It’s long been an incredible irony of the animal world. The biggest animals that have ever lived on Earth—whales the size of Boeing 737s—sustain themselves by feeding on small fry: tiny animals like zooplankton and krill. Gulping and filtering giant mouthfuls of seawater, the massive mammals consume tiny creatures by the millions, and now scientists have learned the ocean behemoths eat orders of magnitude more than experts had ever suspected.
An innovative study published today in Nature has found that, on average, the world’s baleen whales eat three times more krill, tiny fish and animal plankton than previous estimates. Some of the marine mammals can devour up to almost a third of their massive body weight on a big feeding day. All that feasting means that whales also produce a lot more poop, a critical fertilizer at the base of the marine food chain. And the study suggests, today’s diminished populations, and their correspondingly small poop production, might be a key reason some of world’s marine ecosystems exist as only a shadow of their former abundance.
Researchers previously had a hard time getting a handle on how much a 30- to 100-foot whale ate because underwater feeding was tough to observe. Based on stomach examinations and computer models of whale metabolisms, earlier estimates suggested that most whales might eat up to 5 percent of their body weight on a feeding day. But the new study actually tracked and observed the eating habits of hundreds of living baleen whales in real time to discover that they can eat an estimated 5 to 30 percent of their body mass per day.
A North Pacific blue whale, for example, eats some 16 tons of krill, shrimp-like crustaceans just an inch or two long, on a feeding day during the foraging season—that’s about the weight of a city bus. North Atlantic right whales and bowhead whales eat 5 and 6 tons of small zooplankton respectively.
Because whales devour small sea creatures in enormous numbers, you might think that these hungry giants could make life in the sea scarcer. In fact, scientists theorize, just the opposite may be true. The more krill that whales eat, the more krill and other species can be found in some parts of the oceans. That’s because the more whales eat, the more they poop, releasing nutrients like iron into the ocean water to fertilize the growth of phytoplankton, which in turn serve as a primary food source in the marine food web. “How do you get phytoplankton to grow better? They’re just plants, so you fertilize them,” says co-author Matthew S. Savoca, an ecologist at the Hopkins Marine Station of Stanford University. “And how do you fertilize plants in the open ocean? That’s exactly what the whales do.”
For this study, Savoca and colleagues, including Nicholas Pyenson, curator of fossil marine mammals at the Smithsonian’s National Museum of Natural History, measured the feeding habits and rates of 321 individual baleen whales from seven different species between 2010 and 2019. Technology, creativity and hard work were used to merge information on three key aspects of feeding. The scientists measured how often a whale fed, how much that whale could consume based on its mouth size and how much food was available in each swarm the whale devoured.
The team attached high resolution tags that Savoca likens to “whale iPhones” to the animals with suction cups. The devices featured GPS that tracked location and accelerometers that measured telltale feeding movements, like distinctive lunges. The tags enabled the team to see where and how often whales were feeding—actions they confirmed using video cameras on the devices.
The team also flew drones over 105 of the whales and measured each whale’s size and, crucially, the size of its mouth. This information was used to determine how much ocean water, and potential food, each whale could filter each time it fed.
And the third piece of info was crucial—a measurement of how much food was actually in each mouthful of seawater. The scientists pursued feeding whales in small boats, armed with fisheries acoustics devices that sent out pulses of sound and used the echoes to estimate the density of the prey swarms being devoured. “This is not unlike how toothed whales, dolphins and sperm whales, find food with echolocation,” Savoca says.
“So we have from the tag the number of times the whale feeds per hour or per day, we have a really good estimate of the size of the whale’s mouth from the overhead drone images, and then we have the rough density of the krill swarm that the whale is feeding on using these sonar type technologies.”
The researchers showed that blue, fin and humpback whale populations in the waters between British Columbia and Mexico eat an estimated six million metric tons of food each year.
“The results of this study suggest that the influence of whales on marine ecosystems might be greater than we realized,” says Joe Roman, a conservation biologist at the University of Vermont not involved in the research. “This study gives us a better view of just what was lost on a population and ecosystem level.”
In the 20th century, whalers killed an estimated three million whales, severely affecting the ocean ecosystem in ways that scientists are still trying to understand. The larger whale appetite estimates in the new study suggest that prior to the whaling era the mammoths in the Southern Ocean alone ate 430 million tons of Antarctic krill every single year, leading to a lot of poop. Today, all of the krill living in the Southern Ocean add up to only about half of that amount.
“In the open ocean, the vast areas of former whale feedings grounds, it’s now a degraded ecosystem. It’s like a semi-arid land environment that used to be a rainforest before,” says Victor Smetacek, a plankton ecologist at Germany’s Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research who wasn’t involved with the research.
With far fewer whales in today’s waters, the role of their enormous appetites in shaping ocean ecosystems has likely been drastically reduced. Whaling records show that about one million krill-devouring whales were killed in the Southern Ocean, and today Southern Ocean krill exist in far smaller numbers than when sailors of the pre-whaling era described it as coloring surface waters red with its abundance. Scientists have theory about how iron-rich whale poop can explain this ‘krill paradox.’
“Krill is a gigantic iron reservoir,” explains Victor Smetacek, “The whales tapped this gigantic iron reservoir, and every year let’s say one-fourth of that reservoir is recycled, it goes into phytoplankton, the krill pick it up [by eating phytoplankton] and, again, the whales eat the krill. The blue whales and krill support each other in this exclusive relationship. That’s the reason why the krill population plummeted after the whales were taken out. They need each other,” Victor says.
Industrialized whaling utilized steam power, harpoon cannons, radio, aircraft spotting, onboard processing and other advances to become frighteningly efficient. During 50 or 60 years of the 20th century, the lifetime of one whale, some 90 to 99 percent of all the blue whales on Earth were killed.
Today while nations like Norway, Japan and Iceland continue to whale, others subscribe to the International Whaling Commission ban designed to help global populations recover. With varying levels of success, humans are also trying to protect whales from other sources of mortality like entanglement with fishing gear and shipping collisions.
Savoca notes that while we may not understand all the impacts of bringing whales back, just as we haven’t identified all the consequences of losing them, the study is another line of evidence revealing that just a few hundred years ago regions like the Southern Ocean were far, far richer ecosystems than we know today.
“We can recover that system and whales are key component of that,” he says. “There’s a good amount of evidence that, on the whole, with more whales we’ll see more productivity, more krill and more fish, not less. But whether we actually do see that for many hundreds of years in the future really depends on the choices we make in the next few decades.”