By Ivan Noble
BBC News Online science staff
Hungry killer whales deprived of their traditional food by commercial whaling have turned on seals, sea lions and sea otters, scientists in the US suggest.
Killer whales: Skilled hunters
Killer whales fed mainly on giant whales in the past but numbers have fallen by 86% since the last world war.
Deprived of their giant prey, they have turned on smaller mammals, leading to dramatic falls in population size.
The American team came up with their theory after examining decades of climate and marine population records.
The collapses of harbour seal, fur seal, sea lion and sea otter populations in the North Pacific are part of a domino effect, say Alan Springer of the University of Alaska Fairbanks, Jim Estes of the University of California, Santa Cruz, and their collaborators.
The chain of events began with the killing of hundreds of thousands of baleen and sperm whales in the North Pacific between 1946 and 1979.
Hungry killer whales had to look elsewhere for a meal and they turned to harbour seals, the scientists suggest in a paper published in the Proceedings of the National Academy of Sciences.
The harbour seal population collapsed over a period from the early 1970s until the early 1980s.
Fur seals followed between the mid 1970s and the mid 1980s, then sea lions between the late 1970s and the 1990s.
The collapse of the sea otter population began in the 1990s and continues today, they say.
Chain of destruction
But the chain of destruction does not end with mammals, it seems.
The lack of sea otters has allowed sea urchin populations to explode. These tiny creatures have then gone on to eat their way through kelp forests.
The theory is controversial. Many scientists do not believe that the predatory activities of killer whales by themselves account for so much destruction.
But Alan Springer and his colleagues are confident that their theory fits the facts.
"The message is that overfishing and massive extraction can lead to food web impacts that are unexpected and unintended," he says.