y for spawning and rearing, has been reduced or altered significantly, and therefore reproductive success has been lowered. reds- etitian and/or p Another potential reason for decline is comp lion by exotic species. A large number of exotic species has been intro- duced into the Colorado basin and, therefore, may have added to the demise of the humpback chub (Miller 1961;Holden et al. 1974). Another reason for decline may be hybridization (Hinckley 1973; Holden et al. 1974). The relatively frequent occurrence of probable ood humpback chubs in recent col- hybrids in relation to the number of g o en and lections suggests a gradual "swamping" of the genetic stock (H l,~ Stalnaker 1970, 1975; Holden 1977). Some authors have suggested the habitat modification, especially that resulting hybridization is caused b~ from dams is the 1960s (Hinckley 1973; Johnson 1976). Other authors olden et al. 1974) have suggested that the hybridization occurred before major alteration., Regardless, hybridization is small, isolated populations well cause the demise of such populations, or at least the loss of ~' osed er basin, and prop pure genetic stock. Recent alterations in the upp alterations, especially flow depletions, may increase the hybridization potential and therefore speed the demise of the humpback chub. 9