How Whale Hunting Changed the Ocean

"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.

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