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The Colorado squawfish Ptychocheilus lucius was listed as an endangered species in <br />1967 (U. S. Department of Interior, 1973). Colorado squawfish formerly were widespread <br />and abundant throughout warmwater reaches of the Colorado River basin, but are presently <br />restricted to the upper basin upstream of Lake Powell in the Green, Colorado, and San Juan <br />rivers and their larger tributaries (Behnke and Benson 1983; Platania et al. 1991). Decline <br />of Colorado squawfish has been attributed to introduction of nonnative fishes that compete <br />with, or prey upon early life stages (Miller 1961; Williams et al. 1989, Carlson and Muth <br />1989). Mainstream dams constructed throughout the Colorado River basin beginning in <br />1913 have also negatively affected distribution and abundance of endemic Colorado <br />squawfish . (Holden 1979; Tyus 1991 a). Low summer water temperatures caused by <br />hypolimnetic releases from these dams may curtail reproduction of Colorado squawfish and <br />reduce growth and survival of young fish (Vanicek et al. 1970; Holden 1979; Hamman <br />1981; Marsh 1985; Kaeding and Osmundson 1988). <br />Previous laboratory studies have evaluated the effect of temperature on hatch of Colorado <br />squawfish embryos, but not on larval survival. Hamman (1981) induced spawning of <br />hatchery Colorado squawfish at 18-19°C and hatched < 3% of embryos at 12-13°C and 20- <br />21 °C. Embryos from wild Colorado squawfish that were artificially produced by hormone <br />injections or spawned naturally in raceways had hatch rates of 30-90%, when incubated at <br />temperatures of 20-24°C. Marsh (1985) incubated Colorado squawfish embryos at 5, 10, <br />15, 20, 25 and 30°C and found hatch was highest (2%, and 27% in two trials) at 20°C. He <br />also documented lower hatch (0%, and 9%) and highest incidence of abnormalities at 25°C, <br />3 <br />