Abstract
Killer whales (Orcinus orca) were studied for three consecutive years in the Crozet Archipelago, Southern Indian Ocean, and 76 individual whales belonging to 18 pods were photo-identified. At Crozet a pod is typically composed of a single matriarchal unit. The population was made up of 21 % adult males, 34 % adult females, and 45 % juveniles, and was declining at a 4 % annual rate. This unusually high rate of decrease may be explained by the strong reduction of the Elephant seal (Mirounga leonina) population that took place over the last 25 years in the Crozet Archipelago. A comparison with the data available for other localities suggests that the average pod size of killer whales is dependent on the kind of prey they more generally hunt. Our observations also indicate that, within a population, pods may temporarily associate in a way to adjust their pod size to the body size of the prey. Such a behaviour appears to be unique among mammalian predators.
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