Mercury levels in fish, particularly deep-sea varieties, are only going to get higher, and for that you can thank power polluters such as China and India.

According to a study published August 25 in the journal Nature Geoscience, mercury pollution from power plants in China and India is making its way into fish in waters near Hawaii.

University of Michigan researchers say that the mercury produced by coal-burning power plants in these countries travels thousands of miles through the air before rainfall deposits it on the ocean floor near Hawaii.

From there, it ends up in Pacific Ocean fish, such as tuna and swordfish, who kindly pass it on to the humans who eat them.

So yep, you can buy local produce, but this means you are going to have to worry about pollution in China if you like tuna wrapped around your chopped celery.

“The implications are that if we're going to effectively reduce the mercury concentrations in open-ocean fish, we're going to have to reduce global emissions of mercury, including emissions from places like China and India,” lead author Joel Blum, an environmental scientist at UM, told CBS News. “Cleaning up our own shorelines is not going to be enough. This is a global atmospheric problem.”

The toxic form of mercury found in the tissues of these fish, methylmercury, can damage the heart as well as the central nervous system and immune system. It is especially harmful to fetuses and young children.

In the study, the scientists sought to build on earlier research that showed that fish that feed at lower ocean depths are more likely to have high levels of mercury contamination. They tested tissue samples of nine species that live in a region called the North Pacific Subtropical Gyre, which is near Hawaii, and feed at various depths.

“We found that predatory fish that feed at deeper depths in the open ocean, like opah and swordfish, have higher mercury concentrations than those that feed in waters near the surface, like mahi-mahi and yellowfin tuna,” Brian Popp, a professor of geology and geophysics at the University of Hawaii at Manoa, told CBS. “We knew this was true, but we didn't know why.”

Previously, scientists believed that methylmercury forms at the surface layer of the ocean. But this study showed that sunlight actually destroys that process.

Instead, they found that up to 80 percent of methylation — the process through which microbes convert inorganic mercury into its toxic organic form, methylmercury — occurs at depths of about 165 to 2,000 feet.

In separate studies, researchers have predicted that mercury levels at depths of 660 to 3,300 feet could double by 2050.

It's almost impossible to know how much mercury you're actually getting when you buy fish, however, because levels are not listed on packaging. The Environmental Protection Agency simply recommends “limiting” consumption of fish that are known to have higher levels of contamination. How nice and vague of them.

“What's really astonishing about those [EPA] guidelines is that you can have 50 servings of salmon or 100 servings of trout for one serving of swordfish and get the same mercury,” Agus told CBS. In response, stores are starting to test the levels themselves, he said, so they have something to guide customers.

He also emphasized the need to push for international controls of mercury in power plants “so it doesn't cause a problem for all of us.”

How 'bout you get on that chemical threat, John Kerry?

Want more Squid Ink? Follow us on Twitter or like us on Facebook, and follow Samantha Bonar at @samanthabonar.

Advertising disclosure: We may receive compensation for some of the links in our stories. Thank you for supporting LA Weekly and our advertisers.

LA Weekly