The largest animals that have ever lived on Earth swallow much more food than scientists thought, according to a new study of filter-fed whales that reveal how important their eating habits can be for nutrient recycling in the ocean.
Baleen whales such as blue whales, fin whales and humpback whales consume on average about three times more each year than previously estimated, researchers in Nature. A blue whale in the eastern North Pacific, for example, can 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 lead author of the new study. “That’s about 70 to 80 thousand Big Macs. Probably decades of our dining is a day for them. So that’s pretty remarkable.”
Savoca first became interested in how much whales eat a few years ago because he wanted to know how much pollution they could ingest along with their food. To his surprise, he says, the only numbers he could find on whales’ prey consumption came “actually not from living, breathing whales in the wild.”
Instead, researchers had made guesses based on extrapolations from smaller animals’ caloric needs. Or they had simply inspected the stomach contents of whales that had been hunted and relied on a snapshot of time that might not fully reflect how much a whale actually takes over a day or a year.
A new way to count calories
Savoca realized that scientists could get more accurate estimates by using an underwater device that could measure the size and density of swarms of shrimp-like krill – the mainstay of these whales’ diets. This type of device emits sound pulses that bounce off the swarms and return.
He and his colleagues collected data on over 300 tagged whales while the huge animals fed in krill swarms by swallowing in water to filter the krill from. The size of each whale determined how large a mouthful of krill-filled water it could become at once, and the researchers followed the whales’ movements to see how often they took another sip.
In dense swarms of krill, Savoca says, the whales feed at levels that are hard to believe. “Blue whales can throw themselves into a prey area 200 times a day,” he says. “Golden-backed Whales do it maybe 500 times a day.”
After all this eating, there is a beating. Only recently have scientists realized that whale excrement contains high levels of iron, a valuable resource in the ocean. Whales’ fecal tabs disperse nutrients close to the sea surface, increasing the growth of phytoplankton, small life forms at the bottom of the marine food web, which are eaten by krill. The krill are, of course, 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 almost complete loss of whale-recycled iron from the largest species.” Researchers estimate that baleen whales recycled 12,000 tons of iron a year before whaling, compared to 1,200 tons today.
These results are similar to estimates from a 2016 analysis that suggested that iron recycling of large baleen whales in the Southern Ocean was reduced 10 times between 1900 and 2008. But this study also looked at iron recycling of zooplankton and other small creatures, which are far more numerous than whales ever were, concluding that compared to their nutrient recycling work, the whales’ contribution was probably “insignificant”.
Maria Maldonado of the University of British Columbia in Vancouver, who conducted the study in 2016, maintains that “the major recyclers of iron in the ecosystem are not the whales.”
More whales, more krill?
Yet some researchers believe that the killing of more than a million baleen whales around Antarctica during the 20s.th century, and the loss of all their fecal manure, is linked to the subsequent dramatic declines in the krill population.
Populations of pre-whale whales would annually eat twice the amount of Antarctic krill found in the southern ocean today, according to the new report’s calculations.
Before whales were decimated by hunting, observers described these 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 at the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research in Germany. “The last swarms were seen in the early 1980s.”
He believes the whales historically “maintained the krill swarms by recycling iron.” In his opinion, it is worth doing tests to add iron to the ocean to promote the growth of phytoplankton, which would then feed the krill and ultimately give a boost to the puppy populations, which apparently have to eat more than scientists had ever expected. .
Such experiments can be controversial, he says, but “when people start to understand that the whales themselves were in the process of iron fertilization and that we just wanted to emulate the whales, I hope they would come.”
Maldonado opposes this idea, saying that the disappearance of krill may 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 not done well, says Asha de Vos, a marine biologist and founding director of the conservation research group Oceanswell in Sri Lanka.
“I would be careful,” she says, noting that whales could be helped in other ways, such as protecting them from ship attacks or net intrusions. “We also need to start tackling these issues and not just look for a quick fix.”