Unprecedented and potentially harmful levels of mercury have been found in the tissues of coastal wolves in southeast Alaska that hunt sea otters.
A recent study found that some coastal wolves are at severe risk of toxic effects from mercury. As sea otter numbers increase, more coastal wolves are switching to a predominantly marine diet that exposes them to mercury contamination.
Mercury is a naturally occurring element in the Earth’s crust that can be mobilised by natural and human processes. Microbes in aquatic environments transform mercury to methylmercury, which builds up in the tissues of aquatic species, becoming more concentrated in species at higher trophic levels. Because of this, apex predators at the top of marine food webs, such as polar bears, can accumulate high mercury concentrations.
Wolves are terrestrial predators but eat marine prey in coastal areas. As sea otters, themselves apex predators, become more abundant following their reintroduction to southeast Alaska, coastal wolves have learned to hunt them.
When a GPS-collared wolf (wolf 202006) died on Pleasant Island in Icy Strait, southeast Alaska in 2020, scientists investigating her death tested tissue samples for viruses, disease, algal toxins and contaminants. “That’s when we found very high concentrations of mercury, and that was the trigger for this whole study,” explains Dr Gretchen Roffler, a wildlife research biologist with the Alaska Department of Fish and Game.
The study, published in Science of the Total Environment, compared mercury levels of coastal wolves on Pleasant Island to wolves on the nearby mainland with a predominantly terrestrial diet, and to wolves of the Interior with a completely terrestrial diet.
Stable isotope analysis was also used to investigate the prey species being consumed.
The livers of coastal wolves contain concentrations of mercury 7 times higher than wolves consuming more terrestrial prey, and 278 times higher than Interior wolves with an entirely terrestrial diet.
In an unexpected twist, stable isotope analysis revealed that in 2022 the mainland wolves, which historically consumed terrestrial mammals such as moose, deer and mountain goats, switched to a predominantly sea otter diet.
Mounting evidence suggests that more wolves in Alaska are taking advantage of increasingly abundant sea otter prey. “And that means we’re going to start to find more wolves with these high mercury concentrations,” explains Dr Ben Barst, an assistant professor at the University of Calgary.
What does this mean for the coastal wolves of Alaska? As scientists work to answer many of the questions raised by these findings, Roffler is hopeful. “Wolves have shown incredible plasticity and the ability to survive all kinds of calamities in the past. This might be just another one.”
Main image: male wolf on Pleasant Island with a sea otter head in its mouth. Credit: ADF&G
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