A monumental predator-prey event in Norway’s coastal waters has captured scientists’ attention. It marks the largest observed marine feeding spectacle and highlighting significant dynamics in the food chain. Leading the research, Professor Nicholas Makris from MIT’s Department of Mechanical and Ocean Engineering, alongside his team, witnessed this unprecedented event where vast shoals of cod pursued spawning capelin, reshaping how scientists perceive these fish behaviours.

Capelin’s Spawning Journey and Role in the Ecosystem

Every February, billions of capelin—a small Arctic fish—migrate south from the Arctic ice edge to Norway’s coast to spawn. These migrations play a critical role, providing essential nourishment for seabirds, whales, and predatory fish like the Atlantic cod, and maintaining balance within the Arctic ecosystem. During the spawning season, cod take full advantage, building energy reserves that sustain them until the next migration cycle. The delicate predator-prey balance typically regulates itself naturally, but changes to this balance can have profound effects.

Innovative Sonic Imaging Maps Fish Movements

Makris’s team employed an advanced sonic imaging method called Ocean Acoustic Waveguide Remote Sensing (OAWRS) to observe these interactions on a large scale. This technology projects sound waves deep underwater to map fish populations in real-time over extensive distances. In an advance on prior techniques, they also used multispectral acoustic mapping, which distinguishes fish species by identifying their unique swim bladder resonances. Cod and capelin, for example, emit different resonant sounds—making it possible to differentiate them within large shoals.

Unprecedented Predator-Prey Formation Observed

On February 27, 2014, the capelin began moving in loosely formed clusters near the coast. As dawn approached, the capelin congregated into a dense shoal, spanning over six miles and amassing approximately 23 million fish. Reacting to this movement, nearly 2.5 million cod formed their own shoal, closing in on the capelin and consuming an estimated 10 million fish in hours. Shortly after this event, the formations dissolved, and the fish scattered.

Climate Change Concerns for Marine Populations

The implications of such large-scale predation events raise questions about the sustainability of marine species. Makris pointed out that as Arctic ice continues to retreat, capelin face longer journeys to spawning grounds, which could heighten their vulnerability. The study underscores how shifts in predator-prey dynamics due to environmental stresses could significantly impact species central to marine ecosystems.