Eventually, gaseous mercury attaches to water droplets and returns to earth in rain and snow. That doesn't mean a silvery mercury rain is falling on Wisconsin lakes. Quite the contrary, as actual concentrations of mercury in water are very low. One DNR study near Trout Lake in northern Wisconsin found significant concentrations of mercury in fish but had an extremely difficult time detecting it in lake water. For reasons not completely understood, precipitation-borne mercury in some lakes and rivers transforms into methylmercury, a form easily absorbed by plankton.
Insects eat the plankton, minnows eat the insects, and bigger fish dine on the minnows.
As the front section of the advisory explains, in waters where one or more fish are listed for mercury, the odds are other predator fish of similar or larger sizes are tainted too. Stan Nichols, an officer in Dane County's Yahara Fish Club who fishes Lake Monona regularly, says the DNR has distributed fish advisories and made presentations to the club on contaminates in fish over the last four years. Yet even walleyes under 10 inches hold enough mercury that the DNR advises pregnant women to eat no more than one meal per month of them. The board approved the petition and sent it to the DNR, which will present new rules to achieve these goals in March. If the rules are approved, Wisconsin will be the first state to enact mercury emission limits.