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Mercury levels in fish are rising despite a decrease in overall mercury emissions. Getty Images
  • Experts say mercury levels are increasing in many of the fish in our diet.
  • Climate change and overfishing are among the causes as these factors cause larger fish to eat more smaller fish that contain mercury.
  • Health professionals say people should still include some fish in their diet. They also urge consumers to help push action on issues such as climate change.

Despite efforts over the decades to reduce levels of mercury in the air and water, a combination of climate change factors and overfishing has caused mercury levels in many of the fish we eat to continue to rise.

In a new study in the journal Nature, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences and the Harvard T.H. Chan School of Public Health reported that from the 1970s through the 2000s, methylmercury levels in Atlantic cod rose 23 percent due to overfishing.

Their model also predicted a 56 percent increase in mercury levels in Atlantic bluefin tuna from 1969 to now as a result of increased seawater temperatures.

Methylmercury is a powerful neurotoxin that can affect neurological development as well as cause cognitive and neuromuscular damage in adults.

“Consumers should be aware of the risks of eating mercury, especially pregnant women,” Janilyn Hutchings, a food scientist and certified food safety professional at StateFoodSafety, told Healthline. “Mercury poisoning can cause skin disorders, impaired senses, and birth defects. Experts recommend that pregnant women avoid fish that often contain high levels of mercury as well as other foods that could contain mercury.”

The real head-scratcher among these findings is that mercury levels continued to rise while human-made mercury emissions declined by 30 percent from 1990 to 2010, according to a 2016 study.

The reason, the researchers explained, has to do with both how fish eat and how they move.

Fish at the top of the food chain — the ones on the Food and Drug Administration (FDA) do-not-eat list — contain higher levels of mercury due to bioaccumulation.

In other words, they eat a lot of fish with small amounts of mercury in them over the course of a lifetime. Those small amounts accumulate into larger amounts as they eat both greater volumes of fish or subsist on a diet of fish that contain a lot of mercury.

“This doesn’t mean you need to avoid seafood altogether,” said Christopher Shade, PhD, founder and chief executive officer of the nutraceutical detoxification company Quicksilver Scientific. “Because methylmercury biomagnifies in aquatic food chains, eating seafood at the low end of the food chain, such as wild salmon and sardines, significantly limits your mercury exposure.”

Bioaccumulation of mercury isn’t a straight line from the bottom of the food chain to the top. It depends on what a particular fish species eats.

Overfishing has impacted mercury levels in certain fish due to changes in their overall diets. Because when one of the fish they eat is overfished, they may then switch to a different diet of fish that are either higher or lower in mercury content, thereby raising or lowering their accumulated mercury.

Hence, Atlantic cod had high levels of mercury until their primary food source, herring, were overfished. This caused cod’s mercury levels to drop. When the herring population came back, so did the cod’s mercury content.

Where climate change factors into the equation is much simpler.

Fish have to swim harder in warmer water. So as water temperature rises, fish have to consume more calories.

And more calories means more bioaccumulation of mercury among bigger fish as they eat more food to survive.

“Climate change is going to exacerbate human exposure to methylmercury through seafood, so to protect ecosystems and human health, we need to regulate both mercury emissions and greenhouse gases,” Elsie Sunderland, PhD, lead study author and a professor of environmental science and engineering at the Harvard T.H. Chan School of Public Health, told the Harvard Gazette.

“It is important also to remember that fish is a very healthy food overall and when people switch away from fish in their diet, they generally pick less healthy alternatives. We can all agree less methylmercury in these fish in the future would be a good thing,” Sunderland said.

“I think that it is important for consumers of fish not to panic over this information,” Shawn Gerstenberger, PhD, dean of the University of Nevada Las Vegas School of the Community Health Sciences, told Healthline. “We are aware that fish are an excellent source of protein and omega-3 fatty acids, which are well known to be good for you and can play a critical role as one part of a healthy diet.”

“Many of the health issues or concerns arise when people focus on one type of food and fail to eat a balanced diet,” he noted.

So most fish are not off-limits, but experts say if you want to keep enjoying fish such as tuna and cod, you can lobby for tighter regulations on fishing, action to stop climate change, and efforts to reduce pollution.

“Government bodies may choose to revise legislation to help with overfishing or limit the amount of mercury allowed in safe fish,” Hutchings said. “In addition, everyday consumers can help prevent pollution from getting into the ocean, which will help lower the amount of chemicals and toxins that the fish could potentially eat.”