Northern fur seal rookery on the shores of Bogoslof Island, an active volcano on the Aleutian chain. In the background, the volcano's vent steams in this photo from August 2019. Credit: NOAA Fisheries/Paul Hillman taken under NOAA Fisheries Permit #14327.

In August 2023, marine mammal biologists from the Alaska Fisheries Science Center returned to Bogoslof Island for the first time in 4 years. Bogoslof is home to one of the three northern fur sealbreeding colonies in Alaska that together comprise the Eastern Pacific stock. This expedition was part of a routine survey, part of our efforts to estimate the northern fur seal population.

The Bogoslof population is relatively new-fur seals started breeding here in 1980. Remarkably, those initial animals have exploded into the tens of thousands. This is a welcome contrast to the overall decline witnessed at the Pribilof Islands rookeries. However, the 2019 Bogoslof survey was preceded by volcanic eruptions, which yields a fascinating case study. How do tens of thousands of northern fur seals react to drastic changes to their summertime home?

Bogoslof's Eruption Timeline

Bogoslof is the tip of a 6,000-foot underwater volcano, which lies 61 miles northwest of Unalaska Island in the Aleutian Chain. It erupted out of the sea in 1796. In 1883, another major eruption produced Fire Island, a rocky spire just to the north.

Map showing location of Bogoslof Island relative to Alaska and Russia. This 1910 photo shows an eruption between Bogoslof Island (left) and Fire Island (right). Credit: Courtesy of Alex Wetmore.

Over the following 40 years, new peaks came and went. Eruptions formed land bridges between Bogoslof and Fire Island that exploded or eroded away over time. In 1927 and 1992, two new prominent domes formed, which still border the northern shore. Outside of the 18-day eruption that formed the 1992 dome, Bogoslof enjoyed decades of relative stability from 1927 to 2016. The island measured about three-quarters of a mile long and a quarter-mile wide.

Left image is a satellite view of Bogoslof Island in 2015 with labels highlighting pre-existing features and sections affected by the 2016-2017 eruption. On the right shows a clear summer day on Bogoslof in 2005 with wildflowers. The prominent features from left to right are the 1927 dome, Fire Island, and the 1992 dome. Credits: Left image: courtesy of Alaska Volcano Observatory. Right image: NOAA Fisheries/Paul Hillman.

Tundra grass, putchki (wild celery), and wildflowers colonized the land, likely brought by migrating birds. Tens of thousands of puffins, kittiwakes, and murres established nesting colonies, and Steller sea lions dotted the shoreline. More recently, in 1980, northern fur seals established a new rookery and the population took off. Fast forward a few decades and Bogoslof completely changed.

Northern fur seals on Bogoslof Island. The larger animals are adult females with black pups in the background. Credit: NOAA Fisheries/Maggie Mooney-Seus taken under NOAA Fisheries Permit #14327.Satellite imagery shows a violent eruption on May 28, 2017. The island below is already much larger than before this eruptive period started on December 20, 2016. Credit: DigitalGlobe.

Starting in December of 2016, 70 explosive events over the course of 9 months rocked Bogoslof and sent a mile-high stream of ash and smoke into the sky. Air travel was disrupted to and from Dutch Harbor-our nation's largest fishing port by volume. When the dust settled, the island was three times larger and there was not a blade of grass in sight. The entire island's surface was made up of new rock, ash, a smoking mountain, and bubbling pools.

State of Bogoslof on January 10, 2017. Credit: Alaska Volcano Observatory/Chris Waythomas.State of Bogoslof on April 20, 2018. Credit: Alaska Volcano Observatory/Chris Waythomas.

Since 2017, Bogoslof has been continually smoking. Over the past couple of years, winter storms have pounded the shoreline, eroding away much of the perimeter. Today, the island's size is closer to its pre-2016 eruption state, but constantly changing. Signs of unrest continue, with more than 100 earthquakes recorded in October 2023. Eruption may not be imminent, but volcanologists are keeping a close eye.

Bogoslof Island changes from 2021 to 2023. Credit: Alaska Volcano Observatory/Chris Waythomas.

Fur Seals Trends in the Eastern Pacific

The Alaska Fisheries Science Center's Marine Mammal Lab monitors northern fur seal populations in the United States. Specifically, we track the Eastern Pacific and California stocks. They breed on the Pribilof Islands and Bogoslof Island in Alaska, and on San Miguel Island and South Farallon Island in California. Population estimates are derived from pup production; current methods began in the 1960s. This and other long-term data series are critical for understanding population trends.

Northern fur seal pups on Bogoslof Island in 2023. Credit: NOAA Fisheries/Burlyn Birkemeier taken under NOAA Fisheries Permit #23283.

In the Eastern Pacific stock, the overall population has been dropping, due to a 3.2 percent per year decline on the Pribilof Islands. Since 2018, there have been fewer than 100,000 pups born annually on the two main Pribilof islands of St. Paul and St. George-less than half from just 20 years earlier. But Bogoslof has been a shining star. The population increased sevenfold in the same time frame, at a rate of 9.2 percent per year.

Northern fur seal pup production in the Eastern Pacific stock from 1994 to 2022. No data available for Bogoslof between 1997 and 2005 and official data is not yet available for 2023. No 2020 data available for the Pribilofs due to the COVID-19 pandemic.

During 2016 and 2017, Bogoslof was erupting and the landscape completely changed. Did this impact the fur seal growth?

Continue reading in part 2 tomorrow, which focuses on how our scientists track the fur seal population on Bogoslof and the effects of the recent eruptions and subsequent erosion. It also looks at the importance of this breeding colony to the overall health of the Eastern Pacific stock.