FORAGING BEHAVIOR OF ANTARCTIC FUR SEALS DURING PERIODS OF CONTRASTING PREY ABUNDANCE

1. Foraging behaviour of Antarctic fur seals rearing pups at Bird Isla nd, South Georgia, was assessed using at-sea activity patterns measure d by electronic time-depth recorders. Information was obtained for a t otal of 75 individuals and 191 foraging trips to sea over five reprodu ctive seasons from 1988/89 to 1992/93; this included one season (1990/ 91) of low prey abundance. A method was developed to divide the diving record up into logical units or bouts which differed from past method s used for defining bouts of behaviour. 2. Foraging trips were signifi cantly longer in 1990/91 than in the other years. There were significa nt differences between years in the proportion of time spent foraging when at sea and in the distribution of foraging through the day and ni ght. These differences probably represent behavioural responses to cha nges in prey distribution and abundance and were reflected in the freq uency of occurrence of different types of foraging behaviour. 3. Four types of foraging bout were recognized using a cluster analysis. Type I (short) bouts were of short duration (17 min) and occurred mainly du ring daytime and at dusk. They probably represented exploratory behavi our. Type II (long) bouts occurred mainly at night and were of long du ration (80 min). They increased in frequency in 1990/91 when food was scarce and 61-73% of time spent foraging was in these bouts. Type III (shallow) bouts occurred mainly at night, were of short duration (12 m in) and represented feeding close to the surface, possibly in associat ion with other, surface-feeding krill predators. Shallow bouts account ed for 8-14% of time spent foraging. Type IV (deep) bouts were of medi um duration (19 min) and represented feeding at greater depth (40-50 m ) than other bout types. They were most abundant around dawn. 4. Mean dive duration during bouts exceeded the theoretical aerobic dive limit on > 30% of occasions for short, long and deep bouts. There were posi tive correlations between mean dive duration and surface interval dura tion for most of these bout types in most years. This suggested that l ong dives incurred a cost in terms of the amount of time spent at the surface between dives. 5. The study demonstrated that female fur seals invest a significantly greater effort in foraging during periods of l ow prey abundance by both increasing the time spent foraging and by in creasing activity during foraging. This could represent a 30-50% incre ase in the costs of foraging during years of low food abundance.