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The Colorado squawfish Prychocheilus lucius was listed as an endangered species <br />in 1967 (U. S. Department of Interior, 1973). Colorado squawfish formerly were <br />widespread and abundant throughout warinwater reaches of the Colorado River basin, but <br />are presently restricted to the upper basin upstream of Lake Powell in the Green, <br />Colorado, and San Juan rivers and their larger tributaries (Behnke and Benson 1983; <br />Platania et al. 1991). Decline of Colorado squawfish has been attributed to introduction <br />of nonnative fishes that compete with, or prey upon early life stages (Miller 1961; <br />Williams et al. 1989, Carlson and Muth 1989). Mainstream dams constructed throughout <br />the Colorado River basin beginning in 1913 have also negatively affected distribution and <br />abundance of endemic Colorado squawfish (Holden 1979; Tyus 1991 a). Low summer <br />water temperatures caused by hypolimnetic releases from these dams may curtail <br />reproduction of Colorado squawfish and reduce growth and survival of young fish <br />(Vanicek et al. 1970; Holden 1979; Hamman 1981; Marsh 1985; Kaeding and <br />Osmundson 1988). <br />Previous laboratory studies have evaluated the effect of temperature on hatch of <br />Colorado squawfish embryos, but not on larval survival. Hamman (1981) induced <br />spawning of hatchery Colorado squawfish at 18-19°C and hatched < 3% of embryos at <br />12-13°C and 20-21°C. Embryos from wild Colorado squawfish that were artificially <br />produced by hormone injections or spawned naturally in raceways had hatch rates of 30- <br />90%, when incubated at temperatures of 20-24°C. Marsh (1985) incubated Colorado <br />squawfish embryos at 5, 10, 15, 20, 25 and 30°C and found hatch was highest (2%, and <br />27% in two trials) at 20°C. He also documented lower hatch (0%, and 9%) and highest <br />incidence of abnormalities at 25°C, and no hatch at the other temperatures. Optimum <br />9 <br />