Fourth International Orca Symposium and Workshop - CEBC - CNRS

Evidence for flexibility in vocal behaviour has been demonstrated for transients in
Monterey Bay; the killer whales are silent while hunting small-sized marine mammal prey but
are very vocal while hunting and feeding on grey whale calves. It is possible that the vocal
activity increases to attract more whales to participate in the hunt for larger sized prey.
Observations of transient killer whales in BC and Alaska also suggest that the transient killer
whales which have been thought to occur in small groups may be living in larger social units
that are in acoustic contact. They forage in small groups and start vocalising after kills to join
for social behaviours.
The group size of transients may vary depending on the size of prey. When feeding on
harbour seals, the groups tend to be small. Sea lions are larger and also more dangerous for
the killer whales to catch and therefore the group size is often larger when sea lions are
caught. Although harbour porpoises, Dall’s porpoises and white-beaked dolphins are not very
large, the transients need to operate in larger groups while hunting them since these small
cetaceans are swift and able to evade predation unless pursued by several whales.
The size of the prey can also be viewed in terms of mechanistic constraints. A smaller
whale can turn quicker than a larger whale, and a small fish can turn faster than any whale.
All strategies of hunting and feeding vary with many factors relating to prey size and
therefore the prey size and behaviour (for example schooling) will determine foraging
behaviour
Relatively little is known about potential age/sex related differences in the feeding
behaviour of killer whales. In the marine mammal hunts where killer whales partially strand
themselves, such behaviour is not performed by adult males, probably due to their larger body
size which gives them a greater risk of getting permanently stranded. However, in New
Zealand, where killer whales feed on rays in very shallow waters, both sexes participate in
feeding. Adult females seem to be the ones taking tuna from longlines in the Gibraltar strait,
while both sexes are engaged in this behaviour in New Zealand. It has been suggested that
adult males would have an important function in feeding behaviours where deep and/or long
dives are needed. In Norway killer whales occasionally chase up herring schools from 150-
300 meters depth. However, the dive data collected in the satellite tracking study does not
indicate that the adult males would make longer or deeper dives than subadult females. On the
contrary, the deepest and longest dives have been made by 8-10 year old females.
In almost all marine mammal kills by transients, food sharing is observed. This is often
detected by observing gulls following different group members as they tear up prey and leave
scraps behind. In New Zealand killer whales are observed sharing food (including several
observations of adult males sharing food with young calves) regardless of the age/sex of the
animal killing the prey or the type of prey. The killer whales feeding on herring schools in
Norway stun their prey with tailslaps before feeding; other whales than the ones performing
the tailslaps also feed on the stunned prey.
When killer whale prey species are known, research projects studying the foraging
ecology of killer whales benefit from cooperation with scientists studying the behaviour of the
prey species. In the Norwegian study, cooperation with herring biologists has been of vital
importance in understanding the behaviour and habitat use of killer whales. One important
aspect of the biology of the prey species is their sensory abilities; especially knowledge about
hearing and vision are important in understanding the techniques killer whales use while
hunting
FOURTH INTERNATIONAL ORCA SYMPOSIUM AND WORKSHOPS
SEPTEMBER 23-28 2002, CEBC-CNRS, France
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