When wolves on Pleasant Island, off Alaska’s southeastern coast, arrived in 2013, they quickly exhausted the local deer population. Rather than return to the mainland, they adapted. Their new prey: sea otters. Up to 70% of their diet now consists of the charismatic marine mammals, according to new research. But this dietary shift has come with an invisible cost—mercury poisoning, reports Bobby Bascomb.
The findings, led by Gretchen Roffler of the Alaska Department of Fish and Game, suggest that wolves with heavily marine-based diets are accumulating toxic levels of mercury, a heavy metal that bioaccumulates up the food chain. The warning signs emerged after the death of an emaciated collared wolf. Tests revealed what Roffler described as “unprecedented” mercury concentrations in the animal’s liver.
The culprit is methylmercury, a neurotoxin formed when airborne mercury—often originating from coal combustion and other human activity—settles into aquatic ecosystems. There, it infiltrates invertebrates like mussels and sea urchins, which sea otters consume in large quantities. Wolves feeding on otters absorb the mercury stored in their tissues.
“It moves efficiently through a food web,” explains Ben Barst, a co-author of the study and a toxicologist at the University of Calgary. Initially skeptical of the results (“at first we thought the instrument was malfunctioning”), Barst’s team reran the tests. The levels matched those seen in polar bears.
Further sampling confirmed that mercury exposure is not confined to one wolf. Barst and Roffler analyzed two packs—one on Pleasant Island and one on the adjacent Gustavus Forelands. The latter, with continued access to deer and moose, also showed signs of shifting toward a marine diet rich in otters. Sea otter populations have boomed under legal protections, becoming what Roffler calls “super abundant wolf prey.”
As the otters recolonize their former range, more wolves may be tempted to trade deer for shellfish-fed carnivores—with toxic consequences.
