Robin W. Baird and Lawrence M. Dill
Canadian Journal of Zoology 73:1300-1311. 1995.
We studied the occurrence and behaviour of so-called transient
killer whales (Orcinus orca) around southern Vancouver
Island from 1986 to 1993. Occurrence and behaviour varied seasonally
and among pods; some pods foraged almost entirely in open water and
were recorded in the study area throughout the year, while others
spent much of their time foraging around pinniped haul-outs and other
near-shore sites, and used the study area primarily during the
harbour seal (Phoca vitulina) weaning/post-weaning period.
Overall use of the area was greatest during that period, and energy
intake at that time was significantly greater than at other times of
the year, likely due to the high encounter rates and ease of capture
of harbour seal pups. Multi-pod groups of transients were frequently
observed, as has been reported for "residents", but associations were
biased towards those between pods which exhibited similar foraging
tactics. Despite the occurrence of transients and residents within
several kilometres of each other on nine occasions, mixed groups
were never observed and transients appeared to avoid residents.
Combined with previous studies on behavioural, ecological and
morphological differences, such avoidance behaviour supports the
supposition that these populations are reproductively isolated.
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