The seals are at rest in June. Life is pretty quiet on the beach, but there are always a lot of invisible things going on.

The beach that was covered in female seals in May isn’t as crowded as it was. There’s plenty of sand between them. Fewer but larger, the subadult males have arrived on the beach. A few mature adult bulls are already on the beach. The rest will return in July and August.

They are on the beach to molt their skin. Their skin peels off once a year, and they spend four to six weeks on the beach looking terrible. Tap a Friends of the Elephant Seal docent, a guide in a blue jacket, and ask to touch some of the shed skin. They carry samples to share with the public.

Although the seals seem oblivious to the visitors who watch them from the boardwalk, they can’t avoid the land-based issues that affect their ocean home. The skin that is peeling off them is heavily contaminated with mercury.

The mercury rained down on the oceans from the air, carried there from coal-burning plants. It’s a heavy metal element, but takes on an organic form in the ocean that enters the food chain easily, methyl mercury. Top predators such as elephant seals biomagnify it as it goes up the food chain. From microscopic plankton to small fish and so on up, it concentrates more at every step. Mercury in their skin and blubber can be a million times more concentrated than the surrounding seawater.

Researchers at Ano Nuevo found the mercury level in the water during the molting season 17 times higher than what it is at other times of the year. That led them to look at where the mercury was coming from: the seals themselves. The molted skin takes with it some of the mercury that the seals have accumulated from the prey they eat in the ocean. Their blood and muscles have higher concentrations of mercury than would damage the nervous system of a human.

Methylmercury is most concentrated in the deep ocean, where elephant seals feed, especially at 1,500 to 2,000 feet. They accumulate more mercury in their blood and muscles than other marine mammals that feed closer to the surface and the continental shelf.

Methyl mercury is a human neurotoxin. It’s difficult to study the effects of mercury in humans. It’s nearly impossible in a wild population. Seals appear unaffected, although it can affect reproduction, development, behavior and nervous system functioning. As yet, no one knows what effect this may be having on the seals.

Patrick Robinson, UC Año Nuevo Island Reserve Director, points out that handling the skin poses no danger to humans. “The take-home is that although they have high levels relative to other critters, it’s not affecting them. It’s high enough that it is measurable compared to surrounding areas.”

Mercury winds up in the ocean from industrial emissions, mostly from coal burning plants. The amount of mercury in the marine environment has increased two- to four-fold over preindustrial levels. Researchers from Harvard University and the U.S. Geological Survey found that the Pacific Ocean’s mercury levels have already risen about 30 percent over the last 20 years. Mercury levels are predicted to rise by 50 percent within the next few decades as emissions from coal-fired power plants and other sources increase.