Killer Consequences: Has whaling driven orcas to a diet of sea lions?

From the 1970s to the 1990s, populations of sea otters and some pinnipeds, including Steller sea lions and fur seals, took a mysterious nosedive in the northern Pacific. A new study floats a surprising explanation: These creatures became choice entrées for killer whales after industrial whaling wiped out the great whales that killer whales had been eating.

A leading explanation for the disappearing pinnipeds had been that global warming or overfishing caused food shortages, says Alan M. Springer of the University of Alaska in Fairbanks. But several recent reviews of sea lions and their habitats indicate that these animals are not wanting for food. That observation spurred Springer and his colleagues to investigate whether the problem lies with pinnipeds themselves becoming food. The scientists present their hypothesis in an upcoming Proceedings of the National Academy of Sciences.

Killer whales eat a wide range of ocean critters, from salmon to sperm whales. Although marine biologists debate about how often great whales end up on an orca’s platter, they agree that killer whales are the most significant natural predators of these massive cetaceans.

Except, of course, for people.

Using data collected by the International Whaling Commission, Springer’s team surmised that by 1969, whalers essentially wiped out the fin, sei, and sperm whales within 370 kilometers of the Aleutian Islands and the Gulf of Alaska coast.

Then, the numbers of smaller marine mammals began declining, Springer says. The harbor seal population crashed in the late 1970s. In the following years, fur seals and Steller sea lions disappeared in droves. In the 1990s, sea otter numbers plummeted.

Those trends point to killer whales, says Springer. He proposes that these spry predators would first hunt the fat, docile harbor seals. Once those prey dwindled, orcas would settle for the smaller fur seals and aggressive sea lions. Finally, killer whales would snack on otters. In what may be a further iteration of this ecological cascade, sea urchins–a favorite food for otters–have thrived (SN: 10/17/98, p. 245).

“Monkeying around with the system could lead to things you would have never predicted would happen,” Springer says.

By estimating the number of otter and pinniped deaths during the declines and determining how many of these creatures would be required to feed a killer whale, Springer and his colleagues calculated that the population crashes could have occurred even if the orca population had shifted its diet less than 1 percent.

“I think it’s the best hypothesis out there” for these declines, says Jeremy B.C. Jackson of the Scripps Institution of Oceanography in La Jolla, Calif.

Andrew W. Trites of the University of British Columbia in Vancouver questions whether great whales ever constituted major portions of orcas’ diets. He adds that in computer models of marine ecosystems, removing great whales had no significant effect on pinnipeds.

Springer acknowledges that the mystery of these declines may be hard to solve with confidence, since much of the relevant data disappeared with the missing animals.

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