Ocean fish biomass 10 times higher than thought

A team of researchers has found that the abundance of mesopelagic fish in the ocean that could be at least 10 times higher than previously thought.

With a stock estimated at 1,000 million tonnes so far, mesopelagic fish dominate the total biomass of fish in the ocean.

Mesopelagic fishes, such as lantern fishes (Myctophidae) and cyclothonids (Gonostomatidae), live in the twilight zone of the ocean, between 200 and 1,000 metres deep.

The results are based on the acoustic observations conducted during the circumnavigation of the Malaspina Expedition - a scientific exploration that took place during a five-year voyage around the globe.

"Malaspina has provided us the unique opportunity to assess the stock of mesopelagic fish in the ocean," said Carlos Duarte, a researcher with the Spanish National Research Council (CSIC).

Mesopelagic fish are the most numerous vertebrates of the biosphere, but also the great unknowns of the open ocean, since there are gaps in the knowledge of their biology, ecology, adaptation and global biomass.

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," added Xabier Irigoyen, researcher from AZTI-Tecnalia and KAUST (Saudi Arabia).

Mesopelagic fish come up at night to the upper layers of the ocean to feed and they go back down during the day in order to avoid being detected by their predators.

This behaviour speeds up the transport of organic matter into the ocean, the engine of the biological pump that removes CO2 from the atmosphere.

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, according to the study published in Nature Communications.