Scientists have discovered new truths about mercury contamination in fish.

According to new research from the University of Michigan and the University of Hawai'i, scientists have learned that fish experience mercury contamination in deep ocean and expect mercury levels to rise in the Pacific Ocean the coming decades.

"In the next few decades there will be changes in mercury concentrations in the Pacific Ocean, and those changes are likely to be different for surface waters than for deep waters," Brian Popp, co-author of a new study published Aug. 25 in the scientific journal Nature Geoscience, said.

The team at the University of Michigan came to the conclusion by using a mass spectrometer to measure the ratios of stable isotopes of mercury in nine species of marine fish that feed at different depths. At the University of Hawai’i, scientists sampled fish from various depths, measured the amount of mercury in their muscle tissues, and determined their position in the marine food chain.

Researchers found that 80 percent of methylmercury, the most toxic form of mercury, is produced at deeper ocean depths by clinging to sinking organic matter. Mercury is also deposited into the Pacific Ocean by rainfall, lead author Joel Blum said in a press release.

"A few years ago we published work that showed 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," Popp said. "We knew this was true, but we didn't know why."

According to the United States Environmental Protection Agency, mercury poisoning can impair a person’s memory, ability to learn, and motor movements. It can also damage the heart and immune system. Even in small quantities, mercury can cause birth defects in the womb and in breast-fed newborns.

Scientists predict mercury levels will double at intermediate depths, 660 feet to 3,300 feet, in the North Pacific Ocean, by 2050. They point to the growing industrialization in Asian countries like India and China – both rely on coal-burning power plants that are known sources of mercury pollution.

"This study reinforces the links between mercury emitted from Asian countries and the fish that we catch off Hawaii and consume in this country," Blum said, adding that the study highlights the growing need to stem mercury pollution across the globe.

"Cleaning up our own shorelines is not going to be enough. This is a global atmospheric problem," Blum said.

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