As their name suggests, northern fur seals boast dense, luxurious coats—and historically, that made them a prime target for hunters. Exploitation by hunters in Russia and North America drastically depleted northern fur seal populations, which can be found in the Pacific Ocean, all the way from California to Japan. The animals are now protected under the Fur Seal Treaty and the Marine Mammal Protection Act, but they continue to struggle. For instance on St. Paul Island, the largest of Alaska’s Pribilof Islands and a major fur seal breeding ground, pup production was on the decline as of 2016.
But there is one northern fur seal population that, much to scientists’ surprise, is thriving in an unusual location. As Dan Joling reports for the Associated Press, a fur seal breeding ground is booming on Bogoslof Island, which comprises the tip of an active undersea volcano.
Bogoslof sits in the eastern Bering Sea, around 30 miles north of Alaska’s Aleutian Islands. As recently as 2016 and 2017, the volcano erupted, sending plumes of ash into the sky and spewing rocks onto the island landscape. Even on calmer days, fumaroles—or vents in the Earth’s surface—release volcanic gases and send mud geysers spurting into the sky.
“What with the small geysers and boiling mud pots, it’s pretty steamy,” Chris Waythomas, a research geophysicist with U.S. Geological Society at the Alaska Volcano Observatory, who studied Bogoslof’s activity in 2018, tells Sabrina Imbler of Atlas Obscura. Each fumarole, Imbler notes reaches around 212 degrees Fahrenheit in temperature, certainly hot enough to injure mammalian bystanders.
And yet, the northern fur seals that come to the remote island to have their babies seem to be doing just fine. The animals were first sighted on Bogoslof in 1980, marking the first time that northern fur seals had been observed anywhere other than the Pribilof Islands in the eastern Bering, according to the National Oceanic and Atmospheric Administration. Joling reports that in 2015, “biologists estimated an annual growth rate of just over 10 percent to approximately 28,000 pups on the island. The 2019 estimate likely will be more than 36,000 pups.”
Abundant food supply could explain why northern fur seals have chosen to breed on Bogoslof, in spite of the dangers. In the surrounding deep waters, they hunt squid and smoothongue, a type of fish. Tom Gelatt, leader of a NOAA Fisheries group that studies northern fur seals, tells Joling that females on Bogoslof return to their pups after foraging more quickly than mothers on the Pribilofs, which may mean that Bogoslof babies are getting more food and growing to a larger size. The island is also closer to the seals’ winter feeding grounds, which perhaps makes it easier for pups to get there safely.
It is not clear why northern fur seals on the Pribilof Islands haven’t rebounded from the days when they were severely over-hunted—the animals are still classified as “depleted” under the Marine Mammal Protection Act—but a number of factors could be at play, among them climate change, competition with commercial fisheries, and predation by killer whales. And while the Bogoslof population is faring well, its future is not entirely secure. As Waythomas tells Imbler, Bogoslof is vulnerable to wave erosion, and a “couple of big storms could remove a lot of the island.”
But for now, at least, the seals seem to have adapted to the precarious nature of their volcanic home. “I get the sense that they’re pretty tuned into their surroundings,” Waythomas says. “When things start to get going, they head into the water.”