Fourth International Orca Symposium and Workshop - CEBC - CNRS

coastal waters. A satellite tracking study conducted in Norway indicates that the killer whales
follow the yearly migration route of Norwegian spring-spawning herring. However, visual
observations were not made of the killer whales during summer months and it is possible that
the killer whales were preying on one of the predator species (fish, marine mammals) that
follow herring to their feeding grounds during summer.
One of the most important findings since the previous Orca Symposium has been the
documentation of a mixed diet (i.e fish and marine mammals) for killer whales in the coastal
waters of Argentina, Brazil, the Crozet Islands, New Zealand and Norway (for one group
only). In all these areas further observations are needed for better documentation of how
common a mixed diet is and to investigate the possibility of some killer whales specialising in
marine mammals.
In short, it is clear that there are both killer whales with very specialised diet and killer
whales with a more varied diet. In addition, regardless of the degree of
specialisation/generalisation in the population, group specific specialisations have been
documented in many of the populations studied. The behaviour, especially the acoustic
behaviour, of the populations with a mixed diet should be one of the focuses of future studies
(more details in behaviour chapter).
It should also be stressed that it is not sufficient to divide the prey of killer whales into
just these two broad categories; fish and marine mammals. Killer whales do also feed on other
type of prey (for example pelagic sharks). In addition, the behaviour and sensory abilities
differ greatly among marine mammal and fish species.
One of the obvious benefits of specialising on certain type of prey is in being able to
develop and refine foraging techniques in a way that makes these populations very efficient
hunters. However, the cost of being a specialist might be that the population is not flexible in
adjusting to changes in prey abundance, ie. such populations are not likely to switch their prey
type.
The workshop participants suggested that predictability and abundance of prey could
explain why some killer whales are specialised in their choice of prey while other populations
are not. It was suggested that modelling prey abundance and occurrence for areas where there
are specialists and areas where there are generalists might give us a better understanding of
the underlying mechanisms.
How to determine prey species?
As the previous discussion shows, being able to identify killer whale prey species is
important. In principle there are three different ways of studying the diet of killer whales;
stomach content analysis of dead animals, observations of foraging killer whales and analysis
of stable isotopes or fatty acids from biopsies.
Identifying prey species through observing feeding killer whales can be very
challenging. Identity of prey species is relatively easy to establish when feeding occurs close
to the surface (eg. carousel feeding on schooling fish) or on the shore (eg. killer whales
hunting on pinnipeds in Crozet and Patagonia). Observing killer whales feeding on fish or
marine mammals below the surface is much more demanding. Underwater videocameras
FOURTH INTERNATIONAL ORCA SYMPOSIUM AND WORKSHOPS
SEPTEMBER 23-28 2002, CEBC-CNRS, France
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