Mercury Levels in Tuna Aren’t Decreasing

Mercury levels in tuna have remained largely unchanged for over 50 years, a new study has found.

The study, published in the ACS’ Environmental Science & Technology Letters, found that levels of the chemical element haven’t changed in the fish since 1971.

Mercury, or methylmercury, is a toxic chemical found in many fish species that people eat across the world. It is largely built up from fish feeding on prey that is contaminated with the chemical, such as smaller fish or crustaceans. It comes from some natural sources, such as volcanoes and geothermal activity, as well as human-related activities like coal combustion, mining and the incineration of waste.

“This study illustrates that tuna mercury concentrations remain overall stable between 1971 and 2022 globally, except in the northwestern Pacific where they increased significantly in the 1990s,” study author Anaïs Médieu of the French National Research Institute for Sustainable Development told Newsweek.

“This contrasts with the global decline of mercury emissions to the atmosphere since the 1970s, and likely reflects the large amount of legacy mercury, which is historically emitted mercury that has been accumulated over decades or centuries in subsurface and deep oceans,” Médieu said. “Our hypothesis is that, although we reduce mercury emissions in the environment, the surface ocean will still be fuelled by this legacy mercury from subsurface waters, probably for decades.

“This implies that [we] will need to continue, and even accelerate, the global emission reduction efforts initiated by the Minamata Convention, and that we will need to to be patient before seeing a decline of mercury concentrations following emission reduction measures.”

Too much of the chemical can cause mercury poisoning, which can affect the nervous system, sometimes causing problems with fertility and blood pressure, as well as developmental delays in young children.

For this reason, scientists wanted to track how much mercury could be found in tuna, which is one of the most popular seafoods in the world. To do this they looked at data on mercury levels in 3,000 samples from fish caught in the Pacific, Atlantic and Indian Oceans. They specifically looked at skipjack, bigeye and yellowfin tuna as these three tropical species account for 94 percent of global tuna catches. In addition, these species don’t undergo transoceanic migrations, so any contamination found in the fish likely reflects the waters they swim in.

The amount of mercury in these samples remained largely unchanged from 1971 to 2022. However, the researchers also noted that mercury levels decreased in the atmosphere over the same period.

The team theorized that the static mercury levels in tuna may be caused by upward mixing of “legacy” mercury from deeper in ocean water into the shallower depths where tropical tuna swim and feed. The legacy mercury could have been emitted decades previously and does not yet reflect the effects of decreasing emissions in the air.

“The aim of this study is not to say that mercury concentrations in tunas are high or not, but to evaluate if reducing mercury emissions in the environment has induced changes in tuna mercury levels over the past decades,” Médieu said. “Evaluating the risks of methylmercury exposure associated [with] fish consumption is a complicated question because it implies knowing the concentrations in consumed fish, but also the frequency of consumption, as well as the person that is exposed. (Young children and pregnant women are at higher risk). These aspects are not investigated in this study and would therefore need further investigation.”

The researchers found that it would take another 10 to 25 years for airborne emissions to influence the amount of mercury in the oceans. This would then only be seen in tuna populations decades later.

Although further research will be needed, these findings suggest that mercury emissions must be drastically reduced. They also highlight the need for longer-term and continuous monitoring of mercury levels in oceans.