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The blue marble

Fish in the twilight cast new light on ocean ecosystem

New research, published this week in Nature Communications (Irigoien et al. 2014), shows that the fish biomass in the global ocean has been largely underestimated, by at least 10 times and most likely 30 times. The reason for this is the gross overlook of the biomass of mesopelagic fish, the most abundant fish stock in the ocean. This paper, led by Prof. Xabier Irigoien, Director of the Red Sea Research Center at KAUST, Saudi Arabia, with our participation as coauthors from Australia brings about a revolution in our understanding of the ocean and provides embarrassing evidence of how little we know of the oceans as yet. In this piece we explain why.

Lantern Fish (Photo: Joan Costa/CSIC)

As sonars were developed to enable the detection of submarines during WWII, a layer generating diffuse echoes was detected at ocean mid-depths in 1942. This layer was called the “Deep Scattering Layer” (DSL). The DSL was found to be a dynamic layer, as much of the acoustic targets - as the objects returning echoes are referred to in marine acoustics - would migrate to the surface at sunset, remain near the surface during the night, and return to depth during the daytime. It was not, therefore, a layer of accumulated particles or turbidity but one of living organisms. Whereas initial hypotheses pointed at gas-filled swim bladders of fish as responsible of causing the echoes, efforts to catch fish by towing nets through the DSL returned only zooplankton, or small planktonic crustaceans. Decades later, additional results showed that indeed the DSL was mostly populated with fish, called mesopelagic fish, as well as squid and zooplankton.

Echogram showing the position, as a thick blue-green bad at about 400 m depth, of the Deep Scattering Layer in the Subtropical Atlantic, showing how lantern fish return from surface to the DSL during sunrise (the dots moving down on a diagonal, as time progresses towards the right). Image courtesy of Prof. Santiago Hernandez-Leon, Univ. Las Palmas de Gran Canaria.

The layer where the DSL sits is known as the mesopelagic layer, as it sits below the mixed layer of the ocean, reaching down to 100 m or so, and within the thermocline region, the region where temperature declines to depth, extending down to 1,000 m depth. The mesopelagic region is also called the twilight zone, as there is some dim light, insufficient to fuel photosynthesis, but still enough for organisms to still see. Indeed, the organisms inhabiting the twilight zone have highly developed sensorial capacities, with very large eyes relative to their body size, as the lanternfishes (family Myctophidae) in the photo, and, in some cases, special organs, called phosphors, that emit light and serve to attract prey and communicate among themselves. They have also extremely developed sensorial capacities to detect pressure and, therefore, motion around them. Other important components of the mesopelagic fish community are the very scary, but fortunately small, dragonfishes (family Stomiidae), small bioluminescent fish characterized by disproportionately large fang-like teeth. Another mesopelagic fish, the small mesopelagic fish genus Cyclothone sp., called lightfish because of its bioluminescent, are likely the most abundant vertebrate on earth.

However, the extraordinary sensorial capabilities of mesopelagic fish imply that they cannot be captured with nets, whether research or commercial. In a nice piece of research one of the coauthors of the paper, Stein Kaartvedt, placed an echo-sounder facing upwards in a Norwegian fjord and demonstrated how mesopelagic fish detected the net approaching at about 100 m away and then spread out to avoid the net as it swept through the mesopelagic layer, with the fish taking about 2 h to return to position. Of course the net would have either no fish or very few, as only the sick or dead animals are captured. As a consequence the abundance and biomass of mesopelagic fish has been greatly underestimated (Kaartvedt et al. 2012).

The Malaspina Expedition, a spanish circumnavigation cruise including Australian collaborators (The University of Western Australia and CSIRO) led by one of us (C.M. Duarte) sailed the oceans between December 2010 and July 2011, including calls to Fremantle and Sydney Harbours with one of us (S. Agustí), as chief scientist on board. It offered an opportunity to assess, for the first time, the global biomass of mesopelagic fish in the DSL using advanced acoustics, the only efficient way to estimated their biomass.

The results obtained, reported in the newly released paper in Nature Communications, showed that the median biomass of mesopelagic fish between 40oN and 40oS, the band covered by our survey, ranged between 11,000 and 15,000 million tons, compared to a global fish biomass of only 1,000 tons estimated to-date. This is a revolutionary step toward our understanding of the ocean ecosystem, calling for a much larger role of fish in the functioning of the ocean ecosystem and its basic biogeochemical cycles, such as carbon budgets and transport, than previously thought.

Moreover, the strong relationship between fish biomass and satellite-derived primary production revealed that the transference of carbon up the food chain was far more efficient than previously thought. This explains the underestimate of fish biomass, as this was based, for mesopelagic fish, on calculations based on primary production and assumed efficiency in transferring this through zooplankton into mesopelagic fish.

The main implications are (1) that the oligotrophic gyres of the ocean, which at 70% of ocean surface represent the largest ecosystem on Earth, should no longer be considered oceanic deserts, as they support a vibrant food web supporting fish with a higher efficiency than coastal food webs do; (2) fish play a far more important role in the carbon budget of the ocean than previously thought, with the diel migrations of mesopelagic fish accelerating the downward transport of carbon required to sequester atmospheric CO2; (3) this large biomass in the ocean is largely unexploited, moreover the stock of mesopelagic fish is likely to be stronger than it was in the past as their main predators, such as tuna and swordfish are now overexploited; and (4) the large fish stock of mesopelagic fish is largely safe from fishing pressure, since they are extremely skilled as avoiding conventional fishing gear.

Our ignorance on the abundance, biomass, ecology and biology of the extraordinary mesopelagic fish, the most abundant vertebrates on Earth, reveals how little we still know about the ecology of the open ocean. This offers an exciting invitation to explore the open ocean and contribute in the century of the oceans to better understand and conserve the jewel of our blue marble.

*This piece has been coauthored with Prof. Susana Agustí, coauthor of the research article, also at the UWA Oceans Institute and the School of Plant Biology, The University of Western Australia * References

_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. Agusti, D. Aksnes, C.M. Duarte, and S. Kaartvedt. 2014. Large Mesopelagic Fish Biomass and Trophic Efficiency in the Open Ocean. Nature Communications Nat. Commun. 5:3271 doi: 10.1038/ncomms4271.

Kaartvedt, S., Staby, A., & Aksnes, D. L. (2012). Efficient trawl avoidance by mesopelagic fishes causes large underestimation of their biomass. Mar Ecol Prog Ser, 456, 1-6._

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