With a currently-estimated stock of one billion tons, mesopelagic fish dominate the total biomass of fish in the ocean. Nevertheless, a team of researchers, with the participation of the Superior Council of Scientific Research (CSIC), has discovered that their abundance may be at least 10 times larger. The results, published in the Nature Communications journal, are based on acoustic observations that took place during the circumnavigation of the Malaspina expedition.

Researchers from the Department of Biology of Organisms and Systems of the University of Oviedo were part of the work conducted onboard the Malaspina. Professor José Luis Acuña is one of the authors of the publication that now gathers some of the main conclusions reached by the project.

Mesopelagic fish, such as lantern fish (Myctophidae) and Cyclothones (Gonostomatidae) live in the zone of penumbra of the ocean, in depths of between 200 and 1000 meters. They are the most numerous vertebrates of the biosphere, but also largely unknown in the open ocean, since there are many blind spots concerning their biology, ecology, adaptation and global biomass.

Throughout the 32,000 nautic miles covered during the circumnavigation, the scientist onboard the Malaspina, a project led by the researcher of the CSIC Carlos Duarte, made measurements between 40ºN and 40ºS, between 200 and 1,000 m deep, during the day.

"Malaspina has offered us a unique chance to evaluate the stock of mesopelagic fish in the ocean. Until now, we only have the data provided by trawling. Recently, it has been discovered that these fish are able to detect the nets and escape, making trawling a limited tool for counting their biomass", explains Duarte.

Organic carbon transport

"The fact that biomass of mesopelagic fish and, therefore, the total mass of fish is at least 10 times larger that what we previously tought has important consequences for our understanding of the flow of carbon in the ocean and how what we until now considered oceanic desserts work", remarks Xabier Irigoien, researcher of AZTi-Tecnalia and KAUST (Saudi Arabia) and lead researcher.

Mesopelagic fish go to the upper layers of the ocean at night to feed, while they descend again during the day to avoid being detected by predators. This behavior accelerates the transportation of organic matter towards the interior of the ocean, the engine of the biological bomb that removes CO2 from the atmosphere, because instead of slowly sinking from the surface, it is quickly transported to depths of between 500 and 700 meters and it is released as feces.

"Mesopelagic fish accelarate the flow by actively transporting organic matter from the upper layers of the body of water, where the majority of the organic carbon coming from the flow of sedimentary particles is lost. Its role in the biogeochemical cycles of the ocean ecosystems and the global ocean have to be reconsidered, since it is probable that they are inhaling between 1% and 10% of the primary production in deep waters", says Irigoien.

The release of materials coming from the surface may partially explain, according to the scientists, the unexpected microbian breathing registered in these deep layers of the ocean. Mesopelagic fish would act, therefore, as a link between plankton and superior predators, and they would have a key role in the decrease of oxygen at the depths of open ocean.

The Malaspina expedition is a Consolider-Ingenio 2010 project managed by the CSIC and funded by the Ministry of Economy and Competitiveness. Malaspina includes around 50 research teams, including 27 Spanish teams from the CSIC, the Spanish Institute of Oceanography, 16 Spanish unviersities, a museum, the AZTI-Tecnalia research foundation, the Spanish Army, and several Spanish universities. The total funding, which has had the collaboration of the CSIC, the IEO, the BBVA Foundation, AZTI-Tecnalia, several Spanish universities and public research organism, amounts to around 6 million Euros.

Authors of the Article

Irigoien, X., T. Klevjer, A. Røstad, U. Martinez, G. Boyra, J.L. Acuña, A. Bode, F. Echevarria, J.I. González-Gordillo, S. Hernandez-Leon, S. Agustí, D. Aksnes, C.M. Duarte y S. Kaartvedt. 2014. Large Mesopelagic Fish Biomass and Trophic Efficiency in the Open Ocean. Nature Communications. DOI: 10.1038/ncomms4271.