Hakai Magazine

Coastal science and societies

northern resident killer whales at sunset
Killer whales keep busy day and night—but what exactly they get up to at night is not well known. Photo by Interfoto/Alamy Stock Photo

What Do Killer Whales Do at Night?

An ongoing tracking study seeks to understand what killer whales do under the cover of darkness.

Authored by

by Larry Pynn

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Scientists know much about the imperiled, fish-eating resident killer whales that live off the west coast of North America, but some facets of these marine mammals’ lives are mysterious. For instance, what do they do at night?

Think about nocturnal animals and bats and owls probably come to mind. Most animals, humans included, are diurnal and on the go during the day. Killer whales, however, follow a diel cycle—they’re active both day and night. The puzzle for researchers is to determine how the whales’ behavior—their foraging, socializing, traveling, resting, and sleeping habits—changes from day to night.

Understanding the intricacies of whale behavior is difficult at the best of times, even more so when they slip below the ocean’s surface at night. “We hear them on the hydrophones at night,” explains Sheila Thornton, a research scientist with Fisheries and Oceans Canada who is overseeing a study into the shadowy lives of killer whales. “They’re active in their vocalizing, but we want to take that one step further and see what they’re actually doing.”

In August, four researchers took to the water in Telegraph Cove, off the northeast coast of Vancouver Island, British Columbia, armed with the latest in whale-tracking technology. Their targets were the threatened northern resident killer whales, whose population is growing but still totals only an estimated 309 individuals across their entire range from southeast Alaska to southern Washington State.

Armed with decades of data on the whales’ movements, along with real-time information from other researchers and whale watching boats, Thornton’s team had little problem locating the whales. As evening approached, they moved close to the pod and used a six-meter pole to attach digital acoustic recording tags just below the whales’ dorsal fin using four suction cups.

Thornton and her team use a pole to attach a digital acoustic recording tag to a northern resident killer whale. Video courtesy of DFO

Thornton says that compared with other types of tracking tags, which penetrate the skin, the suction cup tags are low impact. Occasionally, a whale might slap its tail or roll in response to the intrusion, but most just dive and come back up, she says.

Each tag is about the size of a large chocolate bar, and provides detailed, three-dimensional data about the whales’ movements. The tags show when and how a whale is swimming or diving, and even whether a hunting attempt is successful.

Hydrophones on the tags provide another level of detail—right down to the crunch of a captured chinook salmon and the sound of a whale rubbing itself on the beach. “You can hear the pebbles rolling, and vocalizations of the animals, and vessels in the background,” Thornton says.

After several hours, the suction cups lose their grip and the tag floats to the surface, emitting a radio signal that helps the researchers find it. “It can be very challenging,” she says. The signal can echo off the land, turning an archipelago into a pinball machine. “You can spend a lot of time waving an antenna around.”

Thornton’s team tagged 17 whales this past summer, obtaining more than 70 hours of data—half at night. The study is expected to continue for another two years; analysis of the information is a huge undertaking that should yield a treasure trove of information on killer whale behavior.

Previous research using less sophisticated technology showed that southern resident killer whales—a distinct, endangered population—swim slower and dive less often at nighttime. Robin Baird of the Cascadia Research Collective in Washington State suggests that the southern residents use more than echolocation to hunt their prey.

“When they’re catching these large, evasive fish, such as chinook, they also probably use vision, at least in the final moments of the chase,” he says. “When it’s dark, they’re just less likely to forage.”

Thornton’s ongoing research will determine how Baird’s results stack up against the northern residents, while shedding light on how killer whales are affected by noise from nearby vessels. “It’s quite surprising the way just one vessel can obliterate their vocalizations,” she says.

Farther south, researchers with the US National Oceanic and Atmospheric Administration are undertaking similar studies on the southern resident killer whales.

Thornton says the results of this and related studies will be useful for informing agencies such as Transport Canada on whether restrictions on vessel activity could mitigate impacts on the struggling killer whales.