The sources for much of our contamination are not well understood and may originate beyond Minnesota's borders. Once in a lake, mercury is converted to methylmercury by bacteria and other processes. Fish absorb methylmercury from their food and from the water passing over their gills. Mercury is tightly bound to proteins in all fish tissue. There is no method of cooking or cleaning fish that will reduce the amount of mercury in a meal.
Fish absorb PCBs and dioxin from water, suspended sediments, and food.
PCBs and dioxin 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 or dioxin in a fish meal. Larger, older fish and fish that eat other fish accumulate more contaminants than smaller, younger fish that eat less contaminated prey. Contaminants are not usually detected in panfish such as bluegill and crappies. Minnesota has one of the most extensive fish monitoring programs in the United States.