Influence of environment, morphology, and instrument size on lactating northern fur seal Callorhinus ursinus foraging behavior on the Lovushki Islands, Russia

Food limitation may be one of the causes of declines in northern fur seal populations on the Pribilof Islands. This hypothesis could be tested by comparing foraging behavior from decreasing Pribilof fur seal populations and an increasing population, such as on the Lovushki Islands, Russia, but factors other than prey availability that differ between sites may also influence behavior. Therefore, we evaluated such factors, including lunar cycle, weather, seal body size, and size of recording instruments, by studying 41 lactating northern fur seals on Lovushki Island over 4 summer breeding seasons using instrument packages of various sizes. With greater moonlight, seals increased foraging trip duration, dive depth, dive duration, and time spent on the bottom of dives but decreased descent rate and diving bout duration. Larger females made shorter shore visits, spent a greater proportion of time at sea diving, and had longer dive bouts than smaller females. Tags with larger frontal surface areas and higher drag caused seals to dive longer and to descend and ascend faster during dives but did not affect foraging trip durations or mass change rates. Seals, therefore, appeared capable of compensating for instrument effects on the scale of individual dives. Although lactating seals from Lovushki Island appeared to spend less foraging effort than seals from the Pribilofs, future studies should control for methodological factors and local environmental conditions before concluding whether food limitation could explain differences in population trajectories.