Mercury Levels in Ocean May Increase: Are Fish Safe to Eat?

Are you eating mercury? You might be. This common industrial toxin can travel through the food change after settling in the ocean. Now, scientists have discovered how the global mercury cycle collides with ocean fish at different depths in the water, revealing exactly how many toxins we might be ingesting.

"A few years ago we published work that showed the 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," said Brian Popp, co-author of the new paper, in a news release. "We knew this was true, but didn't know why."

Organic and inorganic mercury that's dissolved in water has a nutrient-like profile. There are lower concentrations at the surface and higher concentrations at depth. Why this is, though, has remained a mystery. This particular toxin accumulates in higher concentrations in fish that are higher up the food chain in a process called bioaccumulation. Yet these concentrations are raised even higher depending on the depth at which these predatory fish feed.

In order to find out exactly why fish that feed at depth might have greater concentrations of mercury, the scientists used a highly sophisticated mass spectrometer to measure the stable isotopic compositions of mercury in nine species of marine fish. These fish fed at different depths and included six predator fish and three prey fish.

So what did the researchers find? It turns out that chemical reactions driven by sunlight destroy up to 80 percent of monomethylmercury in the well-lit upper depths of the central North Pacific Ocean near Hawai'i. The scientists also discovered that a significant amount of monomethylmercury must be formed and enter the food web in oxygen-poor, deeper water. The findings that mercury is being converted to its toxic, bioavailable form at depth is crucial for understanding mercury levels at intermediate depths in the North Pacific in the coming decades.

"The implication is that predictions for increased mercury in deeper water will result in higher levels in fish," said Joel Blum, one of the researchers, in a news release. "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."

The findings reveal a little bit more about mercury in the food chain. In particular, it allows scientists to determine which marine fish are likely to have lower mercury concentrations. This could lead to safer eating habits for humans, who might choose one fish over another for these concentrations.

The findings are published in the journal Nature Geoscience.