MORE than 300 southern right whales, most of them young calves, have been found dead in the last five years in the waters off Argentina's Patagonian coast — one of the most important breeding grounds for the species.

Possible causes being examined include biotoxins — naturally occurring poisons which include the venom of some snakes and spiders and the 'flesh-eating' bacteria Necrotizing fasciitis — disease, environmental factors, and lack of prey, particularly the tiny krill which make up the bulk of the southern right's diet. Another theory put forward has been the effect of gulls, which can act like parasites, gouging skin and blubber from the whales' backs.

And at a special meeting this week in Patagonia, sponsored by the International Whaling Commission, experts have been looking at the results of tests on samples taken from beached whale calves, which have shown “unusually thin” blubber, said the US-based Wildlife Conservation Society (WCS), which described the die-off as “a perplexing and urgent mystery”.

“We need to critically examine possible causes for this increase in calf mortality so we can begin to explore possible solutions,” said Marcela Uhart, one of the WCS scientists who first discovered the problem. “Finding the cause may require an expansion of monitoring activities to include the vast feeding grounds for the species.”

Southern right whales are one of three species of right whales, so called because fishermen considered them the 'right whale' to hunt, because they are slow swimmers, easy to approach, live close to shore and float when dead.

In the first half of the 1800s about 45,000 right whales were killed, driving them close to extinction, before they became protected in 1937.

Since then the southern right whale — which weighs up to 90 tonnes when fully grown — has been a conservation success, numbers rebounding to about 7,500, in populations off South America, South Africa, Australia and some oceanic islands. Numbers of the Northern Atlantic right whale and Northern Pacific right whale have recovered less well, to a few hundred each, according to the UK-based Whale and Dolphin Conservation Society.

Part of the concern about the recent die-off is that the dead whales have been found around the Peninsula Valdes, where one third of the global population of southern right whales is thought to use the protected bays for calving and nursing between the months of June and December.

“Peninsula Valdes is one of the most important calving and nursing grounds for the species found throughout the southern hemisphere,” said Howard Rosenbaum, director WCS's ocean giants programme, and a member of the International Whaling Commission's scientific committee.

— The Guardian, London