By Richard Black
Environment correspondent, BBC News website
A whale conservation success story, the recovery of the eastern Pacific gray whale, may not be quite what it seems.
Gray whales have one of the most spectacular migrations known
Since the end of commercial whaling, numbers rose to about 20,000, thought to be the level they had been at before hunting began.
But a new study using genetic methods, reported in the journal PNAS, suggests pre-hunting numbers were much higher.
The scientists say climate change may be altering the whales' supply of food significantly.
Earlier this year, researchers noted signs that grays were showing distinct signs of malnutrition when they arrived at their winter breeding grounds along Mexico's Baja peninsula.
They raised the idea that this might be connected with climate change. But the prevalent theory was that numbers had risen beyond the maximum level which the ecosystem could support.
The new research challenges that idea.
"I think that when we see large-scale issues in the population, such as starving or malnourished whales, we should be looking to long-term climatic changes in their feeding grounds," said Liz Alter from Stanford University, US.
A few hundred years ago, three separate populations of gray (or grey) whales lived in the oceans.
The Atlantic stock is thought to have perished in the 17th Century, perhaps despatched by a combination of environmental change and early hunting. Later, the eastern and western Pacific populations almost followed suit.
The western stock, which lives along the eastern coast of Russia, is now close to extinction once more.
Numbers may be as low as 120; development of oil and gas installation and entanglement in fishing nets are the principal threats.
But the eastern Pacific gray has supposedly seen rude health. It was taken off the US endangered species list in 1994, with numbers each year hovering about 20-25,000, which historical records from the whaling industry and computer models of population indicated was around the historical level.
The new genetic analysis, which Liz Alter's group has published in Proceedings of the National Academy of Sciences (PNAS), challenges this view.
By looking at variation in the animals' DNA, the team concludes there were once 76,000-118,000 grays in the Pacific.
Even if those numbers were split between the eastern and western stocks, this indicates that the population of the eastern gray today is well below the historical level.
However, Greg Donovan, head of science at the International Whaling Commission (IWC) and editor of the Journal of Cetacean Research and Management, suggested these estimates should be treated with some caution.
"Using genetic methods to estimate pre-whaling abundance is fascinating but still in its infancy, and IWC scientists believe there are a number of issues still to be resolved with it," he said.
The Stanford researchers acknowledge that further analysis should be done to confirm their findings. In particular, they would like to have samples from the critically endangered western stock, but given its parlous health, this would clearly be a sensitive issue.
Confirmation could have implications for traditional, or subsistence, whaling.
Aboriginal groups in Chukotka in the Russian northeast are permitted to hunt 124 eastern grays each year to provide meat for their communities.
The Makah tribe of Washington State near Seattle is allowed a further five. Its hunting is currently suspended pending a domestic US legal settlement, though one gray was killed just this weekend, apparently without permission of tribal elders.
If historical numbers were much higher, that could imply the grays are not as robust as believed, which could lower these hunting quotas still further.
However, Dr Donovan said current quotas are reliable.
"We have taken scientific uncertainties into account, including how many whales the environment can support, in our calculations of catch limits for the eastern gray whale," he said.
"We believe that conservation efforts should focus on the western population, which numbers only about 120 animals."