Contaminants can reach rivers and lakes from local sources such as improperly stored wastes and abandoned dumps. If a local source is identified, it may be possible to clean it up and decrease the contamination of the lake or river. However, contaminants can reach remote and pristine lakes from the atmosphere. 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.