Whalers have plucked giant whales from the sea for much of the last century, reducing their numbers by up to 99 percent for certain species. Some scientists thought that krill — the tiny crustaceans that many whales eat in gargantuan gulps — would explode in number as a result, mostly free from the feeding pressure of the largest animals that have ever lived.

But that didn’t happen. Instead, Antarctic krill numbers have dwindled since the mid-20th century, by more than 80 percent in areas heavily trafficked by whalers. Many other consumers of krill, like seabirds and fish, have suffered too in the absence of the crustaceans and their giant eaters.

Now, scientists have a clearer idea why this happened: whale poop, or rather, the lack of it.

A new study finds that baleen whales, including blue and humpback whales, eat on average three times as much krill and other food as previously thought, and more food in means more poop out. Paradoxically, the collapse of the krill may stem from fewer whales excreting iron-rich, digested krill, denying these ecosystems some crucial nutrients they need to thrive. Phytoplankton blooms, which sustain krill and many other parts of the food web, rely on that iron. Restoring whale populations to prewhaling levels could help bolster these ecosystems and even store more carbon in the ocean, researchers report in the Nov. 4 Nature.

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“It’s hard to know what role whales play in ecosystems without knowing how much they’re eating,” says Joe Roman, a marine ecologist at the University of Vermont in Burlington who wasn’t involved in the research. Whale food intake was coarsely understood before, he says, and this study will “allow us to better understand how the widespread depletion of whales has impacted ocean ecosystems.”

Assessing the precise diet of Boeing 737–sized creatures that gulp down hordes of centimeter-long invertebrates far below the surface of the ocean is not a trivial undertaking. Previous estimates relied on dissections of dead whales or inferring whales’ metabolic needs based on their size. “These studies were educated guesses, and none were conducted on live whales in the wild,” says Matthew Savoca, a marine biologist at Hopkins Marine Station of Stanford University in Pacific Grove, Calif.

But new technology gave Savoca and colleagues “a chance to answer a really basic biological question about some of the most charismatic animals on earth.”

The researchers needed to know three things: how often whales feed, how big of gulps they take and how much food is in each gulp. Using sophisticated sensors suction-cupped to the backs of 321 individuals of seven whale species, the researchers could tell when the whales lunged for prey, a reliable sign of feeding. Aerial drones snapped photos of 105 whales, which the researchers used to estimate gulp size. Finally, sonar mapping revealed the density of krill in key feeding areas.

Combining these datasets together revealed a more detailed look at feeding than ever before, says Sarah Fortune, a marine ecologist at Fisheries and Oceans Canada in Winnipeg. Savoca and his colleagues “measured all the things you need to measure to get an accurate estimate of consumption,” for baleen whales, she says.

It turns out that, on average, baleen whales eat about three times as much food as earlier estimates suggested. For example, a blue whale can put down 16 metric tons of krill in a day, the researchers found. Energetically, that’s equivalent to around 10 million to 20 million calories, or about 30,000 Big Macs, Savoca says. 

Whales aren’t eating that much every day. The animals go for months without a bite when migrating vast distances. But the sheer volume of food that they consume, and then excrete, suggests that whales shape ocean ecosystems to a larger degree than previously thought, Savoca says, making their loss that much more impactful.

That’s because one role whales play is that of nutrient cycler. By feeding on iron-rich krill in the deep and returning some of that iron to the surface in the form of poop, whales help keep this crucial element in the food web. Excessive whaling might have broken this iron cycle. With less iron at the surface, phytoplankton blooms shrink, krill numbers crash and the ecosystem becomes less productive, Savoca says. 

Before industrial whaling killed millions of whales in the 20th century, the researchers estimate that baleen whales in just the Southern Ocean alone, a key feeding area, consumed 430 million metric tons of krill each year, more than twice the biomass of all krill that’s found in those waters today (SN: 3/4/21). Even with today’s diminished populations, researchers estimate that whales prevent approximately 1,200 metric tons of iron from being lost each year, left to drift down to the dark deep of the Southern Ocean.

Whales are likely not the only factor behind the staggering loss of krill, Savoca says, but the evidence suggests that “whales play a role here, and when you wholesale remove them, the system becomes, on average, less productive.” 

Some whale populations are rebounding (SN: 11/18/19). If whales and krill could be brought back to their early 1900s numbers, the productivity of the Southern Ocean could be boosted by 11 percent, the researchers calculate. That increased productivity would translate into more carbon-rich bodies, from krill to blue whales, which together would store 215 million metric tons of carbon annually, the equivalent of taking more than 170 million cars off the road for a year, the team suggests.

“Whales are not the solution to climate change,” Savoca says. “But rebuilding whale populations would help a sliver, and we need lots of slivers put together to solve the problem.”