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e <br />Toxicant Exposure <br />Exposures were conducted following prescribed methods for <br />renewal-acute toxicity tests (ASTM 1990). Range-finding, 24-h, <br />and 96-h exposures were conducted using 1-L glass beakers <br />containing 0.75 L of toxicant solutions. Exposure concentrations <br />were assigned to ten replicate beakers using a randomized block <br />design. The behavioral reaction to fright pheromone is <br />facilitated by social interactions between fish; thus, five fish <br />were placed in each exposure beaker, but the experimental unit <br />was a beaker. Colorado squawfish were randomized to each <br />replicate of four exposure-concentration treatments: three <br />toxicant concentrations, and a dilution-water control. Toxicant <br />solutions were renewed every 24 h. The dilution factor was 0.5. <br />Cool-white fluorescent lamps were the only source of <br />illumination, and a 16:8-h light:dark photoperiod was maintained. <br />Fish were not fed within 24 h of, or during, the exposure <br />periods. Survival and behavior were monitored at least daily. <br />Behavioral Assay <br />The response by Ostariophysan fishes to fright pheromone has <br />been described (Hara 1971; Pfeiffer 1982; Smith 1982; Smith and <br />Lemly 1986). Fright pheromone is released from specialized cells <br />in the epidermis of Ostariophysan fishes by mechanical damage. <br />Fright pheromone elicits a characteristic behavioral response <br />known as a fright reaction in receiving conspecifics that is <br />easily identified by human observers. Olfaction is the main <br />5 <br />