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Products, Glen Burnie, Md.) and a commercially prepared flake <br />diet (TetraMin; TetraWerke, Melle, Germany) twice daily. <br />Experimental animals were acclimated to exposure conditions for <br />48 h before toxicant exposures were initiated. Age of Colorado <br />squawfish ranged from 185 to 305-d (after hatching). Total <br />length and wet weight ranged from 25 to 44 mm, and 0.14 to <br />0.68 g, respectively. <br />Physical and Chemical Conditions <br />Dilution water for the toxicant exposure was diluted well <br />water prepared to match the average hardness in July of the Yampa <br />River. Average water quality was estimated by calculating a <br />grand-mean hardness from two low (1981 and 1989), medium (1980 <br />and 1982), and high (1984 and 1985) water years (Table 1; data <br />from streamflow-gaging station near Maybell, Colo.; USGS 1980; <br />USGS 1981; Steinheimer et al. 1982; Duncan et al. 1984; Ugland <br />et al. 1985; Ugland et al. 1989). This locality is approximately <br />95 river-km upstream of one of two confirmed spawning sites for <br />Colorado squawfish. Target water quality characteristics were: <br />hardness, 124 mg/L as CaCO3; alkalinity, 100 mg/L as CaCO3; and <br />pH, 8.3. Dilution water was vigorously aerated for approximately <br />48 h while being heated to a exposure temperature of 20±2 °C. <br />Dissolved oxygen, hardness, pH, alkalinity, and specific <br />conductance were measured daily. Water temperature was measured <br />continuously. Measured dilution water characteristics for all <br />exposures had the following ranges: dissolved oxygen, <br />3 <br />