Known as “the twilight zone", the mesopelagic begins where only 1% of light reaches and ends where there is no light at all. Typically, this means that the mesopelagic extends from ca. 200–1000 metres below the water's surface.
Mesopelagic organisms (mainly fish and squid) have long been proposed as a potentially harvestable resource and estimates suggest that the mesopelagic realm could contain a total fish biomass on the order of 2-19.5 gigatonnes, roughly equivalent to 100 times the annual catch of all existing fisheries.
As a result, and in the context of feeding an ever-growing human population, there is an increasing interest in their use for human consumption, fishmeal, as a source of dietary supplements for humans, and to bio-prospect pharmaceuticals.
However, the level of research activity on mesopelagic resources is much less than on other topics currently considered important in marine science. The mesopelagic community are an extremely important component of pelagic food-webs and play a vital role in carbon cycling. And while there is a renewed interest in the possibility of commercially exploiting mesopelagic resources (e.g. European Union's “Blue Growth Strategy"), some countries (e.g. United States) have adopted a precautionary approach towards the exploitation of mesopelagic resources until the ecosystem consequences of any proposed fishery are carefully assessed and management plans are in place.
The latest issue of ICES Journal of Marine Science takes a closer look at the opportunities and risks associated with the exploitation of mesopelagic resources.
Manuel Hidalgo and Howard Browman introduce the themed set, highlighting that most of the research included argues that the science to support knowledge-based management of mesopelagic resources is still lacking, particularly with respect to the effect that extracting significant biomass might have on the pelagic ecosystem and the flow of material and energy through it. They both emphasize the need for more research to support the sustainable management of mesopelagic resources.
"Research is needed to: accurately quantify the biomass of mesopelagic resources, their nutritional and genetic composition, their links to other components of the food web, to other oceanic realms and to biological and chemical oceanographic processes and cycles; develop technology to support efficient extraction of mesopelagic species with minimal bycatch; increase our knowledge and understanding of the possible ecosystem consequences of extracting mesopelagic organisms; conduct socioeconomic receptiveness studies and develop market opportunities; and to face the challenge of commercially exploitating a resource that is mostly beyond national jurisdiction."
Roland Proud, co-author of the paper, From siphonophores to deep scattering layers: uncertainty ranges for the estimation of global mesopelagic fish biomass, also encourages researchers to look at the current challenges. "It is crucial that over the next few years researchers utilize available funding (for example, EU BG3 SUMMER and MEESO projects that have ca. €12 million available for the next four years) to solve some of the many challenges that have been identified in this themed set. Breakthroughs will likely come from the development of methods that combine vessel-based (e.g. acoustics and trawling) and in-situ observations (e.g. optics, acoustics and eDNA) to reduce uncertainty in biomass estimates. Through international collaboration between biologists, ecologists, acousticians and modellers, such estimates could be used to assess the mesopelagic components of ecological models and inform both conservation and fisheries management."
This issue, Mesopelagic resources - opportunites and risks, is free to access for all readers.
A recent ICES workshop explored the development of practical survey methods for measurements and monitoring in the mesopelagic zone (WKMESOMeth). Ciaran O'Donnell, co-chair, noted that this represents a starting point on the development of research in this area through existing survey programs. The workshop reflects many of ICES science priorities: observation and exploration, emerging techniques and technologies, impacts of human activities, seafood production, and conservation and management science.
Drawing by Bas Kohler.