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after 96-h exposure. Comparison of ECSOs for copper showed that <br />Colorado squawfish were approximately 1.6 times more sensitive to <br />24-h exposure than to 96-h exposure. This pattern of decreased <br />olfactory sensitivity to toxicant as duration of exposure <br />increased was also observed for zinc. During 96-h exposure to <br />zinc, sensitivity to the toxicant was reduced to such an extent <br />that no inhibition of olfactory ability was detected. In the <br />highest 96-h exposure concentration (1492 µg/L), a positive <br />response to fright pheromone was elicited from fish in 8 of 10 <br />replicates compared to 9 of 10 control replicates. Positive <br />responses to fright pheromone were elicited from fish in all <br />replicates of the two other 96-hour zinc exposure concentrations. <br />Probability values for chi-square goodness-of-fit tests <br />ranged from 0.1 to 0.7 indicating that logistic regression models <br />adequately described the relationships between olfactory <br />inhibition and toxicant concentration (Table 2; Figure 1). <br />Logistic regression analyses yielded equations that allow <br />calculation of the estimated probability of olfactory inhibition <br />from 24-h or 96-h exposure to any concentration of copper or <br />zinc, based on the experimental conditions (Table 2). <br />Behavioral assay and SEM confirmed that Colorado squawfish <br />can regenerate olfactory receptor cells after exposure to <br />66.5 µg/L copper for 96 h. Fish in 8 of 9 replicates showed a <br />positive reaction to fright pheromone when they were re-assayed <br />after a 2 week recovery period. Scanning electron microscopy <br />11 <br />