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TOLERANCE OF DISSOLVED SOLIDS BY ENDANGERED COLORADO RIVER FISHES 599 Thomson (1974) concluded that, together with other environmental factors, salinity prefer- ences may serve a variety of functions in guiding Leuresthes sardina into desirable nursery areas. McInerney (1964) proposed a mechanism whereby juvenile Pacific salmon (Oncorhynchus spp.) are able to use estuarial salinity gradients as one of the directive cues in their seaward migration. Historically, juvenile bonytails may have been guided by their high TDS preference to preferred habitat areas in a manner similar to that reported for L. sardina. These areas since have been lost or reduced in the Colorado River system due to reservoir construction and al- tered flow regimes. It is also possible that the high TDS prefer- ence of the bonytail served as a mechanism to isolate it from the Colorado squawfish, histor- ically the system's only predator. The TDS con- centrations preferred by bonytails would be avoided by the majority of Colorado squawfish (Table 2). Clemens and Finnell (1957) found a strong correlation between the abundance or absence of certain fish species and the level of salinity in a brine-polluted stream in Oklahoma, indicating that fish distribution could be affect- ed by TDS concentration. Given that bonytails were once the most abundant native fish in the Colorado River system, but are now the rarest, these ideas warrant further investigation. As noted earlier, temperature in the gradient device was not controlled. Temperature is a fac- tor that must be considered in TDS preference and avoidance determinations because low tem- perature hinders osmoregulation and thus in- fluences tolerance to salinity (Davis and Simco 1976). For example, the work of Wikgren (1953), Houston (1962), and Houston and Mad- den (1968) suggested that the TDS concentra- tion avoided by the humpback chub might have been higher if this species had been tested at 14-18 C instead of 12 C. Because of the tem- perature-osmoregulation relationship, our es- timates of TDS avoidance may be high for win- ter management of these Colorado River species. Temperatures in the upper river system drop to as low as 1.0 C during winter, but TDS con- centrations remain high. More work is needed to quantify the relationship of temperature and TDS avoidance with these species. The TDS avoidance concentrations of all three species were high enough in general to suggest that other water uses would be severely affected before increasing salinities of the upper Colorado River system would affect survival of these endangered fishes. This conclusion is ten- able for two reasons. First, the United States has a treaty with Mexico to deliver water that is no greater than 115 ± 30 mg/liter above the TDS of the Colorado River as it enters the Im- perial Dam in California (Johanson 1977). The standard for TDS concentration in the river at Imperial Dam has been set at the flow-weighted average of 879 mg/liter (EPA 1976). This stan- dard effectively places a legal limit on TDS con- centration that is within the range preferred by these fish. Second, in the lower Colorado River basin, it has been estimated (USDI 1981) that each mg/liter increase in average TDS concen- tration in the annual discharge will result in an annual loss of $450,000 in 1980 dollars to downstream users. Approximately one-third of these users are farms and the remainder are municipalities and industries. A doubling of the TDS concentration of the Colorado River may have no effect on juvenile endangered fishes but would be catastrophic to the 12 million water users of the lower Colorado basin. Nevertheless, problems could arise for fish in localized situations where saline oil-shale-pro- cessing waters enter tributaries of the main riv- er system. Fox et al. (1978) found that TDS concentrations of oil-shale-process waters ranged from 1,750 to 24,500 mg/liter and av- eraged 6,800 mg/liter. Tributaries polluted with such high TDS concentrations may be avoided by these species resulting in a loss of habitat. This experiment has been but one segment of a long-term project to study the habitat pref- erences and needs of the endangered fish of the Colorado River system, and is by no means de- finitive. More work needs to be done on the effects of increasing TDS concentration on dif- ferent life stages. Although juvenile fish appear to be tolerant to elevated TDS concentrations, eggs and larvae may be more sensitive. Our con- clusions should be applied only to juvenile fish. Acknowledgments We thank Harold Tyus for reviewing an early draft of the manuscript and Becky Smith for performing chemical analyses. Funds for this project came from the United States Fish and Wildlife Service Colorado River Fishery Prcj-