Fatal strandings of southern right whales around Argentina's Valdes Peninsula have soared in recent years, and worried scientists are not sure why, the International Whaling Commission heard Friday.
From 1971, when systematic monitoring began, only a relative handful of whale deaths were reported over the next three decades.
Starting in 2003, however, the mortality rate began to soar: from 31 that year, to 47 in 2005, 83 in 2007, 95 in 2008 and 79 last year, the IWC's scientific committee reported.
"Over 90 percent of the deaths have been of first-year calves," the scientists said.
The Valdes Peninsula is one of the most popular whalewatching venues on the planet, attracting some 200,000 eco-tourists every year hoping to see the huge mammals -- which grow up to 17-metre (56-foot) long -- in their element.
It is also a critically important breeding and nursery ground for right whales.
Three causes, possibly in combination, have been fingered as possible culprits.
One is reduced availability of food for adult females, notably small crustaceans called copepods and krill. Poor feeding conditions lengthen the normally three-to-five year reproduction cycles, studies have shown.
High concentrations of biotoxins and the spread of an infectious disease are also suspects.
"The fact that the strandings are happening where they do is not that surprising because it is a breeding ground," said Vincent Ridoux, a marine biologist with the French delegation.
"What we don't know is why they are dying," he told AFP.
If females are undernourished it would severely impact calves, which normally consume about 125 litres of milk every day.
Weakening might also explain "an extremely strange phenomenon": kelp gulls that alight on the backs of young whales at the water's surface and feed on their backs, creating lesions vulnerable to toxins or viruses.
"We don't know to what extent they contribute to mortality, but they are the only visible sign of abnormality," Ridoux said, adding that he was unaware of similar cases in other whale populations.
As a precaution, the ICW has asked local authorities to try to keep the gulls at bay.
NGOs and the government have also stepped up aerial surveys to hunt for more clues as to what is killing the whales.
Scientists estimated that there were about 3,200 mature female southern right whales in 2007, double the number 10 years earlier. The population is not considered threatened, according to the International Union for the Conservation of Nature (IUCN).
Created in 1946, the IWC is the only international body that oversees the management of global whale populations.
It's scientific committee is charged with studying all of the environmental impacts affecting the animals worldwide, including toxins, noise pollution, climate change, ocean acidification and ship strikes.
So-called "by-catch" -- where whales are snared in fishing nets set for other prey -- are perhaps the most serious short-term threat to the marine mammals besides large-scale whaling.
From 1994 to 2006, Japan and South Korea each caught more than 1,000 minke whales in their coastal waters this way, according to government statistics.
DNA analysis suggests that the real number of whales killed in the same waters by by-catch is likely twice as high.
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