Nearly all fish contain trace amounts of methyl mercury but longer-lived, larger fish that feed on other fish (like tuna) accumulate the most.
Illness began to appear on the shore of the Minamata Bay about two years after mercury-containing waste from a plastics factory was diverted into the bay. Since the late 1950s, nearly 1,080 Japanese have died and 2,871 have been recognized as seriously affected by Minamata disease, caused by eating fish polluted with methylmercury. The concentration of alkylmercury in fish and shellfish that led to this poisoning was in the range of 5 to 20 mg/kg calculated as mercury. Those who became sick gave a history of eating fish between 0.5 and 3 times per day. It has been estimated that the intake of mercury in fatal cases was at the rate of about 1.64 mg per person per day.
Pregnant women are warned that eating such fish can lead to irreparable brain and heart damage for babies exposed to mercury in the womb. A study, published in the February 2004 Journal of Pediatrics, documented neurological irregularities among 1,000 mothers and their children in Denmark's Faroe Islands, where the local diet includes large amounts of mercury-contaminated fish. Researchers linked the abnormalities to problems with irregularities in heart function that affect the body's oxygen supply.
Mercury poisoning in fish may be caused by coal-burning power plants. A U.S. Environmental Protection Agency report estimates that in 1999-2000, approximately 630,000 infants were born with mercury levels higher than those deemed safe. The U.S. Food and Drug Administration cautions pregnant women not to eat shark, swordfish, king mackerel or tilefish. Mercury contamination of the fish supply is so severe that 40 states have advisories warning consumers against eating fish and parts of rivers, for example the North Fork of the Holston and the Shenandoah rivers in Virginia, have been closed to fishing because of mercury contamination. The EPA estimates that as many as 1.6 million women and children may be at risk from mercury poisoning due to their modest fish consumption.