We know more and more about the impacts of climate change, and a new study published in Nature adds another danger to the list: the warming of the oceans appears to cause an increase in methylmercury, the neurotoxic derivative of mercury that accumulates in some fish consumed by humans. .
“This research represents an important advance to better understand how and why ocean predators are accumulating mercury,” explained Elsie Sunderland, a researcher at the Harvard John A. Paulson School of Engineering and Applied Sciences and one of the authors of the article. . His team has developed a model that simulates the effect of various factors, including rising sea temperatures and overfishing , on the accumulation of methylmercury in various species of fish.
The main conclusion is that, although the regulation of mercury emissions since its toxic effect was discovered has successfully reduced the levels of methylmercury accumulated in fish, the high temperatures are causing a new increase in these levels, and will play a role. An important factor to consider when analyzing the amounts of mercury in marine organisms.
Mercury in fish
To locate ourselves, we must remember that, as Pablo L. Higueras Higueras explains in an article recently published in The Conversation , “part of the mercury that reaches the environment can be transformed into its most toxic compound, methylmercury. This complex of the element is soluble and, were it not for an additional question, it would represent a very limited risk, since it never naturally reaches concentrations high enough to represent a real risk. Except in one case: its entry into our food chain through fish ”. Thus, organisms at the top of the food chain have higher levels of methylmercury than those at the bottom.
Due to this problem, an enormous effort has been made to limit mercury emissions to the environment, and health authorities recommend restricting the consumption of fish of species that are high predators, especially pregnant or lactating women and children under the age of 18. 3 years, due to its neurotoxic effects. It is a delicate balance, since fish is a source of omega-3, a fatty acid that our body is unable to produce on its own and that we must consume in order to have a balanced diet. Some studies, in fact, have shown that the benefits of eating fish outweigh the drawbacks.
Factors that explain the accumulation of mercury in fish
To develop their predictive model, the researchers took into account what the fish eat, how much they eat, and also how much they swim. To do this, they collected and analyzed for 30 years, between 1970 and 2000, the stomach contents of two predators from the Gulf of Maine: the Atlantic cod ( Gadus morhua ) and the spiny or spiny dogfish ( Squalus acanthias ).
The results were curious: in 1970, the levels of methylmercury accumulated in cod were 6 to 20% lower than in 2000, while in spiny shark the levels were 33 to 61% higher. Why did it increase in one species and decrease in another?
The answer lies in the type of diet of these species, since both consume herring. Due to overfishing, herring populations in the Gulf of Maine suffered a drastic decline in the 1970s, forcing their predators to change their diet. The cod turned to other small fish such as sardines, which accumulate little mercury, while the spiny shark began to consume species with higher methylmercury content.
In the year 2000, the herring population had recovered, so the studied species returned to consume them: the cod began, therefore, to feed on a species with more mercury content than those that constituted its previous diet, and the spiny shark the opposite happened to him.
These results also give us an idea that it is necessary to analyze the evolution of the diet of the species studied: the recommendations that we can make regarding the consumption of fish should vary as their diet varies, and we have already seen that this in turn can change in time and space.
The tuna, the mercury … and Michael Phelps
Accumulated levels of mercury in tuna are among the highest of all marine fish, and it appears that their position at the top of the food web is not sufficient to explain this fact.
Amina Schartup, lead author of the research, says she found inspiration while watching the Olympics. “The commentators were counting that Michael Phelps consumes about 12,000 calories a day during the competition, and I thought that multiplies the number of calories that I consume by six. If we were fish, Phelps would be exposed to six times more mercury than me ”, explains the researcher.
This means that migratory fish and hunters that swim at high speeds need much more energy than, for example, scavengers, which requires that they consume more calories, which also does not translate into growth. Because of this, the researchers included this factor as one more function in the model.
Water temperature affects mercury levels
We now come to the effect of the warming of the oceans: as the temperature of the water rises, the fish need more energy to swim, and therefore they consume more calories (and accumulate more mercury).
Data from the Harvard team support this hypothesis: The Gulf of Maine has experienced levels of warming that are among the highest in the world. In parallel, the results of the data collected by the researchers showed that, despite the decrease in mercury emissions, between 2012 and 2017 the levels in Atlantic bluefin tuna increased by 3.5% per year.
According to the model, every 1 ° C increase in seawater temperature compared to 2000 would lead to a 32% increase in methylmercury levels in cod and a 70% increase in spiny sharks.
Several simultaneous scenarios
The statistical model has allowed researchers to simulate several possible scenarios. For example, a 1 ° C increase in seawater temperature and a 20% decrease in mercury emissions result in increases in methylmercury levels of 10% in cod and 20% in spiny shark .
On the other hand, a 1 ° C rise in seawater temperature and a collapse in the herring population result in a 10% decrease in methylmercury levels in cod and a 70% increase in spiny shark. .
A 20% decrease in emissions, with no changes in seawater temperatures, lowers methylmercury levels in both species by 20%.
“This model allows us to see all these different parameters at the same time, just as it happens in the real world,” explained Schartup. “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 this trend of reducing methylmercury exposure in the future, we need a two-pronged approach,” adds Sunderland . “Climate change is going to increase human exposure to methylmercury through their consumption of fish and shellfish, so to protect ecosystems and health, we need to regulate both mercury emissions and greenhouse gases.”
