Electronic tags reveal secrets of foraging fur seals
Northern fur seals in the eastern Bering Sea spend most of their lives foraging underwater and breeding on remote islands. To study their behavior, scientists depend on electronic tags to track an individual animal’s movements as it searches for food.
In a recent Consortium study, Brian Battaile (University of British Columbia) and colleagues analyzed the data recorded by electronic tags that were temporarily attached to 83 northern fur seals in the eastern Bering Sea. The study was published in the scientific journal Marine Mammal Science.
By combining the data from different types of tags, the researchers were able to retrace the animals’ movements at an extremely high resolution, revealing an unprecedented glimpse of northern fur seals in the wild.
“Our study was part of a much larger project in the Bering Sea that investigated how walrus, seabirds and northern fur seals find their prey and make a living,” says Battaile. “We used sophisticated tags that collect data 16 times per second, which gave us incredibly detailed information about what the seals were doing.”
Using this data, the scientists reconstructed and compared the foraging tracks of fur seals from two breeding islands in the eastern Bering Sea—St. Paul and Bogoslof Islands. St. Paul Island historically hosted the world’s largest breeding population of northern fur seals, but has been declining at an annual rate of 4.2% since 1998. Just 250 miles south of St. Paul, the small Bogoslof Island population has been increasing at an annual rate of 10.1% since 1997.
“We expected the animals that were going offshore from St. Paul Island to behave similarly to those going offshore from Bogoslof Island,” says Battaile. “We figured they’d be targeting the same types of prey.”
“We found there were two types of fur seals at St. Paul—one that foraged offshore and the other that stayed over the continental shelf. According to our calculations, the behavior of the Bogoslof seals was similar to the on-shelf St. Paul animals, while the offshore St. Paul seals had different foraging behaviors.”
A leading theory suggests that lactating females on St. Paul Island are unable to capture enough calorie-rich food to produce enough lipid-rich milk to support their pups. As a result, the pups cannot store enough fat prior to weaning and have difficulty surviving their first year as they learn to forage. This makes it critically important to better understand the foraging behaviors of lactating females.
“We knew the St. Paul animals traveled further away from land than the Bogoslof animals, but the new tags showed us that the distances swum were much greater than ever reported. This tells us that the animals from St. Paul are working even harder to obtain food than anyone previously thought―and corroborates the nutritional stress theory.”
Each increase in technology brings a better understanding of the lives of marine mammals. New generations of electronic tags and improved statistical techniques have brought new insights into the foraging habits and movement of northern fur seals—but not without challenges.
“Marine mammal research has often been at the forefront of technological developments. In our case, the resolution of the data we collected and our analysis of fur seal foraging behavior was at a finer-scale than anyone has ever been able to do before. Similar technological advances should allow oceanographers to collect finer-spatial scale data on chlorophyll concentrations and sea-surface temperatures to match our fur seal data,” says Battaile. “Then we will be able to put all the puzzle pieces together to get the complete picture of how fur seals are exploiting the Bering Sea and why the feeding conditions are so different between the two fur seal islands.”