Il. Boyd et al., FORAGING BEHAVIOR OF ANTARCTIC FUR SEALS DURING PERIODS OF CONTRASTING PREY ABUNDANCE, Journal of Animal Ecology, 63(3), 1994, pp. 703-713
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.