Mercury levels in yellowfin tuna caught off the coast of Hawaii have not changed in 27 years, despite a considerable increase in atmospheric mercury during this time, according to a new study. The findings suggest that the high levels of mercury that have been found in tuna and other ocean fish may not be coming from pollution, but from natural sources.
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 François 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.
Unfortunately, scientists are not yet able to measure methylmercury in ocean surface waters, so Morel and his coworkers approached the problem from a different angle. They measured methylmercury levels in yellowfin tuna caught off the coast of Hawaii in 1998 and compared the numbers to a similar study from the same area in 1971.
They found no change in methylmercury levels in the tuna over that 27-year period.
The researchers predicted that mercury in the surface waters should have increased by up to 26 percent during this time, according to a computer model. The model took account of the change in atmospheric mercury, the sub-equatorial Pacific waters and the potential for mixing in the"thermocline" — a transition layer in the ocean where temperature changes rapidly.
The findings imply that the high levels of methylmercury in these fish are not coming from increased pollution, but from a natural source. The specific source is not yet clear, Morel says, but he suggests it could be hydrothermal vents and deep ocean sediments.
The research should also extend to other ocean-going predatory fish, like swordfish and sharks, according to Morel, which could mean that whatever is passing the mercury up to the tuna is probably doing the same to these other fish.
Morel is more cautious, however, about extending the findings to coastal fish. Bluefish, for example, run up and down along the eastern coast of the United States feeding on the continental shelf, and they may be taking up human pollution there. Lake fish are also a different situation, Morel says, since scientists have established a strong link between pollution and mercury levels in lakes.
The U.S. EPA and the U.S. Tuna Foundation provided support for this research.
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