Killers in the Dark – the night-time behavior of mammal-eating killer whales.

Predation by mammal-eating killer whales could be one of the main forces controlling populations of Steller sea lions, fur seals and other marine mammals in the North Pacific. As active, warm-blooded animals, killer whales have tremendous appetites.

listen

DTAG recording of a tagged killer whale chasing and capturing a Dall's porpoise

above: Female transient T065 during an attack on a Dall's porpoise.

Killer whales that specialize on eating marine mammals consume the approximate equivalent of one harbor seal per whale per day. Multiply this by killer whale populations that number in the hundreds in some areas and you have a predatory force to be reckoned with.

Understanding the effects of killer whale predation requires a sound understanding of the prey preference and hunting behavior of mammal-eating killer whales. But studying killer whale predation in the wild is challenging. During the day, researchers rely on short glimpses into the lives of the animals when they break the surface to breathe. But during night, researchers have had to rely on best guesses.

Whether mammal-eating killer whales continue to hunt during nighttime has important implications on the extent to which predators control prey populations, especially in high-latitude areas where daylight is extremely limited for parts of the year. Unlike fish-eaters, mammal-eating killer whales do not emit echolocation clicks to search for prey. Instead, they may rely on visual cues to locate marine mammals or listen for sounds generated by the prey.

Hunting would have to stop after nightfall if killer whales only rely on their excellent vision to find prey, offering their prey a reprieve from predation and a chance to forage or travel with relatively little risk of being eaten. If, on the other hand, killer whales rely primarily on passive listening for prey sounds to find prey, they may hunt preferentially at night when darkness would make them less detectable.

In a recent study funded by the North Pacific Universities Marine Mammal Research Consortium, researchers Volker Deecke, Patrick Miller and Ari Shapiro set out to study the predatory behavior of mammal-eating killer whales in Southeast Alaska. The team used DTAGs, a novel digital recording device developed at the Woods Hole Oceanographic Institution by senior engineer Mark Johnson and his team to learn what the animals are doing at night. This tag is about the size of a cell-phone and is attached to the back of a killer whale with four suction cups using a 7m carbon fiber pole. With deployment times of up to 16 hours, the tags record the precise movements of tagged individuals and any sounds this animal makes or hears. Successful predation events can be identified easily from distinctive crunching sounds generated as the killer whales feed on a prey carcass.

During the 14 day research trip in early July 2006, the team tagged 13 individual killer whales obtaining over 100 hours of on-animal data.

 “This success came as a huge surprise for us” notes Volker Deecke. “In the beginning we weren’t even sure if the animals would allow us to get close enough to deploy the tags. But they didn’t seem to be bothered by the tagging procedure or by the tags themselves”.

Seven of the tags remained on the animals overnight. All four tags analyzed so far recorded predation events during the hours of darkness, surprising given that nighttime lasts for only about 4 hours in Southeast Alaska in July.

According to Deecke, “These results show that killer whales have no problems finding marine mammal prey in the dark, which suggests that they rely primarily on acoustic, not visual cues. We were impressed by the efficiency and swiftness with which they manage to capture and subdue even fast or large prey species – one group of 4 animals launched 3 attacks on Dall’s porpoises during a 12 hour tag deployment — and all of them were successful!

The big question now is whether predation rates actually increase after nightfall or remain the same”. To answer this, the team plans to conduct an additional field season this summer. Stay tuned for the final results.

The researchers are grateful to Mark Johnson and Alessandro Bocconcelli for supplying and calibrating the DTAGs. Research assistants Michiru Main and Michael deRoos provided instrumental help with tag deployment and data collection day and night. They especially thank Captain Jim Eastwood of the F/V Charles T, and his wife Gayle for looking after the team and for spending many nights at the helm following killer whales.  Graeme Ellis also deserves special thanks for his his extensive help with the photographic identification of tagged individuals.

11 April 2007