The mesopelagic zone, a little-understood depth of ocean also called the twilight zone, turns out to have more fish than previously thought. Much more.

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A recent Spanish expedition to circumnavigate the globe brought back new results of a kind of deep-sea census, published Friday in the journal Nature Communications. Using updated techniques, scientists found that 10 times more fish live between 650 and 3,300 feet below the surface of the ocean than previous estimates have concluded. Instead of 1,000 million tons of biomass, the new estimate suggests there are about 10,000 million tons. Already, animals living in the mesopelagic zone accounted for a majority of all the world's fish. The new study places them firmly in the lead.

But how did previous scientists get the number so wrong? It seems the old method involved combing the seas with fishing nets tied to boats, a fishing strategy known as trawling, to count them and extrapolate the total. "It has recently been discovered that these fishes are able to detect the nets and run, which turns trawling into a biased tool when it comes to count its biomass," says lead researcher Carlos Duarte in a statement. The Malaspina Expedition, the 2010 scientific journey around the world, used acoustic observation for its research. "Malaspina has provided us the unique opportunity to assess the stock of mesopelagic fish in the ocean," Duarte said.

The ocean has five main zones (see the photo below). The mesopelagic zone is the second deepest, called the twilight zone because light only penetrates its depths a little bit. By the third-deepest zone, the bathypelagic zone, there's no light at all. Lanternfish and the bioluminescent cyclothonids are among the residents of the mesopelagic zone. Many of the fish that live in this zone feed off the feces and corpses of fish that live above them.

One of the goals of the Malaspina expedition was to learn more about this zone and the oceans in general. Hundreds of scientists embarked in 2010 on the 32,000-mile trip from Spain to study global oceanic changes. As part of it they took acoustic measurements of the mesopelagic zone. "The fact that the biomass of mesopelagic fish (and therefore also the total biomass of fishes) is at least 10 times higher than previously thought has significant implications in the understanding of carbon fluxes in the ocean and the operation of which, so far, we considered ocean deserts," said Xabier Irigoien, another researcher on the project.

By "carbon fluxes," Irigoien means that oceanic matter is moving between the zones faster than scientists would've guessed. That's because mesopelagic fish don't always stay in their zone. Sometimes they go up into the top level, the epipelagic zone, or sunlight zone, to hunt for food. But during the day they return to the depths to avoid being eaten themselves. Once they get back to relative safety, they relieve themselves. Hence, carbon fluxes.