EFFECTS OF FLUCTUATING AND CONSTANT TEMPERATURES ON EARLY DEVELOPMENTAND SURVIVAL OF COLORADO SQUAWFISH

A laboratory study was conducted to determine the effects of four cons tant temperatures (18, 22, 26, and 30-degrees-C) and three fluctuating temperatures (diel fluctuations of +/- 2.5-degrees-C around 18, 22, a nd 26-degrees-C) on early development and survival of Colorado squawfi sh Ptychocheilus lucius, which is listed as an endangered species by t he U.S. Department of the interior. Average hatch in constant and fluc tuating temperatures was 72% at 18-degrees-C, 67% at 22-degrees-C, 62% at 26-degrees-C, and 38% (constant temperature only) at 30-degrees-C. There was no significant difference in hatch between constant and flu ctuating temperatures. Average survival of larvae to 7 d posthatch in constant and fluctuating temperatures was 68% at 18-degrees-C, 64% at 22-degrees-C, 83% at 26-degrees-C, and 13% (constant temperature only) at 30-degrees-C. Survival of larvae at 30-degrees-C may have been con founded by a relatively low hatch of embryos and poor condition of lar vae. Survival of larvae was 10-31% higher in fluctuating than in const ant temperatures. Incidence of abnormalities was 2-22% at 18-26-degree s-C and 100% al 30-degrees-C. Differences in abnormality rates were no t detectable between constant and fluctuating temperatures. Times to s tart of hatch, swim bladder inflation, and exogenous feeding were shor ter at higher temperatures. First feeding occurred about 31 h earlier in fluctuating temperatures than in constant temperatures. Differences in lengths of larvae at hatching and on day 7 posthatch at the variou s test temperatures were small and not considered ecologically signifi cant. Tolerance of a relatively wide range of high water temperatures by Colorado squawfish embryos and larvae may reflect the historically variable Colorado River environments in which the species evolved. Low summer water temperatures caused by mainstream dams have eliminated C olorado squawfish from portions of its historic range in the Colorado River basin. Water temperatures that more closely reflect historic reg imes are necessary to restore self-sustaining populations of Colorado squawfish in those areas.