On the small Russian island of Mednyi in the North Pacific Ocean, where the foxes survive almost exclusively on sea birds and seal carcasses, researchers said they believe mercury has played a key role in the decline, the BBC reported Tuesday.
The fox population on the island dropped in the 1970s and many of the surviving animals are in poor condition and have low body weight, researchers said.
Writing in the journal PLoS One, the researchers said hair samples from the foxes and the food they eat showed significant levels of mercury, they said.
"They have high levels, compatible with the food, and it could explain the state of the foxes there," said Gabor Czirjak from the Leibniz Institute for Zoo and Wildlife Research.
"We know it is in the marine environment and it is causing exactly the type of symptoms that were found in this population," he said.
A comparison with fox populations that live inland, that survive on non-marine birds and rodents, found much lower levels of mercury in the inland animals, researchers said.