Climate Change Could Expose More Humans to Mercury in Seafood

The levels of harmful methylmercury (MeHg) found in fish may have spiked over the past three decades. That's according to scientists, who believe global warming could be to blame.

Researchers created models to estimate that between the 1970s and 2000s, the levels of the substance in Atlantic bluefin tuna increased by 56 percent; and 23 percent in Atlantic cod. The authors of the paper published in the journal Nature pinned the climb to rising seawater temperatures after 1969.

The toxin can be passed on from a mother to her fetus and has been linked to long-term neurocognitive problems in children, which can last into adulthood.

In the U.S., 82 percent of the MeHg that people are exposed to comes from seafood in the diet. As much as 40 percent originates in fresh or canned tuna.

MeHg occurs when microscopic organisms in soil and water combine mercury with carbon. This can enter the food chain, with predators at the top accumulating higher concentrations than other animals.

The researchers looked at data on the ecosystem and MeHg levels of water, sediment, and animals in the Gulf of Maine in the northwestern Atlantic Ocean. The stretch off the east coast of North America has been fished commercially for over 200 years.

The industry has lead to bigger populations of herring, lobster, and cod. This, in turn, has changed the shape of food webs, and the prey that predators can feast on.

Over-fishing of herring led to cod eating fewer small clupeids (the ray-finned fish family herrings are a part of) and more big herring, lobster, and macroinvertebrates like insects, worms and snails in the 2000s compared to the 1970s.

sushi, maki, japanese, stock, getty, food
A stock image of sushi, including tuna, salmon, shrimp, and crab. Methylmercury is finding its way in more cod, Atlantic bluefin tuna and swordfish thanks to climate change, scientists believe. Getty

Highlighting how changes to the availability of food affects species differently, the spiny dogfish ate more squid and cephalopods when there was a shortage of herring. Concentrations of MeHg in these fish was higher in the 1970s than 30 years later, the scientists believe.

Meanwhile, the Gulf Stream has moved northward and changes to ocean circulation have caused seawater to warm. The authors think this has lead to high levels of MeHg in fish who live in freshwater and estuaries.

The findings could have implications for the three billion people who eat seafood worldwide, they said.

Senior author Elsie Sunderland, professor of environmental chemistry at Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Harvard T. H. Chan School of Public Health (HSPH) commented in a statement: "This research is a major advance in understanding how and why ocean predators, such as tuna and swordfish, are accumulating mercury.

Their model also estimated what could happen to MeHg levels in cod and spiny dogfish if seawater warmed, and levels of MeHg if seawater fell.

A rise in 1 C was predicted to cause a 32 percent rise in concentrations of MeHg in cod, and 70 increase in spiny dogfish.

Co-author Amina Schartup, former research associate at SEAS and HSPH, commented: "Being able to predict the future of mercury levels in fish is the holy grail of mercury research.

"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 big fish."

Dr. Oliver Jones, Associate Professor of Chemistry, RMIT University in Melbourne, Australia, said: "There are some limitations to the study in that it reuses published (not new) data for the Gulf of Maine, so the conclusions may not hold true for other areas. A lot of data in the models (which are themselves very complex) are estimated or extrapolated so there is likely to be some error in the final result.

"Nevertheless," she said, "the work is clever and detailed, and shows the importance of fully characterizing natural systems before you attempt to understand them."

Professor Andy Smith, senior scientist at the U.K.'s Medical Research Council Toxicology Unit, who was also not involved in the work, said: "The conclusions are based on modeling with extrapolations of incomplete data sets and restricted to the Gulf of Maine. Although it is not possible to see immediate implications for human consumers, it underlines the importance of continuing pressure to decrease global exposure to environmental mercury."

Addressing the potential harms, Elizabeth Lund, an independent consultant in nutrition and gastrointestinal health, said: "Although methylmercury in fish is a potential cause for concern current evidence points toward the benefits of fish consumption, including in pregnancy, outweighs any risk.

"However pregnant and nursing women should heed current guidelines to avoid certain species of fish high up the food chain such as swordfish. The main point of this paper seems to be to say the current regulations should not be relaxed or levels may go up."