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The mesopelagic zone provides a critical link between surface waters and the abyss: what happens here will affect food supply and climate resilience throughout the ocean. Spanning the water column between roughly 200 and 1,000 meters, it comprises only 20% of the ocean by volume yet is home to 50-90% of the planet’s fish biomass.
Many species such as tunas, swordfish, sharks, marine mammals, and seabirds rely on mesopelagic-dwelling fishes, squids, jellyfish, and shrimp. The daily migration of many mesopelagic species to surface waters forms the largest migration of animals on the planet, acting as a “biological pump” for moving an estimated 2-6 gigatons of carbon from the surface to deep water, double the emissions produced by cars worldwide annually. About 90% of the carbon that gets into the mesopelagic remains there as consumable organic carbon, with the rest sinking to the deep ocean where it could be locked away for hundreds or thousands of years. The unique migration and biomass of mesopelagic species renders the mesopelagic zone crucial for climate regulation.
The abundant mesopelagic fish biomass is also spurring interest in commercial extraction. Pressure is rising as current stocks of small pelagics, such as anchoveta, critical inputs to fishmeal and fish oil production, are declining or shifting distributions due to climate change. The high concentration of omega-3 fatty acids and huge numbers of species such as Lanternfish (quadrillions) make them attractive for fish meal/oil for the aquaculture, pet food, and health food supplement industries. Yet such fisheries could undermine ecosystem health and climate resilience essential to a regenerative blue economy.
The mesopelagic environment is already feeling the impacts of climate change, with warming and deoxygenation documented globally. Toxic sediment plumes released into the mesopelagic zone from any future deep sea mining could add to these climate and fishery threats. Additionally, the impact of marine carbon dioxide removal (mCDR) approaches aiming to draw down carbon from the surface to the deep ocean—through modifications to the chemical carbon pump (e.g., ocean alkalinity enhancement) or the biological carbon pump (e.g., ocean iron fertilization)—are critically understudied, but of significant concern.
It is thus imperative that action is taken to promote and protect the vital ecological and climate services of mesopelagic communities within EEZs and the high seas and that we do not trigger perverse impacts on natural carbon sequestration processes. Pathways for large-scale protections include a precautionary pause on expansion of mesopelagic fisheries or other activities so that a suite of research, assessment and conservation measures can be put in place under the UN Food and Agriculture Organization, regional fishery management organizations, the Convention on Biological Diversity, the London Protocol and the BBNJ Agreement. An important catalyst can be international resolutions by the IUCN World Conservation Congress and the United Nations General Assembly. Such resolutions could drive increased awareness and ambition, spur subsequent implementation actions and support collaborative research, capacity development, and consideration of further governance mechanisms needed.
WHOI, 2020. Value beyond View: https://twilightzone.whoi.edu/value-beyond-view