The Illinois Fish Contaminant Monitoring Program advises people to limit the number of
predator fish they eat because of methylmercury in fish tissue. Predator fish (bass, walleye,
muskellunge, northern pike, etc.) eat other fish or aquatic animals. The advisory applies to
predator fish from any surface water body in the state and especially targets women of
childbearing age, pregnant and nursing women, and children younger than 15 years old. Recent
studies have shown that methylmercury adversely affects the developing nervous system of
children and fetuses and could result in lower IQ, abnormal muscle tone, and slowed motor
function (Source: Illinois Department of Public Health Web site at http://www.idph.state.il.us/envhealth/fishadv/fishadvisory_qa.htm).
Small amounts of mercury occur naturally in the environment. Mercury also comes from burning
coal or incinerating medical and municipal waste, and from some industries. According to a U.S.
Environmental Protection Agency (USEPA) emissions inventory, these human activities are the
largest source of airborne mercury in Illinois. The USEPA, along with state environmental
protection agencies, has determined that the atmospheric deposition of mercury is the single
largest cause of surface water impairment in USEPA Region V, which includes Illinois.
However, the report does not explain how mercury in precipitation becomes methyl mercury in
Methylmercury is a toxic and very persistent form of mercury that forms in lake and river
sediments. Rainfall and runoff provide a way for methylmercury to enter lakes and streams. It
then bioaccumulates in fish tissue as small fish consume tiny organisms containing
methylmercury, large fish consume small fish, and so on, until the largest predator fish contain
methylmercury levels deemed harmful for human consumption. Eating contaminated fish is the
most important source of human exposure to methylmercury.
The Illinois State Water Survey is home to the National Atmospheric Deposition Program, which
operates the Mercury Deposition Network (MDN). That network measures the amount of
mercury in precipitation (rain, snow, and ice pellets). Additional information is available on the
MDN Web site (http://nadp.isws.illinois.edu/mdn/).