Levels of highly toxic mercury contamination in Atlantic bluefin tuna are rapidly declining, according to a new study. That trend does not affect recommended limits on consumption of canned tuna, which comes mainly from other tuna species. Nor does it reflect trends in other ocean basins. But it does represent a major break in the long-standing, scary connection between tuna and mercury, a source of public concern since 1970, when a chemistry professor in New York City found excess levels of mercury in a can of tuna and spurred a nationwide recall. Tuna consumption continues to be the source of about 40 percent of the mercury contamination in the American diet. And mercury exposure from all sources remains an important issue, because it causes cognitive impairment in an estimated 300,000 to 600,000 babies born in this country each year.

The new study, published online on November 10 by Environmental Science & Technology, links the decline directly to reduced mercury emissions in North America. Most of that reduction has occurred because of the marketplace shift by power plants and industry away from coal, the major source of mercury emissions. Pollution control requirements imposed by the federal government have also cut mercury emissions.

Progress on both counts could, however, reverse, with President-elect Donald Trump promising a comeback for the U.S. coal industry, in part by clearing away such regulations.

For the new study, a team of a half-dozen researchers analyzed tissue samples from nearly 1,300 Atlantic bluefin tuna taken by commercial fisheries, mostly in the Gulf of Maine, between 2004 and 2012. They found that levels of mercury concentration dropped by more than 2 percent per year, for a total decline of 19 percent over just nine years.

Although the researchers were aware of the decline in the amount of mercury entering the atmosphere over North America, it came as a surprise when this improvement also showed up in the flesh, says study co-author Nicholas Fisher, a marine biogeochemist at Stony Brook University. Atlantic bluefin tuna are big, fast-moving predators at the top of the food chain and live on average 15 to 30 years. Those traits make them perfectly suited to accumulating mercury and other environmental contaminants. “We could as easily have expected it to take a century” for the fish to show signs of recovery, Fisher remarks. The contrary finding “tells me we don’t just have to wring our hands about the high level of mercury in these fish. There is something we can do about it and get pretty quick results.”

The study is also surprising because mercury is long-lived in the environment, says Noelle Eckley Selin, an atmospheric chemist at Massachusetts Institute of Technology, who was not involved in the study. Whereas surface waters reflect recent changes in mercury emissions, past emissions from early in the industrial era can persist for centuries in the deep ocean.

The level of mercury contamination has also decreased in Atlantic coast bluefish, according to a 2015 study. But that assessment was based on less reliable data, and in a much shorter-lived species, observes Carl Lamborg, a biogeochemist at the University of California, Santa Cruz, who was not involved in either study. The bluefin tuna finding “shows that even a fishery that we would have thought had a certain amount of chemical inertia can be cleaned up if you stop putting mercury into the system,” he says. Bluefin tuna is a highly prized fish, sold mostly in Japan, where a single fish went for $118,000 at auction early this year. Both Atlantic and Pacific varieties are listed as endangered on the International Union for Conservation of Nature Red List of Threatened Species.

The new study comes as worldwide mercury emissions continue to rise, particularly in the Pacific, source of much of the tuna and other seafood consumed in the U.S. That increase of about 3.8 percent per year results largely from increased reliance on coal-fired power plants in China, India and other Asian countries.

Despite President-elect Trump’s campaign pledge to revive the coal industry in this country, economic factors, including competition from inexpensive natural gas, make a U.S. coal comeback unlikely. Even U.S. Senate Majority Leader Mitch McConnell, who for eight years blamed the Obama administration for the demise of coal, began to walk back the idea that Republican control in Washington, D.C., would make much difference: “Whether that immediately brings business back is hard to tell,” he told a Kentucky audience on November 11, “because it’s a private sector activity.”

In any case, the Electric Power Research Institute (EPRI), a utility-funded nonprofit, projects that mercury emissions from U.S. power plants will be about 85 percent lower in 2017 than in 2010. Planned retirements of coal-fired power plants, combined with pollution control upgrades already installed to comply with Environmental Protection Agency regulations, will drive the expected decline. Those regulations face continuing legal challenges but “you can’t ‘un-pay’ for the controls you have already put in,” says Leonard Levin, an EPRI air quality analyst.

The larger concern, Selin notes, is Trump’s plan to abandon the Obama administration’s major climate change initiatives. These initiatives include the Clean Power Plan, which would set a gradually decreasing national limit on carbon pollution from electric utilities, and the recently signed 2016 Paris agreement, in which 193 nations committed themselves to individual limits on carbon pollution. The U.S. is also party to the Minamata Convention on Mercury, a treaty designed to reduce mercury emissions worldwide that is likely to go into effect in 2017. (The name comes from Minamata, Japan, site of one of the most horrifying environmental disasters of the 20th century, with severe birth defects and other disorders resulting from mercury contamination in seafood.)

“These issues are linked,” Selin says. “More coal burning gives you more carbon dioxide and more mercury, and these are things that connect very directly with people’s health in the United States. The future of mercury emissions really depends on energy sources in Asia,” and it requires “U.S. involvement to encourage” energy production from sources other than coal. Without that global shift away from coal, all the progress on mercury so far achieved in North America ultimately could be in vain.