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fungal infection and oxygen deprivation. Differences in hatch and survival at 18-26°C and <br />30°C may partly reflect the fact that these treatments were done in different years with <br />embryos from different broodfish. <br />High hatch and survival of larvae were observed in all treatments except 30°C, a <br />temperature rarely recorded in river channels in the upper Colorado River basin. Summer <br />water temperatures of 30°C were found in the former range of Colorado squawfish in the <br />lower Colorado River (Dill 1944). However, it is likely that Colorado squawfish there <br />spawned earlier in the spring when water temperatures were lower. Although little is <br />known about the reproductive ecology of Colorado squawfish in that area, spring spawning <br />is also supported by collection of 32 mm (likely age-0) Colorado squawfish in May in <br />Arizona (Sigler and Miller 1963). <br />Hatch was highest at 18°C (regimes combined) and lowest at 26°C, but survival to 7 d <br />post-hatch was lowest at 18°C and highest at 26°C. If overall reproductive output (product <br />of % hatch and % survival of larvae) to 7 d is considered, the "optimum temperature" for <br />reproduction by Colorado squawfish may be 18-26°C. The lower temperature limit for <br />incubation is unknown but survival 'was < 3% among embryos incubated at 12-13°C <br />(Hamman 1981). The upper temperature limit for incubation is probably near 30°C. Our <br />results indicate reproduction is precluded at 30°C by the high rate of abnormalities. Marsh <br />(1985) found no hatching of Colorado squawfish embryos spawned artificially at 18°C and <br />transferred to 10, 15, or 30°C; however, he did not acclimate his embryos, which increased <br />the likelihood of temperature shock and subsequent embryo mortality. <br />10 <br />