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Even though several studies have found that eating a good amount of fish could lead to better health, the risk of consuming excess amounts of mercury can be extremely detrimental. Mercury is a toxin created industrially that infects the marine environment. By seeping into marine animals, mercury then becomes a health risk for humans who love to chow down on their seafood. In a new study, researchers from the University of Michigan and the University of Hawaii at Manoa School of Ocean and Earth Science and Technology (SOEST) outlined how mercury affects fish at different depths in the ocean.

"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 yellow fin tuna," said Brian Popp, professor of geology and geophysics at UH Manoa according to a press release. Popp is a co-author of this study. "We knew this was true, but we didn't know why."

For this study, the team of researchers looked at marine ecology and used biogeochemistry to map out mercury contamination at different levels of the ocean. The researchers used a mass spectrometer in order to measure the levels of stable isotopic compositions of mercury in nine species of marine fish. These fish, which included six predator species and three prey species, ate at different depths of the ocean. The researchers found that fish that fed further into the ocean waters had higher content of mercury.

The researchers concluded that mercury levels increased further down the ocean waters because of the availability of sunlight. The researchers stated that their chemical analysis revealed that sunlight was responsible for destroying around 80 percent of monomethylmercury in water that was closer to the surface. This analysis applied to the water source in the central North Pacific Ocean by Hawaii. In addition, the researchers stated that monomethylmercury was formed in the oxygen-poor regions of the water, which are the deeper parts of the ocean.

"The implication is that predictions for increased mercury in deeper water will result in higher levels in fish," said Joel Blum of the University of Michigan, the lead author on the new paper and a professor in the department of earth and environmental sciences. "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 researchers hope that this research could provide more information regarding the safety of the fish humans consume. The new paper was published in the scientific journal, Nature Geoscience.