Because of a lack of Steller sea lions and harbor seals, large numbers of sea otters are being eaten by hungry killer whales in western Alaska waters, according to findings in the October 16 issue of the journal Science. In "Killer Whale Predation on Sea Otters Links Oceanic and Nearshore Ecosystems,"a U.S. Geological Survey biologist and his colleagues at the University of California, Santa Cruz, reported on sharp reductions in sea otter numbers in the North Pacific rim.
"We estimate that between 40,000 and 45,000 sea otters have died since 1990 from killer whale predation in roughly 3,300 kilometers of shoreline," said Jim Estes, a biologist at the Western Ecological Research Center. "This unusual behavior of killer whales toward sea otters ultimately raises questions about the health of our oceans."
In a chain of events possibly initiated by overfishing, said Estes, some killer whales have shifted from ocean feeding to coastal. Their usual diet is Steller sea lions and harbor seals, but populations of these marine mammals recently declined in the North Pacific. While no one knows for certain why these populations crashed, it is believed to be linked to a shift in abundance and kinds of food fish, making food a problem for fish-eating species. Estes said other researchers have reported the fish change possibly resulted from a combination of three factors -- expanding fisheries, increases in ocean temperature, and a depletion of large whales.
After ranging into coastal waters, the killer whales found a substitute prey in sea otters. Fewer sea otters, said Estes, allowed sea urchins to increase in numbers and strip coastal kelp forests. Estes and his colleagues are concerned about how events occurring far out to sea are profoundly affecting shorelife. This dramatic change in how ocean and coastal systems are being linked may affect numerous marine and coastal species, the authors concluded.