Increased neurotoxins in seafood are yet another risk identified by researchers at Harvard along with the existing risk of the spread of tropical diseases northward, and the loss of crucial nutrients in crops as scientists continue to reveal different other ways climate change poses a threat to human health.
The commonly eaten fish such as tuna, cod, and dogfish contains methylmercury in increasing amount because of overfishing and rising temperature of the oceans, according to a new study, published in the journal Nature. There are government regulations reducing mercury emissions from industry seeping into the water over the decades.
“You would expect that as mercury was reduced in seawater, then it would go down in all the fish,” lead author Amina Schartup, who was a research associate at the Harvard School of Engineering and Applied Sciences, told NBC News. “But there was variability. We were trying to tease out the different factors contributing to those differences.”
Methylmercury, as the Centers for Disease Control and Prevention warns, is a harmful toxin that affects the heart and blood vessel system, disturbs immune function, and mainly causes damages to the nervous systems of pregnant women and their babies.
The 30 years of data from the Gulf of Maine were used by the researchers for the study, and they developed a food web model to show how mercury levels in certain fish are affected by warming waters, overfishing, and other environmental factors.
The overfishing in the Gulf of Maine and especially of herring by the 1970s forced the herring predators such as cod and dogfish to look for new prey. Cod switched to eating smaller fish, and the dogfish started to eat squid and other cephalopods containing a high level of mercury.
The mercury levels in dogfish increased highly, but it dipped in cod. With the rebound of fishery stocks, both the fish started eating mostly herring. After switching back, there was a 23 percent mercury increase in cod while the mercury levels got lowered in dogfish. The model predicts that with the additional 1 degree Celsius warming of the oceans over the 2000 temperature, mercury levels would jump by 70 percent in dogfish and 32 percent in cod, The Harvard Gazette reported.
The researchers also examined what increases the mercury levels of Atlantic bluefin tuna as there was no change in prey and mercury emissions from humans were also decreasing.
The model showed that in the 1990s, decrease in mercury levels in the tuna cooled the water temperatures in the Gulf of Maine temporarily. According to The Harvard Gazette, oceans in the Gulf of Maine are warming faster than most places as climate change has steadily been warming the ocean. As the temperatures increased between 2012 and 2017, mercury levels in tuna rose 3.5 percent every year, and it could probably rise by 30 percent over 2015 levels by 2030.
Two reasons were identified behind this by the research team.
Firstly, tuna are higher on the food chain and use more energy compared to other fish, so they eat more prey that already contains mercury, Schartup said. Secondly, fish need to use more power to swim, which, in turn, increase their calorie intake that warms the waters. As the model suggested, in total, the warming of their habitat increased the mercury levels in tuna by 27 percent.
However, the researchers do not have any intentions to scare people away from seafood.
“It’s not that everyone should be terrified after reading our paper and stop eating seafood, which is very healthy, nutritious food,” researcher Elsie Sunderland, a professor of environmental chemistry at Harvard University, told Al Jazeera. “We wanted to show people that [climate change] can have a direct impact on what you’re eating today, that these things can affect your health … not just things like severe weather and flooding and sea-level rise.”