Minke whales are predominant prey of killer whales in Northwest

According to Tara Stevens, who will earn her doctorate at the University of Rhode Island Graduate School of Oceanography later this year, about 200 killer whales are believed to live in the area, most of which are seen in the summer months around Newfoundland.

"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," explained Stevens, who published a field guide to whales and dolphins in Atlantic Canada in 2013. "Their strategy is to drown the animal. We would see sometimes 10 or 20 killer whales jumping on a minke to force it under water to drown it."

In addition to her own field observations, the remoteness of the area necessitated that she gather sighting reports from fishermen, tour boat captains and others in the area who are interested in whales. Her data indicate that killer whales also prey on dolphins, porpoises, seals and other prey. Stevens said it is difficult to determine if they may also be eating fish because that takes place entirely under the water and out of sight of observers.

"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."

Although it is unknown whether killer whales in the North Atlantic are prey specialists, as they are in the Pacific, Stevens believes it is likely that the distribution and movement patterns of the whales are linked to those of their prey. For instance, some killer whales that remain year-round in Newfoundland and Labrador have been sighted within the pack ice feeding on breeding seals.

Stevens will present the results of her research Feb. 22, 2016 at the biennial Ocean Sciences meeting in New Orleans, sponsored by the American Geophysical Union, the Oceanography Society, and the Association of the Sciences of Limnology and Oceanography. She is being advised by Jack Lawson, a research scientist with the Canada Department of Fisheries and Oceans, and URI Marine Scientist Emeritus Robert Kenney.

Stevens is creating a catalog of photographs of killer whales in the northwest Atlantic in an effort to identify individual whales so she can track their movements and learn about their social interactions. She has also collected biopsies on some animals for DNA and health analyses.

"The population dynamics seem to be similar to those of the transients in the Pacific," said Stevens. "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."

She also noted that social interactions among the whales is variable.

"They transfer between groups often and hang out with individuals of other groups," she said. "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."

In addition to the killer whales living around Labrador and Newfoundland, Stevens said there are additional groups around the British Isles, Norway and the Arctic Canadian islands. She wonders whether those animals may occasionally interact with the Newfoundland whales.

"There are still a lot of questions to answer," she said. "It would be great to have satellite tags on a few individuals to learn about finer scale movement patterns and to see with greater resolution who they interact with, where they spend their time, what they do in winter, and whether there are any fish-eating killer whales."