At-sea behavior of juvenile male northern fur seals (Callorhinus ursinus)

Abstract

The at-sea behavior of juvenile male northern fur seals, Callorhinus ursinus (L., 1758), captured at two haul-out sites on St. Paul Island, Alaska, during the 1999 and 2000 breeding seasons (July–September) was studied. To compare at-sea locations, dive behavior, and changes in body mass, 31 juveniles between the estimated ages of 3–6 years were captured, instrumented, and released. Individuals behaved like central-place foragers by making trips to sea and returning to the Pribilof Islands. Trip durations ranged between 8.74 and 29.81 d, whereas distances from departure site ranged between 171.27 and 680.68 km (maximum straight-line distance). Differences in maximum straight-line distance traveled and trip duration were not observed when comparing years or departure site. Diving tended to reflect patterns associated with different bathymetric domains; shallow nighttime diving was common in ~3000 m deep waters, whereas deeper diving was generally observed in <200 m deep waters. Proportion of body mass gained over a single trip to sea averaged 27.8% (range 3%–65%, n = 19). Mass gain was similar between individuals that dove in shallow waters (over the continental shelf; 10.9 ± 1.8 kg (mean ± 1 SE), n = 11) versus individuals that dove in pelagic waters (8.5 ± 1.0 kg, n = 8). These results demonstrate that the at-sea behavior of juvenile males can extend farther from the Pribilof Islands when compared with previous reports of parturient female at-sea behavior, thus revealing important variation within this species.