Madison, Wisconsin: Wisconsin Department of Natural Resources
Summary
Forty-three lakes throughout Wisconsin were sampled in 1985-86 to determine the water and sediment chemistry characteristics that were associated with elevated concentrations of mercury in walleyes (Stizostedion vitreum vitreum (Mitchill)). Mean mercury concentrations for each of three different length classes of walleyes increased as the parameters lake pH, alkalinity, calcium, conductivity, or chlorophyll-a decreased. Low values for these parameters characterized most lakes in northern Wisconsin. Mean mercury concentrations exceeded the Wisconsin health standard of 0.5 µg Hg/g wet weight of fish for all walleye length classes in lakes with pH values less than6.0, for walleyes greater than or equal to15.0 inches in lakes with pH 6.0-6.9, and for walleyes greater than or equal to20.0 inches in all lake pH categories. Apparently the older, larger walleyes in hard-water as well as soft-water lakes can acumulate enough mercury to warrant concern. Sediment mercury concentrations were generally less than or equal to0.2 µg/g dry weight for all study lakes, but sediment mercury and organic matter were higher in lakes with pH values less than7.0 than in lakes with pH greater than or equal to7.0. Models were developed and tested to predict mercury concentrations in a 17-inch walleye for each lake. The best model derived from our study and tested on an independent dataset used alkalinity and calcium as independent variables. Clearly, walleyes from soft-water, poorly buffered, low pH lakes have the highest concentrations of mercury, but the reasons for these higher concentrations require further study.