Mercury pollution is decreasing, but not in tuna. This could be the reason.

In the 1960s and 1970s, the horrors of mercury poisoning in Japan and elsewhere shocked the world into limiting emissions of the toxic metal. Since then, mercury pollution from human activities such as coal burning and mining has declined in many parts of the world.

But when a team of French researchers analyzed thousands of tuna samples between 1971 and 2022, they found that mercury levels in the fish remained virtually unchanged.

That’s likely because “legacy” mercury that has accumulated deep in the ocean circulates to shallower depths where tuna swim and feed, the researchers say. a study published this month in the journal Environmental Science & Technology Letters.

Using models, they predicted that even with the strictest mercury regulations, it would take another 10 to 25 years for mercury concentrations in the ocean to decline. Decreases in mercury levels in tuna would not follow for decades.

The conclusion: the world’s fight to curb mercury pollution is far from over.

“Our research shows that we need to cut emissions significantly to even hope for a decline in the coming decades,” said David Point, an environmental chemist at France’s National Research Institute for Sustainable Development and one of the authors of the new study.

Mercury is a naturally occurring element, but human activities such as mining and burning fossil fuels cause the majority of mercury pollution worldwide. From the air it eventually settles, with much of it ending up in the oceans. Along the way, microorganisms convert mercury into a highly toxic form that accumulates in fish and shellfish.

Most people with mercury in their bodies get it from eating contaminated seafood, and even in small amounts this can damage the brain of unborn children and have toxic effects on the human nervous system, digestive system and immune system. The Environmental Protection Agency estimate that more than 75,000 newborns in the United States may be at increased risk for learning disabilities due to mercury exposure in the womb.

The heavy toll of mercury poisoning came to public attention after thousands of people developed neurological and other illnesses in Minamata, Japan, following decades of exposure to mercury in industrial wastewater that had poisoned local fish. (The story was the subject of a 2022 film starring Johnny Depp.)

Given the global scientific consensus on the health risks of mercury, most countries of the world have signed the 2013 Minamata Convention, committing to eliminate its use. Last year, the EPA said it was further tightening standards for mercury and other toxic air pollutants from power plants.

Still, the new research suggests that the Earth is slowly healing.

In an extensive effort that began more than a decade ago, scientists collected previously published findings and combined them with their own data on the mercury content of nearly 3,000 samples of tuna caught in the Pacific, Atlantic and Indian Oceans between 1971 and 2022. They looked specifically to tropical tuna – skipjack, bigeye and yellowfin tuna – which make up 94 percent of the world’s tuna catch.

They found that, in contrast to a global decline in mercury emissions since the 1970s, mercury levels in tuna remained virtually unchanged. In striped specimens caught in some parts of the Pacific Ocean, mercury levels rose, reflecting increases in mercury emissions from Asia, they said.

The persistently high levels of mercury in tuna were related to mixing in the ocean, which brings up mercury that has been lurking in the ocean’s depths for decades. Yet the complexities of that process are not yet fully understood. One question: How will climate change, which is rapidly warming the world’s oceans, affect the way mercury circulates?

Not a single tuna sample exceeded health standards; The health effects depend on who consumes the tuna (pregnant women, infants and children are particularly vulnerable) and how often they eat tuna, a low-fat, nutrient-rich protein source with its own health benefits.

Joel D. Blum, a professor of earth and environmental sciences at the University of Michigan who was not involved in the study, said the paper was consistent with best practices and current knowledge about how mercury moves in the world’s oceans behaves. “The data set presented in this paper is the largest I know of,” he said.

Environmentalists and public health advocates say the Minamata Treaty has a major loophole: It allows the trade and use of mercury in small-scale gold mining, a major source of mercury pollution. It is now thought that gold mining belongs to the world largest source of man-made mercury emissions.

Countries needed to take clear action to strengthen restrictions on mercury, including setting a deadline for ending its use in industries such as gold mining, said Lee Bell, technical adviser to the International Pollutants Elimination Network, a nonprofit based in Sweden.

“Business as usual clearly means contaminated tuna well into the next century,” he said.