The researchers also compared sea otter populations at two sites on Adak Island, one an open bay and the other an area inaccessible to killer whales. From 1993 through 1997, sea otter numbers were stable in the protected Clam Lagoon, while in the adjacent Kuluk Bay they declined by 76 percent.
According to calculations based on the team's observations in one section of the Aleutian Islands, killer whales must have killed 6,788 sea otters per year between 1991 and 1997 to account for the observed decline in the population. Taking into account the number of hours of field observations by the research team during this period, the number of those attacks they could expect to have observed is 5.05, which is very close to the actual number of six observed attacks.
The researchers ruled out other possible causes of the sea otter decline, such as disease, toxins, and starvation, Estes said.
"It took about two years for me to become convinced that killer whales were responsible, as we dismissed one possibility after another and gradually gathered direct evidence pointing to killer whales," Estes said.
Terrie Williams, an associate professor of biology at UCSC and coauthor of the paper, analyzed the caloric value of sea otters, which are a less nutritious food source than killer whales' usual prey because they have very little blubber. Williams estimated that a single killer whale would need to eat 1,825 otters per year to meet its energy requirements. These figures suggest that as few as four killer whales feeding exclusively on sea otters could have driven the population decline observed over a large area of the Aleutian archipelago.
Sea otters were nearly extinct by the early 20th century due to overhunting for
the fur trade. Pr
Contact: Tim Stephens
University of California - Santa Cruz