Changes in subantarctic fur seal pups' activity budget and diving behaviours throughout the rearing period

Abstract

Time spent in the water and diving behaviour of subantarctic fur seals (Arctocephalus tropicalis (Gray, 1872)) at Amsterdam Island in the southern Indian Ocean were investigated using time–depth recorders throughout the pup-rearing period. The main finding of this study, contrary to the findings of previous studies conducted on other fur seal species, was the decrease in the mean daily amount of time spent in the water throughout the pup-rearing period. The decrease in the amount of time spent in the water by older individuals compared with younger ones is likely to be related to the reduction of the pups' energy expenditure with a decreasing provisioning rate by their mothers. No change in the number of dives or the amount of time dedicated to diving relative to the time spent in the water could be detected throughout the pup-rearing period. The mean maximum dive depth increased with pup age and decreased with pup mass, and maximum dive duration was also found to increase with pup age. The positive effect of pup size on dive rates was the only relationship suggesting that pups' growth performance may act positively on the ontogeny of their diving capabilities. The large interindividual variation in the amount of time spent in the water and in dive rates, and the absence of any clear relationship with pup mass and pup growth performance, suggest the existence of alternative strategies: either pups invest more energy in developing their swimming and diving abilities, to the detriment of the body fat stores, or reduce their activity level to enhance body fat stores to increase their postweaning survival rate.