11 September 2007Catherine Brahic
Starvation may be impeding the recovery of the Pacific Gray whale population, say researchers.
The Gray whale population was thought to have recovered from commercial whaling, but now a new genetic study suggests the marine mammals once numbered between three and five times the 22,000 population estimated today.
If true, the findings could imply that the world's oceans are no longer able to support the same number of whales that they once could, says Stephen Palumbi of Stanford University in California, US, who led the study.
Previously, it had been thought that thin, starving whales – as have been observed recently in Mexico – were a consequence of the population exceeding its historical ecological limits, rather than the oceans running out of food.
Palumbi speculates that a depletion in the whales’ natural prey in the oceans could be down to natural variation or the warming of waters due to climate change.
"Our results might be telling us that whales now face a new threat – from changes to the oceans that are limiting their recovery," says Palumbi. "Decades ago, whales were the first creatures to tell us that we were over-fishing the oceans. Maybe now they trying to tell us the oceans are in deeper trouble."
Genetic mutations
Palumbi and his colleagues obtained DNA samples from 42 Gray whales Eschrichtius robustus, and sequenced it at 10 different points to see how much genetic variation there was between individuals. This gives scientists an idea of how large a population once was.
"Genetic variation builds up in populations because of random mutations," explains Palumbi. The larger the population, the more random mutations occur in individual whales.
"In a small population, inbreeding tends to strip the variation away," he adds. It takes a long time for that to happen – the rule of thumb is that it takes at least 10,000 generations to strip away the variation of a population that originally comprised 10,000 individuals.
From the samples, the researchers determined that the Gray whale populations would have averaged between 78,000 and 118,000 over the past tens of thousands of years – rather than the current estimated population of 22,000.
Most of the existing population roams the eastern Pacific – there are only about 100 Gray whales left in the western region, making them some of the most endangered whales on the planet.
However, even if historically, the whales were evenly distributed between the American and Asian Pacific coasts, with 48,000 on either side on average, this would still mean that the current eastern Gray whale population is now half of what it was.
Malnourished and emaciated
"It might be that the eastern Gray whales are on their way to recovery, in which case our study would suggest that we can expect them to keep growing," Palumbi told New Scientist. But in April 2007, the conservation group Earthwatch reported that eastern Gray whales were arriving at their breeding ground in Mexico malnourished and emaciated.
For Palumbi, the logical conclusion is that the oceans are no longer able to feed as many Gray whales as they once did. He says there could be any number of reasons for this, ranging from natural variation of the ocean’s food supply, to human effects on the oceans. He also points to a 2006 study led by Jackie Grebmeier of the University of Tennessee at Knoxville, US, which showed that warming seas in the Arctic are displacing crustaceans on which the whales feed.
Depleted numbers of the whales, says Palumbi, is bad news for other species too. As Gray whales feed, they stir up sediment by "bulldozing" the ocean floor for food – this feeds animals throughout the marine food chain.
Palumbi and his colleagues have calculated that 96,000 Gray whales would have resuspended 12 times more sediment each year than the Yukon River, the biggest river in the Arctic.
For example, says Palumbi, "96,000 Gray whales would have helped feed over a million seabirds a year".
The researchers are recommending that the eastern Gray whales still be considered depleted and that the US Marine Mammal Protection Act should be amended to allow just 207 killings by humans a year, instead of the 417 currently allowed for the US.
http://environment.newscientist.com/article/dn12623-starving-whales-point-to-depleted-oceans.html
