Large fish contain more and more methylmercury, a compound toxic to humans when consumed in high doses. This is due in particular to global warming.
Methylmercury is the most toxic organic form of mercury. At high doses, it is harmful to the central nervous system of humans, in particular during its development in utero and during early childhood. Men are mainly exposed to it by eating fish and seafood. And according to an American study published in the journal Naturethe level of this toxic compound increases worryingly in fish such as cod or even tuna Atlantic red, particularly due to global warming.
To reach these conclusions, the researchers analyzed the equivalent of 30 years of data on the ecosystem of the Gulf of Maine in the Atlantic Ocean. They first looked at the diets of two marine predators, Atlantic cod and spiny dogfish, from 1970 to 2000. For cod, methylmercury levels had increased by 6 to 20% since 1970. On the contrary, levels of this toxic compound have dropped by 33 to 61 percent in spiny dogfish, the scientists observed.
Because in the 1970s, the population of herring, their favorite prey, declined considerably in the Gulf of Maine due to overfishing. As a result, cod began to prey primarily on shad and sardines, smaller fish that generally have very low concentrations of methylmercury. So his levels went down. At the same time, spiny dogfish turned to squid, which themselves predators, have much higher levels of methylmercury in their blood than herring. Logically, this increased the presence of the toxic compound in dogfish. But in the 2000s, the herring population returned to normal and gradually, methylmercury concentrations in cod increased again, while those in dogfish decreased.
The Gulf of Maine is warming abnormally fast
But changes in “available food” are not the only factors influencing the levels of toxic compounds found in large fish, say the researchers. The ton, for example, is a migratory species that swims very quickly. It therefore uses a lot of energy and needs to eat more to maintain speed and agility.
Finally, there is another key factor: global warming. However, according to researchers, the Gulf of Maine is one of the fastest warming water bodies in the world. “The Gulf Stream’s northward migration and decadal oscillations in ocean circulation have resulted in unprecedented warming of seawater in the Gulf of Maine between 1969 and 2015, placing this region in the top 1% of documented seawater temperature anomalies”, detail the researchers.
And the warmer the water, the more energy fish need to swim, which means they eat more small fish and ingest more methylmercury. Between 2012 and 2017, researchers found that the Atlantic red tone increased in methylmercury levels by 3.5% each year.
“We need to regulate both mercury emissions and greenhouse gases”
On the strength of these results, the researchers were able to create a model predicting the increase in methylmercury levels in sea fish: for a spiny dogfish weighing 5 kg, a 1°C increase in sea water could lead to a “70% increase in tissue concentrations (methylmercury)”. For cod, it would be 32%. “Being able to predict the future of mercury levels in fish is the holy grail of mercury research. It has been so difficult to answer this question because, until now, we did not fully understand why the of methylmercury were so high in the big fish,” says Amina Schartup.
“If we want to continue to reduce methylmercury exposure in the future, we need a two-pronged approach,” adds Professor Elsie Sunderland, senior author of the study. “Climate change will exacerbate human exposure to methylmercury from seafood, so to protect ecosystems and human health, we need to regulate both mercury emissions and greenhouse gases,” warns- she.
Limited fish consumption for pregnant women and infants
In France, the National Health Security Agency (Anses) has looked into the subject several times since 2002. At present, it considers that “the consumption of fish does not present a health risk with regard to the risk from methylmercury”. “Indeed, the population’s intake of methylmercury is below the tolerable daily intakedefined by the World Health Organization, she says on her website.
Thus, in view of the various advantages linked to the consumption of fish, it encourages eating oily fish twice a week and diversifying the species consumed. However, it recommends that pregnant and breastfeeding women and children under 30 months avoid the most contaminated fish such as shark, swordfish or marlin. It is also suggested that they limit the consumption of fish “likely to be highly contaminated” to “150 g per week for pregnant and breastfeeding women and 60 g per week for children under 30 months”.
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