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Introduction <br />Many contaminants are known to disrupt chemoreception in <br />fishes. Short-term sublethal exposure to naphthalene, copper, <br />lead, mercury, nickel, silver, zinc, and extremes of pH, inhibit <br />olfactory ability (Sutterlin 1974; Brown et al. 1982;` Hara et al. <br />1983; Brand and Bruch 1992; Klaprat et al. 1992). Olfactory <br />receptors are not protected from contaminant exposure by external <br />membranes, but come into direct contact with water-borne solutes. <br />Toxic substances may alter perception of olfactory cues through <br />several modes of action: damaging organelles and enzyme systems, <br />direct interaction with membrane receptor sites, or masking <br />biologically important chemical signals. Olfaction facilitates a <br />variety of ecological interactions between fishes and disruption <br />of sensory ability may affect success of feeding, mating, <br />predator avoidance, or homing and migration. <br />The Colorado squawfish (Ptychocheilus lucius) is the largest <br />minnow in North America and historically occurred throughout the <br />Colorado River Basin (Behnke and Benson 1983; Tyus 1991). <br />Colorado squawfish populations have declined as a result of <br />interactions with introduced fishes, construction of reservoirs, <br />and other management practices (Carlson and Muth 1989). In <br />response to the rapid decline and threat of extinction, the <br />Colorado squawfish was listed as a federally endangered species <br />in 1967. Although scarcity and federally endangered status has <br />made study of wild Colorado squawfish difficult, information on <br />1 <br />