Fur seals find unlikely refuge: on an active volcano

 A plume from Bogoslof Volcano in 2017 (Photo from Trever Shaishnikoff/USGS)
A plume from Bogoslof Volcano in 2017 (Photo from Trever Shaishnikoff/USGS) (KTUU)
Published: Nov. 26, 2019 at 6:24 AM AKST
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In 1796, there was only water.

Now the 300-acre Bogoslof Island - located some 30 miles northwest of Unalaska - has become the unlikely refuge for an iconic species that has been in serious decline elsewhere in the Bering Sea: the Northern fur seal.

The island emerged around 1796 from an underwater stratovolcano, a type of volcano known for periodic, explosive eruptions that can produce even, conical mountains.

The most recent period occurred in 2016 and 2017, nearly tripling the size of the island as tephra - fragments of rock ranging from boulders to ash - accumulated from underground eruptions, while submarine heat lifted or lowered the ground around the island.

Annotated photograph of Bogoslof Island showing the cumulative effects of 2016-17 eruptive activity. A layer of fine muddy appearing ash drapes most of the landscape and covers pre-existing vegetation. The dashed line indicates the area excavated by explosive eruptive activity so far. A prominent zone of upwelling is probably the surface expression of a shallow submarine vent. Photograph taken by Dan Leary, Maritime Helicopters, January 10, 2017.(Credit: Christopher Waythomas, U.S. Geological Survey. Public domain.)

Steam, ash, geysers and heat killed off virtually all vegetation on the island, depriving birds like puffin and place to burrow. So how did the fur seals weather two years of intense volcanic activity?

“What we discovered was that … the population increased, despite the eruptions of the last few years,” said NOAA fisheries biologist Tom Gelatt, who was part of an expedition to the island in September to conduct the first population estimate in four years.

That’s good news for the species, which was one of the main economic engines at the time of the purchase of Alaska in 1867. At the time, Russians, and then American industrialists, led a brutal harvest of fur seals on the Pribilof Islands that killed thousands of the pinnipeds each day for their prized pelts that were sold throughout the world. Native Aleuts were held in slave-like conditions long after slavery was abolished on the mainland.

The industrial harvest decimated the population of fur seals, once estimated at in the millions on the small Pribilof island of St. Paul alone.

While the total population of fur seals in Alaska rebounded to about 2.1 million in the 1950s, it fell again to about 620,000 in 2016, with a large part of the decline beginning in the late nineties.

It's something that has real effects on the people who harvest the seals for subsistence, the Unangax of St. George and St. Paul.

"There might be hundreds of thousands of fur seals, but the number of properly aged male fur seals is not there," said Ilarion Merculieff, a tribal member of St. Paul who consults for St. George, "One time when I was there we had to go to three different rookeries to get 100 seals of the right age and sex."

While a hundred seals sounds like a lot, hunting is done with large groups who divide into different jobs and then share the spoils with friends and family around the islands.

The decline has residents of the Pribilofs frustrated with the fisheries management, which they blame for overfishing pollock, fur seals' primary food source. Merculieff says the tribe has noticed changes in diet of the seals.

"They're less fatty and the taste is different," he said, "The taste and color is more fishy, more like salmon, so they could be intercepting the salmon that people are complaining about is disappearing from the Yukon-Kuskokwim Rivers."

But despite problems with food sources on the Pribilofs, the Bogoslof population, however, has been going in the opposite direction.

A single breeding pair was recorded in the 1980s. In 2005, it was about 56,000.

The 2019 estimate is even more encouraging: 160,000.

To count fur seals, scientists use a method of marking fur seal pups and by shearing a small section of fur, then using the ratio of marked to unmarked on further samples to come up with a population estimate. This year, scientists also used drone surveys which they hope to compare to traditional methods of counting. (Photo from NOAA)

It’s an astounding feat, considering the difficulty of the terrain.

“You can dig down in some of the sand and it's still warm, but the area of the island that's still active is in the center and there's no fur seals in that area,” said Gelatt of NOAA.

He said that active volcanic activity led the research team to decide to stay in their research vessel, the Tiglax, instead of camping out on the island as they had in the past.

“The island is still active, there's still bubbling mud, coming out there's still geysers coming up, the ground is quite warm in some areas,” he said.

The activity turned much of the island from green grassy vegetation to a moonscape of ash and rock.

Image from NOAA

“When we were there I don't think we saw any green vegetation. It's still a brand new piece of ground and even the existing island that was there previously, of course, is covered with all the materials, so there's no vegetation at all coming out,” said Gelatt.

On top of that, the deep seafloor that abruptly drops off from the steep sides of Bogoslof volcano is thousands of feet deep, where they are foraging is drastically different than the mere hundreds of feet around the Pribilofs, which sit on the continental shelf. There, fish like walleye pollock are prolific and are the main source of food for the seals.

But the seals seem to be thriving on Bogoslof, despite the different terrain, consuming deepwater fish and squid that require deeper dives, but seem to be more abundant.

Gelatt says that with satellite telemetry, scientists can track the amount of time it takes for a nursing mother to go out to retrieve food that it will turn into milk that can feed a nursing pup.

“On the Pribilof Islands, that period of time ranges somewhere between four and seven days, even eight days in a poor season. On Bogoslof in the 90s when we first started attaching satellite tags, that period of time was less than one day,” said Gelatt.

Since then, the time for feeding trips has increased to about three or four days, suggesting that the ecosystem is approaching its carrying capacity, but he still says that it’s made for healthier pups.

“The result of that is that in the fall when the pups ween and are off on their own, the Bogoslof pups are likely quite a bit heavier which is better for survival, so their survival is better.”

Fur seal pups on Bogoslof (Image from NOAA)

While the tougher foraging means that there isn’t room for unlimited growth, the pup population on Bogoslof has already surpassed the population on St. George, the smaller of the two Pribilof islands. It’s also been a biologists’ dream to watch a habitat - and the populations that go with it - emerge from nothing.

“It's really a classical ecological study. I mean it's exactly what you'd predict in terms of ecology and resource depletion that as the population increases, there's competition and so something gives and so either you have to have more food or you have to work harder to get it,” says Gelatt.

But for hunters on the Pribilofs, a population increase on Bogoslof is cold comfort.

"It doesn't matter because the margin of increase there is still insignificantly small compared to the overall decline," said Merculieff.

Bogoslof mud (Photo from NOAA)

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