On a small island near Gustavus, a wolf pack has decimated the local deer population – and started feeding on sea otters instead.
The shift underscored coastal wolves’ adaptability. But then one died.
“We found her in a hole, under a tree,” Gretchen Roffler, a wildlife research biologist with the Alaska Department of Fish and Game. “She had lost about a third of her body weight. She was emaciated.”
The researchers sent the carcass to a wildlife veterinarian, who did a series of tests on the animal’s liver, muscle, brain and kidneys. The results ruled out diseases and factors ranging from canine distemper to algal toxins. What they did find were elevated levels of mercury.
“They were many orders of magnitude higher than other wolf liver tissues that had been analyzed in other parts of the world,” Roffler said during an interview this week in Haines.
The finding kicked off a sweeping research project that examined different wolf packs’ reliance on marine prey for food, and how that diet might affect their mercury levels – and long-term health.
In a study published late last month in the journal Science of the Total Environment, researchers concluded that two wolf populations near Gustavus are increasingly relying on marine prey – specifically sea otters – for food. And that in some cases, like the wolf who died, the shift is resulting in potentially toxic mercury exposure.
“The ability of wolves to switch their diets from terrestrial prey to marine prey, just shows how resilient they can be. But we now know that their reliance on marine prey can also lead to the risk of toxicity,” Roffler said.
The researchers looked at a handful of areas including Pleasant Island — the island where wolves are now largely eating sea otters instead of deer. They also looked at an area of the Gustavus mainland, where wolves mostly eat terrestrial animals – namely, moose – but have started adding more marine animals to their diet.
Using hair and muscle tissue samples archived over the last 20 years, they concluded that marine-heavy diets can have dangerous consequences. That was especially the case when compared to samples taken from wolves in two other areas – Douglas Island near Juneau, and the Interior – where wolf diets are predominantly terrestrial.
Roffler said some mercury is not uncommon in predators with marine diets, like fox and polar bears.
“The thing that was unusual in our study was the severe mercury concentrations in these wolf tissues,” she said.
The study has implications for wolves and other predators beyond the Gustavus area – particularly as sea otters proliferate along Alaska’s coasts.
See otters were hunted to local extinction during the fur trade in the 1800s. But then, in the 1960s, the state reintroduced them to the region. In the time since, the marine mammal has recolonized some areas, including around Glacier Bay National Park.
“We can assume that as sea otters continue to recolonize parts of their former range and grow in numbers, that wolves and other terrestrial predators will start using them as prey,” Roffler said.
But why is mercury present in marine environments – and animals – in the first place?
Report co-author Ben Barst, an assistant professor at the University of Calgary who studies ecotoxicology, said mercury ends up in the ocean after humans release it through activities like coal combustion and gold mining.
“It can be a vapor in the atmosphere, can travel for long distances, and then eventually it’s deposited even very far away from its original emission sources. It’s deposited in rain and snow and other types of precipitation,” Barst said.
In Southeast, there’s another potential source: Glacier runoff. It holds mercury, and is increasing with climate change.
No matter the source, once mercury enters an aquatic environment, microbes convert it into a new form that easily makes its way into living organisms. Think: mussels, clams and sea urchins.
“You get all this mix of minerals in there. And of course, it’s going to go in the ocean and of course, the sediment. The clams and everything else, crabs, bury themselves in it,” said Chilkat Valley local and marine mammal hunter Tim Ackerman. “The sea otter are going to dig those up and consume them.”
By the time an otter becomes wolf prey, it can deliver a big dose to the apex predator.
“We see this in other instances with fish. You know, small fish are getting eaten by larger fish, which are getting eaten by the biggest fish, and they tend to have the highest mercury concentrations,” Barst said.
The researchers initially assumed that high concentrations of mercury in wolves could be unique to the Gustavus and Pleasant Island area, Barst said. More research is needed to determine whether that’s the case; there’s uncertainty about the contribution of glaciers and how wolf diets might fluctuate over time.
But Barst said it’s possible the trend could play out elsewhere as sea otters proliferate – and predators increasingly tap into marine food webs.
“We’re trying to get a handle on, are the concentrations of mercury that we’re seeing in Pleasant Island wolves, are those the highest that we’re going to see?” Barst said.
