The biomass of one class of fish has been estimated by researchers to be ten times greater than previously believed.

Researchers studied populations of mesopelagic fish, which live between 650 and 3300 feet under the ocean surface. Species of mesopelagic fish include lantern fish and cyclothonids. This variety of animal is the most numerous of all known species with a backbone. With a biomass estimated at one billion tons before the new study, they make up a majority of the biomass of wild fish. Still, due to the depths at which they live, much of how the creatures live remains a mystery.

Crew members aboard the Malaspina Expedition traveled over 36,000 miles, studying life far beneath the ocean surface using sound waves. The crew, led by Carlos Duarte, took readings from latitudes ranging from 40 degrees north to 40 degrees south. They found mesopelagic fish had the ability to avoid nets. Trawling was essential to earlier counts of population sizes, and having the skill to avoid such nets may have caused earlier population measurements to be too low.

"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," Duarte said.

This new finding could have a profound impact on how scientists understand the underwater carbon cycle. Mesopelagic fish were observed traveling to upper layers of the ocean at night, in order to feed. When daylight comes, they swim back down to greater depths, in order to avoid predators. When they do so, they transport carbon, in the form of feces, up to half a mile underwater. The large population of fish could lock up carbon, deep beneath the ocean surface, at a far greater rate than previously believed.

Xabier Irigoien from AZTI-Tecnalia and KAUST (Saudi Arabia), is an oceanographer and headed the research analyzing the data collected on the expedition.

"Their role [mesopelagic fish] in the biogeochemical cycles of ocean ecosystems and global ocean has to be reconsidered, as it is likely that they are breathing between one percent and 10 percent of the primary production [of carbon] in deep waters," Irigoien said.

Through their role in the marine carbon cycle, mesopelagic fish would serve a critical role in the balance between plankton and top-level predators. They would also be responsible for the consumption of large quantities of oxygen in deep water.
The Spanish National Research Council (CSIC) assisted the Malaspina Expedition in their study of fish populations.

Details of the study was profiled in the journal Nature Communications.