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07261 Tra nsadioixs of the 1111"rmt Fisheries Societe 112:595-600,1983 Concentrations of Total Dissolved Solids Preferred or Avoided by Endangered Colorado River Fishes R. PIMENTEL AND R. V. BULKLEY Utah Cooperative Fishery Research Unit, Utah State University Logan, Utah 84322 Abstract juveniles (age 0-1) of three endangered Colorado River fishes were subjected to a gradient of total dissolved solids (TDS) to determine the concentrations that they preferred or avoided. Preferred and avoided TDS concentrations, respectively, for juveniles of each species were: Colorado squawfish Ptychocheilus lucius, 560-1,150 mg/liter and greater than 4,400 mg/liter; humpback chub Gila cypha, 1,000-2,500 mg/liter and greater than 5,100 mg/liter; and bonytail Gila elegans, 4,100-4,700 mg/liter and less than 560 mg/liter or greater than 6,600 mg/liter. Colorado squawfish and humpback chubs selected TDS concentrations that were similar to those in waters they now inhabit, but bonytails selected concentrations four times higher. Future increases of TDS in the Colorado River system would degrade municipal, industrial, and agri- cultural water uses before these endangered fishes would be affected. Received November 1, 1982 The Colorado River is highly developed and regulated because it serves as the major water supply for the arid southwestern United States. Reservoirs and agricultural diversions, among other developments, have increased concentra- tions of total dissolved solids (TDS) in the river water because they enhance evaporation and soil leaching. The problem causes grave con- cern to states in the Colorado River drainage, and to the federal governments of the United States and Mexico. There has been a concerted effort through desalination plants and reservoir management to control TDS concentrations in the river. The upper Colorado River basin, however, is entering into a new era of energy and mineral extraction. Oil-shale-process waters, saline groundwaters associated with oil shale, tar sands, mineral deposits, and the additional dams needed to provide water for the devel- opment of these resources all have the potential to greatly increase TDS concentrations in the waters of the basin (Fox et al. 1978; Greer 1981). The upper Colorado River drainage (com- posed mainly of the Green and Colorado rivers) contains several species of native endangered and threatened fishes including the Colorado squawfish Ptychocheilus lucius, humpback chub Gila cypha, and bonytail Gila elegans. The pres- ent status of these species was reviewed by Behnke and Benson (1980). The original range of the Colorado squawfish, the largest minnow Accepted June 5, 1983 endemic to North America, was the entire Col- orado River system but today this species is found only in main stems of the Green and upper Col- orado rivers. The humpback chub was not known to science until 1946 and was never a common fish in the system because of its ap- parent habitat restrictions. It occurs in river sections of deep swift water typically found in steep canyons. Present distribution is in the Green, Little Colorado, and upper Colorado rivers. The bonytail was once the most abun- dant fish in the main river channels of the entire Colorado River system. Today it is the rarest fish in the system and in the last 5 years has been found only in Lake Mohave and Desola- tion Canyon of the Green River. Some spawn- ing of this species in reservoirs has been re- ported but no survival of the young fish has ever been documented. The bonytail is apparently closest to extinction of these three species. Little information other than that reported by Behnke and Benson (1980) is available on the habitat requirements and biology of these fishes, and virtually nothing is known about the effect of elevated TDS concentrations on them. The objective of our study was to determine concentrations of TDS preferred or avoided by young of the year or yearlings of the three species. Information gained will assist energy and water developers in assessing and reducing the effects of their projects on these species. 595