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exposure period by determining the average blotted wet mass and average total length (TL) of <br />fish that survived. Average mass was measured to 0.0001 g; TL to 0.1 mm. <br />Experimental design and exposure system for acute tests <br />To compare sensitivity of razorback sucker and flannelmouth sucker to dissolved <br />selenium, 96-hour renewal-acute toxicity tests were conducted using prescribed methods <br />(ASTM 1990b; USEPA 1991). Fathead minnow were also tested to provide a basis for <br />comparing results to other investigations. Experimental treatments were assigned to replicate <br />beakers (n=4) using a randomized, balanced 6x2 factorial design, as described above, with six <br />target exposure concentrations (control, 10, 20, 40, 80, and 160 mg/L selenium). Fifteen larvae <br />were randomly assigned to each beaker. Tests were conducted using 1-L polyethylene beakers <br />containing 0.50 L of exposure solutions. Depth of test solutions was 7.3 cm. Test solutions were <br />renewed every 24 h. Cool-white fluorescent lamps were the only source of illumination (530 lx), <br />and a 12:12-h light:dark photoperiod was maintained. <br />Larvae were offered live <_ 24-h-old brine shrimp nauplii before and during toxicity tests. <br />During a test, brine shrimp nauplii were introduced into exposure beakers once daily at least 2 <br />hours before renewal. Survival was monitored at 6, 12, 24, 48, 72, and 96 h after start of the <br />exposure period. <br />Physical and chemical conditions <br />Water temperature in the ELS test was measured continuously during the exposure <br />period. A test temperature of 201 °C was maintained using a water bath. Alkalinity, hardness, <br />7 <br /> <br />