Climate change worryingly increases the presence of mercury in fish (and we know it thanks to Michael Phelps)

Eating fish is not only safe, it is also healthy, but will it always be? This is the question asked by a group of researchers from Harvard University, who have studied how overfishing and, above all, global warming, are causing an increase in the levels of mercury present in fish.

The presence of mercury in fish, in the form of methylmercury, is not a recent concern. As explained by the Spanish Agency for Consumption, Food Safety and Nutrition, the toxic effects of mercury have been known since ancient times, although it was not until 1968, as a result of the contamination of Minamata Bay (Japan) by a mercury spill from a chemical industry, when its toxicity was related to the consumption of contaminated fish.

Methylmercury affects the developing central nervous system, hence the fetus and young children are the most sensitive to this metal. Since 1977 there are maximum limits for mercury in fishery products established at the national level. And the European Food Safety Authority itself recommends limiting the consumption of species with a high content of methylmercury - which are, as a general rule, the largest predatory fish: tuna, swordfish, pike, cod ... -, especially pregnant women and kids.

The results reveal an increase of up to 23% in the levels of the metal in the species studied

But these recommendations could get tougher sooner rather than later. According to the new study, warming oceans are leading to an increase in methylmercury in many widely consumed species, including cod, Atlantic bluefin tuna and swordfish.

The investigation, which has just been published in the magazine Nature, analyzes more than 30 years of data on mercury concentrations in the ecosystem of the Gulf of Maine, in the northwestern Atlantic Ocean. The results reveal an increase of up to 23% in the levels of the metal in the species studied - cod and spiny dogfish - between the years 1970 and 2000. But the worst is yet to come.

Tuna is one of the fish that accumulates the most mercury.

It is not easy to calculate the accumulation of mercury

Researchers have developed a new model that simulates how environmental factors, including rising sea temperatures and overfishing, affect methylmercury levels in fish. And his conclusion is not promising. While regulating mercury emissions has successfully lowered methylmercury levels, high temperatures are causing those levels to rise again. Climate change will play an important role in the methylmercury levels of marine life in the future, although it will affect each species differently.

Organisms at the top of the food chain have higher levels of mercury than those at the bottom

“Being able to predict the future of mercury levels in fish is the holy grail of mercury research,” explains Amina Schartup, first author of the article, in the presentation note. "That question has been so difficult to answer because, until now, we didn't have a good understanding of why methylmercury levels were so high in large fish."

Methylmercury has long been known to accumulate through the food chain - organisms at the top of the food chain have higher levels of methylmercury than those at the bottom. But to understand all the factors that influence the process, you must understand how fish live.

These animals do practically nothing other than eat and swim, but within this behavior there are more variables than it seems.

The change of diet in fish, promoted by climate change, has varied the accumulation of the metal in them. In the 1970s, the Gulf of Maine was experiencing a dramatic loss in the herring population due to overfishing. Cod and spiny dogfish, the two species studied, eat herring. Without it, each turned to a different substitute. The cod ate other small fish like shad and sardines, which are low in methylmercury. However, spiny fish substituted herring for foods with higher methylmercury content, such as squid and other cephalopods. When the herring population recovered in 2000, cod returned to a diet high in methylmercury, while spiny dogfish returned to a diet low in methylmercury. Researchers have also discovered that the size of the mouth of each species influences the accumulation of the metal - another variable that must be included in the model.

The hypercalotic diet of Michael Phelps inspired the researchers.

How Phelps inspired the study

Another factor that influences the accumulation of mercury is the calorie expenditure of fish, the variable most closely linked to climate change, which Schartup could not identify until he found inspiration in an unexpected place: the Olympics.

As the waters warm, fish use more energy to swim, which requires more calories

"I was watching the Olympics and the TV commentators were talking about how Michael Phelps consumes 12,000 calories a day during competition," explains Schartup. “I thought, that was six times more calories than I consume. If we were fish, he would be exposed to six times more methylmercury than me. "

And something similar happens among tunas. Large predators and fast-moving fish use more energy, requiring a higher intake of calories, and thus mercury.

"These Michael Phelps-style fish eat a lot more for their size, but because they swim a lot, they don't have compensatory growth to dilute their body load," explains Schartup. "So you can model that as a function."

And this is where warming seawater makes things worse: As the waters warm, fish use more energy to swim, which requires more calories.

Although the presence of mercury decreases, if you increase the temperatures, the concentration of this in the fish will increase.

A future full of mercury

The Gulf of Maine is one of the fastest warming ocean regions. Researchers found that between 2012 and 2017, methylmercury levels in Atlantic bluefin tuna increased 3.5 percent per year despite declining mercury emissions.

Based on their model, the researchers say that a one degree Celsius increase in seawater temperature compared to 2000 will lead to a 32 percent increase in methylmercury levels in cod and a 70 percent increase in the dogfish.

“We have shown that the benefits of reducing mercury emissions are sustained, regardless of what is happening in the ecosystem. But if we want to continue the trend of reducing methylmercury exposure in the future, we need a two-pronged approach, ”explains Elsie Sunderland, co-author of the paper. “Climate change is going to exacerbate human exposure to methylmercury through fish, so to protect ecosystems and human health, we need to regulate both mercury emissions and greenhouse gases. It is also important to remember that fish are a very healthy food in general and when people eliminate fish from their diet, they generally choose less healthy alternatives.

Images | iStock / Nature / Marco Paköeningrat

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