Foraging ecology of subantarctic fur seals Arctocephalus tropicalis breeding on Amsterdam Island: seasonal changes in relation to maternal characteristics and pup growth

Abstract

The distribution and availability of marine resources are directly affected by seasonal spatial changes in physical and oceanographic features. They are thus likely to influence maternal for- aging provisioning patterns, efficiency, and subsequent pup growth rate of central place foragers such as otariid seals. While previous studies have documented foraging locations and diving activity of female otariids in relation to oceanographic features, few have focused on species characterised by a long pup-rearing period. The present study investigated seasonal changes in foraging parameters in relation to the following oceanographic features and maternal characteristics: foraging grounds (using satellite tags, geolocation, GIS and kernel estimation techniques), at-sea activity budget (using time-depth recorders), and the foraging success and diet of female subantarctic fur seals Arcto- cephalus tropicalis breeding on Amsterdam Island (southern Indian Ocean). The seals mainly ex- ploited the subtropical front, but also exhibited large differences in seasonal distribution, from short trips in restricted foraging areas during summer to widely distributed foraging grounds during the winter. This is consistent with a seasonal shift in diet and an increasing proportion of time being dedi- cated to diving and resting. This increase in foraging trip duration throughout the season paralleled decreasing rates in maternal mass gain and pup growth, suggesting a decrease in food availability. During the summer, maternal mass gain rate and pup growth were related to foraging tactics (percent of time spent diving and in the core foraging area), whereas maternal characteristics such as body length were more important during the latter months of the pup-rearing period. However, no influence of age and thus no apparent advantage of female experience was detected in this study.