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

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

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

Using modeling, they predicted that, even with the most stringent mercury regulations, it would take an additional 10 to 25 years for mercury concentrations to start falling in the ocean. Drops in mercury in tuna would follow only decades after that.

The takeaway: The world’s fight to tame mercury pollution is far from over.

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

Mercury is a naturally occurring element, but human activities like mining and burning fossil fuels cause the bulk 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 builds up in fish and shellfish.

Most people with mercury in their bodies get it from eating contaminated seafood, and, even in small amounts, it can harm the brains of unborn children and have toxic effects on the human nervous, digestive and immune systems. The Environmental Protection Agency estimates that more than 75,000 newborns in the United States may have increased risk of learning disabilities linked to mercury exposure in the womb.

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

Given the global scientific consensus over mercury’s health risks, most of the world’s countries signed the 2013 Minamata Convention committing themselves to eradicating its use. Last year, the E.P.A. said it was further strengthening standards on mercury and other toxic air pollutants from power plants.

Yet the new research suggests that the earth heals slowly.

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

They found that, in contrast to a global decrease in mercury emissions since the 1970s, mercury levels in tuna remained virtually unchanged. In skipjack caught in some parts of the Pacific, mercury levels rose, mirroring an increase in mercury emissions from Asia, they said.

The stubbornly high levels of mercury in tuna had to do with ocean mixing, which is churning up mercury that’s lurked for decades in the ocean’s depths. Still, the complexities of that process aren’t yet fully understood. One question: How will climate change, which is rapidly warming the world’s oceans, affect the way mercury circulates?

No single sample of tuna exceeded any health standards; health effects depend on who is consuming the tuna (pregnant women, infants and children are particularly vulnerable) and how often they eat tuna, a low-fat, nutrient-rich source of protein with its own health benefits.

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

Environmentalists and advocates for public health say that the Minamata Convention has a big loophole: It allows for the trade and use of mercury in small-scale gold mining, a significant source of mercury pollution. Gold mining is now thought to be the world’s largest source of human-caused mercury emissions.

Countries clearly needed to step up to strengthen restrictions on mercury, including setting a deadline for ending its use in industries like gold mining, said Lee Bell, a technical adviser for the International Pollutants Elimination Network, a nonprofit group based in Sweden.

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