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Life on an Active Volcano: Fur Seals Adapt to a Changing Landscape on Bogoslof Island - Part 2

March 13, 2024

Volcanic eruptions at Bogoslof have changed the landscape dramatically. It’s home to a thriving breeding ground of northern fur seals. Scientists monitor the fur seal population every 4 years to understand how they respond to such a dynamic environment.

Aerial view of Bogoslof Island in June 2022 showing the impact of the volcanic eruption from 2019 Aerial survey imagery from June 2022 along with a white outline showing the former size in 2019. Erosion has changed the island dramatically since it tripled in size after the eruptions in 2016-2017. Credit: NOAA Fisheries/Alexey Altukhov and Burlyn Birkemeier.

Monitoring Fur Seals on Bogoslof: Post-Eruption

After the eruptions in 2016 and 2017, it was 2 years before Bogoslof was deemed stable enough to allow scientists back on land. This tracked with our goal of surveying this population every 4 years. These surveys are typically conducted with support from the U.S. Fish and Wildlife Service aboard their research vessel the R/V Tiĝlax̂. The previous census was 2015 and the estimated pup production was 27,750.

Two scientists lay on a volcanic beach in the looking out across the water to another beach full of seals. In the background, there is a volcano smoking
NOAA biologists Rolf Ream and Rod Towell perform re-sights during the 2019 mark-recapture census of pup production on Bogoslof Island. They stay low to minimize disturbance to the fur seals. In the background is the new peak formed during the 2016-2017 eruption. Vents continue to steam. Credit: NOAA Fisheries/Paul Hillman taken under NOAA Fisheries Permit #14327.

In 2019, nobody knew what to expect, but given the increase in habitat, hopes were high that the population would continue to grow. And what we found did not disappoint. Recounted biologist Rod Towell, “On Bogoslof, we’re seeing a redistribution of the land. All of the area where we’re seeing fur seals now didn’t exist prior to the eruption. And it’s hard to see a real impact that the volcanic activity may have had on the fur seals. They seem to be doing really well. Our estimated pup production in 2019 was 36,015."

By 2023, this dynamic island changed again. Whole sections of the island are gone. Would this yield a different story?

Beach with large hill looking out towards the ocean and seabirds
Looking south along Bogoslof Island’s western shore in 2019, with Castle Rock on the left. Sea birds nest everywhere and northern fur seals line the coastline. In 2023, most of the low-lying shoreline to the right of Castle Rock was gone and the ocean was breaking along the western face of Castle Rock. Credit: NOAA Fisheries/Maggie Mooney-Seus taken under NOAA Fisheries Permit #14327.

Towell summarizes, “The island has changed considerably in size since 2019 and the fur seal distribution seems to be different too. But these animals are amazing. I mean, this is already a harsh environment without the volcanic activity and subsequent erosion. And they just persevere, no matter what nature throws at them.”

Aerial view of gray beach occupied with seals. Large hills and ocean in background.
Aerial view looking north from the southern end of Bogoslof Island on August 7, 2023. Individual black specks are northern fur seals. Large groupings are territories established by a single male, in which he holds many females. Prominent features from left to right: Castle Rock, new peak from 2016-2017 eruption, 1927 dome, 1992 dome, and Fire Island. The 1992 dome is now separated from the island by the sea. Credit: NOAA Fisheries/Rod Towell & Burlyn Birkemeier under NOAA Fisheries Permit #23283.

The 2023 estimate is not available yet, partially because the survey was only performed with aerial drones. Relative to previous surveys, the data needs to be analyzed more thoroughly and verified with the mark-recapture method. Anecdotally, the team notes that the population seems to be doing fine.

Based on 2018–2019 estimates, Bogoslof accounted for approximately a quarter of the pup production in the Eastern Pacific stock. The rest come from the Pribilof Island rookeries. However, the population on the Pribilofs declined another 4.2 percent between 2018 and 2022. While we don’t have the official 2023 estimate, Bogoslof’s pup production is certainly robust. And the increases observed over the past decade have helped to stabilize the declining trend for the entire Eastern Pacific stock. 

Behind the Pribilof’s St. Paul Island, Bogoslof has cemented its status as the second largest colony in the Eastern Bering Sea. That’s pretty incredible for an island this small, and which didn’t exist as a fur seal rookery until a few decades ago.

Explosion of Fur Seals: Good Sign for Ocean Health

The fur seal population explosion on Bogoslof is a good sign. Pup production is heavily influenced by ocean conditions. The dramatic rookery growth indicates that the feeding grounds around Bogoslof are productive and healthy. This may be contrary to the Pribilof Islands, where fur seals are declining.

To better understand these connections, our scientists collect tracking information and biological samples alongside every population estimate.

Satellite and VHF Tagging

In August 2023, we tagged 11 fur seal mothers with satellite instruments that track movement, depth, and time.

Three scientists on a gray beach with large hills in the background. Two scientists hold a fur seal in place while putting on a satellite tag.
NOAA biologists Carey Kuhn and Brian Fadely prepare to affix a satellite tag onto an adult female fur seal. Credit: NOAA Fisheries/Rolf Ream taken under NOAA Fisheries Permit #23283.

From this, we can understand feeding trip duration, distance traveled, feeding depth, location, amount of time nursing a pup between foraging trips, etc. Compared to mothers on the Pribilofs and Bogoslof, these trips are much shorter for Bogoslof mothers. Again, this supports a healthier feeding ground around Bogoslof than the Pribilofs. Continuing observations will help us detect changes due to climate change and other factors.

We also installed VHF receivers on Bogoslof to listen for VHF flipper tags that are deployed on the Pribilofs and Bogoslof. Tracking animals that return next year or that migrate between the two sites is valuable. Fur seal mothers will return to the same rookery—even the same part of the beach—year after year. Female pups have also returned to the same place they were born to be mothers themselves.

Scientist on a beach with a yellow tub installing scientific equipment. Boat on the ocean in the background.
NOAA biologist Jeremy Sterling installs a VHF antenna with the R/V Tiĝlax̂ in the background and northern fur seals along the shoreline. Credit: NOAA Fisheries/Brian Fadely taken under NOAA Fisheries Permit #23283.

We also use visual observation to record flipper tags, but the benefit of the VHF receivers is that they are listening for animals all year round. This is critical for monitoring animals on Bogoslof since our team only surveys there every 4 years.

Biological Sampling

When our scientists get hands-on with a northern fur seal, they maximize the opportunity. We collect blood and hair samples to study animal health, disease, heavy metal consumption, plus stable isotopes and scat (aka poop) to study diet. Whiskers can also yield a whole story behind an animal, such as their history of foraging, motherhood, and stress.

Gloved hands holding a seal flipper while blue gloved hands draw blood.
Drawing blood from a northern fur seal’s hind flipper. Credit: NOAA Fisheries/Maggie Mooney-Seus taken under NOAA FisheriesPermit #14327.

New in 2023: eDNA Sampling

New in 2023 is the addition of environmental DNA sampling at Bogoslof, also known as eDNA. We sampled fur seal scat and the water column offshore of the rookeries.

To understand eDNA research in a nutshell, we collect the scat or water sample. We perform some extraction and chemical fixing on the sample, and then sequence the DNA. For scat samples, the DNA sequences reveal what animals are present, and therefore what that fur seal has eaten. Likewise, analyzing water samples illustrates what animals are living around Bogoslof or nearby.

eDNA sampling widens the breadth of research we do on northern fur seals, and our understanding of them and the environment they live in.

Scientists holds scientific equipment while scientist in the foreground gets a water sample into a plastic bag
NOAA biologists Tonya Zeppelin and Carey Kuhn pour a water sample from a Niskin bottle into a sterile plastic bag. The sample is filtered to extract eDNA. Credit: NOAA Fisheries/Brian Fadely.

Aerial Surveys on Bogoslof—Future for Pup Counts?

Also new for 2023 was gathering a full aerial mosaic of the entire rookery on Bogoslof Island. This method began in 2019 as a pilot study, and has since been used on the Pribilof Islands, and now again on this return survey of Bogoslof.

Particularly striking on Bogoslof is the dynamically different shoreline and size of the island. And, in turn, the distribution of fur seals.

Animation showing how Bogoslof Island changed from 2019 to 2023
Mosaics of aerial survey imagery from the summers of 2019, 2022, and 2023. Credit: NOAA Fisheries/Alexey Altukhov and Burlyn Birkemeier.

Northern fur seals live on land in very tight groups, and there are lots and lots of animals on Bogoslof. The traditional mark-recapture method to count pups is invasive, and requires researchers to break up those groups and disturb the rookeries.

Scientists at the Alaska Fisheries Science Center are analyzing these high-resolution aerial images to determine if aerial surveys can replace the invasive hands-on method. Currently, we rely on counting individual pups. We’re also exploring artificial intelligence and machine learning as ways to make the process more efficient and accurate. 

Aerial view of a rookery of seals with yellow dots imposed on the seals for counting.
Aerial survey image with yellow dots indicating each fur seal pup counted. Credit: NOAA Fisheries/Rod Towell & Burlyn Birkemeier under NOAA Fisheries Permit #23283.

With such a large and increasing population, an automated solution is critical. It would also be a valuable development to apply to other populations and species.

Shifting Sands but Still Climbing

This Bogoslof fur seal population is remarkable. For someone who has been studying northern fur seals for over 30 years, Rod can’t wait for the Bogoslof survey years. He reflects, “To come to Bogoslof and see the production that goes on, the mass of animals here is just phenomenal. It’s akin to seeing the great animal congregations around the world, like in the Serengeti or Antarctica.”

A group of seals on the beach ready to dive into the ocean.
Northern fur seal rookery on the southern spit of Bogoslof Island. Credit: Maggie Mooney-Seus under NOAA Fisheries Permit #14327.

As eruptions continue to expand and shrink the shoreline on Bogoslof, it seems one thing will be constant—the northern fur seals. It’ll be exciting to see where this population will be after the next survey in 2027, and beyond. Stay tuned.

Bogoslof Island is part of the Alaska Maritime National Wildlife Refuge, managed by the U.S. Fish and Wildlife Service. The NOAA Alaska Fisheries Science Center has obtained a special use permit from the U.S. FWS to conduct our research here.

Last updated by Alaska Fisheries Science Center on March 18, 2024

Research in Alaska