Californian orcas used specialised tactics for hunting whale calves

Orcas that live in the deep, open waters off the coast of California have specialised tactics for hunting whales and other marine mammals, which involve working together over vast distances to coordinate attacks.

Different populations of orcas (Orcinus orca), or killer whales, have developed distinct behaviours as a result of their environment. Some mainly eat fish and squid, while one type in the north-east Pacific, known as transient killer whales, primarily feeds on other marine mammals. This type can be divided into two groups, one that feed close to the shore and another that hunt in deep water, but less is known about the latter group.

To learn more about how these animals hunt, Josh McInnes at the University of British Columbia in Canada and his colleagues compiled observations and survey data of orcas around Monterey Bay in California from between 2006 and 2021.

In total, the team identified 183 individual orcas that were only spotted in open water and primarily hunted California sea lions (Zalophus californianus), grey whale (Eschrichtius robustus) calves and northern elephant seals (Mirounga angustirostris).

Unlike coastal populations, open water orcas have nowhere to corner their prey, so they have had to come up with specialised hunting methods.

Monterey Bay features a deep submarine canyon with steep sides where grey whales often feed on crustaceans. The transient killer whales in this area seem to use the canyon walls for hunting, says McInnes. “That was one of the most surprising parts of the study.”

By patrolling these canyon walls, the orcas can spot and chase mother whales across large distances until their calves get tired. Once the calves are separated from the mother, the orcas drown them by stopping them from coming up for air.

Away from the edges of the canyon, in more exposed waters, groups of orcas tend to spread out so they are over a kilometre apart and zigzag on long dives. Once an orca locates potential prey, they signal others in the area to come and hunt it together. This often involves ramming the prey animal with their bodies or heads and flicking their tails to propel it into the air.

“It’s very unique, especially for this area at least,” says McInnes. “We’re constantly learning about killer whales. Every study that comes out is extremely valuable for our understanding of their ecology as well as their conservation needs.”