Killer Whales Develop a Taste For Sea Otters
by Ned Rozell
This column is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Ned Rozell is a science writer at the institute.
Sea otters are getting harder to find along the western part of the Aleutian chain. Their population has dropped from about 53,000 animals in the early 1990s to only 6,000 today. Some biologists think the missing otters of western Alaska have disappeared to an unlikely place--the bellies of killer whales. Researchers say the actions of people may have caused this unusual switch in the diet of killer whales.
Jim Estes, a wildlife research biologist who works for the U.S. Geological Survey at the University of California, has watched sea otters in Alaska since the 1970s. On his 1990s cruises to the Aleutians, he and other biologists noticed a 25 percent decline in sea otters each year. At first, Estes didn't consider killer whales as a reason for the sea otter decline. Killer whales mostly eat sea lions, seals, and other marine mammals that spend most of their time far offshore, away from sea otters.
When he was on a cruise from Attu to Dutch Harbor in the early 1990s, Estes and his colleagues saw killer whales where they hadn't before, observations that later became a clue to the disappearance of the sea otters. "We were seeing killer whales near the beach all the time," Estes said during a phone interview from his office in Santa Cruz, California. "All of us commented on how peculiar that was."
The whale sightings in shallow waters frequented by sea otters coincided with a nosedive in the population of harbor seals and Steller sea lions, but Estes said he was skeptical about the killer whale-sea otter connection. One reason for his doubt was that in several decades of going to sea and observing otters, he had never actually seen a killer whale eat a sea otter.
No one had published a scientific paper on killer whale predation on sea otters until Brian Hatfield, also of the U.S.G.S. in California, gathered anecdotes for a paper published in the October 1998 Marine Mammal Science. Researchers doing wildlife surveys following the Exxon Valdez oil spill and biologists studying otters in the Aleutians witnessed killer whales attacking sea otters.
One method the whales used was to breach near floating otters and land on top of them, presumably eating the stunned otters underwater because the animals never returned to the surface. Hatfield concluded that the lack of reports of killer whales eating sea otters may be due to the fact that killer whales have only recently shifted their diets to include sea otters, possibly because of the decline of Steller sea lions and harbor seals.
Estes said he wasn't convinced killer whales were eating sea otters until he and Tim Tinker, also from Santa Cruz, did a study in which they compared two populations of sea otters at Adak Island. The number of sea otters in Clam Lagoon remained stable from 1993 to 1997, while sea otters in nearby Kuluk Bay disappeared at a rate five times greater. Clam Lagoon is an area uniquely protected from killer whales by a narrow channel only three or four feet deep, while Kuluk Bay is open coastline that offers otters no protection from killer whales.
One killer whale with a taste for sea otters could eat more than 1,800 sea otters a year, Estes said. Why the killer whales changed their diet is still a matter of speculation, but people may be the culprits. In a recent Anchorage Daily News article, director Andy Rosenberg of the National Marine Fisheries Service said a link may exist between overfishing of pollock and the decline of the Steller sea lion, a killer whale prey species that has declined more than 70 percent since the 1960s. Estes said the cause of the killer whales' change in menu may also be a natural warming of the ocean or some other change in ocean ecology.