On the Trail of the Kodiak Killers

Nestled against the rugged Alaskan coast between the Gulf of Alaska and the eastern Aleutian Islands, Kodiak Island is a natural waypoint for marine life. The breakwater at Kodiak Harbor provides a convenient haul-out for dozens of local Steller sea lions, attracting boats full of curious tourists. But the noisy haul-out has also attracted a more unusual element to the Kodiak scene: a small but highly specialized group of killer whales dubbed the “Kodiak Killers” by the scientists who seek to understand them.

Craig Matkin is one such scientist. Working with the Alaska SeaLife Center and as Executive Director of the North Gulf Oceanic Society, a non-profit organization specializing in marine mammal research and conservation, Matkin has spent more than 20 years studying killer whales in Prince William Sound. His research focuses on documenting the potential and real effects of transient (mammal-eating) killer whales on marine mammal populations and, more broadly, on entire ecosystems. For the past four years, he has worked with the North Pacific Universities Marine Mammal Research Consortium to study the role of killer whales in the decline of Steller sea lions and fur seals in the western Aleutians.

“The Kodiak Killers are a group of six whales that spend part of their winters around the city of Kodiak,” Matkin says. “They are unusual because they seem to specialize in taking Steller sea lions—it is the only prey we have ever seen them eat. Of the six whales, the adult male is a particularly efficient predator. He is extremely adept at subduing juvenile sea lions but also takes adults on occasion.”

In addition to the 25-foot long male, the other members of the group include two adult females, two juveniles born in 2002 and a calf born in 2005.

Dietary Traditions
While many generalizations have been made about Alaska’s transient killer whales across their range, their diet and behavior appear to vary from area to area. In the summer and fall in the eastern Aleutians, for example, fur seals are emerging as the most important prey for killer whales, while Steller sea lions are infrequently hunted. This picture is quite different a short distance away, near False Pass at the end of the Alaska Peninsula, where other transients specialize in hunting migrating gray whales in spring. In Prince William Sound and Kenai Fjords the threatened population of transients known as the AT1’s displays rather different predatory habits, preying mainly on harbor seals and Dall's porpoises.

“Many of these killer whale populations are small and may need to develop very different habits in order to survive,” Matkin says. “A number of researchers believe the transient killer whales pass on these different hunting traditions through matrilineal groups. This system of learning and teaching appears complicated, and it is not too surprising that you have to look from area to area and group to group to understand the bigger picture.”

The fact that the Kodiak Killers are successfully building their numbers is testament to their success at hunting Steller sea lions, which appear to be a considerably more challenging prey than harbor seals and fur seals. Matkin estimates that despite the Kodiak Killers’ unpredictable appearances—a few dozen days between February and April each year and occasional sightings in Kenai Fjords throughout the year—they collectively take an average of two juvenile sea lions (weighing about 150kg each) for every 36 hours they are present. Based on data from feeding studies of captive whales, he says this rate of consumption is probably about average for a group of wild killer whales.

But does such consistent predation present a threat to Kodiak’s Steller sea lions? Matkin says there is not enough information to be certain. “When a declining population is trying to recover, any animal removed is one less animal that could potentially produce offspring and help the population recover,” he says. “The question is whether it is stopping recovery. If the reproductive potential of the animals is high enough, it could offset the losses to predation.”

The Big Picture
Data on killer whale predation in Alaska is spotty at best. In fact, in western Alaska only the population in Kenai Fjords has been studied thoroughly enough to determine whether those killer whales pose a significant threat to Steller sea lion recovery (current studies suggest they do not). In other areas of sea lion decline, the picture is far less clear. For his part, Matkin is combining his own first-hand behavioral observations with population data derived from photo identification work. These data will help him to estimate how much prey a population of killer whales needs to eat, based on its size and predatory preferences.

This naturally requires a consistent field presence, which sometimes has exhilarating results. Matkin recalls observing a recent encounter between the Kodiak Killers and a tour boat in Kenai Fjords: “Apparently a juvenile Steller sea lion surfaced next to the tour boat, and all of a sudden the big male killer whale came right up under the bow and sucked it down.

“Other members of the group appeared behind the tour boat, and the male brought the sea lion over to them,” he continues. “We dropped a hydrophone in the water and heard this racket of vocalizations and crunching bone, and this fine rain of pulverized blubber boiled to the surface. Then the big male surfaced ten feet behind our boat with a whole flipper and scapula in his mouth, waving it at us as if to say, ‘Are you getting all this?’”

Matkin and colleagues are getting it. Although much remains to be learned, their combined research on the Kodiak Killers and other transient killer whale populations in Alaska is slowly unraveling the complexities of life atop the marine food web.

April 3, 2006