Mystery at Sea | EcoCenter: Oceans | Smithsonian
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(Cheryl Carlin)

Mystery at Sea

How mercury gets into tuna and other fish in the ocean has scientists searching from the coast to the floor

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In the United States and many places around the world, people get a majority of their mercury intake from ocean fish—particularly tuna. Fish has some health benefits, but too much mercury consumption can cause developmental defects in young children. Scientists understand how mercury makes its way into freshwater species, but because oceans are so much larger and deeper, they aren't sure the process is the same.

This uncertainty was underscored in May of 2006, when the San Francisco Superior Court ruled that tuna companies do not have to include mercury warnings on cans. In large part, the decision hinged on whether mercury found in ocean fish originated from man-made industry, such as coal-burning factories that emit the gas, or from a natural location, such as the sea floor. In the court's opinion, two things were clear: No one really knows where ocean fish contract their mercury. And the little that is known suggests it does not come from human pollution.

"One of the big questions is, where does the mercury in tuna fish and ocean fish come from? Because that's where most people get their mercury," says senior scientist Cynthia Gilmour of the Smithsonian Environmental Research Center in Edgewater, Maryland. That big question holds big implications for public health. If mercury in fish comes mostly from the atmosphere, then emission regulations and other efforts might over time make fish safer to eat. If ocean fish get their mercury from the natural environment, however, educating women about the health effects of mercury on unborn and young children might be the only influential option. "It's pretty important to know that," Gilmour says, "and we don't know."

That's not the case in freshwater sources, where the process is well-studied. Rain washes mercury down from the air onto rivers, lakes and watersheds. Micro-organisms convert it into a harmful form, methylmercury. Small fish consume the microbes, large fish consume the small fish, and eventually the toxin lands in kitchens. This chain of events can happen rapidly. In research published online last week in Proceedings of the National Academy of Sciences, Gilmour and her colleagues found that mercury appeared in lake fish as soon as two months after it had landed on the water surface. The amount of mercury emitted into the atmosphere has tripled, by some estimates, during the past century of industrial activity. As a result, most researchers say with confidence that decreasing man-made mercury emissions will, in time, make fish from some lakes and rivers safer to eat.

In oceans, however, scientists aren't sure mercury follows that path. The high cost of research ships and the sheer size of the sea make marine data collection a lengthy procedure. In addition, much work on ocean mercury done before about 1980 is potentially spoiled by contaminated instruments. "We don't have much data for the ocean. It's surprisingly sparse," says biogeochemist William Fitzgerald of the University of Connecticut. But within the past decade, scientists have made a push to fill this void in understanding. The work is "finally getting through in a broad way," he says.

As a result, researchers are just starting to piece together the big picture. They generally agree that three places produce this methylmercury: vents on the ocean floor, coastal areas and water columns near the surface. Vent mercury, likely thousands of years old, would be produced independent of human activity. Methylmercury from the coast or surface, however, likely would be the result of industrial pollution. The proportional impact of each avenue is much less clear.

"Right now, I'd say nobody has found a source of methylmercury in the ocean that can easily account for what we find in terms of methylmercury in open ocean fish," says geochemist François Morel of Princeton University. "It's been hard to figure out where it's coming from, where's it's going. Now we're beginning to understand."

In 2003, Morel and some colleagues measured mercury levels of yellowfin tuna caught near Hawaii in 1998 and compared them with measurements taken by other researchers from tuna caught in 1971. Mercury from industrial emissions would settle near the surface, so if that's where methylmercury in ocean fish is produced, then the 1998 fish should have noticeably higher amounts of mercury, the researchers proposed. Instead, Morel's group found no difference at all between the two fish samples, they reported in the journal Environmental Science and Technology.

The researchers concluded that the methylmercury in tuna came not from atmospheric emissions but rather from a natural source—hydrothermal vents at the bottom of the ocean. Though tuna live in the upper part of the ocean, they could possibly contract vent mercury by eating fish that spend time in the deep sea.

The findings produced strong reactions in the research community. Some argue that the two tuna populations aren't comparable. Yellowfin tuna have been heavily fished since 1971, and fishing pressure can alter the mercury levels in certain fish stocks, says aquatic toxicologist James Wiener of the University of Wisconsin-LaCrosse. Others believe that mercury in the atmosphere hasn't drifted out far enough into the ocean yet to measure a change.

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