Northbound movement of northern fur seal (Callorhinus ursinus) and their response to the oceanographic features

Abstract

Northern fur seals (NFS; Callorhinus ursinus) are a highly migratory otariid species that leave breeding grounds in autumn (October–November), travel south to winter foraging grounds, and return to breeding colonies in early spring (March–April). NFS migrate south to regions with sufficient food, and their movement is hypothesized to be correlated with several oceanographic features. Although southward migration patterns are well-documented, fine-scale information on northbound migration remains limited owing to extended pelagic wintering periods and associated tracking challenges. Using satellite tags, we tracked five juvenile and subadult male NFS from a primary wintering ground in the Sea of Japan to examine their movement patterns, migration pathways, and behavioral responses to oceanographic features. Migration was divided into stay and northbound transit phases based on movement patterns. Slower movements with frequent course changes dominated the stay phase, whereas faster and more directed movements characterized the northbound transit phase. Foraging behavior was concentrated in continental shelf-break regions, where bathymetric features enhance prey availability. Foraging was most frequent within water temperature ranges of 8 °C–13 °C, corresponding to the highly productive Kuroshio-Oyashio Transition Zone. In contrast, transit behavior predominated at water temperatures below 2 °C and above 13 °C, likely reflecting physiological constraints. NFS also exhibited transit behavior near anticyclonic eddy edges, suggesting they use high-velocity regions to reduce travel costs. This study provides valuable insights into the northbound migration strategies of NFS overwintering in the Sea of Japan, demonstrating how they use specific oceanographic features for efficient foraging and movement during migration.