Southern right whales have grown thinner in the fall, when they swim from the Southern Ocean waters to the coast of South Africa, because their food is disappearing with melting sea ice due to global warming.

This is explained in a study carried out by researchers from Denmark, Iceland or South Africa, and published in the journal ‘Scientific Reports’.

In the month of June, when winter hits the southern hemisphere and the sea around Antarctica freezes over, the southern right whale (‘Eubalaena australis’) swims north. Many of them gather in the bay outside the city of Hermanus in South Africa.

There, the warmer water is perfect for mating or raising newborn calves. However, there is no food for the whales. Throughout the winter, right whale mothers use their blubber reserves to produce milk for their calves.

It is therefore extremely important that the whales eat plenty and fatten up in the cold Antarctic waters all summer long, but there seems to be not enough food and the whales that come ashore in South Africa are thinner than before.

Since researchers began measuring right whales in the 1980s, they’ve gotten increasingly skinny.

“Right whales are 25% skinnier than they were in the 1980s. This is bad for the whale population, because it means that newborn whale calves are at greater risk of dying. Fortunately, right whales in the ocean Antarctica are not in danger of extinction, but if this continues they could become so”, according to Fredrik Christiansen, principal investigator of the Department of Ecoscience of the University of Aarhus (Denmark).

When winter comes and the whales leave Antarctica and swim north, they have to make do for several months without food, consuming the blubber stores they have built up during the warm, light summer season.

Throughout the summer, right whales swim under the sea ice, opening their mouths to absorb seawater, krill, and water fleas. The beards inside its mouth are a kind of giant filter that selects the small animals from the salt water, allowing them to eat large amounts of food without using a lot of energy.

“But the large shoals of krill are shrinking and this means the whales can’t fatten up before winter like they used to,” says Christiansen, adding: “Krill shoals live on phytoplankton, which thrives best in cold waters offshore. “Antarctica. Here, like plants on land, they transform sunlight into energy. Rising sea temperatures mean there is less phytoplankton, less krill, and therefore less food for the whales.”

Instead, the whales forage for food further north, where there is another, less energy-rich form of krill.

To find out the weight of the whales, researchers have devised a method based on photographs taken by drones. “Right whales like to lie on the surface of the sea. This makes them easy to photograph from above. When the drone has taken some pictures and we know the height of the drone, we can calculate the size of the animal,” explains Christiansen.

However, to know the weight of the whale it is necessary to know its volume, not just the length and width. Because scientists like Christiansen have observed many right whales along the sea surface over the years and thus have been able to measure their size, scientists now know the relationship between the length, width, and volume of those animals.

“We calculate the volume using the drone photographs. When we know the volume, we more or less know the weight. In this way, we can see that the whales have become thinner in the last 30 years and that is serious. The weight of the mothers has a huge impact on their calves,” says Christiansen.

30 or 40 years ago, the southern right whale had calves every three years on average when it arrived off the coast of South Africa, but this is no longer the case as it is now difficult for them to fatten up during the summer.” “This has been reduced to every five years. This means that the population is growing significantly more slowly”, emphasizes Christiansen.

Right whales were given this name because they were considered ‘suitable’ for hunting as early as the 14th century. For hundreds of years, they were hunted in both the North and South Atlantic.

The oil from the blubber of the whales was one of the most important sources of energy. Train oil, as it used to be called, became the fuel for lamps, both for interiors and for public lighting. Their demand was also one of the most important reasons Denmark colonized Greenland in the 18th century.

Around 1900, train oil was replaced by another, more efficient energy source: crude oil. Black gold mined from underground meant that whaling was no longer profitable.

The southern right whale is one of the species that benefited from the end of whaling. For more than 100 years, the population has been allowed to grow large and healthy again. And this is not only good for these animals, but also for the entire ecosystem of the Southern Ocean.