Predators typically hunt prey, but occasionally, the predator itself becomes the prey. This is the unlikely scenario that new research in and near Glacier Bay is beginning to reveal. There, biologists have discovered that an apex terrestrial predator, wolves, are eating a top marine predator, sea otters.
This unexpected relationship was detected when researchers began analyzing wolf scat samples to study the wolves’ diet and discovered that they contained significant amounts of sea otter remains.
Alaska Department of Fish & Game biologist Gretchen Roffler has collected wolf scat from 12 study sites throughout southeast Alaska to learn more about the wolves’ diet. Along with collaborator Taal Levi at Oregon State University, Roffler found that wolves eat a variety of things, including typical fare of deer, moose, goats, and even birds and salmon. However, on Pleasant Island (in Icy Strait just outside of Glacier Bay), Roffler determined that sea otters are the wolves’ predominant food source. This surprising discovery has led to more questions than answers—specifically, are the wolves scavenging dead otters or preying upon live ones? If the former is true, then what would cause the otters to die off in such large numbers? If the latter is true, then how would a wolf be able to even catch a sea otter? After all, wolves hunt on land, and sea otters live in the sea.
Why would wolves eat otters?
To understand how this scenario developed, we first need to step back in time about 100 years. The ice that had once filled Glacier Bay was retreating, revealing a barren land and seascape that was being recolonized by plants and animals. Larger terrestrial mammals like goats, bears, and wolves were some of the first pioneers to recolonize this newly exposed land. In the late 1960s, moose started moving in, and wolves adapted to them as a major prey source.
Around this same time, sea otters were re-introduced at several sites along the outer coast of southeast Alaska after being hunted out by Russian fur traders. By the late 1980s, sea otters had found their way to the mouth of Glacier Bay, where they feasted on abundant delicacies like clams, crabs, and urchins while protected from hunting inside the national park. It wasn’t long before the population there exploded. Twenty years later, the sea otter population in Glacier Bay had the highest density and fastest population growth in all of southeast Alaska. During most of this time period, the wolves in this area are believed to have behaved like typical wolves—preying upon ungulates and the occasional fish or bird.
But a few years ago, something interesting happened—at least two of the wolves that called Glacier Bay home took a half-mile swim from the mainland near Gustavus over to Pleasant Island—an oblong piece of land just seven miles long by four miles wide with a maximum elevation of 200 feet. There, they found a healthy deer population that had been unbothered by glacial effects or any predators (except humans) for at least 100 years. Within about three years, the wolves had nearly wiped out the island’s deer population, leaving the wolves hungry for the next protein option.
Enter the sea otters.
By most estimates, sea otters have reached or exceeded the area’s carrying capacity—meaning, there is not enough food to sustain the population at its current level. This would put otters in a weakened state, requiring more rest, which could lead to increased competition for prime resting places like kelp beds. It is possible that they are resorting to resting at locations where they are more vulnerable to wolves, like near-shore intertidal areas or land.
Roffler has yet to gather definitive proof that the wolves are, indeed, preying upon live sea otters (versus scavenging dead ones), but she is gathering clues. On a recent survey in Glacier Bay, she found a relatively high concentration of otter remains, some of which were very fresh. The remains were just inside the beach fringe, well above the high tide line, indicating that they hadn’t been merely washed ashore but had been dragged there. (Birds often take their prey there, to avoid having to share their catch, but it is unlikely that even a bird as large as a bald eagle could drag a 50-pound or heavier sea otter carcass that far.) Scattered among the otter remains was plenty of wolf scat. Perhaps the most important clue was that just a few hundred yards offshore, on a small intertidal islet, was a group of about a dozen sea otters resting in ankle-deep water. At least one otter was completely hauled out on the rocks. A hungry wolf might spot an otter in such a position, swim across, and nab it. The unsuspecting sea otter, although formidable in the sea, would be completely vulnerable to a wolf on land.
Still, all of these clues, though compelling, don’t prove that wolves are preying on live sea otters. Until recently, the only way to confirm this behavior would have been through direct observation, and, so far, no one has witnessed it. But now, Roffler has a tool that can help. By placing a game camera pointed at the intertidal islet in question, she hopes to catch a wolf in the act. Time will tell if indeed the predator has become the prey in this intriguing story that began with an unexpected discovery in a pile of poop.