Because contaminants may produce harmful effects when consumed over a period of time, the ISDH advises that limited amounts of these fish be consumed. Once in a lake, mercury is converted to methylmercury by bacteria and other processes. Fish absorb methylmercury from their food and from water as it passes over the gills. Mercury is tightly bound to proteins in all fish tissue, including muscle. There is no method of cooking or cleaning fish which will reduce the amount of mercury in a meal.
Fish absorb PCBs from water, suspended sediments, and food.
PCBs concentrate in the fat of fish and in fatty fish such as carp and catfish. Cleaning and cooking a fish to remove fat will lower the amount of PCBs in a fish meal. Larger, older fish, and fish which eat other fish, accumulate more contaminants than smaller, younger fish, which eat less contaminated prey. Contaminants are not usually detected in panfish such as Bluegill and Crappie. PCBs and pesticides tend to be stored in the fat of fish.