Mercury is associated with fish because eating fish is the primary way that people, at least most Americans, are exposed to this toxic but natural element. And most of the fish we eat is marine fish—seafood.

So it might seem surprising that the majority of scientific research on mercury takes place not in the marine realm but in freshwater and terrestrial environments. How can this be? If our exposure to mercury—which is particularly harmful to pregnant women and children—is from ocean fish, why aren’t we researching mercury in the ocean?

Three years ago, a group of top mercury researchers thought the same thing, and in response they formed the Coastal and Marine Mercury Ecosystem Research Collaborative (C-MERC), coordinated by the Toxic Metals Superfund Research Program at Dartmouth College and funded by the National Institutes of Health. Nearly 70 scientists reviewed current knowledge – and knowledge gaps relating mercury contamination of the world’s marine fish.

They found that mercury pollution is ubiquitous throughout the global oceans, from the Arctic to the tropics, from San Francisco Bay to the Gulf of Maine. There is a “zone” in the middle depths of the open ocean (between 100 and 1,000 meters) where mercury is converted to the more toxic and biologically active methylmercury. Bioaccumulation through the food web means that fish feeding in these waters can have nearly 100 million times more mercury in their flesh than the surrounding water.

Researchers also analyzed of how cuts in mercury pollution can result in cleaner, healthier fish.

The researchers found that they could classify estuaries and oceans based on the dominant source of mercury. For example, the atmosphere might be the source of half of the mercury in the Gulf of Maine or Gulf of Mexico, but it accounts for approximately 90 percent of the mercury in the open ocean.

And the open oceans are home to large tuna and swordfish, which together account for more than half of the mercury intake from seafood for the overall U.S. population, according to Dr. Elsie Sunderland of Harvard University.

Estimating the sources of mercury in marine systems allowed the scientists to model how mercury levels in fish would respond. Model estimates from the report indicate that methylmercury concentrations in marine fish will decline roughly in proportion to decreases in mercury inputs, though the timing of the response will vary. For some bays and estuaries, the majority of mercury can be coming from rivers draining watersheds with contaminated industrial sites, wastewater treatment plants, and stormwater runoff.

“The impact of mercury released to coastal waters from watersheds via rivers has been a somewhat underappreciated aspect of the problem. Yet, these bays and estuaries can be important sources of fish for local anglers, thus controls on these sources can have substantial local benefits,” explained Dr. Charles Driscoll of Syracuse University.

C-MERC’s research findings are especially timely, as the U.S. and other nations recently convened at the fifth session of the United Nations Environment Programme’s Intergovernmental Negotiating Committee on January 13-18, 2013 in Geneva, Switzerland, who are working to prepare a legally binding instrument to control mercury releases to the environment. The C-MERC findings suggest that any steps this body makes toward controlling mercury emissions will impact seafood with a relatively rapid response time.

Because really, isn’t that what we all want—to be able to take our sustenance from the sea, free of potential harm?

For more information, please, to read peer-reviewed papers from C-MERC and a summary report,Sources to Seafood: Mercury Pollution in the Marine Environment(produced by Maine Sea Grant).