Foraging behaviour of Antarctic fur seals during periods of contrasting prey abundance

1. Foraging behaviour of Antarctic fur seals rearing pups at Bird Island, South
Georgia, was assessed using at-sea activity patterns measured by electronic time-
depth recorders. Information was obtained for a total of 75 individuals and 191
foraging trips to sea over five reproductive 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
methods used for defining bouts of behaviour.
2. Foraging trips were significantly longer in 1990/91 than in the other years. There
were significant differences between years in the proportion of time spent foraging
when at sea and in the distribution of foraging through the day and night. These
differences probably represent behavioural responses to changes in prey distribution
and abundance and were reflected in the frequency 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 during daytime and at
dusk. They probably represented exploratory behaviour. Type II (long) bouts occurred
mainly at night and were of long duration (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 min)
and represented feeding close to the surface, possibly in association with other,
surface-feeding krill predators. Shallow bouts accounted for 8-14% of time spent
foraging. Type IV (deep) bouts were of medium 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 positive correlations
between mean dive duration and surface interval duration for most of these bout
types in most years. This suggested that long 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 low prey abundance by both increasing the time spent
foraging and by increasing activity during foraging. This could represent a 30-50%
increase in the costs of foraging during years of low food abundance.