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.