What if whales returned to every ocean?

Whales are the biggest animals in the sea.

Once there were many more of them. What if whales came back to every ocean?

Blue whales, humpbacks, sperm whales and fin whales once swam in far greater numbers than today. In the 1900s, hunting removed most of them.

Now many whale populations are growing again. So here is a thought experiment: what if whales returned to every ocean, as many as there used to be? What would change for the tiny plankton, the fish, the seabirds and even the air? You can follow three paths: whale poop, carbon sinking to the deep, and a sea full of whale sounds.

Industrial whaling in the 20th century removed millions of large whales. Populations protected since then are recovering unevenly; some remain endangered.

This thought experiment asks what a return to pre-whaling numbers might mean for ocean ecosystems. It draws on field measurements of fecal plumes, tag-based estimates of how much whales eat, and research on carbon storage. Many of the system-wide effects are calculated from models and remain ideas rather than observations.

People hunted lots of whales long ago. Then most whale hunting stopped.

Now some whales, like humpbacks, are growing in number again.

NOAA Fisheries says commercial whaling cut whale populations by more than 80 percent around the world. Humpback whales were hit especially hard: before a final ban on commercial whaling in 1985, most humpback populations had dropped by more than 95 percent.

Since then, humpbacks have been increasing across much of their range. Blue whales off the U.S. West Coast and gray whales in the eastern North Pacific have also responded to protection. But whales still face dangers from fishing gear, ship strikes and underwater noise.

NOAA Fisheries: commercial whaling depleted whale populations by more than 80 percent globally. Before the International Whaling Commission's final moratorium on commercial harvest, in effect since 1985, humpback populations had mostly fallen by more than 95 percent.

Humpbacks are now increasing throughout much of their range; of 14 distinct population segments, four remain endangered and one threatened (2016 listing). Threats include entanglement in fishing gear, vessel strikes, vessel-based harassment and underwater noise. Blue whales off the U.S. West Coast and Eastern North Pacific gray whales show positive responses to conservation.

Let's pretend the oceans are full of whales again.

Whales eat lots of tiny animals. Then their poop feeds tiny sea plants. More whales might mean more food for everyone!

Imagine whales back in their old numbers, spouting across every ocean. Whales eat enormous amounts: a 2021 study found baleen whales eat about three times as much as scientists had thought, and a blue whale can gulp about 16 metric tons of krill in a day.

All that eating means lots of poop, full of iron and other nutrients. That poop feeds phytoplankton, the tiny drifting plants that feed krill and much of sea life. So scientists think more whales might mean more plankton, more krill, and more carbon taken out of the air. These are ideas being studied.

Savoca and colleagues (Nature, 2021) tagged 321 whales of seven species, used drones to photograph 105, and measured prey with sonar. They found baleen whales eat about three times earlier estimates; a blue whale can take about 16 metric tons of krill in a day.

The team estimated that before whaling, Southern Ocean baleen whales ate about 430 million metric tons of krill a year, while less than half that much krill lives there today. They calculated that restoring whales and krill to early-1900s levels might raise Southern Ocean productivity by 11 percent. These are model estimates, not observed outcomes.

Whales eat deep down. Then they come up to breathe and poop near the top.

The poop feeds tiny sea plants that need sunlight. Scientists call this the whale pump.

Phytoplankton need sunlight and nutrients, such as nitrogen and iron, to grow. Sunlight is only near the surface, but many nutrients sink into the deep.

Whales help bring them back. Many whales feed deep down, then rise to the surface to breathe, where they release floating plumes of poop and pee. Scientists call this the whale pump. A 2010 study found that whales and seals in the Gulf of Maine bring more nitrogen to the sunlit surface each year than all the region's rivers combined.

Roman and McCarthy (PLoS ONE, 2010) sampled humpback fecal plumes on Stellwagen Bank and found nitrogen concentrations far above surrounding water. They estimated that whales and seals replenish about 23,000 metric tons of nitrogen a year to the Gulf of Maine's euphotic zone, more than all rivers combined, and that before commercial whaling this recycling was likely more than three times atmospheric nitrogen input.

NOAA describes the vertical whale pump and a horizontal whale conveyor belt: nutrients consumed on feeding grounds are released along migration routes up to 12,000 miles long.

When a whale dies, it sinks to the deep sea floor.

Many deep-sea animals feed on it for years. Its body keeps carbon down deep for a very long time.

NOAA Fisheries explains that whales store carbon in their huge bodies. When a whale dies, its body often sinks to the seafloor, an event called a whale fall. The carbon can stay down there for hundreds to thousands of years, while deep-sea animals feed on the body. The Monterey Bay Aquarium Research Institute says one whale fall can bring as much food to the seafloor as several thousand years of falling bits called marine snow.

NOAA also says it's hard to put an exact number on how much carbon whales store.

NOAA Fisheries lists three routes: biomass storage in long-lived bodies, whale falls that sequester carbon for hundreds to thousands of years, and nutrient supply that stimulates carbon-capturing phytoplankton blooms. It also says it is difficult to put an exact number on whales' share of blue carbon storage.

MBARI notes a single whale fall can deliver as much organic material as several thousand years of marine snow. Savoca's team calculated that restored Southern Ocean whales and krill might store about 215 million metric tons of carbon a year; like other system-wide estimates, this comes from calculation, not measurement.