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Natural Born Killers

Natural Born Killers

As sea ice melts, orcas are staying in the North longer. That’s bad news for narwhal.
By Elaine Anselmi
May 03
2017
From the May 2017 Issue

Killer whales, those smooth, almost cartoonish looking black and white mammals are the top predator in the waters they inhabit. And while they’re most common off Canada’s west coast, orca are getting more and more comfortable farther north.

Whalers’ logbooks going back to the 1800s recorded some orca sightings in Davis Strait and Baffin Bay, but over the last few years the reduction of sea ice has allowed them to move into Hudson Bay and linger in the waters of North Baffin Bay —where the majority of the world’s narwhal population spend the summer months.

“In the Antarctic it appears that killer whales cannot stay in the colder waters more than a few weeks before they have to return to warmer waters  to moult their skin,” says Steve Ferguson, marine biologist with Fisheries and Oceans Canada. “We have no evidence of this in the Arctic where the killer whales stay for months.” The orca move out quickly once the sea ice starts to form, but that time is coming later and later every year.

 

How they hunt

Inuit hunters’ accounts of orca attacking narwhal were recorded across Nunavut between 2007 and 2010 for a study Ferguson lead, published in the online journal Aquatic Biosystems. The details are rather gruesome.

Travelling in pods of up to 20, killer whales move slowly and silently through Arctic waters as they approach narwhal, creating as little wake as possible. As they get closer, the orca pick up speed and herd narwhal, driving them into deep water and away from the shelter of the shore and ice. They circle, keeping the narwhal penned in and stationary.

 Throughout the 105 accounts, killer whales bit, bashed, and batted narwhal around with their tails like soccer balls. Pieces of narwhal were gnawed off and tossed back and forth. One hunter reported two orca biting at a narwhal and pulling it apart to eat its mid-section, leaving the rest. “They may ‘play’ with their prey to teach their young how to hunt,” says Ferguson. “They may kill more than they need and then eat prime parts and leave the remains, which in anthropomorphic terms appears wasteful.” Carcasses were found with broken ribs and chunks of skin missing, but maqtaq left uneaten by the killer whales.

 

Where to hide

For narwhal and other slow-moving marine mammals, the best defence for evading killer whales is hiding out in shallow inlets and bays or among the sea ice.

 “Narwhal are very good in ice whereas killer whale are not familiar behaviourally with sea ice and they have a large dorsal fin that gets in the way in sea ice,” says Ferguson. “As we lose sea ice, narwhal lose this advantage.”

Another study Ferguson worked on (released this year) looked at the way narwhal are responding to the growing threat of predation. In Admiralty Inlet, a long fiord running south from Lancaster Sound on Baffin Island, narwhal tended to move closer to shore when orca were within about 100 kilometres. But not all narwhal go down without a fight. One hunter shared a story of a narwhal spearing an orca with its tusk. The two became locked together and both died as a result.

Among the marine mammals studied, seals (ring, bearded, harp and hooded) were the most common prey of killer whales, followed by narwhal. But for all those reports, Ferguson says there is still limited data on what this means for narwhal numbers. “We have no evidence that the population size of narwhal is decreasing or being greatly affected by killer whale predation,” he says. “This may change as we continue to lose sea ice but so far things seem okay.”