PALO ALTO, Calif. - One of the great success stories of the ocean, the return of the Pacific gray whale, may have been based on a miscalculation, scientists reported Monday in a study based on whale genetics.
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What was assumed to be a thriving whale population actually is at times starving from a dwindling food supply, said study co-author Stephen Palumbi, a Stanford University marine sciences professor. And global warming is a chief suspect.
Scientists may have underestimated the historical number of gray whales from Mexico to Alaska, according to the study published Monday in the Proceedings of the National Academy of Sciences. And that may have led to a misdiagnosis of what is behind surprising die-offs over the past few years and the appearance of many so-called "skinny" whales.
Earlier this month the National Marine Fisheries Service reported that at least 10 percent of gray whales returning to one of their four main calving and breeding lagoons off Baja California showed signs of being underfed. Some of the whales even had bony shoulder blades.
"This is a hint of a problem," Palumbi said. "Our antennas should be up. Our antennas should be asking if the ocean is capable of supporting life the way it used to."
The study concludes that the original Pacific gray whale population hundreds of years ago may have been far higher than currently thought - closer to 100,000 whales than conventional estimates of 20,000 to 30,000.
The scientists base that on how diverse the population of whales once was - information they gleaned by examining differences in the DNA of 40 whales. They studied 10 spots on the whale's genetic blueprint.
The diversity of genes in this group of whales indicates there had to be about 100,000 whales centuries ago, the scientists reported.
If the whale population was five times higher than originally thought, that makes recent problems with the whale look far worse.
Gray whales were the first marine mammal to bounce back and get off the endangered species list in 1994. Scientists had figured that a population of about 20,000 whales was normal, so in 1999-2000 when some whales started dying off, the experts figured it was just the result of the ocean reaching its normal "carrying capacity." There was just not enough room for more whales, so nature thinned out the herd, they figured.
But Palumbi said his genetic analysis shows the oceans were once more crowded with gray whales. He said anecdotal historical evidence supports that. One historical document claimed a count of 1,000 whales a day seen on the California coast.
French explorer Jean-Francois La Perouse sailed into Monterey Bay in the 1700s and complained "there were so many whales, that they stunk up the air with their breath," Palumbi said.
While some scientists said Palumbi's work makes sense, it left a nagging question for fisheries biologist Jeff Breiwick, who works on the gray whale census for the National Marine Mammal Laboratory in Seattle: What happened to the 80,000 gray whales for which there is no evidence?
Breiwick and others have used computer models and historical documents to estimate the level of whale hunting since the 1600s. To explain the extra whales living and then dying would have meant about three whales a day being killed for four centuries.
"Where's the evidence of that mortality?" Breiwick asked.
Palumbi said the answer is in Asia. The gray whales in his study are eastern Pacific gray whales. Western Pacific gray whales look identical and can only be identified by genetic tests, he said.
Palumbi figures that the 100,000 whales of centuries ago would includes both types of whales, and the massive decline can be explained by Asian whaling. The western gray whales number only in the hundreds and are on the precipice of extinction, he said.
Getting the eastern Pacific whales back to nearly 20,000 is "a great success story, don't get me wrong," Palumbi said. "It's not a success story that's finished yet."
If the whales aren't at their natural limit, then some other problem is harming them, Palumbi said. And that has to do with a food shortage and global warming, researchers theorize.
The gray whale relies on massive numbers of small crustaceans that live in the Arctic regions, Palumbi said. That food supply may have been cut because of warmer waters, he said.
Frances Gulland, director of veterinary science at the Marine Mammal Center in Sausalito, Calif., examined some of the whales that died in the 2000 and agreed with Palumbi that the new estimates on the past population indicate that something bad is happening now.