GRANTS PASS, Ore. - A cascading decline in seal, sea lion and sea otter populations in the North Pacific may have been triggered by industrial whaling after World War II that forced killer whales to look for new sources of food, a group of scientists suggests.
The industrial hunting depleted the populations of humpback, gray, sperm and other great whales that made up the killer whales' usual prey, leading them to feed on the smaller marine mammals instead.
"If our hypothesis is correct, either wholly or in significant part, commercial whaling in the North Pacific Ocean set off one of the largest and most complex ecological chain reactions ever described," the scientists wrote in a recent article in the Proceedings of the National Academy of Sciences.
The hypothesis goes against past theories that declines in Steller sea lions and other marine mammals off Alaska dating to the 1970s were caused by a lack of food due to overfishing by humans. That research led to limits on commercial pollock fishing that remain in effect.
The "abrupt and huge loss" of the population of great whales "was important to some portion of the killer whale population," said Alan Springer, a research associate professor at the University of Alaska Fairbanks who was one of the authors.
"Without that food available to them, they didn't entirely switch their diet. They just increased the proportions of other species of marine mammals," said Springer. "These smaller species just weren't able to sustain this increased level of predation.
"Likely they are now preying on something else, because sea otters were the last to go."
Based on International Whaling Commission records, the authors figure that between 1949 and 1969, Japanese and Russian whalers took at least 62,858 whales, amounting to 1.8 million tons of killer whale food, within 200 miles of the Aleutian Islands and Northern Gulf of Alaska.
Although gray whale and humpback whale populations have recovered, the overall population of great whales - including sperm, fin and sei whales - in the area remains about 14 percent of what it was before the whaling started, the scientists wrote.
They suggested that killer whales chose their new prey based on the biggest bang of nutrition for the smallest buck of energy expenditure.
Harbor seals, slow-swimming and loaded with blubber, were first on the shopping list, and began to decline in the early 1970s. A few years later came fur seals. Sea lions, big but harder to catch, were next, and declined in the late 1970s. Last came sea otters, which were easy to catch, but amounted to just a snack. They declined in the late 1980s.
Bruce Mate, a professor at Oregon State University's Hatfield Marine Science Center in Newport who specializes in whales, said the conclusions in the Oct. 2 article made ecological sense.
"I like this approach. I think it's very logical," said Mate. "But it may not be the full explanation. I always tell people that animals are like parts of a watch to an ecosystem. Every one has a specific function. But they interact with each other a lot."
Jim Estes, a research scientist for the U.S. Geological Survey and adjunct professor at University of California Santa Cruz, acknowledged that the conclusions were speculative and there is room for more work.
He suggested that studies using modern techniques for analyzing chemical isotopes in the bones of killer whales or samples from their blubber could show whether they switched from eating whales to seals and sea lions.
Killer whales are big eaters, consuming up to 243,000 calories a day. It would take only a small change in their eating habits to account for the decline, the scientists said. This could have happened two ways.
One would be for virtually all the estimated 3,900 killer whales living near the Aleutians to make a small shift in their diet. The other would be for a few killer whales in the most affected areas to shift their prey species.
As few as 27 killer whales, preying just on sea lions, could account for their decline, the scientists estimated. Six killer whales feeding exclusively on sea otters would account for their decline.