The FDA also recommends that small children, women who are nursing, or who are considering becoming pregnant avoid those fish as well. The Environmental Protection Agency, which regulates noncommercial fish, also includes freshwater fish caught by family or friends, which may be contaminated with mercury. According to the FDA, samples taken of fresh tuna showed mercury levels of up to 1.30 parts per million, which was lower than the fish in the higher-risk group. Samples of tilefish, by comparison, showed mercury levels up to 3.73 parts per million. The highest range of canned tuna samples were 0.75 ppm.
Mercury occurs naturally in the environment.
But in addition, about 40 tons of mercury are released into the atmosphere annually in the United States as a byproduct of fossil-fuel burning power plants, according to the NAS report. That mercury ends up in oceans, lakes and streams, where it is converted into methylmercury by aquatic microbes. Those microbes eventually are eaten by fish, who are in turn eaten by bigger fish. As a result, large predators such as tuna and shark often contain high levels of methylmercury. The typical American consumer eats less than a third of an ounce of fish per day, and is at low risk of mercury exposure.