Mercury Levels in Tuna Are Just as High as They Were in 1971

Mercury levels in tuna have stayed steady since the early 1970s, according to a new paper.

Despite a global reduction in mercury pollution in the intervening fifty years, the international team of researchers say marine fish are still just as contaminated with the toxin, likely due to legacy mercury still circulating in the oceans.

The findings were published in Environmental Science & Technology Letters.

Steady mercury

Mercury pollution enters the seas via industrial discharge, land runoff and atmospheric deposition. Once there, microbes convert the element into methylmercury, a toxic cation that is then consumed by sea life such as plankton.

As the plankton is eaten by small fish, which are in turn eaten by larger fish, the levels of methylmercury accumulate up the food chain.

Sitting near the top of their food chain, tuna fish end up with a notable quantity of methylmercury in their bodies, which can constitute a minor health risk to humans if consumed in exceptionally large portions (three cans or more a day). 

Since the implementation of several environmental policies in the late 20th century and the decline of polluting industries such as mining in key areas, some environmental scientists had hoped ocean methylmercury levels would have declined. But this hasn’t been the case.

Now, an international team of researchers has compiled previously published data plus their own records on total mercury levels from nearly 3,000 tuna samples of fish caught in the Pacific, Atlantic and Indian Oceans between 1971 and 2022. They specifically looked at three species of tropical tuna: skipjack, bigeye and yellowfin, which account for 94% of global tuna catches.

As none of the species undergo transoceanic migrations, the researchers reasoned that any methylmercury contamination found in the animals’ muscles would likely reflect the waters they swam in.

After standardizing the data to allow for comparison across decades and regions, the researchers observed that tuna mercury concentrations remained stable worldwide from 1971 to 2022, except for an increase in the northwestern Pacific Ocean in the late 1990s. This likely resulted from concomitant increasing mercury emissions from Asia. Airborne mercury pollution, however, did decline over the same period.

The researchers posit that the stubbornly static mercury levels in tuna may be caused by upward mixing of old mercury from deeper in the ocean water into the shallower depths where tropical tuna swim and feed. This “legacy” mercury, they say, could have been emitted years or even decades prior and doesn’t yet reflect the effects of decreasing emissions in the air.

The team performed some mathematical simulations to test this theory. All three models predicted that even the most restrictive emission policy would take 10 to 25 years to influence oceanic mercury concentrations, with drops in tuna mercury levels following decades later.

While the researchers recognize their forecasting does not consider all variables in tuna ecology, they insist their findings point to a need for a worldwide effort to aggressively reduce mercury emissions and a commitment to long-term and continuous mercury monitoring in ocean life.

Reference: Médieu A, Point D, Sonke JE, et al. Stable Tuna Mercury Concentrations since 1971 Illustrate Marine Inertia and the Need for Strong Emission Reductions under the Minamata Convention. Environ. Sci. Technol. Lett. 2024. doi: 10.1021/acs.estlett.3c00949

This article is a rework of a press release issued by the American Chemical Society. Material has been edited for length and content.