(CN) — Baleen whales, including iconic species such as the blue whale and humpback whale, measure closer in size to commercial jets and large buildings than to other animals alive today. With a large frame comes a large appetite — and no one works a seafood buffet quite like a baleen.
Until the whaling industry decimated the species’ numbers in the 20th century, that was likely an all-you-can-eat buffet, but today’s whales have to take what they can get, and that’s having a knock-on effect throughout Earth's oceans. Reducing a predator’s numbers typically increases the numbers of its prey, but in this case the opposite is true.
Baleen whales primarily feed on krill, which themselves primarily eat phytoplankton. Each depends on the other — whales eat the iron-rich krill and redeposit that iron in the upper levels of the ocean through their feces where it can fuel the phytoplankton blooms that krill depend on. Without whales redistributing iron near the ocean’s surface, krill numbers decline, and so do whale numbers, leading to a vicious cycle in all three groups.
Researchers from the Smithsonian’s National Museum of Natural History investigated baleen whale feeding habits for a new study published Wednesday in the journal Nature and found that previous studies underestimated the baleen’s food intake by a factor of three — and thus underestimated their importance to the oceans as a whole.
Three times the food intake means approximately three times the iron-rich excrement, which in turn powers phytoplankton blooms in the upper levels of the ocean. In addition to feeding the krill that baleen whales depend on, these blooms can also absorb significant amounts of planet-warming carbon, and could play a major role in mitigating climate change if phytoplankton numbers are restored to past levels.
“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, and author of the study. “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.”
To determine just how much food these colossal creatures can ingest, a team of scientists crunched data collected between 2010 and 2019 from 321 previously tagged whales from seven species in the Atlantic, Pacific and Southern oceans. The tracking tags are stuck to the subject whales’ backs via suction cup and record camera, microphone, GPS and movement data, and when paired with drone photographs and sound detection equipment allowed researchers to look for patterns indicating feeding activity.
The authors estimated baleen whales’ median daily prey consumption to be between 5% and 30% of body mass per day. With all that food coming in, something must be going out, and they calculated that baleen whales in the Southern Ocean recycled 12,000 tons of iron per year before commercial-scale whaling took place, compared to a mere 1,200 tons per year today — a 10-fold reduction in the vital nutrient.
Without enough iron to sustain the plankton, krill populations surprisingly shrunk in many areas where whale populations had been hardest hit. The decline in krill populations following a decline in their predators’ numbers is known as the “krill paradox,” according to Matthew Savoca, a marine ecologist at Stanford and lead-author of the study, which is especially noticeable in regions that once experienced heavy whaling.
“This decline makes no sense until you consider that whales are acting as mobile krill processing plants,” Savoca said. “These are animals the size of a Boeing 737, eating and pooping far from land in a system that is iron-limited in many places. These whales were seeding productivity out in the open Southern Ocean and there was very little to recycle this fertilizer once whales were gone.”
The authors hypothesize that restoring whale populations could have a ripple effect reversing lost marine productivity and leading to phytoplankton absorbing more carbon from the atmosphere. They estimate that 215 million or more tons of carbon could be absorbed and stored by returning to 19th century population levels of whales, krill and phytoplankton.
“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,” Pyenson said. “That system is still there, and helping whales recover could restore lost ecosystem functioning and provide a natural climate solution.”
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