Understanding how northern fur seals (Callorhinus ursinus) navigate the North Pacific is crucial not only for marine science but also for managing rising tensions between wildlife and fisheries. A new study by the University of Hawaiʻi at Mānoa, Hokkaido University, and led by the Wildlife Research Center of Kyoto University reveals that the seals' northbound migration is shaped by a sophisticated interplay of prey distribution and ocean current dynamics—offering key insights into a previously understudied part of their life cycle.
Northern fur seals are long-distance migrants, traveling seasonally between southern wintering areas and northern breeding colonies to avoid ice and access rich feeding grounds. Along Japan's Sea of Japan coast, they are increasingly seen interacting with fishing operations—damaging gear and removing fish from nets—making it vital to understand their movement patterns.
While southward migrations have been better documented, their return journey in spring has remained elusive due to challenges like tag loss and tracking limitations.
Abundant prey, energy conservation
A northern fur seal's spring migration toward northern breeding colonies is influenced by two critical factors: locating abundant prey and utilizing ocean currents to minimize energy expenditure.
"Specific seafloor features may concentrate prey resources, and water temperature data suggest that animals may be foraging in the Kuroshio-Oyashio Transition Zone, where prey availability is high," said Heping Li, lead author and doctoral student at Kyoto University.
"Seals also showed a high tendency to travel near the edges of anticyclonic eddies, where high-velocity regions may reduce travel cost," added Ryo Dobashi, oceanography graduate student in the UH Mānoa School of Ocean and Earth Science and Technology .(SOEST)
Movement patterns revealed two distinct phases: in the stay phase, south of 43° N, seals exhibited slower movements with frequent course changes, while in the northbound transit phase, when they were north of 43° N, their movement became faster and more directed. Results revealed that foraging activity was concentrated along continental shelf-breaks and within particular temperature ranges (8–13°C).
"Our study filled key gaps in understanding the northbound migration of juvenile and young adult male northern fur seals," said Li. "These findings enhance our understanding of how fur seals respond to oceanographic conditions during this critical phase of their annual life cycle."
