ANCHORAGE, Alaska — Alaska’s northern fur seal population for three decades has been classified as depleted, but the marine mammals are showing up in growing numbers at an unlikely location: a tiny island that forms the tip of an active undersea volcano.
Vents on Bogoslof Island continue to spew mud, steam and sulfurous gases two years after an eruption sent ash clouds into the path of jetliners passing over the Bering Sea. Still, northern fur seal moms find the remote island’s rocky beaches perfect for giving birth and mothering pups.
“The population growth of northern fur seals on Bogoslof has been extraordinary,” said Tom Gelatt, who leads a NOAA Fisheries group that studies northern fur seals. Geographically speaking, the island is not a particularly unusual place for the seals known for their thick coats to hang out.
But why the seals chose volatile Bogoslof over the dozens of other uninhabited Aleutian Islands is unclear.
“The surface is covered with these big, ballistic blocks, some as big as 10 meters (33 feet) in length that were exploded out of the vent,” said Chris Waythomas, a U.S. Geological Survey research geophysicist at the Alaska Volcano Observatory. “They litter the surface. It’s pretty wild.”
The eastern Bering Sea population of northern fur seals numbers about 635,000, with their main breeding ground on St. Paul Island, 240 miles northwest of Bogoslof. The animals were first spotted on Bogoslof in 1980, and NOAA researchers have since conducted periodic checks on the population.
In 2015, biologists estimated an annual growth rate of just over 10% to approximately 28,000 pups on the island. The 2019 estimate likely will be more than 36,000 pups, Gelatt said.
Seals elsewhere stay on beaches, but on Bogoslof — which is about a third the size of New York City’s Central Park — they are never far from signs of volcanic activity.
Food in the nearby deep water could be a factor in the seals’ behavior. Bogoslof’s seals eat squid and northern smoothtongue, a deep-water fish that looks like a smelt. Seals on St. Paul, the largest of the Pribilof Islands, forage on the shallow continental shelf for walleye pollock, a fish targeted by commercial fishermen.
Females with pups on Bogoslof return from foraging faster than Pribilof mothers, possibly allowing their pups to receive more meals and wean at a larger size, Gelatt said.