Mercury levels in tuna remain almost unchanged since 1971, study finds

Tuna is one of the most popular seafood in the world. But this protein-rich fish can accumulate high levels of methylmercury by feeding on contaminated prey, like smaller fish or shellfish. Despite efforts to reduce mercury emissions into the environment, researchers report in ACS’ Letters on environmental science and technology that levels in tuna appear to be unchanged since 1971. They warn that more aggressive emissions reduction targets are needed to begin to bring down mercury levels in tuna.

Environmental protection policies have helped reduce mercury pollution from human activities such as coal burning and mining around the world. However, people can still be exposed to methylmercury, and unborn babies and young children are most at risk.

Methylmercury is a particularly toxic chemical that affects the nervous system and is expected to be the primary form of mercury in tuna contamination. The researchers therefore sought to determine whether declining air emissions led to lower concentrations of mercury in the oceans, particularly methylmercury found in food sources at the top of the food chain, such as tuna.

Anne Lorrain, Anaïs Médieu and David Point worked with an international team of researchers to study mercury trends in tuna over the past 50 years. They also wanted to simulate the impact of different environmental policies on mercury levels in the oceans and in tuna in the future.

The researchers compiled previously published data and their own data on total mercury levels from nearly 3,000 tuna muscle samples from fish caught in the Pacific, Atlantic and Indian Oceans from 1971 to 2022.

They specifically studied tropical tuna: skipjack, bigeye and yellowfin tuna. These three species represent 94% of global tuna catches. Because they do not undergo transoceanic migrations, any contamination found in the animals’ muscles likely reflects the waters in which they swim.

After standardizing the data to allow comparison across decades and regions, the researchers observed stable concentrations of mercury in tuna worldwide from 1971 to 2022, except for an increase in the northwest region. Pacific Ocean in the late 1990s. However, during the same period, airborne mercury declined globally.

The team hypothesized that the static levels in tuna could be caused by an upward mixing of “legacy” mercury from the depths of ocean water into the shallower depths where tropical tuna swim and nurse. nourish. Existing mercury could have been emitted years or even decades ago and does not yet reflect the effects of reduced air emissions.

The researchers’ mathematical models that simulate three increasingly restrictive environmental policies support their theory. Models predict that even the most restrictive emissions policy would take 10 to 25 years to influence mercury concentrations in the oceans, and that a decline in tuna emissions would follow decades later.

Although the researchers acknowledge that their predictions do not take into account all the variables of tuna ecology or marine biogeochemistry, they say their results highlight the need for a global effort to more aggressively reduce carbon emissions. mercury and a commitment to continued, long-term monitoring of mercury in ocean life. .