We examined relations between mercury concentrations in walleyes Stizostedion vitreum and the characteristics of clear-water Wisconsin lakes, which spanned a broad range of pH values (5.0-8.1) and acid-neutralizing capacities (-9 to 1,017 jieq/L). Total concentrations of mercury in axial muscle tissue of walleyes (total length, 25-56 cm) varied from 0.12 to 1.74 jig/g wet weight.Concentrations were greatest in fish from the eight lakes with pH less than 7.0; concentrations in these fish equaled or exceeded 0.5 /ig/g in 88% of the samples analyzed and 1.0 Mg/g in 44%. In the five lakes with pH of 7.0 and above, concentrations exceeded 0.5 Mg/g in only 1 of 21 walleyes. Multiple regression revealed that lake pH and total length offish accounted for 69% of the variation in mercury concentration in walleyes. Regression models with total length and either waterborne calcium or acid-neutralizing capacity as independent variables accounted for 67% of the variation in concentration. The observed differences in fish mercury concentration between the low-pH and high-pH lakes could not be logically attributed to differences in growth rate or diet among the walleye populations. Moreover, it is improbable that mercury influxes to the low-pH lakes were greater than those to the high-pH lakes, because of the close proximity and spatial interspersion of low-and high-pH lakes. We attributed the observed pH-related trend in mercury concentration in walleyes to variation among lakes in within-lake processes that affected the production and bioavailability of methylmercury.
Methylmercury is a known neurotoxin at high blood levels (> 400 micrograms/l) and is thought to cause neurologic symptoms at substantially lower levels in susceptible adults and infants. Given that levels of methylmercury in fish in northern Wisconsin lakes can be high (> 1 ppm, FDA standard) and Chippewa Indians take large amounts of fish from these lakes, the extent of their exposure to methylmercury was investigated. Using tribal-maintained registries, 465 Chippewa adults living on reservation were selected randomly and were invited to participate; 175 (38%) participated in the study. In an effort to characterize nonrespondents, 75 nonrespondents were selected randomly and were followed up aggressively. An additional 152 volunteers who were selected nonrandomly also participated in the study. Subjects completed a questionnaire about fish consumption patterns and had blood drawn for mercury determination. Sixty-four persons (20%) had blood mercury levels in excess of 5 micrograms/l (i.e., upper limit of normal in nonexposed populations); the highest level found was 33 micrograms/l. Fish consumption was higher in males and the unemployed. Blood mercury levels were highly associated with recent walleye consumption (p = .001). Methylmercury levels in some Wisconsin Chippewa were found to be elevated, but were below the levels associated with adverse health effects. We recommend a continuation of efforts to limit exposures in this high-risk population.
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