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Considering the myriad threats plaguing whales around the world—from fishing ropes to heavy metals and plastic pollution to ship strikes—the belugas that live in the Beaufort Sea seem to be doing pretty well. Their population is holding steady at about 40,000, and there aren’t too many boats and humans crowding them—yet.

But there’s one troubling sign that all is not well. Over the past two decades, the belugas have been getting smaller and skinnier. New data suggests a frightening possibility: as the whales’ condition worsens, they may be less and less able to hold enough oxygen in their bodies to dive, restricting their ability to hunt. Since climate change is causing the availability of their prey to dwindle, this creates a vicious circle: less food means smaller and scrawnier belugas, and small, scrawny belugas can’t dive as long. Shorter dives mean less food … and repeat.

Emily Choy was working on her doctorate at the University of Manitoba when she discovered this downward spiral. Choy and her colleagues have a long-standing partnership with Inuit beluga hunters along the Beaufort Sea, who allowed them to collect blood samples and measurements from 77 animals between 2014 and 2016.

Choy was looking for a connection between body size and diving ability. That connection showed up when she measured how much oxygen belugas of different sizes can hold in their bodies.

Belugas, like humans, use hemoglobin and other proteins to store dissolved oxygen in their blood and muscles. Belugas’ proteins are different than humans’ and they have more of them, allowing them to stay underwater longer. Choy found that smaller belugas, the ones in worse condition, have proportionally much less hemoglobin than their beefier brethren. The difference means that smaller, thinner belugas embark on dives that are about three minutes shorter than bigger whales. While dropping from 17 minutes on average to 14 might not sound like a lot, those three minutes can be very important.

“Belugas’ main prey is Arctic cod, and they’re a sea-ice-associated fish,” Choy explains. In other words, the fish move with the ice. So as the water warms and sea ice cover shrinks, Arctic cod are shifting northward away from the belugas’ coastal habitats. This, along with expected decreases in Arctic cod populations, means that belugas will find it increasingly difficult to feed. Choy says the whales’ condition already tracks with the cod—in years with lots of sea ice, belugas are strong and fat; in years with little sea ice, belugas are gaunt.

Perhaps more importantly, the fattest, juiciest cod take more effort to hunt. These big fish live up to half a kilometer down in the dark, cold water. And as biologist Robert Michaud, president of the Group for Research and Education on Marine Mammals in Tadoussac, Quebec, says, those three lost diving minutes come out of hunting time. “Travel time down and back to the surface is fixed,” he says. “So all the deductions will be in your chase time.”

If there are fewer cod to catch, every second counts.

Michaud studies a different population of the playful white whales in Quebec’s St. Lawrence estuary. That population is small and getting smaller and it’s not clear why. Michaud thinks Choy’s study is valuable because it identifies a specific threat to the Beaufort Sea whales thanks to the detailed data. “It reminds me that we rely on long-term data sets to understand what’s going on in our world,” he says.

Choy agrees long-term monitoring is vital. For her study, it was only possible because of the hunters and their communities. However, this means her samples are biased because the hunters tend to take bigger adult males. But it still hints at a scary pattern­—one the hunters already knew. “The community had told us for years that the whales seemed to be getting smaller,” Choy says.

If her study is right, this worrying trend may continue.