Reduced nearly to extinction by the early 20th century, sea otter populations grew again under the protection of the International Fur Seal Treaty, reaching near maximum numbers in some areas by the 1970s. When a recent abrupt and unexpected decline in sea otter numbers occurred over large areas of the Aleutian archipelago, the scientists, who had been studying otters there since the 1970s, sought to discover why. In time, they eliminated disease, toxins and starvation -- three causes typically responsible for large die-offs in wildlife.
There was no sign of disease in the sea otters, and few sea otter carcasses were found washed up on the beaches as would be expected in die-offs by these more typical causes. Additionally, the researchers found a significant increase in the number of sea urchins, a principal food of sea otters.
"Lacking sea otter remains, we had to ask ourselves what could explain these disappearances without a trace," said Estes.
Having ruled out other causes, the scientists began to investigate killer whales
attacks on sea otters. Such attacks were first witnessed in 1991. The scientists
contrasted the widespread declines of sea otters in Kuluk Bay, in the
west-central Aleutian archipelago, with those of Clam Lagoon, an adjacent area
uniquely inaccessible to the whales. In Clam Lagoon, sea otter numbers held
steady from 1993 to 1997, while in Kuluk Bay, they dropped during the same
period by 76 percent. By tagging sea otters, the biologists determined that the
marked animals did not move between the two areas. Yet the rate of disappearance
over a two-year period of tagged sea otters in Kuluk Bay (65 percent) was more
than five times as great as at Clam Lagoon (12 percent). The re
Contact: Catherine Haecker
United States Geological Survey