Ocean oxygen decline threatens deep-sea fish populations and ocean health, new study warns 

The ongoing decline in ocean oxygen levels is intensifying due to climate change, significantly impacting marine ecosystems, including mesopelagic fish populations. This could disrupt marine food webs, fisheries, ocean health, and the ocean’s capacity to store carbon. 

These are the findings of an international study led by the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona (ICTA-UAB), Spain, which investigated how marine ecosystems responded to past episodes of ocean deoxygenation. To do so, researchers studied fossil remains of lanternfish—one of the most abundant and important fish families of the deep ocean—preserved in seabed sediments of the Eastern Mediterranean, dating back more than 10,000 years. The findings have been published in the journal Communications Earth & Environment (Nature Portfolio). 

Lanternfish are very small, deep-sea fish of the family Myctophidae, named for their ability to produce light through bioluminescent organs. Regardless of their small size, with an estimated biomass of 600 million tons, lanternfish are extremely abundant in the global ocean, possibly making them the most abundant vertebrates on Earth by weight. During the day, lanternfish live in the dark mesopelagic zone (200–1000 meters deep) to hide from predators, while during the night they swim to the ocean surface to feed on zooplankton. Owing to both their great biomass and diel vertical migration, lanternfish play a very important role in climate regulation, and the ocean food webs, connecting the surface with the deep ocean. Therefore, lanternfish are widely considered a good indicator group for mesopelagic ecosystem health. 

However, the fossil record shows that lanternfish were largely absent during periods of extreme oxygen depletion. They only reappeared—and in large numbers—once oxygen levels increased again, around 6,000 years ago. 

The team, including researchers from Scripps Institution of Oceanography (USA), Woods Hole Oceanographic Institution (USA), Biodiversity Research Center at Academia Sinica (Taipei, Taiwan), McGill University (Canada), Freie Universität Berlin (Germany), and Heidelberg University (Germany), used fossil otoliths to reconstruct past fish populations. The Eastern Mediterranean Sea, which has alternated over the past ten thousand years between well-oxygenated and highly anoxic conditions, provided a unique natural archive for studying the biological impacts of deoxygenation events. 

“The case of lanternfish clearly illustrates what may happen on a larger scale if ocean deoxygenation continues. If a group with such massive biomass disappears, other marine species are also likely to be at risk,” warns Sven Pallacks, main author of the study. 

The twilight zone —mesopelagic zone, located between 200 and 1,000 meters deep— plays a key role in Earth’s climate system, primarily through its influence on the global carbon cycle. The results suggest that mesopelagic ecosystems are particularly vulnerable to oxygen loss. Their collapse could destabilize ecological balances in the ocean, impair its role in global carbon cycling, and pose a threat to marine biodiversity and global food security.