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Colorado rivers. According to a 1978 U.S. Bureau of Land Management report on salinity problems in the Colorado River, irrigated agricultural lands in the Grand Valley region of Colorado return 8 tons of salts to the Colorado River for each acre under irrigation. Overgrazed rangelands in the basin, characterized by greatly accelerated erosion and high salt content, can contribute up to 30 tons of salts to the rivers per acre of watershed. Salinity concentrations have doubled and tripled in some sections of the Colorado River in comparison to virgin flow conditions. Such changes in the flows and water quality exert influ- ences downstream on channel structure and fish habitat. The loss of great numbers of bonytail chubs from the areas inhabited by squawfish must have severely depleted the potential food supply of squaw- fish, and may be a major cause of the reduced growth rates of squawfish in recent times. Prospects for the Future Realistically, the prospects for restoring the abundance of bonytail chubs to a semblance of their former numbers in any part of the original range must be viewed as dim. This species is now the rarest of the native fishes and the species in most imminent danger of extinction. Biologists have been attempting to obtain live specimens from lakes Mohave and Havasu to hold in a hatchery for artificial propagation. Captive propagation may prove to be the only way this species can be maintained. Unless the factors caus- ing the elimination of bonytail chubs are understood, and some action is taken to modify or eliminate these factors, the restoration of the bonytail chub in its historic range cannot be expected from stocking hat- chery reared fish. Even if the factors causing elimination from a river section became clearly under- stood, it is not likely that remedial action would be possible. For example, the dismantling of Flaming Gorge Dam to restore the original flow and tempera- ture regime of the Green River must be considered beyond the realm of possibility. If bonytail chubs occur in an area, it is likely that an occasional specimen would be caught by fish- ermen fishing for catfish. If fishermen become familiar with the appearance of bonytail chubs, and if a specimen believed to be of this species is caught, it should be photographed before release and reported to the Colorado Division of Wildlife. There have been no verified records of the bonytail chub in Colorado for many years. The discovery of a population would be a significant event. 1982 UPDATE Intensive sampling by federal and state agencies Experimental hybridization between the three and private consulting firms throughout the Colorado chub species were carried out at the Willow Beach, and Green rivers in 1981 and 1982 failed to find a Arizona, National Fish Hatchery. All hybrid combina- viable population of bonytail chub. The only site tions among the three species proved to be fully where fish sampling obtained specimens bearing a fertile, confirming their close genetic relationships. resemblance to bonytail chub was Coal Creek Rapids Bonytail chub still occur in Lake Mohave- Spe- of the Green River in Gray Canyon, Utah. Detailed cimens from Lake Mohave were taken to Willow Beach examination of these specimens, however, indicated Hatchery to initiate artificial propagation of the they were the result of hybrid combinations between species. Large numbers of bonytail chub have been humpback, bonytail, and roundtail chubs. Evidently, produced at the Willow Beach and Dexter, New Mexico, the last remnants of the bonytail chub species in the national hatcheries. Some of these young were used upper basin is in the process of being "absorbed" in laboratory studies to determine environmental into populations of humpback chub and roundtail chub preferences and tolerances. In November, 1981, by hybridization. 42,000 hatchery-reared bonytail chub were stocked into Lake Mohave to supplement the natural population. 23