Biomass of Mesopelagic Fish in the Ocean 10x Higher Initial Estimates

Until recently, scientists estimated mesopelagic fish total around 1 billion tons in the world's oceans, dominating the total biomass of fish on Earth. However, a team of researchers used acoustic observations to measure fish biomass and have concluded that their abundance could be at least 10 times higher, in a study published in Nature Communications.

Mesopelagic fishes live between the 200 and 1,000 meters (656 and 3281 feet) deep in the ocean. They are the most numerous vertebrates of the biosphere, but the great unknowns of the open ocean mean that there are gaps in the knowledge of their biology, ecology, adaptation and global biomass.

Researchers of the Malaspina Expedition, a project lead by Spanish National Research Council (CSIC) researcher Carlos Duarte, took measurements between 40°N and 40°S, from 200 to 1,000 meters deep, during the day.

"Malaspina has provided us the unique opportunity to assess the stock of mesopelagic fish in the ocean. Until now we only had the data provided by trawling. 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," said Duarte.

Transport of organic carbon

Head researcher of this study, Xabier Irigoien, stated, "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."

Mesopelagic fish come to the upper layers of the ocean to feed and night and then swim back down during the day to avoid predators. This behavior transports organic matter in the ocean, which helps to remove CO2 from the atmosphere by transporting carbon down 500 to 700 meters (1,640 to 2,297 feet) deep and then releasing it as feces.

Irigoien added, "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."

Mesopelagic fishes would act therefore as a link between plankton and top predators, and they would have a key role in reducing the oxygen from the depths of the open ocean, according to a press release announcing the findings.