Once in a lake or river, mercury is converted to methylmercury by bacteria and other processes.
Fish absorb methylmercury from their food and from the water as it passes over their gills.
Mercury is tightly bound to proteins in all fish tissue, including muscle. There is no method of cooking or cleaning fish that will reduce the amount of mercury in a meal. Nationwide, mercury is represented in more fish consumption advisories than any other chemical. Methylmercury in the water and sediment is taken up by tiny animals and plants known as plankton. Minnows and juvenile fish eat large quantities of plankton over time Large predatory fish consume smaller fish, accumulating methlymercury in their tissues.