Annual spikes in mercury along the California coastline have been puzzling scientists for over two decades. Now, researchers think they know what's causing these toxic increases: the fur of molting elephant seals.

Researchers have traced methylmercury — a particularly toxic form of mercury — found in the waters around California's coastal Año Nuevo State Park back to elephant seal fur, according to a study published this week in Proceedings of the National Academy of Sciences. If the study's findings are confirmed, it's possible that seals who dwell closer to human food sources are leaching mercury into the environment there, too — but scientists have yet to determine what that could mean for our species.

mercury can cause nervous system disorders, kidney damage, and death

In humans, mercury can cause nervous system disorders, kidney damage, and death. The chemical element can enter water when it's absorbed in the air, or dumped near water sources in industrial areas. When it reaches the ocean, tiny microbes convert it to methylmercury — a neurotoxin that's easily absorbed by the animals that eat those microbes. As it moves up the food chain, it accumulates in increasingly large quantities, which is a huge problem given that mercury doesn't break down easily in the environment. That's why scientists are so concerned with its presence in the ocean; when mercury is out in the open seas, it accumulates in fish that humans like to eat. But today's study shows something different: a mechanism by which methylmercury from the deep ocean makes its way back to shore.

In the study, researchers analyzed seawater taken from the shoreline at Año Nuevo State Park — a park that serves as a breeding and molting ground for elephant seals that feed out in the open seas. The scientists found that during the molting season, methylmercury levels are 17 times higher than those recorded at comparison sites where seals don't gather. When they analyzed samples taken during the breeding season, however, they got a different result: methylmercury levels were only twice as high as those recorded in the comparison sites.

seal feces aren't the problem

The difference between the breeding season and the molting season indicates that the fur — which is full of mercury — is likely the culprit, says Jennifer Cossaboon, an environmental health researcher at San Diego State University and a co-author of the study. Other animals don't tend to hang out in the same area as the seals, so it's probably not coming from other species. And seal feces likely aren't to blame either, because seals at Año Nuevo fast during both seasons — which means they don't poop all that much.

This raises an important question: if the seals aren’t defecating much, what’s causing the mercury increase when they breed? "We speculate that it could be elevated because of the pups that are born then," Cossaboon says. Methylmercury can cross the placental barrier, so seal pups born to contaminated mothers become contaminated in the womb. Scientists know this because when the pups are born, they have high concentrations of methylmercury in their natal coat — a coat that they shed shortly after they're born.

"Really hard to measure health effects or behavior effects in seals."

That’s pretty much all the researchers know at this point. They didn’t measure methylmercury in small shore-dwelling animals so it’s unclear whether the neurotoxin that the seals are "shedding" can get into human food sources. The researchers also aren't sure what the methylmercury does to the elephant seals. "We know what levels indicate that [methylmercury] could have a health effect in humans, but it's really hard to measure health effects or behavior effects in seals, who have a different physiology," Cossaboon says.

Answering these new questions may help scientists get a better handle on the ecosystem effects of human pollution. Mercury is already a problem in some of the fish species that humans eat, and scientists think that's only going to get worse, Cossaboon says. So knowing about seal fur adds another piece to the puzzle. "If seals contain such high concentrations, and they're affecting ecosystems the way that it seems that they are, that's something we should be aware of."

As for how humans should address mercury in the environment, Cossaboon says there's only one solution: "cutting emissions."