However, some fish contain high levels of methylmercury, a form of mercury that can harm an unborn child s developing nervous system. Mercury occurs naturally in the environment and it can also be released into the air through industrial pollution. It falls from the air and can get into surface water, accumulating in streams and oceans. Bacteria in the water cause chemical changes that transform mercury into methylmercury, which can be toxic.
Fish absorb methylmercury from water as they feed on aquatic organisms.
Nearly all fish contain trace amounts of methylmercury, which is not harmful to humans. However, long-lived, larger fish that feed on other fish accumulate the highest levels of methylmercury and pose the greatest risk to people who eat them regularly. These include shark, swordfish, king mackerel and tilefish. Although the primary danger is to unborn children, it is prudent for nursing mothers and young children not to eat these fish as well. As long as you select a variety of other kinds of fish while you are pregnant or may become pregnant, you can safely enjoy eating them up to 12 oz.