The purpose of this work was to determine if environmentally-relevant concentrations of chlorpyrifos and copper, two commonly detected chemicals in Western surface waters, can negatively impact the biological health of salmonids. Both compounds are highly neurotoxic to fish, but each with distinct biological target sites and mechanisms of action. We used common biochemical and physiological indicators of toxicity, and correlated these effects with potentially significant behavioral alterations. For chlorpyrifos, the mechanism of toxic action is the inhibition of acetylcholinesterase (AChE) throughout the peripheral and central nervous system. Here, we conducted biochemical assays of AChE activity in brain and muscle tissues after exposing steelhead trout (Oncorhynchus mykiss) and coho salmon (0. kisutch) to chlorpyrifos for 96 hours. We then correlated the AChE inhibition with behavioral impairment in swimming and feeding activities. In juvenile steelhead and coho exposed to 0.6-2.5 ��g/L chlorpyrifos, AChE activity was inhibited between ~10-65%. This biochemical indicator was significantly correlated with changes in behavioral patterns. Spontaneous swimming rates were reduced ~30-80% in the exposed fish, and strikes at food items (brine shrimp) were reduced ~10-70%. For copper and some other neurotoxicants, the olfactory nervous system is a sensitive target site in fish. The highly-developed olfactory system in salmonids is particularly susceptible to toxic insult by dissolved chemicals since receptor neurons are in direct contact with the aquatic environment. Here, we used electrophysiological techniques to record odor-evoked responses from the sensory epithelium and the olfactory bulb as direct measures of olfactory function in juvenile coho salmon. In fish exposed to copper, chlorpyrifos, or esfenvalerate for 7 days, field potentials recorded from the sensory epithelium and the olfactory bulb showed reduced or obscured olfactory responses to two classes of odorants, which activate non-overlapping populations of receptor neurons. To determine if this reduced sensory input can subsequently alter or diminish olfactory-mediated predator avoidance behaviors, paired physiological and behavioral tests were conducted on juvenile coho exposed to copper. In fish exposed to 2-20 ��g/L copper for 3 hours, olfactory sensitivity was reduced by ~50-9O%. When these fish were presented with a predatory alarm cue (conspecific skin extract), fish with reduced olfactory function increasingly failed to exhibit antipredator behavior. In the following experiments, we show that chlorpyrifos and copper can impair the biochemical and physiological biology of salmonids at environmentally-relevant concentrations, and that these sublethal effects can alter potentially important behavioral patterns. / Graduation date: 2004