Food for thought: Tuna may pick up toxins from Asia on its way to your supermarket.

RODALE NEWS, EMMAUS, PA—We’ve all heard about mercury in seafood, and we know it’s best to avoid the neurotoxin. But exactly how it finds its way into fish, particularly deep-sea fish like tuna, has been a bit of a murky mystery. A new landmark study documents for the first time how increased mercury emissions from human sources, particularly in Asia, move through ocean currents and contaminate seafood far from the coast. The study was published in the journal Global Biogeochemical Cycles this month.

“This study gives us a better understanding of how dangerous levels of mercury move into our air, our water, and the food we eat, and shines new light on a major health threat to Americans and people all across the world,” says U.S. Environmental Protection Agency Administrator Lisa P. Jackson. “With this information in hand, plus our own mercury efforts, we have an even greater opportunity to continue working with our international partners to significantly cut mercury pollution in the years ahead and protect the health of millions of people.”

THE DETAILS: U.S. Geological Survey researchers sampled water at 16 different spots between Hawaii and Alaska and found that mercury levels in mid-depth waters, but not surface waters, were 30 percent higher than mid-1990s readings. Their computer simulations show that mercury emissions from Asia, originating in coal-fired power plants, cement manufacturing plants, metal-smelting operations, and trash incinerators, fall into the ocean when it rains and are carried to the North Pacific Ocean. There, decomposing bacteria in the water mix with mercury to create methylmercury, a highly toxic form of mercury that gets into the food chain and can hinder brain development in fetuses and young children.

One unexpected finding from this study is the significance of long-range transport of mercury that originates in the western Pacific, off the coast of Asia, by ocean currents. Study coauthor David Krabbenhoft, PhD, USGS research hydrologist, says researchers were surprised to find that mercury emissions from off the coast of Asia made it to the North Pacific in just 2 to 4 years. “It appears the recent mercury enrichment of the sampled Pacific Ocean waters is caused by emissions originating from fallout near the Asian coasts. The mercury-enriched waters then enter a long-range eastward transport by large ocean circulation currents,” he explains.The study authors predict that if mercury emissions continue as projected, there will be a 50 percent increase in Pacific Ocean mercury levels by 2050.