Kyoto, Japan -- Northern fur seals are long-distance migrants, moving seasonally between breeding colonies and wintering grounds. After leaving breeding colonies in autumn, they travel south for winter foraging, returning in early spring. This allows them to avoid sea ice and harsh conditions in northern waters while relocating to ice-free areas.
The southward migration of these seals has been relatively well-documented. One of their primary wintering areas is the Sea of Japan, where they can find abundant prey. They have also been reported to damage fishing gear and remove catch directly from nets, potentially competing with local fisheries.
The seals' northbound migration from wintering grounds back to breeding colonies in spring remains poorly understood, however. Factors such as tag loss, limited battery life, and the difficulty of capturing animals at sea pose particular challenges.
Recently, however, a team of researchers at Kyoto University succeeded in tracking some of the seals to analyze their behavior in an effort to better understand their northbound migration routes.
"Understanding how environmental conditions shape the northern fur seals' migratory strategies and habitat use is critical to clarifying their ecological role in the Sea of Japan," says corresponding author Heping Li.
The research team used satellite tracking to collect seal movements and behavior during their northbound migration. Between 2017 and 2020, they attached satellite tags to the dorsal fins of five juvenile and subadult male northern fur seals. Younger male seals are not constrained by the mating schedule, allowing them to remain at sea for longer periods and forage to meet their energy demands. The team was able to track the seals for more than three weeks.
The results revealed that foraging activity was concentrated along continental shelf-breaks and within the temperature range 8-13°C, corresponding to the Kuroshio-Oyashio Transition Zone. Temperature along with depth concentrate prey resources in this zone. The seals also showed a high tendency to travel near the edges of anticyclonic eddies, high-velocity regions that help them reduce the energy cost of long-distance travel.
These findings provide important new insights into the seals' northbound migration routes and enhances our understanding of how they respond to oceanographic conditions. Furthermore, it demonstrates how long-term monitoring is essential to understand how these marine mammals interact with changing ocean environments.
"As top marine predators, fur seals may shift their distribution and habitat use in response to environmental changes driven by climate variability and human activities," says Li. "These shifts could lead to increased competition with coastal fisheries and with other marine predators for shared resources."
This study offers a scientific foundation to support sustainable marine resource management and help mitigate conflicts between northern fur seals and coastal fisheries. In particular, identifying when and where intense foraging occurs may assist stakeholders in developing evidence-based strategies for coexistence.
