The Southern Ocean that encircles Antarctica has, for decades, been home to a mystery known as the “krill paradox.” It goes like this: tiny shrimp-like krill have been in decline there for decades, even though the population of one of their chief predators—whales—fell dramatically during the 20th century. With fewer whales, shouldn’t krill numbers boom? Now, scientists think they have an answer: Whale poop.
Thanks to an elaborate decade-long investigation involving whale-mounted trackers, drones and sonar, scientists have found that the planet’s largest animals eat and poop far more than previously thought. This gluttony has the potential to ripple through entire ecosystems—including krill—as the whales vacuum up vast amounts of nutrients, then spread them back into the water as feces.
“Just this idea that if you remove large whales, there’s actually less productivity and potentially less krill and fish is amazing,” said Jeremy Goldbogen, a Stanford University biologist who supervised much of the work.
The insight came from a simple question: How much do baleen whales eat?
The animals, which can grow as large as a commercial airliner in the case of the blue whale, feed on some of the smallest animals in the sea by sucking vast amounts of ocean water through comb-like structures in their mouths called baleen. The baleen catches krill or small fish such as anchovies, which the whales then swallow.
But estimates of the total mass of food these giants consume has been wracked by uncertainty and guesswork. The whales are too big to study in captivity, and much of their feeding happens out of sight, underwater in the open ocean. Scientists instead extrapolated the whales’ metabolic needs based on smaller animals, and estimated prey consumption by measuring the stomach contents of dead whales.
In 2010, scientists launched an effort to get a clearer picture of these whales’ appetites. Using suction cups, they attached 321 motion trackers to 7 different species of baleen whales spanning the Pacific, Atlantic and Southern oceans. These devices allowed scientists to record the movements of the whales, including distinctive swimming behavior when feeding. All told, they recorded more than 70,000 feeding events over 10 years.
Scientists also flew drones over 105 whales, taking photos to calculate each whale’s size and how much water it filtered in a single gulp. Finally, they followed feeding whales aboard small boats and used sonar to measure the density and size of clouds of krill or fish on which the animals were feasting. The effort involved 17 researchers from 13 different universities and government agencies in Europe, Africa, and the U.S.
The results were stunning. These whales, it turns out, eat approximately triple the amount of food previously estimated. For the eastern North Pacific blue whale, that amounted to roughly 16 metric tons per day—the equivalent of eating 3 adult African elephants. To grasp the difference from previous studies, the researchers noted that in 2008 scientists predicted that each year whales off the west coast of North America ate approximately 2 million metric tons in total. The new study found that individual blue, fine and humpback whales in the area ate that amount, according to the study, published today in the journal Nature.
“Think of these large whales as mobile krill processing plants,” said Matthew Savoca, a marine ecologist and postdoctoral fellow at Stanford University who was the study’s lead author.
To understand the implications for ecosystems, Savoca and colleagues looked to the Southern Ocean. There the whaling industry had decimated whale populations during the 20th century, killing more than 1.5 million baleen whales. Based on estimated whale numbers there around 1900, those whales each year ate 430 million metric tons of krill a year, double the total mass of krill found in that ocean at the end of the 20th century. The results suggest krill numbers in that earlier era were far greater than today, to feed so many whales without being wiped out.
The researchers suspect the answer is in whale poop, or, more specifically, the iron contained in the poop. The growth of phytoplankton in the Southern Oceans is limited by a lack of iron, so having more than a million additional whales defecating relatively close to the ocean surface would be like having fertilizer machines crisscrossing the region. At pre-hunting numbers, Antarctic minke, humpback, fin and blue whales combined would have injected as much as 1500 metric tons of iron back into the environment, much of which otherwise would have sunk to the ocean floor as krill died.
The iron boost would have driven much bigger plankton blooms—approximately 215 million metric tons of additional plant growth, equal to 11% of today’s Southern Ocean plankton production, the scientists estimate. That, in turn, would have supported larger numbers of krill, which feed on the plankton.
“Our results say that if we restore whale populations to pre-whaling levels seen at the beginning of the 20th century, we’ll restore a huge amount of lost function to ocean ecosystems,” said Nicholas Pyenson, curator of fossil marine mammals at the Smithsonian’s National Museum of Natural History, who took part in the research. “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.”
As world leaders gather today in Glasgow, Scotland for a global climate change summit, the findings also hint that more voracious whales could even put a dent in greenhouse gas pollution. The additional plankton growth from earlier whale numbers in the Southern Ocean, said Pyenson, would have absorbed as much carbon as forest ecosystems covering a continent.
Photo: Humpback whales Antarctic. Duke University Marine Robotics and Remote Sensing under NOAA permit 14809-03 and ACA permits 2015-011 and 2020-016.