In the largest predation event documented to date, researchers witnessed capelin congregating near Norway’s coastline, which attracted a massive swarm of cod that consumed more than 10 million fish within a few hours. The research team aspires to use their methodology to observe large-scale interactions among different fish species and monitor at-risk keystone species.
The saying goes, “there is strength in numbers.” However, recent findings suggest that in the ocean, fish swimming in schools may not always survive together. In fact, having more fish present can make them a more attractive target for predators.
This phenomenon was captured by oceanographers from MIT and Norway as they explored the waters off Norway during the peak spawning season for capelin, a small Arctic fish akin to an anchovy. Each February, billions of capelin migrate from the Arctic ice edge to the Norwegian coast to lay their eggs. At this time, the area also attracts the Atlantic cod, a primary predator of capelin. Until now, the large-scale dynamics of this predation process had not been thoroughly investigated.
In their publication in Nature Communications Biology, the MIT research team documented interactions between individual migrating cod and spawning capelin across a vast area. They employed a sonic-based wide-area imaging technique to observe random capelin grouping together, eventually forming a massive shoal extending for kilometers. As this ecological “hotspot” of capelin formed, the researchers noted that individual cod started to gather, creating their own considerable shoal. The swarming cod quickly overwhelmed the capelin, feasting on over 10 million fish—an estimated more than half of the assembled prey.
This remarkable event, occurring over just a few hours, marks the largest recorded predation event, both in the number of fish involved and the area it spanned.
Although this incident is not expected to significantly deplete the overall capelin population—since the shoal represented only 0.1 percent of local spawning capelin—it highlights concerns as climate change leads to the retreat of Arctic ice. This retreat forces capelin to swim greater distances to spawn, making them increasingly susceptible to natural predation events like the one observed by the researchers. Given that capelin serves as a crucial food source for many fish, including cod, continuous monitoring of their behavior at a resolution approaching individual fish across vast areas will support efforts to keep the species thriving and maintain ocean health.
“Our findings indicate that natural catastrophic predation events can swiftly alter the local predator-prey balance,” remarks Nicholas Makris, a professor at MIT’s mechanical and ocean engineering department. “This scenario presents no issue for a healthy population with many widely distributed centers. However, as these centers dwindle due to climate change and human impacts, occurrences of significant predation on keystone species could result in serious ramifications for that species and all species that depend on it.”
Co-authors of the study include Shourav Pednekar and Ankita Jain from MIT, along with Olav Rune Godø from Norway’s Institute of Marine Research.
Bell Tones
For their research, Makris and colleagues revisited data collected during a cruise in February 2014 to the Barents Sea near Norway. During this journey, they utilized the Ocean Acoustic Waveguide Remote Sensing (OAWRS) system, a sonic imaging method that uses a vertical acoustic array attached to a boat to send sound waves deep into the ocean in all directions. These waves can travel great distances, bouncing off obstacles or fish they encounter.
Another boat, towed with an array of acoustic receivers, continuously captures the scattered and reflected waves from considerable distances. Researchers can analyze these waveforms to produce real-time maps of the ocean across expansive areas.
Previously, the team generated maps of individual fish and their movements, but they could not differentiate between species. In the latest study, a new “multispectral” technique was applied, enabling the researchers to identify species based on the unique acoustic resonance of their swim bladders.
“Fish swim bladders resonate like musical instruments,” explains Makris. “Cod have large swim bladders that resonate at low frequencies, like a Big Ben bell, while capelin possess smaller swim bladders that produce higher pitch sounds, similar to piano notes.”
By reexamining the OAWRS data for specific frequencies corresponding to capelin and cod, researchers could visualize groups of fish, identify their species, and track their movements over extensive areas.
Observing Movements
The researchers applied the multispectral technique to OAWRS data collected on February 27, 2014, during the height of the capelin spawning season. In the early morning, their mapping revealed that capelin were largely solitary, moving randomly in loose clusters along the Norwegian coast. However, as daylight broke and illuminated the waters, the capelin began descending into darker depths, likely seeking seafloor areas to spawn.
As they sank, the capelin transitioned from individual to group behavior and ultimately formed a massive shoal of around 23 million fish that moved in a coordinated wave over a length of more than ten kilometers.
“We discovered that capelin have a critical density, a concept we’ve derived from physical theories and now observed in nature,” Makris notes. “When they are close enough to each other, they can synchronize with the average speed and direction of nearby fish, enabling them to form a large, cohesive shoal.”
As they observed, the shoaling fish began to swim in unison, demonstrating coherent behavior seen in other species for the first time in capelin. This synchronized migration is believed to conserve energy over long distances by allowing fish to benefit from the group’s collective motion.
However, this cohesive shoal attracted more and more cod, which formed their own group estimated at about 2.5 million fish, according to the researchers’ acoustic mapping. Within just a few hours, the cod consumed 10.5 million capelin over vast distances before both groups dispersed. Makris speculates that such massive, coordinated predation may be common in the ocean, but this marks the first time it has been scientifically documented.
“This is the first observation of predator-prey interactions on such a large scale, depicting a dramatic battle for survival,” Makris states. “We’re witnessing this monumental scale of interactions, with capelin swirling in like a wave in a stadium, instinctively attempting to form a defense, while the predators come together to systematically strike.”
The research team aims to employ OAWRS to explore large-scale dynamics among other fish species in the future.
“It has been repeatedly shown that when a fish population faces collapse, there is often one last large shoal. Once this final dense group disappears, a collapse follows,” says Makris. “It’s crucial to understand what exists before it’s too late, as the challenges are not favoring their survival.”
This research was partially funded by the U.S. Office of Naval Research and the Institute of Marine Research in Norway.
