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temperatures average less than 15°C (59°F)(Ugland et al. 1987), and below Lake Powell for 384 km (238 mi) where summer water temperatures rarely exceed 15°C (59°F)(Carothers and Minckley 1981). In some portions of the historic razorback sucker range, temperatures may now be too cold for survival of fertilized eggs (Marsh, pers. comm. 1996). Marsh (1985) reported an optimal temperature of 20°C for incubation of razorback sucker eggs in the laboratory; hatching success was lower at 15°C and hatching failed completely at 5° and 10°C. Bozek et al. (1990) reported similar results. Chemical Changes Changes in water quality also are associated with an increasing human presence. Environmental contaminants may be introduced from municipal or industrial point source discharges, or from non-point sources associated with agricultural activity or resource extraction. The threat posed by environmental contaminants has not been studied adequately. Preliminary results from one study show that exposing young razorback suckers to agricultural drainage from areas near the Green River can produce mortality in the range of 30 to 50% (Bruce Waddell, US Fish and Wildlife Service, pers. comm. 1993). The specific agent responsible for the mortality has not yet been identified, but trace elements or metals are possibilities. For example, at the Stewart Lake Waterfowl Management Area near Jensen, Utah, concentrations of boron, selenium, and zinc in water, bottom sediments and biological tissues (Stephens et al 1992) were sufficiently high to be harmful to fish and wildlife (e.g, Ostler 1985). Selenium has probably received more attention than other environmental contaminants that may be harmful to razorback suckers. Hamilton and Waddell (1992) reported values of 3.7, 4.7, and 10.6 ~cg/g dry weight for selenium in eggs of razorback suckers collected from fish spawning in the Green River. Waddell and May (1995) reported selenium concentrations of 24-54 ~cg/g dry weight in muscle plugs collected from Green River razorbacks. Concentrations that high have caused poor growth and reproductive failure in other fishes (Gillespie and Badman 1986). High selenium levels also occur in backwater habitats along the Colorado and Gunnison rivers, and preliminary data suggest that the selenium concentrations in some areas are high enough to cause reproductive failure in razorback suckers (S.J. Hamilton, USGS Biological Service, pers. comm. 1996). Because razorback sucker recovery will require the use of some of the large reservoirs in the Colorado River system, it is pertinent to understand threats of environmental contaminants that also may occur. As an example, one area that has been intensely studied is the effect of drainage from Las Vegas Valley on the limnology of Lake Mead, Arizona and Nevada. Contaminants there include organochlorines, polycyclic aromatic hydrocarbons (PAHs), and phenols (Bevans et al. 1996). Using common carp as a surrogate fish for razorback sucker, Bevans et al (1996) found necrotic changes in kidney and hepatopancreas tissues, evidence of long-term exposure to environmental 19