How do you weigh a massive whale?

Dead whales, stranded and decaying on beaches, can sometimes be weighed. But weighing a live whale is nearly impossible.

Or, at least, it was impossible.

Marine scientists have developed an innovative way to accurately estimate the heft of free-living whales by using drones to capture aerial footage of cetacean bodies. Scientists published the research Wednesday in the journal Methods in Ecology and Evolution.

Knowing the weight of whales is valuable for grasping the health, trends, and nutritional requirements of these giant and profoundly intelligent mammals.

"Body weight is a fundamental characteristic in animals and especially whales — they are the largest animals on the planet," said Fredrik Christiansen, lead author of the research and a marine ecophysiologist at the Aarhus Institute of Advanced Studies in Denmark.

The old ways of weighing whales, other than finding dead whales on shorelines, is grim. "Either you kill a whale as part of a commercial or scientific whaling operation," explained Christiansen, "or you can rely opportunistically on whales caught in fishing nets."

But even these limited weighing opportunities are now profoundly rare. Today, hunting whales for profit is outlawed by nearly every nation in the world, barring defiant operations like those in Iceland and Japan.

Original image has been replaced. Credit: Mashable Original image has been replaced. Credit: Mashable

But with this drone innovation, weighing many whales in diverse waters is now feasible.

"It's the first time we’re going to be able to measure free-living whales in the wild," said Rachel Cartwright, a whale biologist who runs the Keiki Kohola Project, an organization that preserves humpback whale habitat in Hawaiian waters. "It's a huge leap forward to be able to do this on wild whales with low impact."

"It's been something that [whale researchers] have been trying to do for a while," added Cartwright, who had no role in the research. "The fact they managed it is really cool."

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Christiansen flew a drone over southern right whales in the remarkably clear waters of Argentina's Península Valdés, where some 1,000 whales congregate to breed. "You can fly a drone in from the beach and measure them," he said.

With vivid, detailed whale images, Christiansen and his team then developed a program that allows them to plug in the whales' length and width to estimate body size. To ensure the estimates were accurate, they compared their results to direct weight records of beached and hunted right whales (from earlier whaling operations).

Their model, after being fed data from the drone, produced results that closely matched the available weight records — especially in newborn calves. "The estimates were spot on," said Christiansen.

Video Credit: FREDRIK CHRISTIANSEN

After successfully estimating the body weights of right whales, Christiansen now seeks to do the same with other species, and there's a critical need. Some whale populations are struggling to find food.

"We're seeing extreme nutritional stress in several populations," said Cartwright, noting certain populations of killer whales, right whales, and humpbacks.

The consequence of insufficient food is often lower reproductive rates, explained Cartwright, as the whales can't support as many offspring. These nutritional stresses — which will be accelerated and exacerbated as the oceans warm and fisheries change — can be spotted earlier in whale species if biologists can monitor drops in weight, she said.

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"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," noted Michael Moore, a senior scientist at the Woods Hole Oceanographic Institution, in a statement.

One critical issue to remember, emphasized Cartwright, is not just anyone can fly a drone over a whale. Every whale is protected by the Marine Mammal Protection Act, meaning that without special authorization (like for scientific research) a drone can't come closer than 1,000 feet of a whale, she said.

If a drone, with permission, is used carefully, there's likely little effect on the cetaceans — though how much of an effect is an area of active research, noted Cartwright.

For drone-flying biologists, the crafts have made a nearly impossible scientific measurement, now possible.

"There’s no real way to weigh whales," said Christiansen. "It's nice to see that this is possible."