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.
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?
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.”
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.
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.
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.
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.
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.
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.
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.”
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.
