The genus of fish that we popularly know as tuna (which includes a dozen species) is one of the most popular foods of marine origin and most consumed by humans in various parts of the planet.
Tuna is rich in protein and is recognized for its flavor and texture, but for decades we have known about the risks that high consumption of this type of product can entail because it sometimes presents high levels of methylmercury, the most common form of organic mercury found in nature. Mercury is present in the tuna we consume because these fish are predators that feed on smaller fish or crustaceans that in turn accumulate these types of contaminants.
The approval and implementation, over the last decades, of national and international regulations to reduce the emission of mercury pollutants into the environment has so far had no direct effects on the expected reduction in the accumulation of methylmercury in tuna. In other words, the tuna that live today accumulate the same amounts of mercury as those caught 50 years ago, when there were practically no regulations to control this dangerous contaminant, according to a study whose results have been published (February 21) in Environmental Science Magazine
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 fetuses and young children are at greatest risk of harm. Methylmercury is a particularly toxic chemical that affects the nervous system and is expected to be the main form of mercury in reference to tuna contamination. For this reason, the authors of the new study set out to determine whether lower atmospheric emissions resulted in lower concentrations of mercury in the oceans, specifically methylmercury found in food sources that are at the top of the food chain, such as the tuna.
Anne Lorrain, Anaïs Médieu and David Point worked with an international team of researchers to investigate 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 caught in the Pacific, Atlantic and Indian Oceans between 1971 and 2022. They specifically examined tropical tuna: skipjack tuna (Katsuwonus pelamis), bigeye tuna (Thunnus obesus) and yellowfin (Thunnus albacares). These three species represent 94% of the world’s tuna catches. Because they do not undergo transoceanic migrations, any contamination found in the animals’ muscles is likely a sign of contamination in the waters in which they swim.
After standardizing the data to allow comparison across decades and regions, the researchers observed stable mercury concentrations in tuna around the world from 1971 to 2022, except for an increase in the northwest Pacific Ocean in the late 1990s. However, during the same period mercury in the air decreased globally. The team theorized that the static levels in tuna may be caused by an upward mixing of “legacy” mercury from the depths of ocean water to the shallower depths where tropical tunas swim and feed. Legacy mercury could have been emitted years or even decades earlier and does not yet reflect the effects of declining air emissions.
The researchers’ mathematical models simulating three progressively more restrictive environmental policies support their theory. Models predict that even the most restrictive emissions policy would take 10 to 25 years to influence ocean mercury concentrations, followed by declines in tuna decades later. While the researchers acknowledge that their forecasts do not consider all variables in tuna ecology or marine biogeochemistry, they say their findings point to the need for a global effort to more aggressively reduce mercury emissions and a commitment to Continuous, long-term monitoring of mercury in ocean life. .
In statements collected by SMC, Joan O. Grimalt, research professor at the CSIC, Department of Environmental Chemistry at the Institute of Environmental Diagnosis and Water Studies (IDAEA-CSIC), indicates that the now presented “is an interesting study that shows that Once the sea is contaminated, recovery to previous levels is not so simple. “The sea has a great storage capacity for pollution, therefore, we can use it as a landfill without it being very noticeable a priori. However, precisely because of the large amount of pollutants it can store, when we want to go back, it is not so simple,” says Professor Joan Grimalt.
Diego Romero, professor in the Toxicology Area of the Veterinary Faculty of the University of Murcia, highlights in statements published by SMC that “the study by Médieu and collaborators shows the evolution of mercury concentration in some species of tuna in more than five decades. It is a solid work that has involved the review of numerous previous investigations and, in conclusion, calls on the authorities to take measures to continue reducing mercury concentrations in marine organisms. If achieved, this reduction will undoubtedly contribute to improve the health of ecosystems and that of human beings”.
“One of the most important reflections that can be read in the article refers to future changes in the climate, since as a consequence of this the kinetics of methylmercury in food webs can be modified. Therefore, it is essential to continue studying the characteristics hydrodynamics and mercury concentrations in tunas and other fish species, but not only at the muscle level, but in all tissues, to be able to know the bioaccumulation and biomagnification in important tissues and organs such as the liver, kidney or brain”, explains Professor Diego Romero.