The wolves on a small island in Alaska have a diet problem. They’ve wolfed down dangerously high levels of mercury—a result of eating sea otters.

Pleasant Island is a mile off the coast of Glacier Bay National Park, in Alaska’s panhandle.

Wolves have decimated the island’s population of deer, which used to be their main prey. So the wolves started eating sea otters. In fact, otters now make up about two-thirds of their diet.

Biologists have been studying the wolves for years. When a pack member died in 2020, they found that its tissues held extremely high levels of mercury—a nasty toxin. So they compared mercury levels of the island’s wolves to the wolves along the adjacent coastline and in the middle of Alaska. The mainland wolves eat mainly moose and deer.

The island wolves have much higher levels of mercury. Some of the levels were the highest ever seen in any wolves anywhere in the world.

Mercury builds up in marine organisms. Larger organisms eat lots of smaller ones, allowing the mercury concentration to grow as it moves up the food web. Sea otters are near the top of the web, so they build up a lot of mercury. And when the wolves eat the otters, they get high doses of mercury as well.            

The population of sea otters on the coast of Alaska and British Columbia has been increasing. And wolves along the coast appear to be incorporating more otters into their diets. So wolf populations could face greater mercury-contamination problems in the years ahead.

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