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spawned earlier in the spring when water temperatures were lower. Although little is known about the reproductive ecology of Colorado squawfish in that area, spring spawning is also supported by collection of 32 mm (likely age-0) Colorado squawfish in May in Arizona (Sigler and Miller 1963). " Hatch was highest at 18°C (regimes combined) and lowest at 26°C, but survival to 7 d post-hatch was lowest at 18°C and highest at 26°C. If overall reproductive output (product of % hatch and % survival of larvae) to 7 d is considered, the "optimum temperature" for reproduction by Colorado squawfish may be 18-2b°C. The lower temperature limit for incubation is unknown but survival was < 3% among embryos incubated at 12-13°C (Hamman 1981). The upper temperature limit for incubation is probably near 30°C. Our results indicate reproduction is precluded at 30°C by the high rate of abnormalities. Marsh (1985) found no hatching of Colorado squawfish embryos spawned artificially at 18°C and transferred to 10, 15, or 30°C; however, he did not acclimate his embryos, which increased the likelihood of temperature shock and subsequent embryo mortality. The higher hatch at lower temperatures and higher post-hatch survival at higher temperatures observed in this study may suggest that these are differences in physiological tolerances of embryos and larvae and may represent adaptions to particular habitats. Colorado squawfish may spawn in the 16-26°C main channel habitat (Kesler et al. 1988; Tyus and Haines 1991). Larvae emerge from the substrate after an undetermined period, drift downstream, and occupy 25-28°C, low-velocity channel margin habitats. Field evidence suggests that larger (older) Colorado squawfish larvae and juveniles prefer backwater habitat (Tyus 1991b; Tyus and Haines 1991, Haines and 16