These endangered whales are disturbingly thin—why that matters

North Atlantic right whale researchers have many reasons to worry: Only 409 of the majestic marine mammals remain, and the threats facing them are formidable. For one, the endangered whales inhabit the busy waters off the Atlantic coast, where they must navigate crowded shipping channels and water columns clogged with fishing gear.

Now, a new study gives scientists another cause for alarm: North Atlantic right whales seem to be in significantly poorer condition than their close relatives, the southern right whales.

While they inhabit different ranges—southern right whales favor the relatively quiet seas south of the Equator, and North Atlantic right whales inhabit the heavily trafficked waters off eastern North America—the two species share similar genetics and the same unfortunate history.

Reaching the length of school buses and weighing up to 70 tons, right whales derive their name from whalers, who deemed them the “right” targets to hunt because they swim slowly, stick close to shore, and float when harpooned. From the 11th to 20th centuries, whaling decimated all three species of right whales—North Atlantic, North Pacific, and southern—with some reduced to 5 percent of their original population.

In 1935 the League of Nations banned the hunting of all right whales, and since then southern right whales have made encouraging steps toward recovery. Their population of more than 10,000 is growing at a healthy rate, with some groups expanding as much as 7 percent a year. The International Union for Conservation of Nature considers them of least concern.

From 1990 to about 2010, the North Atlantic right whale population also rebounded, from 270 individuals to 483 in 2010. But over the past decade their numbers have dwindled, largely due to collisions with vessels and getting stuck in fishing gear. (Read about deaths of North Atlantic right whales in Canada.)

To explore why the two whale species’ paths have diverged, Fredrik Christiansen, a marine ecophysiologist at the Aarhus Institute of Advanced Studies in Denmark, turned to drones to compare their body conditions from above.

Immediately, Christiansen was shocked by how emaciated the North Atlantic right whales looked, calling them “impressively skinny.” In contrast, the southern right whales “looked like a runway…you could basically set up a tent on their backs,” says Christiansen, who has received funding from the National Geographic Society.

Such poor condition, caused in part by exhaustion from dragging around fishing gear, likely explains why the animals are reproducing so slowly, the authors note.

“We think of the species as being in trouble,” notes study co-author Peter Corkeron, leader of the whale research team at the Anderson Cabot Center for Ocean Life.

“But this research shows the individuals are in trouble, too… If there isn’t massive intervention, they’ll be gone in 20 years.”

Size matters

In collaboration with 17 other whale researchers around the world, Christiansen collected aerial photos of 523 North Atlantic and southern right whales at various stages of life: calves, immature whales, adult whales, and lactating females.

Through photogrammetry, the science of making measurements from photos, Christiansen and his team then analyzed the images to compare the lengths and widths of the two whale species.

From there, they used the estimated body volumes to infer an “index of body condition,” or, in less scientific terms, relative fatness.

They found that North Atlantic right whale juveniles, adults, and lactating females were all in significantly worse condition than their southern counterparts.

North Atlantic right whale mothers seemed to be in particularly poor shape, according to the study, published recently in Marine Ecology Progress Series.

Compared with similar-size southern right whales, they weighed 20 percent less on average—a difference of about 10,000 pounds. (Read more about the struggles that right whale mothers face.)

“Incredibly important”

This helps explain the decline of the North Atlantic right whale species, Christiansen says: Bearing a calf takes an enormous amount of energy, and the thinner the whale, the longer they likely need to recover in between births.

Recently, North Atlantic right whales have birthed calves about seven years apart in comparison with every three years for southern right whales. (See 14 jaw-dropping pictures of whales.)

“To assess body condition in a whale you have to measure how fat it is generally, so this study is incredibly important,” says Victoria Rowntree, a right whale biologist at the University of Utah who wasn’t involved in the research.

“You can’t take a whale to the doctor’s office and plop it on the table and take its temperature.”

Why so thin?

The study points to three possible explanations for the North Atlantic right whale’s thinness.

The first is fishing gear: Studies suggest more than 85 percent of North Atlantic right whales have been caught in nets, lines, and other fishing equipment at least once in their lives. Between 2017 and 2020, this gear killed seven whales—or about 2 percent of the surviving population. During the same period, another nine whales were killed by collisions with vessels.

Though fishing gear can kill, many whales survive entanglement and drag the heavy ropes around with them, burning lots of calories as they do. Vessel traffic noise is another source of stress, and thus energy expenditure.

And finally, warming oceans have forced copepods, a tiny crustacean that’s the main staple of the North Atlantic right whale’s diet, to migrate north. This forces the whales, which must eat about 2,000 pounds of food daily, to travel outside of protected areas to feed, making them more vulnerable to vessel strikes and gear entanglement.

Reason for hope

In a result that surprised the study’s authors, North Atlantic right whale calves that were younger than four months were not in poorer bodily condition than southern right calves of the same age. In contrast, juvenile North Atlantic right whales were found to be significantly scrawnier than their southern cousins.

This suggests that if human stressors on North Atlantic right whales and their mothers are reduced, the North Atlantic calves could develop to be healthier.

Several ongoing lawsuits—including one against federal agencies, including the National Marine Fisheries Service and another against the state of Massachusetts—aim to boost protections for North Atlantic right whales, such as reducing entanglement in lobster fishing gear.

“When you see the calves three months [after they’re born], they’re half the size of their mother. You can’t believe she can actually survive nursing this meatball that’s swimming next to her,” Christiansen reflects. “And that’s when you realize that they’re already pushing themselves to their limits.”