Scientists can now weigh wild whales using remote-controlled drones hovering above them.

In the past it has only been possible to measure the weight of free-living whales if they were dead and out of the water or stranded on a beach.

But researchers have released aerial footage of them flying a drone over a mother and baby southern right whale, near Argentina, to work out how much they weigh.

They can do this by taking photographs to work out the length, width and height of the animal.

This can then be compared to past measurements of the density of that particular species of whale's body, to estimate its volume and weight.

Doing this makes scientists more able to study what the ocean giants eat, how their bodies change over time and how certain stresses affect their health.

Scientists from Denmark managed to use a drone (pictured, hovering over two southern right whales) to estimate the weights of whales in the wild – they can weigh up to 200,000lbs

'Knowing the body mass of free-living whales opens up new avenues of research,' said Assistant Professor Fredrik Christiansen, from Aarhus University in Denmark.

'We will now be able to look at the growth of known aged individuals to calculate their body mass increase over time and the energy requirements for growth.

'We will also be able to look at the daily energy requirements of whales and calculate how much prey they need to consume.'

Professor Christiansen and his colleagues tested their technology on 86 different whales off the coast of Península Valdés, a nature reserve in Patagonia.

They used a DJI Inspire 1 Pro, a drone worth about £3,500 and capable of flying more than three miles away from its controller.

Scientists have been largely limited to only weighing wild whales after they're caught in fishing nets, stranded on land or dead, the researchers said

Professor Christiansen and his colleages took aerial photographs of whales off the coast of Península Valdés, a nature reserve in Patagonia, Argentina

WHAT IS A SOUTHERN RIGHT WHALE? The southern right whale is a species of baleen whale which was once hunted to the brink of extinction by humans. Hunting of the ocean giants was banned in 1935 but illegal Russian whaling operations continued until the late 1970s. Southern right whales are huge, measuring up to 56 feet (17m) long for a fully-grown female or 49 feet (15m) for a male. They can weigh up to 198,000lbs (90,000kg; 90 tonnes). The whales are slow swimmers and naturally full of blubber, oil, meat and baleen (whalebone), all of which combined to make them prime targets for whalers. Southern right whales live all over the southern hemisphere of the Earth and migrate every year to warmer waters to breed, often off the coast of South America, South Africa, Australia and New Zealand. Their bodies are mostly black with patches of white on their underbelly and chin, and they lift their huge tails out of the water when starting to dive beneath the surface. Source: Whale and Dolphin Conservation Advertisement

The area had ideal conditions for taking photographs of the animals because the seawater is so clear and big groups of southern right whales travel there to breed.

Southern right whales are huge and can reach up to 56 feet (17m) in length and weigh almost 200,000lbs (90,000kg; 90 tonnes).

Images taken of them from the sky were analysed to work out the exact dimensions of both the top and the bottom of the whale, so the scientists could work out its volume.

Being able to take multiple measurements of living whales and track them over time added another element of the work marine biologists do.

When they were mostly limited to dead samples, measurements may have been distorted by the bloating or deflation which happens to carcasses.

And accuracy could help with other, less obvious, problems, like working out how much medication to give an injured animal.

Co-author of the study, Dr Michael Moore, from the Woods Hole Oceanographic Institution in Massachusetts, said: 'Weight measurements of live whales at sea inform how chronic stressors affect their survival and fecundity, as well as enabling accurate sedative dosing of animals entangled in fishing gear that are aversive to disentanglement attempts.'

Professor Christiansen added: 'The difficulty in measuring body mass reliably in free-living whales, has prevented the inclusion of body mass in many studies in ecology, physiology and bioenergetics.

'This novel approach will now make it possible to finally include this central variable into future studies of free-living whales.'

The team hope their method will be applicable to other species of whale and potentially other marine animals.

Their research was published in the journal Methods in Ecology and Evolution.