— The population of sea lions, seals and otters in the north Pacific Ocean has declined so catastrophically in recent years that scientists fear for their continued survival, and the reasons why remain open to fierce debate. Now, a team of scientists has come up with a creative hypothesis that blames it all on human activities following World War II.
Extensive whaling removed more than half a million great whales from the north Pacific after the war, forcing a subtle change in the dietary habits of the true lord of the ocean, the killer whale, according to this new theory. With fewer baleen and sperm whales to dine on, pods of killer whales that used to take out an occasional great whale gradually turned to other marine animals, setting off an ecological domino effect that has extended to the present.
"If our hypothesis is correct, either wholly or in significant part, commercial whaling in the north Pacific Ocean set off one of the longest and most complex ecological chain reactions every described, beginning in the open ocean 50 years ago" and continuing to this day with the devastation of huge kelp beds off western Alaska, the scientists argue in their report, published in a recent issue of the Proceedings of the National Academy of Sciences.
Looking For Causes
The decline of various species has been blamed on everything from commercial fishing to global climate change, and the situation has become so serious — and so poorly understood — that the federal government has pumped millions of dollars into research. At least $100 million has been spent over the last three years studying the decline of sea lions alone, according to Alan Springer, an oceanographer with the University of Alaska, Fairbanks, who has spent years studying the decimation of various marine animal populations.
Sea lions have declined by more than 80 percent in the last 30 years throughout a huge area stretching from Alaska to Japan, according to the National Marine Fisheries Service.
Similarly, harbor seals, fur seals and sea otters have also declined dramatically in most areas of the north Pacific, despite the fact that these animals differ somewhat in their diet and lifestyles.
Springer, the lead author of the report, says he found the most common explanations for their decline inadequate. Many scientists, perhaps most, believe that commercial fishing has depleted coastal food resources, leading to malnourished populations that are susceptible to diseases. Others believe global climate change has so altered the marine environment that these animals are finding it impossible to cope with shifting resources.
But the evidence for that is hard to come by, according to many scientists, and even the National Marine Fisheries Service describes that hypothesis as just an educated guess.
And it seems reasonable to look for a common cause for these declines, because while the species are different, they also share much in common.
So Springer was particularly intrigued by a report in the journal Science a few years ago that indicated that killer whales were to blame for the decline of sea otters in the Aleutian Islands.
Changing Whale Taste
Springer contacted the author of that report, Jim Estes of the University of California, Santa Cruz, and the two began working together.
"Jim had come to the conclusion, based on a lot of really thorough research in the Aleutian Islands, that predation was the most probable explanation for the collapse of the sea otter population," Springer says. "He had a lot of really solid evidence that pointed in that direction."
Springer and Estes were eventually joined by six other experts, and they began examining the question of whether killer whales might be the culprit in the decline of other marine animals as well.
The working hypothesis was pretty straightforward. When commercial whaling destroyed huge stocks of whales following the war, at least some killer whales might have turned to other sources. They probably knocked off harbor seals first, because they are easy to catch and nutritious, then turned to fur seals, and then sea lions (suggesting a level of desperation because sea lions would be fierce combatants and less nutritious) and finally to bite-sized sea otters. But would the historical record support that?
The researchers found a very tidy fit. Harbor seal populations began collapsing in the early 1970s, and fur seals a bit later in the mid-'70s, and sea lions in the late' 70s, and sea otters in the '90s. Commercial whaling has been banned for years now, but some species have been slow to rebound, and the dietary preference of killer whales may have changed forever.
Many scientists contend that no single predator could possibly have caused such destruction, so Springer and his colleagues looked at the numbers. They found that if the overall killer whale population shifted just one percent of its diet away from whales to smaller marine animals, that would have led to the catastrophic decline we see today.
And not every killer whale would need to make the switch.
"A much smaller number of killer whales would have done the same thing if they had changed their diet so that the bulk of it came from sea lions or harbor seals," Springer says.
And as Estes has shown, killer whales have developed a taste for sea otters, and once the otters were decimated, sea urchins moved into the kelp beds off Alaska and mowed them down, depriving the region of valuable marine habitats.
So if Springer and his group are right, the impact of whaling half a century ago is still rampant.
Problem of Proof
Proving that hypothesis is going to be difficult. Springer hopes that killer whales themselves might eventually prove him right, or wrong. If they did indeed change their diet, that should show up in the isotopic ratios of carbon and nitrogen in their teeth, he says. Or there may be other "bio-markers" that would shed more light on the eating habits of killer whales.
"To some degree, you are what you eat," he says.
Now, how do you get killer whales, unique in the sea because they have no predators, to stop by the local dentist office?
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.