Are the ocean’s greatest battles happening beyond our sight? Off the frozen coast of Norway in February 2024, researchers documented a predation event in the sea that reset the size of underwater battles: over 10 million capelin devoured by Atlantic cod over the course of a few hours. It was not a dispersed skirmish, but an organized coming together of prey and predator, each aggregating into schooled kilometers-long formations.
The observation was facilitated by Ocean Acoustic Waveguide Remote Sensing (OAWRS), a technology that sends low-frequency sound waves down through the water column and detects echoes from fish and other objects over tens of kilometers. By using a multispectral method, scientists sorted species according to the resonance of their swim bladders cod generating low, bell-like tones, capelin generating high, piano-note tones. “Fish have swim bladders that resonate like bells,” said Nicholas Makris, MIT professor and lead author. “Cod have large swim bladders that have a low resonance, like a Big Ben bell, whereas capelin have tiny swim bladders that resonate like the highest notes on a piano.”
During the morning of February 27, capelin spread along the coast started to achieve a critical concentration of 1.4 fish per square meter. Within minutes, density waves propagated faster than the fish could swim, aligning individuals into a massive shoal over 10 kilometers long and containing about 23 million fish. This ordered group behavior, previously theorized but never documented in capelin, likely improves hydrodynamic efficiency for long migrations and may enhance spawning success. But it also made them visible to predators.
Cod, initially scattered, achieved their own critical density of 0.06 fish per square meter in areas coincident with the developing capelin shoal. Similar to the prey, their shoaling spread out as waves of density, covering over the capelin group. In about four hours, 2.5 million cod ate an estimated 10.6 million capelin an areal rate of 3.32 million capelin per square kilometer. Analysis of the cod’s stomach contents validated the predation: each cod had consumed approximately nine spawning capelin, weighing an average of 155 grams.
The incident, which accounted for just 0.1% of the regional spawning stock, highlights how quickly predator-prey relations can change. Makris explained, “Natural catastrophic predation events can change the local predator-prey balance in a matter of hours.” In healthy populations with numerous hotspots distributed in space, such mortality is soaked up. But in climate change- and human activity-stressed ecosystems, the loss of a single large shoal might trigger collapse.
Climate dynamics in the Barents Sea raise the stakes. Retreat of sea ice compels capelin to swim further away from feeding to spawning areas, raising energetic costs and risk of predation. Long-term surveys of the ecosystem reveal that warming and diminished ice cover are forcing a “borealization” of Arctic waters, with boreal species such as cod and capelin moving northwards and coming into contact more strongly with ice-associated species such as polar cod. Models predict that continued ice loss will benefit capelin abundance in the short term through warmer waters, but will diminish polar cod populations, potentially altering trophic pathways and predator diets.
The OAWRS dataset provides unprecedented temporal and spatial detail broad-area imagery refreshed every 50 seconds across a 75-kilometer diameter allowing for quantification of fish densities and movements at scales from interactions between individual animals to ecosystem-level patterns. Such capacity is important for identifying the onset of critical density thresholds that initiate mass behavior shifts, be they for migration, spawning, or predation.
For climate scientists and marine biologists, the 2024 Barents Sea phenomenon is both an ecological cautionary tale and a climate expert’s technical success. It demonstrates the thin line between vulnerability and survival strategy: the same energetic-saving shoaling behavior that coordinates reproduction can, under specified circumstances, serve as a beacon for predators. As the Arctic ocean warms and ice recedes, knowing and keeping ahead of these dynamics will be critical for the management of keystone species and maintaining the balance of high-latitude marine ecosystems.
