The biomass of mesopelagic fish is estimated to be 10 times higher than previously thought, according to a Spanish National Research Council (CSIC) study.

Until now, researchers estimated that there were about 1000 million tons of the fish, which accounted for a massive share of total fish biomass in the world. However, the new study estimates the population of mesopelagic fish to be around 10,000 million tons.

Similar to lantern fish and cyclothonids, mesopelagic fishes live in the "twilight zone, between 200 and 1,000 meters (656 and 3281 feet) deep in the ocean. 

The population of Mesopelagic fish was counted through trawling nets earlier. However, since the fish are adept at dodging the nets, the experts used acoustics like sonar and echo sounders.

"Malaspina has provided us the unique opportunity to assess the stock of mesopelagic fish in the ocean...... 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," Duarte from the University of Australia Ocean Institute said in a press release.

The researchers said understanding the flux of carbon in the depths could also help estimate the biomass of the fish correctly.

Carlos Duarte, CSIC researcher and project lead of the Malaspina Expedition, took measurements between 40°N and 40°S, from 200 to 1,000 meters deep, during the day.

The study also found that the fish rose to the upper levels at night to feed and returned to the lower levels during the day to evade predators. Therefore, the organic matter (Carbon from the feces) released 500 to 700 meters deep down. This sped up the carbon cycle and increased the concentration of inorganic carbon in the lower levels of the ocean.

The amount of inorganic carbon released underneath could help gauge the number of mesopelagic fish in the ocean.

"Mesopelagic fish accelerate the flux for actively transporting organic matter from the upper layers of the water column, where most of the organic carbon coming from the flow of sedimentary particles is lost. Their role in the biogeochemical cycles of ocean ecosystems and global ocean has to be reconsidered, as it is likely that they are breathing between 1 percent and 10 percent of the primary production in deep waters," Xabier Irigoien, researcher from AZTI-Tecnalia and KAUST (Saudi Arabia) and head of this research.

The finding is published in the journal Nature Communications.