Enlarge image

Tuna in a port of Spain

Photo: Carlos Castro / EUROPA PRESS / dpa

The mercury emitted worldwide was absorbed into the oceans over the centuries, where it accumulated in marine animals through the food chain - and ultimately ends up on our plates in tuna, for example. Although mercury pollution from coal burning or mining has recently fallen significantly, tuna is still almost as contaminated with the toxic heavy metal as it was in the early 1970s.

A research group explains in the journal “Environmental Science & Technology Letters” that there is still a lot of mercury deposited at depths of more than 50 meters below the ocean surface, which apparently repeatedly finds its way into the food chain. Tuna is one of the most popular seafood worldwide.

Mercury is a liquid metal that evaporates in small quantities at room temperature. In addition to the elemental form, it occurs in two other forms: as inorganic and organic mercury compounds. Mercury is difficult to excrete and accumulates: If a predatory fish such as tuna regularly eats animals with an increased concentration of mercury, the mercury accumulates in its muscles. Long-lived predatory fish such as halibut, swordfish and tuna are particularly affected.

In Europe, North America and the states of the former Soviet Union in particular, significantly less mercury has been emitted since 1970, write researchers led by Anaïs Médieu from the French Université de Bretagne Occidentale. They analyzed existing mercury measurements in the three species of tuna, which account for about 94 percent of the world's tuna catch. Together with our own current studies, they covered a period from 1971 to 2022.

Result: On average across the three species, the different oceans and the five decades examined, the mercury concentration was around one microgram (millionth of a gram) per gram of fish, despite major fluctuations.

The fact that mercury is probably the problem in water depths of more than 50 meters is shown by the values ​​for bigeye tuna in the southwestern Pacific: the animals are two to three times more contaminated than average - probably because the zone is here of the warm surface water extends to greater depths and the tuna hunt correspondingly deeper. This means that the mercury from greater depths stays in the food cycle.

Main source sea fish and amalgam dental fillings

Methylmercury, a highly toxic organic mercury compound, can damage the central nervous system, especially in unborn children, according to the Federal Environment Agency. Pregnant women are advised not to eat tuna. Infants and small children are also particularly at risk from neurotoxic effects because their development is not yet complete and their nervous tissue is therefore particularly vulnerable.

Possible neurological damage from high levels of stress includes problems with motor skills, speech problems, memory problems and other cognitive impairments. Overall, according to the Federal Environment Agency in Germany, mercury pollution is harmless to health. In addition to sea fish, amalgam dental fillings are a major source of mercury in the human body.