Scientists at the University of Michigan and the University of Hawaii said as much as 80 percent of the toxic form of mercury, called methylmercury, found in the tissues of deep-feeding North Pacific Ocean fish is produced deep in the ocean, most likely by bacteria clinging to sinking bits of organic matter.
And most of mercury found in fish near Hawaii likely traveled through the air for thousands of miles before being deposited on the ocean surface in rainfall, they said.
"This study reinforces the links between mercury emitted from Asian countries and the fish that we catch off Hawaii and consume in this country," UM environmental scientist Joel Blum said.
"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," he said.
Exposure to methylmercury from consuming fish can affect humans, with risks of damage to the central nervous system, the heart and the immune system, the researches said.
"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," the University of Hawaii's Brian Popp said. "We knew this was true, but we didn't know why."
The reason, researchers said, is that while methylation occurs in well-lit near-surface waters, sunlight destroys up to 80 percent of the methylmercury formed there in a process called photochemical degradation.
But deeper in the ocean, they said, there is significant microbial production of methylmercury from inorganic mercury down to a depth of about 2,000 feet, were deep-water marine fish feed.
"The implication is that predictions for increased mercury in deeper water will result in higher levels in fish," Blum said.