Researchers are to launch an investigation into the unexplained deaths of hundreds of young southern right whales, one of the planet’s most vulnerable marine species.

The £740,000 project – jointly funded by Defra, the UK environment department, and the EU – will involve researchers tagging whales and calves, tracking them by satellite and identifying individuals by taking DNA samples. The aim is find out why the carcasses of almost 500 young southern right whales have washed up on Argentina’s Valdés Peninsula, one of the species’ key calving areas, over the past decade.

“There are only a few thousand southern right whales left on the planet,” said the project’s leader, geneticist Jennifer Jackson, of the British Antarctic Survey, Cambridge. “We need to find out what is killing them and we think their sub-Antarctic feeding ground holds the answer.”

Southern right whales (Eubalaena australis) can grow to 18 metres and weigh up to 80 tonnes. They get their name for being the right whale for hunters to pursue, added Jackson. “They swim slowly, float when dead, and yield a great deal of oil,” she said. “They were perfect targets for whalers.”

The species was also highly vulnerable because mother whales are extremely protective of their offspring and swim with them in shallow coastal waters while they are young. There they were easily picked off by harpoon. As a result, the global population, which probably numbered several hundred thousand in the 18th century, crashed and the species plunged towards extinction.

Numbers slowly recovered from the 1940s after a moratorium was agreed – though levels plunged again in 1970s when they were killed, illegally, by Soviet whalers. The species recovered again when that hunting stopped and numbers have now reached around 12,000.

However, scientists at Argentina’s Instituto de Conservacion de Ballenas have recently seen large numbers of dead southern right whales – most of them calves – washing up on the shores of Valdés Peninsula, one the species’ key breeding grounds. “We still do not know why this is happening – though we have ideas,” said Jackson. “Whatever the cause, it is a worrying development, especially for a species that has been so badly depleted in the past.” Jackson highlighted three theories that have been put forward to explain the deaths:

• Lack of food: krill may disappearing in whale feeding grounds at South Georgia;

• Exposure to toxic algae may be responsible for killing calves;

• Kelp gulls: researchers have noted that there has been a rise in attacks by kelp gulls on young whales since the 1970s and lesions on their backs have increased accordingly. These could be damaging the health of calves, triggering the jump in deaths.

The lack of food theory is supported by studies that show unusually warm summers off South Georgia decrease whale calving rates the following year, suggesting whales will struggle as the climate warms. “The trouble is that we know so little about the lives of southern right whales,” said Jackson. “That is why we are going to spend the next two years studying them in great detail.”

Her study, to be carried out in collaboration with researchers at St Andrews University and other groups, will involve spending two summers observing the creatures off South Georgia Researchers will use acoustic devices and drones to investigate whales and DNA samples will be taken to identify individual animals and to determine their hormone levels and reproductive status. Precise numbers of populations will be established while old whaling logbooks will be consulted to estimate how many southern right whales used to inhabit the seas around South Georgia. Surveys of numbers of krill will also be taken.

“The aim is to understand how southern right whales live out their lives and what influences their health and reproduction,” said Jackson.

“Once we have done that we hope we will a much clearer idea of what is happening to them and so be able to do something about that.”