As the largest animals that have ever existed, whales should have a hearty appetite. But until now, researchers never realized how big that appetite really is. A new study reveals baleen whales—14 species that filter feed using comblike mouth structures—eat, on average, three times as much as previously thought. That might seem like bad news for their prey, but the study also suggests the whales are doing the ocean a favor: By lunging after prey and filtering water, they act like plows churning nutrients through small patches of sea. And by feeding at the bottom of the ocean and defecating at the surface, they cycle nutrients through the entire water column.
“Whales have a value beyond just being amazing,” says Shirel Kahane-Rapport, a marine biologist at California State University, Fullerton, who was a Ph.D. student at Stanford University’s Hopkins Marine Station (HMS) when she participated in the study.
HMS ecologist Matthew Savoca was trying to figure out how much plastic whales eat when he realized he first had to answer a much more basic question: How much do they eat, period? He was shocked to realize only estimates existed, and those estimates were rough, based on the stomach contents of beached or killed whales—or metabolic calculations.
So Savoca, Kahane-Rapport, and colleagues used drones, echo-sounding equipment, and suction cup tracking devices to follow 321 whales while they foraged. Their study ran from 2010 to 2019 and included data across the Atlantic, Pacific, and Southern oceans on seven species—including blue whales, humpback whales, and fin whales. Gathering the data required finesse, teamwork, and sea legs, Savoca says. “I can’t overstate how hard it is to do anything on a boat that’s rocking around.”
Researchers set out on small, rigid-hulled inflatable boats in areas with lots of whale action. To attach a tracking device, a team member would stand at the front of the boat holding a 3-meter-long pole, with a gripper stuck to the end containing the device. When a whale took a break at the surface to rest and breathe, the captain would pull the boat alongside the much larger animal, “like a car pulling up alongside a tractor trailer,” Savoca says.
The suction cups, which don’t seem to bother the whales, typically hold for about 1 day. During that time, the tracking device monitors the whale’s movements using video, audio, GPS, and an accelerometer. The accelerometer measures a whale’s bursts of speed as it gulps prey during mealtimes, and the GPS tracks where a whale goes to feed when it’s out of sight. Drone footage from above let researchers precisely measure the size of the whales—and their mouths. Advanced echo-sounding technology on the bottom of the boats provided a picture of how densely packed the prey was, so researchers could calculate how much the predators scooped up with each mouthful. “There is this whole team of people coming together to get the perfect sample from this whale,” Kahane-Rapport says.
On average, the team found that baleen whales consume three times as much food as previous estimates indicated, they report today in Nature. An adult North Pacific blue whale, for instance, eats a daily average of 16 tons of tiny crustaceans known as krill—roughly equivalent to the weight of a city bus. A bowhead whale, meanwhile, is a comparatively dainty eater, consuming about 6 tons of zooplankton per day, or the weight of an elephant. Using their new estimates, the researchers also calculated that—before they were decimated by whaling practices in the 20th century—baleen whales in the Southern Ocean would have eaten twice as much Antarctic krill every year—430 million tons—as currently exist.
The new estimates also support something called the krill paradox: As their biggest predators have disappeared, krill numbers also suffered. In a region of the Southern Ocean hit particularly hard by whaling, for example, krill populations have plummeted since the mid–20th century by more than 80%. The reason, Savoca says, is that whales perform essential ecosystem services, like fertilizing and nutrient mixing, that krill also need to thrive.
These new feeding estimates suggest whales are producing and mixing more nutrients, such as iron, than previously thought. That in turn would encourage blooms of photosynthetic plankton that form the base of the food chain, meaning more krill for whales and more fish for fisheries. The photosynthesis also means more carbon dioxide pulled out of the atmosphere.
“It’s a good reminder that the removal of whales has both direct and indirect impacts on our ocean,” says Asha de Vos, a marine biologist with the Sri Lankan ocean conservation organization Oceanswell who was not involved with the study. The findings “give us baseline estimates of what the world looked like pre–industrial whaling,” she says.