Hawaii and Michigan researchers say they’ve discovered how large quantities of toxic mercury end up in the fish we eat — and their work suggests that levels will rise in the coming decades.

Their study, published this week in the scientific journal Nature Geosciences, indicates that much of the mercury found in predatory fish that feed hundreds of feet below the Pacific Ocean’s surface can be traced back to sources that pump mercury high into the air.

Some of those mercury sources are natural — such as forest fires — but many others are man-made, University of Hawaii researchers say. They include the coal-fired power plants in rapidly industrializing Asian nations such as China and India — upwind of the North Pacific fisheries.

And the emissions from those coal-fired power plants are only expected to increase in the coming years, said Brian Popp, a professor of geology and geophysics at the University of Hawaii at Manoa and co-author of the study, released Sunday.

From the air, mercury makes its way into the oceans through rainfall or contact between the air and the water’s surface, experts say. Plankton take in the mercury and small sea­life eat the plankton and eventually wind up in fish up the food chain.

The research, which was conducted during a two-year period by a team from UH-Manoa and the University of Michigan, measured the mercury isotopes in nine different species of fish — six predators and three prey species — off the shores of Hawaii.

Their isotopes readings traced the mercury in the fish samples back to the air, Popp said.

"It’s got to be coming from the atmosphere," he said Tuesday. "It’s not from sediments" or other sources in the ocean."You have to show the ‘fingerprint’ of the mercury matches" what’s in the air.

The team’s finding that mercury levels are expected to increase is of particular concern in an island state whose residents rely heavily on fish as a part of their diet.

Local and federal health officials have advised for decades that people should eat certain fish in moderation to avoid neurological health problems associated with long-term consumption of mercury.

They also warn pregnant women and nursing mothers not to eat marlin, shark or swordfish because newborns’ exposure to mercury can lead to neurological and developmental disorders.

A 2005 University of Hawaii-led study looking at fetal exposure to mercury found that women in the islands were three times more likely than the national average to have elevated levels of the toxic substance in their umbilical cord blood after delivery.

That doesn’t mean people should avoid fish, which is also one of the more healthful meal options a person can choose, health officials add.

"Everybody’s going to get exposed to it," Alfred E. Asato, a chemical response laboratory coordinator with the state Department of Health, said Tuesday of mercury.

Over decades "it’s almost impossible to isolate a single toxic element with all the diseases," he said. "There are so many potential causal effects, causations for the various symptoms of diseases."

The Health Department recommends that pregnant women and young children eat ahi, ono and opah no more than once every two weeks. It further recommends that pregnant women and young children eat canned tuna, skipjack tuna, grouper, cod, mahimahi and halibut no more than once a week.

This week’s UH study, which linked the mercury in fish to airborne origins, comes after previous independent studies in recent years have shown concentrations of mercury are increasing in the Pacific Ocean as they decrease in the Atlantic, where the U.S. and European countries in the Atlantic region have taken steps to curb industrial emissions, Popp said.

Large predator fish often have larger mercury levels because they’re higher up on the food chain and accumulate more of the element, researchers say.

However, a study led several years ago by UH oceanography graduate student Anela Choy also found that the fish feeding in deeper ocean waters, typically at least 100 meters down, contain higher levels of mercury than those at the surface.

Researchers didn’t know why that was, Popp said.The UH-University of Michigan study released this week also solves that, he said.

It suggests that chemical reactions driven by sunlight destroy up to 80 percent of "monomethylmercury" (the organic mercury that’s absorbed into the ocean food chain) near the water’s surface. The sunlight doesn’t reach the greater ocean depths — where more organic mercury is produced by bacteria and accumulates.

Researchers studied mercury in six predator species that feed at greater depths: swordfish, mahimahi and opah as well as skipjack, bigeye and yellowfin tuna. They examined three prey species that feed at the surface: two types of lanternfish and flying fish.The deep-dwelling predators all had larger traces of mercury than the shallow-dwelling prey — and the mercury isotope was traced back to the atmosphere, Popp said.

"We don’t know" if other processes that don’t rely on sunlight to break down the organic mercury can keep up with all that’s building up at those greater ocean depths, he added.

"The way to control it is to cut it off at the source" — and push for cleaner industrial plants with better filters and "scrubbers" on smokestacks in Asia’s developing nations, Popp said.