The vast appetites of the oceans’ largest whales are a far mightier ecological force than scientists had imagined and boosting their numbers could be a powerful means of fighting climate change.
A study has found that baleen whales, which include blue, fin and humpback whales, consume three times as much food than was previously thought.
It follows that they produce three times as much excrement as well — a vital source of nutrients, particularly iron, that other ocean organisms require to flourish.
The new findings shed new light on how the decimation of whale populations in the 20th century transformed ocean habitats, creating vast areas of ecologically degraded seas. In particular, huge amounts of microalgae, which absorb carbon dioxide from the atmosphere, vanished.
Krill, small shrimp-like crustaceans which blue whales eat almost exclusively, contain a significant amount of iron. This is a scarce nutrient in the open ocean. When whales eat krill they convert the protein into blubber and defecate iron-rich waste.
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This provides a vital source of iron for organisms known as diatoms, microalgae that remove large amounts of carbon from the air through photosynthesis. These diatoms form the base of the marine food chain. They are eaten by krill, which are consumed by whales, which emit the iron and so the cycle continues.
Crucially, because whales need to breathe air, the nutrients remain close to the surface of the ocean where the diatoms live, rather than sinking into the deep.
When the whales are removed, however, the system collapses. The study, published yesterday in the journal Nature, concludes that krill biomass is now a fraction of what it once was even though the number of blue whales that eat it is also lower.
The researchers suggest that industrial whaling led to the disappearance of the “hugely productive ocean pastures dominated by diatoms” that naturalists described in the 1930s. These areas, which covered much of the ocean surface, have reverted to a barren, low-iron state.
“This degraded ecosystem dominates the former whaling grounds, presumably because the hard-working whales are almost completely absent,” the researchers write.
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“Think of these large whales as mobile krill-processing plants,” Matthew Savoca of Stanford University, an author of the study, said. “Each fin whale or blue whale is the size of a commercial airliner. So, in the first half of the 20th century, before whaling, there were an additional one million of these 737-sized krill-processing plants moving around the Southern Ocean eating, pooping and fertilising.”
In recent years scientists have proposed adding iron to the oceans to stimulate the growth of carbon-absorbing algae.
Restoring whale populations could do much the same thing, say the authors of the new study. “Our results suggest the contribution of whales to global productivity and carbon removal was probably on par with the forest ecosystems of entire continents, in terms of scale,” said Nicholas Pyenson of the Smithsonian’s National Museum of Natural History.
“That system is still there, and helping whales recover could restore lost ecosystem functioning and provide a natural climate solution. It may take a few decades to see the benefit, but it’s the clearest read yet about the massive role of large whales on our planet.”
The researchers tracked the movement of 321 whales of seven species, which were tagged with sensors and cameras. They also used acoustic sensors to monitor the densely concentrated prey. Drones were also used to watch more than 100 whales while they fed. These different sources of data were combined to estimate just how much the whales were eating.
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The results suggest that an adult eastern North Pacific blue whale consumes 16 metric tonnes of krill each day during its foraging season.
A North Atlantic right whale eats about five metric tons of small zooplankton daily, and a bowhead whale consumes six metric tonnes of small zooplankton.
The analysis suggests that minke, humpback, fin and blue whales in the Southern Ocean consumed about 430 million metric tonnes of krill annually at the beginning of the 1900s. That is double the amount of krill estimated to be in the entire Southern Ocean today and is more than twice the total global catch from all human wild-capture fisheries combined.
In terms of the whales’ role as nutrient recyclers, the researchers calculate that whale populations, before they were decimated by 20th-century whaling, produced a flow of poo that would have contained some 12,000 metric tonnes of iron in the Southern Ocean — ten times the present amount.
From 1910 to 1970 humans killed an estimated 1.5 million baleen whales in the waters around Antarctica. The number of Antarctic blue whales fell from 125,000 individuals in 1926 to about 3,000 in 2018. “Fifty years after we stopped hunting whales, we’re still learning what impact that had.
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The system is not the same,” Savoca said.
“We’re looking into ways of using this information to restore ocean ecosystems and bring whales back. And hopefully, that will have benefits for everything from biodiversity conservation to fisheries yield to carbon storage.”