For decades, less mercury has been falling out of our skies and into our oceans. That's because coal-fired utilities, waste incinerators, mercury dumping and commercial products contaminated with mercury have all been declining in North America and Europe since the 1970s. The result was that, in general, marine life carried less of the toxic metal in their bodies year after year.
But now that trend might be reversing, according to a new study published in the journal Nature. The work suggests that warming seawater temperatures and changing ocean conditions might be causing toxic mercury concentrations to rise in many fish species including Atlantic bluefin tuna, cod and spiny dogfish.
“It’s a great and important piece of work,” says biologist Celia Chen, director of the Toxic Metals Superfund Research Program at Dartmouth University, who did not work on the study. “The takeaway is even if we don’t reduce [mercury emissions], we are going to be ingesting more mercury in the future if we look at just temperature alone.”
To conduct the study, researchers built a computer simulation that mimics the different fish species. “It’s a virtual fish,” says Amina Schartup, an environmental health scientist at Harvard University and the Scripps Institute of Oceanography and lead author on the study.
The simulation models how different fish grow, their behavior in different ocean temperatures, and what they eat based on their environment. “We laid on top of that how mercury is accumulating in each fish as they grow and eat the mercury from their prey at the same time,” Schartup says. “It’s all mathematical. It’s kind of fun.”
Then, the researchers started changing the seawater temperature in the model, and Schartup’s virtual fish began displaying much higher concentrations of mercury in their bodies. “That’s how we realized, wait a second, temperature has a pretty large impact on mercury levels in fish,” Schartup says. “When you warm them up, they eat more.”
As the virtual fish ate more, they also ingested more mercury. “But they don’t grow fast enough to make up for the additional mercury they are consuming,” Schartup says.
It’s a subtle increase, she says, but it quickly adds up across the food web to predatory fish like tuna. For every 1 degree Celsius in warming, the model suggested cod would see toxic mercury concentrations rise by 30%, and dogfish would see an increase of 70%, if only temperature is taken into account, Schartup says.
By 2030, the model suggested rising temperatures could cause mercury concentrations to rise 30% in Atlantic bluefin tuna, practically reversing the drop in toxic mercury in this fish over the last 30 years. When the team sampled Atlantic bluefin tuna in 2017, they found the average toxic mercury concentration was 809 micrograms of mercury per kilogram of tissue. The Environmental Protection Agency recommends that people avoid eating fish containing over 460 micrograms of mercury per kilogram of tissue.
Cod contain far less mercury than bluefin tuna, which are a vulnerable species. On average, cod contain 110 micrograms of mercury per kilogram of tissue and are considered a "best choice" for fish consumption by the EPA.
The study looked exclusively at cod, dogfish and Atlantic bluefin tuna, but any fish would react similarly to ocean warming, says John Reinfelder, an environmental scientist at Rutgers University who was not involved in the study. “In theory,” he says, “this is potentially very profound considering the climate change we are experiencing.”
Reinfelder cautions that only future observations can confirm if the simulation's predictions are correct. "We'll have to wait and see," he says. "But the finding needs to be taken very seriously."
There are some things that the model – and ocean science in general – still have not addressed, Reinfelder adds. For one, ocean bacteria both convert mercury from human sources into methylmercury, the element’s toxic form, and degrade that toxic mercury into safer forms. Global warming might increase the rate at which bacteria create methylmercury from inorganic mercury, but it might also increase the rate bacteria break down toxic mercury. “We don’t fully understand that balance,” he says.
It’s also unclear how shifts in what fish are eating might change mercury levels for many species in the future, Reinfelder says. Schartup’s model showed that the availability of herring, a favorite meal for cod and dogfish, also affected their mercury levels.
After the collapse of the Atlantic herring population after the 1970s, dogfish switched to eating squid and cephalopods, which are higher in mercury, while cod began eating smaller fish that were lower in mercury. When the herring population recovered, those effects bounced back.
The results don't mean that people should not eat fish, but it may mean that consumers will have to be more careful in the future, Reinfelder says. Mercury is toxic to the human nervous system and can damage the development of fetuses and very young children.
“Fish is still good. It’s a very healthy food,” he says. “But this raises the specter that maybe the low mercury fish won’t be so low. Fish on low-moderate mercury could shift up a notch to a ‘restrict your diet’ list or ‘do not eat’ list. That would be bad for many reasons.”
That scenario could become even more likely if mercury rules relax in the United States, according to Chen, the toxic metals expert from Dartmouth. In recent years, mercury emissions have remained relatively stable, as coal-fired utilities have declined in North America and Europe but increased in Asia. But, she says, recent proposals from the Trump administration may weaken U.S. regulations on mercury. If passed, that could lead to more mercury in the environment.
“If we don’t reduce mercury emissions," Chen says, "we are going to end up having more mercury in our fish.”