The biggest animals to have ever lived on Earth gobble up much more food than scientists thought, according to a new study of filter-feeding whales that reveals just how important their eating habits could be for recycling nutrients in the ocean.
Baleen whales such as blue, fin, minke and humpback whales consume, on average, around three times more each year than previous estimates suggested, researchers report in Nature. A blue whale in the eastern North Pacific, for example, might eat between 10 and 20 tons of food a day.
"That amount of food is somewhere in the range of 20 to 50 million calories," says Matthew Savoca, a researcher at Stanford University and the lead author of the new study. "That is about 70- to 80-thousand Big Macs. Probably decades of our eating is one day for them. So it's pretty remarkable."
Savoca got interested in how much whales eat a few years ago because he wanted to know how much pollution they might ingest along with their food. To his surprise, he says, the only numbers he could find on whales' prey consumption "didn't actually come from living, breathing whales in the wild."
Instead, researchers had made guesses based on extrapolations from the caloric needs of smaller animals. Or, they had simply inspected the stomach contents of whales that had been hunted, relying on a snapshot in time that might not have fully reflected how much a whale takes in over a day or a year.
A new way to count calories
Savoca realized that researchers could get more accurate estimates by using an underwater device that can measure the size and density of swarms of shrimplike krill — the mainstay of these whales' diet. This kind of device sends out pulses of sound that bounce off the swarms and return.
He and his colleagues gathered data on over 300 tagged whales as the huge animals fed in krill swarms by gulping in water to filter out the krill. The size of each whale determined how big of a mouthful of krill-filled water it could get at one go, and the researchers tracked the whales' movements to see how often they went for another gulp.
In dense swarms of krill, says Savoca, the whales feed at levels that are hard to believe. "Blue whales might lunge into a prey patch 200 times a day," he says. "Humpback whales might do it 500 times a day."
After all this eating comes pooping. Only recently have scientists realized that whale excrement contains high levels of iron, a precious resource in the ocean. Whales' fecal plumes spread nutrients out close to the ocean's surface, which boosts the growth of phytoplankton, tiny life forms at the bottom of the marine food web that are eaten by krill. The krill, of course, get eaten by whales.
But this nutrient-recycling system has been disrupted by the mass slaughter of whales over the past two centuries, according to this new report, resulting in "the near-complete loss of whale-recycled iron from the largest species." The researchers estimate that baleen whales recycled 12,000 metric tons of iron per year before whaling, compared with 1,200 metric tons today.
These findings are similar to estimates from a 2016 analysis that suggested iron recycling by large baleen whales in the Southern Ocean was reduced tenfold between 1900 and 2008. But that study also looked at iron recycling by zooplankton and other small creatures that are far more numerous than whales ever were and concluded that compared with those creatures' nutrient-recycling work, the whales' contribution was likely "negligible."
Maria Maldonado of the University of British Columbia in Vancouver, who did the 2016 study, maintains that "the big recyclers of iron in the ecosystem are not the whales."
More whales, more krill?
Still, some researchers believe that the killing of more than a million baleen whales around Antarctica over the 20th century, as well as the loss of all their fecal fertilizer, is tied to the subsequent dramatic declines in krill populations.
Pre-whaling populations of whales would annually eat double the total amount of Antarctic krill that exists in the Southern Ocean today, according to the new report's calculations.
Before whales were decimated by hunting, observers described those seas as being colored red by swarming krill. "Krill swarms at the surface used to be a common sight in the Southern Ocean," notes Victor Smetacek, a researcher with the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research in Germany. "The last swarms were seen in the early 1980s."
He believes that, historically, the whales were "maintaining the krill swarms by recycling iron." In his view, it's worth doing tests of adding iron to the ocean to encourage the growth of phytoplankton, which would then feed the krill and ultimately give a boost to whale populations, which apparently need to eat more than researchers ever expected.
Such experiments can be controversial, he says, but "once people start understanding that the whales themselves were doing iron fertilization and that we would be just mimicking the whales, I am hoping that they would come around."
Maldonado is against this idea and says the disappearance of krill could be due to changes in water temperature or ocean acidification.
An iron fertilization test in the ocean would be complicated and could potentially have unintended consequences if it weren't done well, says Asha de Vos, a marine biologist and the founding executive director of the conservation research group Oceanswell in Sri Lanka.
"I would be cautious," she says, noting that whales could be helped in other ways, such as protecting them from ship strikes or net entanglements. "We need to start tackling those issues as well and not just look for one quick fix."