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~ t <br />life history and behavior has been collected (Tyus 1991). <br />Natural reproduction of Colorado squawfish is preceded by <br />spawning migrations of 100 km or more. The means of navigation <br />used by Colorado squawfish during migration are unknown, but it <br />has been proposed that olfaction is a critical component of the <br />homing processs(Tyus 1985; 1990). ' <br />To investigate the effects on olfaction of two potential <br />environmental contaminants, copper and zinc, we exposed age-1 <br />Colorado squawfish to sublethal concentrations of each toxicant <br />for 24 or 96 h, and then evaluated olfactory ability using a <br />behavioral assay. Toxicant-exposed fish were challenged by <br />introducing a fright-pheromone extract, and behavioral response <br />was used to infer inhibition of olfactory ability. Conclusions <br />of behavioral assays were confirmed with systematic surveys of <br />olfactory receptor cells using scanning electron microscopy <br />(SEM). To verify that Colorado squawfish can regenerate <br />olfactory receptor cells after contaminant exposure, previously <br />inhibited fish were re-tested after a 2 week recovery period and <br />results were confirmed with SEM. <br />Materials and Methods <br />Experimental Animals <br />Colorado squawfish were obtained from Dexter National Fish <br />Hatchery and Technology Center (Dexter, N.M.). Fish were fed a <br />mixture of live < 24-h-old brine shrimp nauplii (Aquarium <br />2 <br />