MERCURY CONTAMINATION OF FISH IN THE OJIBWAY DIET .2. SENSORY EVOKED-RESPONSES IN RATS FED WALLEYE

The Ojibwa people of the upper Great Lakes in the United States have a long history of utilizing walleye caught by spear fishing as a major part of their diets. Walleye (Stizostedion vitreum) have been collected and prepared into fillets using traditional methods, submitted for total mercury (Hg) analysis, and fed to laboratory rats in standard neurotoxicity protocols to determine the human health risks associated with consuming these fish. Wisconsin officials recommend avoiding the consumption of fish containing more than 0.5 ppm Hg Laboratory rodent neurotoxicity bioassays included blending composite fish samples of 0.8, 0.4, and 0.2 ppm total mercury and feeding it to 48 young adult female Long Evans rats for 90 days. Standard behavioral assessments included: clinical neurologic observations, motor activity, and accelerating rotarod Twelve of the 48 rats were surgically implanted for electrodiagnostic evaluations using sensory evoked potentials with auditory and visual stimuli. Auditory (clicks) responses were relatively stable and unaffected by Hg laden diets. However, visual evoked responses at low flash intensities demonstrated a dose related slowing of brain visual processing activity. Methylmercury contamination is known to affect visual systems, and visual evoked brain potentials are apparently sensitive indicators of dietary Hg.