Most of the stories we hear about orcas come from the Northern Pacific Ocean, but orcas are also found in the North Atlantic.
About 200 orcas live around Newfoundland and Labrador, but little is known about their lives. Tara Stevens, a doctoral candidate at the University of Rhode Island (URI) Graduate School of Oceanography is the first to investigate their distribution, movement, and feeding habits. She found that these orcas feed primarily on minke whales.
“We had no idea what they were even feeding on in the Atlantic, but eventually it has became more and more clear that minke whales are the predominant prey source for certain killer whales in our area,” Stevens explained in a news release. “Their strategy is to drown the animal. We would see sometimes 10 or 20 killer whales jumping on a minke to force it underwater to drown it.”
According to data collected from field observations, and reports from fishermen and tour boat captains, she found that orcas also eat other prey, including dolphins, porpoises, and seals.
Pacific orcas can be separated into two categories (transient and resident populations) based on not just their locations, but also by their preferred prey. Transients are known for eating marine mammals, while residents eat fish. It’s unknown whether the Atlantic orcas have distinct feeding preferences like this and, according to Stevens, it might be hard to figure that out.
“We can’t rule out that some animals may be hunting fish,” she said. “We’ve seen some taking halibut and tuna off of longlines. And there used to be a substantial population of killer whales associated with the tuna fishery in the Gulf of Maine, but after that fishery crashed, the fishermen weren’t seeing killer whales any more. We have no idea where they went or what they’re feeding on now.”
From a social standpoint, they seem to behave similarly to the transients in the Pacific.
“They seem to roam around in groups of five or six individuals and don’t have strong fidelity to any particular site. They don’t spend long periods of time in the same area like resident populations do out west,” said Stevens.
“They transfer between groups often and hang out with individuals of other groups. They’ll go off alone and join up with a different group weeks later. Again, that’s like the transients out west. Resident groups of killer whales in the Pacific have a stable social structure where group members never leave, and we don’t see any of that here as of yet.”
To answer the remaining questions, like whether or not any of them eat fish, Stevens said it would be helpful to track a few individuals with satellite tags.
To learn more:
- Read the URI news release: Minke whales are predominant prey of killer whales in northwest Atlantic
Copyright © 2016 by Marine Science Today, a publication of Marine Science Today LLC.