The hunting strategies of killer whales

Killer whales adopt different strategies to catch different prey species. When they hunt dolphins, they close in silently forming a circle round their prey and then move steadily in, closing the circle, driving the hapless victims before them into crowded rings,
observes Nutan Shukla

FOR centuries humans have been in awe of the marine mammal which in English is known as killer whale, in Spanish "assassin", Romans called it ‘orca’ after an underworld ogre (man-eating giant of the folklore) and the Scandinavians know it as ‘blubber chopper’. All the above names highlight its savage reputation as the only whale to devour warm-blooded creatures.

Although given the name ‘whale’, the orca is the largest of the dolphins. It can grow to a length of 30ft and weighs up to 9.5 tonnes. Despite its large size it is one of the swiftest animals in the sea, a quality which gives it an added advantage over its prey. In other words it is one of the sea’s supreme predators.

Living in groups of up to 20 individuals they have matriarchal society where each member is related with one another through the two or more mothers and one or two grandmothers, who dominate the group. Rest of the members are juveniles, youngsters and bulls. However, the bulls are much larger in size and strong but they have inferior status as compared to cows.

The diet of these mammals vary considerably. In the northern-Pacific orcas prey upon sea-lions, elephant seals and other dolphins. Off Japan they feed on cod and halibut; and near the Brazilian coast they hunt stingrays.

In one orca study area, off the coast of British Columbia near Vancouver Island, scientists have identified two kinds of orcas; residents and transients, which look and behave differently. The residents have a slender, rounded dorsalfin, travel in pods of up to 30 individuals, and are most active when the pacific salmon return to their home rivers to spawn. Their lifestyle is determined by the behaviour of their prey.

Salmon swim in large mobile shoals, a food source that is said to be ‘clum-ped’. In order to find a ‘clump’, it is an advantage to have a large group which can spread out and, working together, search a wide area of ocean.

When these creatures set out for hunt, they spread out in a line-abreast search formations, each individual positioning itself about 150ft from its neighbour. The line advances slowly, the whales constantly ‘talking’ to each other in squeals and moans. At the sound of a loud honk, the pod converges on the salmon and herds them towards a cove, trapping them against the shore. Fish on the periphery of the shoal are picked off one by one. Then, as suddenly as it began, the attack is called off. The whales submerge, swim further along the coast, and repeat their herding and feeding behaviour all over again.

Transient whales, which have a stubby, triangular dorsal fin, are less predictable in their behaviour and travel inshore to feed on more thinly dispersed and less abundant sources of food. The transient hunt is more dramatic, the pod of these creatures concentrate on seals and sea-lions instead of fish. A small pod will round up a group of sea-lions, isolate a victim, and then but it with their powerful tails until it is unconscious. This may take a couple of hours, after which the prey is taken below, drowned and eaten. Despite its size, the orca is a cautious predator. A bite or scratch from a large seal could inflict serious damage.

Killer whales adopt different strategies to catch different prey species. When they hunt dolphins, they close in silently forming a circle round their prey and then move steadily in, closing the circle, driving the hapless victims before them into a crowded rings. When the killer circle has decreased to about 50 metres across, the dolphins swimming round nose to tail, three or four of the hunters enter the enclosure and select as many victims as they need to satisfy their hunger. These they maim, biting some across the tail so they cannot swim away.

Orcas not only attack small mammals like dolphins and seals but they also do not spare even the creature like blue whale, the world’s largest living animal. Other whales are also attacked. Grey whales, migrating along the pacific coast of North America, are followed by pods of orcas. Humpback whales, which feed off the coasts of New-foundland and Alaska, are mercilessly pursued. As always, the victims are often the young, the old and the infirm. Orcas exhibit marked divisions of labour while on a hunting trip. In one such attack on blue whale it was observed that some orcas flanked the blue whale on either side, as if herding. Two others went ahead and stayed behind to foil any escape attempts. One group seemed intent on keeping the blue whale underwater to hinder its breathing. Another phalanx swam underneath its belly to make sure it didn’t dive out of reach. The big whale’s dorsal fin had been chewed off and its tail flukes shredded, impairing its movements. The bulls led forays to pull off huge chunks of flesh. This battle continued for full five hours, but miraculously, the whale survived and eventually got away.

Young southern sea-lions on Patagonian beaches may not be so lucky. In this area, orcas have learned to change ashore, coming right out of the water, grap a victim and shuffle back into the sea. Any individual careless enough to stay in the target zone is in mortal danger. The orcas are unexpectedly fast and surprisingly well-practised. During the penguin breeding season, when sea-lions are making offshore feeding forays of their own search of penguins, the orcas have been seen to practise their beach-charging skills on steeply shelving beaches similar to those frequented by the sea-lions during their breeding season. The young whales learn from their elders, the technique standing them in good stead for the whales are able to take 20 sea-lions in an hour.

The sea-lions seem frozen to the spot, mesmerised and confused by the great black-and-white shape that emerges from the water and grabs them on dry land.

This feature was published on March 21, 1999