Nicholas S. Fisher got a research opportunity he couldn’t pass up.

When he embarked on a study of fish two years ago, he didn’t know what he was looking for. All he knew was that a researcher in Massachusetts had samples of nearly 1,300 Western Atlantic bluefin tuna in a deep freeze and was offering them up for investigation.

Today, his team’s findings are being greeted as some of the most positive news in a while related to the lowering of power-plant emissions. Studies of tuna caught in the Gulf of Maine between 2004 and 2012 revealed that levels of methylmercury in their bodies decreased at a rate of 2 percent per year, or nearly 20 percent over a decade.

“The decline is real,” Fisher, a professor of marine sciences at Stony Brook University in New York, said Monday. “The decline is almost in parallel with declines in mercury emissions from coal-fired power plants and the decline of mercury in the air. It appears that the fish are responding almost in real time. We thought that was pretty exciting.”

The decreases occurred as coal-fired power plants began closing in 2008 – with 300 now shut down, according to the National Mining Association. Four years before that date, the carcasses of bluefin tuna, regardless of age, size and sex, had a lot more mercury than four years after. The research was published this month in Environmental Science and Technology.

Make no mistake, the fish still contained dangerously high levels of mercury, a substance that’s especially harmful to pregnant women and children. But the research showed that the benefits of lowering coal emissions as power plants switch to natural gas are almost immediate and measurable.

A high consumption of tuna accounts for more than 40 percent of mercury concentrations in humans, more than any other source, the study said.

Bluefin tuna, which often wind up in cans on grocery shelves and kitchen cupboards, tend to have the highest levels of mercury of any type of tuna. Yellowfin tuna tend to have more moderate levels, and skipjack tuna’s mercury levels are relatively low.

Since tuna are an apex predator, near the top of the marine food chain, mercury in the fish they eat accumulates in their bodies. “Fish acquire about 95 percent of mercury from their diet,” Fisher said, with a much smaller amount absorbed just from swimming in toxic waters.

No research has determined that mercury levels have also dropped in the smaller fish on which bluefin prey. “We can only speculate that that is the case,” Fisher said. But he expects that result would be found.

He would not speculate about mercury levels in fish outside the Gulf of Maine: “In the Pacific, increased emissions in China and Asia could be increasing it. It may be an entirely different story.”

His team would next like to study whether methylmercury that’s also found in the brains of tuna affects their behavior, such as how they swim or relate to each other. Both are things science doesn’t know. For now, Fisher said, he’ll settle for his study’s good news.

“It says that you know you don’t have to look at all the mercury in the seafood and wring your hands and say, ‘Oh my God, there’s nothing that can be done about this,’ ” he said. “It appears that something can be done, and there’s a positive benefit in real time. You don’t have to wait for decades.”