Mercury levels in seafood found to be affected by climate change and ecosystem shifts

Mercury levels in seafood can shift as fish seek new sources of prey and as water temperatures warm due to climate change, according to a recent study.

The new study, published in Nature, illustrates that even as global human-driven mercury emissions are declining, warming oceans and shifting predator-prey relationships caused by human fishing practices could still be major drivers of mercury in seafood. 

Most – 80 percent – of the inorganic mercury (Hg) that is emitted into the atmosphere ends up in the oceans. Microorganisms then convert some of that mercury into methylmercury (MeHg), which is amplified in predatory fish higher up the food chain in a process called bioaccumulation. 

The study looked at concentrations of methylmercury in several marine species in the Gulf of Maine, including Atlantic bluefin tuna, cod, and spiny dogfish.

The study’s model estimates that rising water temperatures on their own would have led to a 56 percent increase in methylmercury concentrations in the tissue of Atlantic bluefin tuna between 1969 and peak levels in 2017. When accounting for both water temperature increases and reductions in atmospheric mercury emissions, the model estimated a 27 percent rise.

“We may be doing a good job at reducing mercury emissions and pushing those emissions down, but a lot of other environmental changes that are happening are compensating for that,” lead author Amina Schartup, a former research associate at Harvard University, where she conducted the study, told SeafoodSource. “We need to pay attention to other environmental factors that are changing at the same time.”

But people shouldn’t be deterred from eating fish and reaping the nutritional benefits, Schartup said. 

“When people switch away from fish to something else, they tend to pick something that is less healthy,” she said.

Consumers worried about mercury levels in their seafood should be confident that seafood remains safe to eat, added Gavin Gibbons, spokesman for the National Fisheries Institute. 

“Mercury does bioaccumulate, so you’ll see more of it in species at the top of the food chain. But that doesn’t mean people should stop eating them,” Gavin Gibbons, a spokesman for the National Fisheries Institute, told SeafoodSource. “In fact, many of those same species are also high in essential nutrients like omega-3s. Cutting back on protein, vitamin D, and omega-3s is definitely not what the doctor ordered.” 

Gibbons noted that the amounts of mercury found in fish are quite small, according to the Food and Drug Administration, with levels for cod averaging 0.111 parts per million. The FDA’s safety threshold is 10 times higher, at one part per million. The study doesn’t address human health, or mercury in humans, Gibbons added.

David Schalit, president of the American Bluefin Tuna Association, argued that concerns about the negative health effects of methylmercury in seafood are invalidated by the latest science. 

“The methylmercury issue has been weaponized and used as a tool to reduce the demand for seafood,” he told SeafoodSource.

The study, which focuses on the forces that cause methylmercury levels to rise and fall in marine creatures, stemmed from a paradox: Even though mercury emissions declined in the Northeast U.S. in recent decades, trends in levels of methylmercury in fish didn’t match. Depending on the species and data source, methylmercury levels were observed to move in all directions.

To figure out why, and to parse the source of changing methylmercury levels, Schartup created a mathematical model of the marine food web.

It turns out that diet played a key role for the differences between cod and spiny dogfish. For cod, methylmercury levels were modeled to be lower in 1970 than in 2000, yet in dogfish they were higher in 1970 than in 2000. The reason, Schartup found, was that the two species chose different substitutes after the collapse of herring populations in the 1970s forced them to find new prey. 

“So you end up with an increase in one predator and a decrease in the other predator based on what they pick,” Schartup said. 

Seawater temperature was also found to affect methylmercury levels in fish.  

Fish that burn more energy – such as tuna, who are high-speed hunters – develop higher concentrations of methylmercury because they’re processing more calories. In a similar fashion, warmer waters cause all marine creatures’ metabolism to quicken, requiring the whole food web to burn more energy, Schartup said. The effect is particularly pronounced in the Gulf of Maine, which is warming faster than almost anywhere else.

Between 2012 and 2017, levels of methylmercury in Atlantic bluefin tuna rose 3.5 percent per year, the study found – a sign of the coming effects of warming waters. In addition, the study’s model predicts that every increase of one degree Celsius in seawater temperature will lead to a 32 percent increase in methylmercury levels in cod and a 70 percent increase in methylmercury levels in spiny dogfish. 

Photo courtesy of Krasowit/Shutterstock