Some coastal wolves in Alaska, U.S., have toxic levels of mercury in their bodies after shifting from a terrestrial diet of deer and moose to a marine diet heavy with sea otters, new research finds.
Mercury is a naturally occurring heavy metal found in the Earth’s crust. However, human activities like burning coal and fossil fuels release mercury into the atmosphere, where it can travel hundreds of miles from its source. When mercury enters aquatic ecosystems, it’s converted into methylmercury, a potent neurotoxin that “moves efficiently through a food web,” Ben Barst, study co-author and assistant professor with the University of Calgary, Canada, told Mongabay in a video call.
Methylmercury “biomagnifies,” accumulating in larger amounts higher up the food chain, making it dangerous for predators like wolves and sea otters. Large sea otters (Enhydra lutris) daily eat roughly 11 kilograms (25 pounds) of invertebrates like mussels, clams and sea urchins, all known to accumulate methylmercury.
Gretchen Roffler, the study’s lead author and a research biologist with the Alaska Department of Fish and Game, first learned of mercury poisoning in wolves (Canis lupus) when she investigated the death of an emaciated collared wolf from another study. Roffler’s tests revealed “unprecedented” levels of mercury in the animal’s liver. So, she sent samples to Barst’s lab for further testing.
The mercury concentration in those samples were so high, “at first we thought the instrument was malfunctioning,” Barst said.
Once mercury levels were confirmed — on par with those observed in polar bears, an apex marine predator — the researchers wanted to know how widespread mercury poisoning is in wolves and where the mercury came from.
They examined archived tissue samples starting from the year 2000, along with samples from recently trapped wolves and hair and blood samples from collared wolves.
They looked at two different wolf packs: one on Pleasant Island in coastal Alaksa and another a mile away on mainland Gustavus Forelands. The island pack moved there in 2013 and within a few years wiped out the island deer population. Instead of swimming back to the mainland they stayed and “switched to a very marine-heavy diet dominated by sea otters; up to about 70% of their diet is sea otters,” Roffler told Mongabay in a video call.
The mainland Gustavus pack, despite having access to deer and moose, also began eating more sea otters around the same time.
Roffler said she believes both packs likely made the switch because sea otters, which are easier and safer to kill than a large moose, have recently become very abundant. Once decimated by the fur trade, sea otter populations have rebounded, thanks to conservation efforts including protection under the Marine Mammal Protection Act. They are now “super abundant wolf prey,” Roffler said.
“We expect this to be a broader geographic trend across the former range of sea otters as they recolonize,” she said.
Banner image: of a wolf, courtesy of U.S. National Park Service.
