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' hatching) at the start of the dietary exposure study and 27 days old at the start of acute toxicity <br />' tests. <br />Fathead minnow <br />Larval fathead minnows were purchased from Aquatic BioSystems Inc. (Fort Collins, <br />Colorado). Experimental animals were acclimated to test temperature and water quality for <br />48 hours before toxicity tests were initiated. Larvae were approximately. 3 days old (after <br />hatching) at the start of acute toxicity tests. <br />Experimental design and exposure system <br />Exposure procedures for the water-versus-dietary exposure study (henceforth "dietary <br />study") were based on prescribed methods for conducting early life-stage toxicity tests with <br />fishes (ASTM 1990a). Experimental treatments were assigned to replicate exposure beakers <br />(n=4) using a randomized, balanced 5X2 factorial design with five target exposure concentrations <br />(0.0, 2.5, 5.0, 10.0, and 20.0 µg/L) and control or selenium-enriched rotifer diet. This <br />experimental design was equivalent to conducting two toxicity tests simultaneously. In one test, <br />the larvae were exposed to five dissolved selenium concentrations and control diet. In the other, <br />larvae were exposed to five dissolved selenium concentrations and food organisms cultured in <br />the corresponding selenium concentration. Twenty larvae were assigned to each exposure beaker <br />(experimental unit). Larvae were transferred from mass cultures to flow-through beakers about <br />24 h before the toxicant metering system was activated. A continuous-flow diluter was used to <br />generate exposure concentrations. The diluter maintained a 0.5 dilution factor and supplied a <br />volume of 34 ml/min to exposure beakers. Beakers were polyethylene vessels having a diameter <br />~ ° <br />