Grey whales in the eastern Pacific appear to be in some trouble, with the cause far from clear, scientists say.

Researchers with the conservation group Earthwatch found that whales are arriving in their breeding grounds off the Mexican coast malnourished.

The same thing happened just after the 1997/8 El Nino event, which warmed the waters and depleted food stocks.

Scientists are not sure whether the current decline is climate related or part of a natural predator-prey cycle.

"We're not really sure what is going on now," said William Megill, a member of the Earthwatch team who also holds posts at Bath University in the UK and the University of British Columbia in Canada.

"We certainly saw in Mexico this winter a very large number of starving whales," he told the BBC News website. "There is currently an El Nino building, and this is a worry."

No fat

There are thought to be between 15,000 and 18,000 grey whales in the eastern Pacific, a population that has been in generally good health since pulling back from the brink of extinction when hunting stopped in the 1940s.

Numbers may be higher now than before the hunting era.

By contrast, the other population, on the western side of the Pacific near Russia, has been in trouble for many years owing to a combination of hunting and, latterly, oil and gas exploration. It may now number as few as 120 individuals.

On the eastern side, whales migrate between their summer feeding grounds to the north, which stretch from the waters near Seattle and Vancouver to the Arctic Bering Sea, and their winter breeding home along Mexico's Baja peninsula.

This is one of the longest migrations of any marine mammal; and at the end of it, in the last few years, Dr Megill's team has found the animals arriving thin and exhausted.

"The animals are starving, their fat has just gone, and there's not a lot of breeding going on," he related.

"They seem to spend their time looking around for food when they should be breeding."

Going down

The cause of this change is not clear. A link with climatic conditions makes sense; warmer waters hold less oxygen, they become less productive, resulting in less of the tiny crustaceans which are the grey whales' favoured food.

This is thought to have caused the slump which followed the 1997/8 El Nino event.

One suggestion, from Dr Justin Cooke, who works with the World Conservation Union (IUCN) on cetacean issues, is that the greys have just become too plentiful.

"No whale population can expand indefinitely," he said, "and these whales seem to have exceeded their historical level so it would be surprising if they continued increasing - they're due for a slump.

"When whale numbers were lower there was enough to go round in poor years, but now numbers are higher and so there's only enough to go round in good years."

William Megill acknowledges that the population could have become unsustainably high.

"Around the year 2000, colleagues looked for mysids (tiny crustaceans) in kelp beds off the Canadian coast, and they found lots of them," he said.

"The last two years, we've stuck cameras down there and seen nothing.

"It could just be the whales ate them all, and what we're seeing is the same thing that happens to wolf and lynx populations when they eat too much of their prey."

But he is concerned that other factors may be involved too, in particular the slow rise in the average temperature of the oceans.

The deepening annual Arctic melt, too, would also deprive the whales of a rich source of food, which accumulates along the edge of the pack ice.

"I'm looking at it and thinking, 'I'm a bit worried about it'," he said, "and what we need to know is what's going on quickly so we can get proper management plans in place.

"It may be a lot more serious than just grey whales - they may just be the early warning sign of changes for the whole Pacific, and we urgently need to know what's going on."