Toxic mercury in the open ocean is increasingly showing up in coastal areas and are brought there from an unexpected source, researchers say: the fur of sea lions and seals.

Absorbed into ocean water from air pollution or dumped as waste from industrial processes, mercury can move up the marine food chain to end up in fish-eating predators at the apex of the chain including marine mammals such as seals, researchers point out.

Researchers at the University of California, Santa Cruz have found that elephant seals release significant amounts of mercury during molting, the shedding of their fur.

Elevated concentrations of toxic mercury have been detected in coastal waters near an elephant seal rookery at Año Nuevo State Reserve on the central California coast, they report in the Proceedings of the National Academy of Sciences.

Mercury that enters the ocean is of particular concern, scientists explain, because bacteria in seawater can convert it into a form known as methylmercury, a neurotoxin that can cause brain damage if consumed by humans.

As early as the 1980s, some researchers suggested marine mammals could act as a source of methylmercury after noticing elevated levels of mercury in mussels at the Ańo Nuevo reserve, a known breeding ground of several species of sea lions and seals.

"At the time, [they] speculated that all the biological material coming from those animals was basically pulsing mercury into the system," says lead author Jennifer Cossaboon, who led the study as a undergraduate at UC Santa Cruz. She is now a graduate student in environmental science at San Diego State University.

However, technology sensitive enough to test that theory didn't exist back then, the researchers say.

New techniques have allowed Cossaboon and colleagues to test mercury concentrations in both seawater and the fur of elephant seals that reside in the Ańo Nuevo area.

Methylmercury levels in the seawater there were as much as eight times higher than the average for other sites along the California coast, suggesting the seals and sea lions could have something to do with it, they report.

Even more telling was the discovery of a spike in those levels during elephant seal molting season.

One of the ways mercury can work its way out of an animal's body is through fur, Cossaboon says.

Mercury in all its forms — but particularly as methylmercury — poses a significant environmental threat, she says.

"Mercury is an element, so it never breaks down and goes away — it just changes forms," she says.