Mystery at Sea

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

(Cheryl Carlin)

(Continued from page 1)

Despite its criticisms, the study led to some important ocean research. To study the impact of vents, a group of researchers led by Carl Lamborg of the Woods Hole Oceanographic Institution in Massachusetts sent a robot down 1.7 miles to collect samples from the Pacific Ocean's Gorda Ridge. In 2006, the researchers published their results—the first ever based on methylmercury in a vent—in the journal Geophysical Research Letters. They concluded that levels of mercury were fairly high in vents, but not quite high enough to support the amount found in fish at the surface.

The findings suggest that while vents might be a source of methylmercury, they likely are not an important one, says Chad Hammerschmidt of Wright State University, a coauthor on the paper. Even Morel, who served as a key witness for the tuna companies in the San Francisco case, now says that vents don't make up enough methylmercury to supply it to surface fish. But this realization in itself, he says, still doesn't explain where the majority of mercury comes from.

For that reason, many researchers are focusing on how methylmercury created in coastal regions could reach fish in the open ocean. Gilmour and Rob Mason of the University of Connecticut are leading a study of how methylmercury accumulates in the ocean shelf and the Chesapeake Bay. They analyzed sediment from nine areas along the mid-Atlantic coast and found evidence of methylmercury production in the continental shelf, as well as in the slope that breaks off below the shelf. The work is not yet complete, but "our results suggest that you can't ignore the edges," says Mason. "What's going on in the shelf seems to be very important."

Methylmercury from the coast might be transported out to sea in several ways. Tuna and other open ocean fish might swim in to the coast, eat contaminated coastal fish and swim back. A study published in Nature in 2005, led by Barbara Block of Stanford University, shows that bluefin tuna spend a lot of time near East Coast feeding areas before swimming far out to sea—even migrating across the Atlantic.

Currents might also wash mercury out from the shore. Some researchers have thought that sunlight would break down the toxic compound before it reached far out to sea, but new evidence about the movement of other metals, such as iron, is starting to challenge that concern, Fitzgerald says.

"There's increasing evidence for the importance of the coastal zone," he says. "That's really exciting. It's been there a long time, and we haven't paid enough attention to it."

Perhaps the biggest question is how much mercury can be converted into methylmercury on the ocean surface. Common wisdom has been that only bacteria living in oxygen-free areas can produce this conversion. However, Mason has done work near the equator in the Pacific Ocean showing that methylation might indeed occur in low-oxygen waters. It remains to be seen whether enough of these regions exist to have a big impact on methylmercury levels in fish.

If it turns out methylmercury can be created near the water surface, emissions regulations might have a direct impact on the amount of mercury in tuna and other fish in the ocean, Mason says. The same holds true if subsequent research supports the idea that methylmercury made in the coastal zone can be transported offshore.

What scientists do know, of course, is that something must account for the mercury found in tuna and other ocean fish. "The reality is that all methylmercury is being probably produced in all three environments"—along coasts, in deep vents and in some ocean surfaces—"but we need more work to parse out this fractionation," Mason says. For now, except in one San Francisco courthouse, the jury is still out.


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