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Cyprinus carpio pituitary (males) and human chorionic gonadotropin (females; Hamman <br />1981). The resulting embryos were shipped to Fort Collins, Colorado, in water at 18- <br />21 °C within 24 h. <br />Four groups of 30 viable embryos each were randomly assigned to each <br />experimental treatment, placed in incubation chambers, and acclimated to test <br />temperatures at about 2°C/h. Embryos were incubated in chambers constructed from 76- <br />mm inside-diameter polyvinyl chloride (PVC) pipe with 560-um nylon screen attached <br />inside and halfway down the 100 mm long pipe section with silicone caulk. Chambers <br />were placed in a bath and filled with water delivered via a fine bore pipette. Holes were <br />drilled at the base of the chamber wall to allow water to flow out. Embryos were <br />incubated at nominal constant temperatures of 18, 22, 26, and 30°C and nominal <br />fluctuating temperatures of 18, 22, and 26°C (diel fluctuation of + 2.5°C). One group of <br />embryos was incubated in each of four test chambers at each of the seven constant and <br />fluctuating temperature treatments. Additional embryos, and larvae were held in a fifth <br />chamber at each treatment so that observations of development and fish length could be <br />made without disturbing fish in the other four incubation chambers. <br />Diel fluctuating temperatures were tested because diel temperature fluctuations of <br />5°C or more are common in the Yampa and Green rivers, Colorado and Utah, during the <br />spawning season of Colorado squawfish (late June to early August, Nesler et al. 1988; <br />personal observation KRB). Our fluctuating temperature cycle mimicked the natural <br />cycle as follows: lowest temperatures occurred from 0100 to 0700 h (e.g., 19.5°C for the <br />22°C fluctuating treatment); warming occurred at a constant rate from 0700 to 1300 h; <br />temperatures were highest from 1300 to 1900 h (e.g., 24.5°C for the 22°C fluctuating <br />11 <br />