Migratory movements of rhinoceros auklets in the northwestern Pacific: connecting seasonal productivities
Spatial and temporal variability in marine biological productivity may drive heterogeneity in seasonal resources available for marine animals in temperate waters. Migratory seabirds are expected to adjust their annual cycle of breeding activities and migratory movements to exploit seasonally available resources efficiently. We studied the movement and trophic position of rhinoceros auklets Cerorhinca monocerata breeding at Teuri Island, Japan Sea, during the nonbreeding and early breeding periods over 2 yr. After breeding, the auklets moved northward from the colony to the Sea of Okhotsk, where phytoplankton blooms enhanced biological productivity in autumn. The birds then moved southward to the southwestern Japan Sea (~1470 km from the colony), where major epipelagic fish and squid concentrations have been reported in winter. Stable isotope analyses suggest that the auklets fed on higher-trophic level prey, including fish and/or squid during the autumn and winter nonbreeding periods. The auklets moved northward and returned to the colony in mid-March. During the early breeding period, the birds foraged close to the colony (~380 km) on lower-trophic level prey including fish and/or krill, which were available during the spring phytoplankton bloom. The timing of the return migration does not match with the northward migration of warm-water anchovy, a profitable prey during summer, but may be related to timing the chick-rearing period to correspond with anchovy arrival. We suggest that rhinoceros auklets follow spatial and seasonal changes in prey availability by a distinctive ‘3-step’ migration (first northward, second southward, third northward) in the temperate marine system of the northwestern Pacific.