The report will appear in the Dec. 15 edition of Environmental Science & Technology, a peer-reviewed journal of the American Chemical Society, the world's largest scientific society.
Nearly all fish contain trace amounts of mercury, but longer-lived predators like tuna, swordfish and sharks generally have higher levels. The U.S. Food and Drug Administration warns pregnant women against eating large amounts of fish to avoid harming an unborn child's developing nervous system.
Mercury enters the environment naturally and through industrial pollution, mostly from coal-fired power plants. Scientists have estimated that the amount of mercury in the atmosphere today is about two to three times what it was 150 years ago.
"People have assumed that the high mercury in fish must be from pollution," says Franois Morel, Ph.D., a professor of geochemistry at Princeton University and an author of the study. "We have about tripled the mercury in the atmosphere, and therefore it should be tripled in the ocean, right? But maybe mercury that occurs in fish is a natural thing, and it may have been there all along."
The first step in exploring this assumption is to clarify the chemical nature of mercury in the environment. "The question is not where mercury is coming from, but where methylmercury is coming from," Morel says. Mercury concentrations in the air are of little concern, but when mercury enters water, microorganisms transform it to a highly toxic form methylmercury that builds up in fish.