A blue whale can grow to weigh 150 tons, nearly as heavy as 14 school buses. You don’t get that big without gobbling up — and pooping out — a whole lot of stuff. As whales eat and excrete, they keep essential nutrients cycling through the top levels of ocean water, where they help power blooms of phytoplankton, microscopic organisms that absorb carbon and are the foundation of the marine food web.
Now, new research published in Nature reveals that baleen whales, the group of giant marine mammals including blue, fin and humpback whales, eat an average of three times more food per day than previously estimated. By eating more food, baleen whales play an even larger role in bolstering nutrient circulation and carbon uptake in the ocean.
“Our results say that if we restore whale populations to pre-whaling levels, 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 and co-author on the new paper. “It's the clearest read yet about the massive role of large whales on our planet.”
In the past, scientists’ best estimates for the quantity of whales’ diets were mostly just educated guesses said Matthew Savoca, a marine ecologist and postdoctoral fellow at Stanford University and lead author of the new paper. But as the world’s oceans get hotter and face staggering species loss, scientists needed a clearer idea of how much baleen whales eat to make predictions and develop solutions for the animals’ survival.
So Savoca, Pyenson and their colleagues turned to a data set that directly tracked the whales’ snacking. Between 2010 and 2019, researchers tagged 321 whales in the Atlantic, Pacific and Southern Oceans with a device Savoca likened to a waterproof smartphone. Each gadget included a camera, accelerometer and GPS tracker, and could measure each whale’s 3D movement, allowing scientists to identify feeding patterns and behaviors in near real-time.
The team also compiled drone photographs of 105 whales to measure how much water — and thus, krill — each whale could filter in its mouth. And the researchers hopped on boats to watch whales feeding in the wild, deploying machines that used sound waves to measure swarms of krill and other prey species the giant mammals like to chow on.
These data helped Savoca, Pyenson and their colleagues paint a remarkably complete picture of baleen whales’ diets. For example, they found that a North Atlantic right whale eats five metric tons (about 11,000 pounds) of plankton each day, while an eastern North Pacific blue whale eats about 16 metric tons daily. The study also estimates that the blue, fin and humpback whale populations living in the Pacific Ocean along the Western United States each require more than 2 million tons of prey annually.
The researchers then investigated how much iron, an essential nutrient that powers phytoplankton blooms in the Southern Ocean, the whales pump back into ocean waters in their feces. Previous research found that whale poop contains nearly 10 million times the amount of iron found in seawater. Using this, the study authors calculated that whales in the Southern Ocean poop about 1,200 metric tons of iron back into the water.
“Blue and fin whales are the size of a Boeing 737, eating and pooping far from land in a system that is iron-limited in many places,” Savoca said.
Because whales breathe air, they keep close to the water’s surface. The iron they poop out stays in the upper levels of ocean water where it can be used by krill, plankton and other microscopic critters that are essential parts of the marine ecosystem.
An estimated 2 to 3 million whales were killed by industrial whaling in the 20th century. In their study, Savoca and Pyenson estimated that whales in the Southern Ocean recycled around 12,000 metric tons of iron before this dire decline, 10 times as much as they do today. The scientists concluded that bringing whale populations back to pre-industrial levels could boost phytoplankton blooms.
And because phytoplankton suck carbon dioxide out of the atmosphere as they grow, they could provide a boon to the global greenhouse gas problem. “Helping whales recover could restore lost ecosystem functioning and provide a natural climate solution,” Pyenson said. “Our results say that if we restore whale populations to pre-whaling levels, we'll restore a huge amount of lost function to ocean ecosystems, which is something close to a natural climate solution.
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