Killer whales put Alaska sea otters at risk
October 19, 1998
By Environmental News Network staff
Killer whales have been eating an unprecedented amount of sea otters in the absence of their traditional food source of Stellar sea lions and harbor seals in western Alaskan waters, researchers report in today's issue of the journal Science.
"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."
Due to a complex web of events, possibly initiated by overfishing, some killer whales have shifted from ocean feeding to coastal, said Estes.
The usual diet of killer whales is Stellar sea lions and harbor seals, but populations of these marine mammals recently declined in the North Pacific. These populations are believed to have crashed because of a shift in abundance and kinds of food fish, making food a problem for fish-eating species.
Other researchers have reported the 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.
The researchers calculated that a killer whale on a steady diet of sea otters could consume as many as 1,825 otters in a year. How many killer whales exist in the Aleutian archipelago is unknown, but as few as four whales on an exclusive otter diet could cause the declines that have occurred, Estes said.
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.
After their investigation eliminated disease, toxins and starvation -- three causes typically responsible for large die-offs in wildlife -- the researchers began to investigate killer whale attacks on sea otters.
"Lacking sea otter remains, we had to ask ourselves what could explain these disappearances without a trace," said Estes.
A comparison study of sea otter populations in one lagoon accessible to killer whales and one lagoon not accessible to the whales in the Aleutian archipelago led them to conclude that the whales were indeed eating the sea otters.
Killer whales and sea otters have existed together for thousands of years, but killer whales are offshore dwellers, whereas sea otters live along the coast.
Estes said that the sea otter is a keystone species of coastal ecosystems, an animal on which the balance of entire ecosystems rests. As the top predator in a food web having a three-level system -- sea otter, sea urchin and kelp -- the sea otter feeds on the sea urchin, a plant-eating animal that in turn feeds on seaweed, or kelp. Without sea otters as predators, sea urchins increase in numbers and devour the kelp forests, Estes said.
In contrast, the killer whale is a top predator in the oceanic ecosystems. By shifting to the sea otter as a food source, the whale makes a fourth level in the sea otter's coastal system and upsets the balance in the coastal food web, Estes said.
"The coastal events we've observed appear to be driven by events out in the ocean," said Estes. "It's like an ecological chain reaction, affecting many different species and many different levels of the food chain."
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