In the vast ocean, the saying “there is power in numbers” may not hold true for fish. Recent findings from MIT and Norwegian oceanographers reveal that congregating in large schools doesn’t always lead to survival; in fact, a higher number of fish can sometimes make them more appealing targets for predators.
This discovery came during an exploration off Norway’s coast, where the capelin—a small Arctic fish akin to an anchovy—spawns. Every February, billions of capelin migrate southward from the Arctic ice sheet to lay their eggs along Norway’s coastline, which also serves as a feeding ground for their primary predator, the Atlantic cod. Until now, scientists had not assessed this predator-prey interaction across extensive oceanic expanses.
In a study published in Nature Communications Biology, the MIT team documented interactions between migrating cod and spawning capelin over vast areas. Using advanced sonic imaging techniques, researchers observed how scattered capelin began to gather and form a massive shoal, stretching for tens of kilometers. This concentration acted as an ecological “hotspot” that attracted cod, leading them to form their own substantial shoal. Within hours, the feeding frenzy resulted in the consumption of over 10 million capelin, representing more than half of the spotted prey.
This significant predation event, the largest on record in terms of participating fish numbers and the spatial area involved, is unlikely to adversely affect the overall capelin population, as it represents merely 0.1 percent of the spawn. However, climate change poses a significant threat with the retreat of Arctic ice, compelling capelin to migrate longer distances to spawn and thus increasing their vulnerability to predators. Capelin are crucial for numerous fish species, including cod, so ongoing tracking of their behavior at nearly an individual fish resolution will be vital for sustaining the species and marine ecosystem health.
“Our research showcases how natural predation events can rapidly alter predator-prey dynamics,” states Nicholas Makris, professor of mechanical and ocean engineering at MIT. “This is manageable for robust populations with diverse ecological hotspots. However, as these hotspots diminish due to climate change and human influences, such predation could have significant repercussions for keystone species and their dependent predators.”
Co-authors of the study include Shourav Pednekar and Ankita Jain from MIT, along with Olav Rune Godø from the Institute of Marine Research in Norway.
Innovative Acoustic Technology
In this groundbreaking study, the team re-evaluated data collected during a 2014 research cruise to the Barents Sea. They utilized the Ocean Acoustic Waveguide Remote Sensing (OAWRS) system, which employs a specialized vertical acoustic array attached to a boat’s underside. This setup emits sound waves that propagate downward and outward into the ocean, reflecting off various marine life and obstacles.
A second boat towed acoustic receivers that continuously captured reflected waves from distances of up to several tens of kilometers, allowing scientists to create real-time maps of ocean activity. While previous mappings focused on individual fish movements, this analysis introduced a new “multispectral” technique to differentiate fish species based on the unique acoustic signatures of their swim bladders.
“Each fish’s swim bladder resonates like a bell,” explains Makris. “Cod have large, low-resonance swim bladders, while capelin possess tiny ones that produce high-pitched sounds.” This innovative approach enabled researchers to visualize fish schools, ascertain their species, and track their movements over extensive areas.
A Shoaling Phenomenon
By applying the multispectral technique to OAWRS data from February 27, 2014, during peak capelin spawning, researchers noticed that in the early hours, capelin moved as individual fish in loose clusters. However, as daylight appeared, the capelin began diving to deeper waters, likely seeking spawning sites along the seafloor. They quickly transitioned from isolated movements to cohesive group behavior, ultimately forming an expansive shoal of around 23 million fish that moved in a synchronized wave, stretching over ten kilometers.
Makris notes, “We identified a critical density among the capelin, which stems from a physical theory we’ve now verified in the wild. When they come close enough, they can synchronize with those around them, resulting in a vast, coherent shoal.” This coordinated swimming behavior, known to occur in various fish species, had never been documented in capelin until now. Such shoaling enhances their energy efficiency during migration by leveraging the collective movement of the group.
However, this massive shoal attracted predators; cod rapidly gathered and formed their own group of approximately 2.5 million fish. Remarkably, in just a few hours, the cod consumed over 10.5 million capelin before both shoals dispersed. Makris believes these monumental predation events are likely more common than previously thought, although this is the first instance documented at this scale.
“This is the first time we’ve observed large-scale predator-prey interactions, akin to a dramatic survival contest,” Makris states. “It’s like watching a wave of capelin surge, much like a crowd in a sports stadium, assembling to form a defensive front, while predators come together to mount a coordinated attack.”
George Rose, professor of fisheries at the University of British Columbia, notes, “This significant study highlights the intricate spatial dynamics between predators and prey in marine ecosystems, revealing scales previously unimaginable. The ability to map multiple species simultaneously with OAWRS offers profound insights into fundamental ecological processes and holds great promise for enhancing current survey methodologies.”
Makris hopes to deploy OAWRS for continuous monitoring of large-scale interactions among various fish species in the future. “When populations teeter on the brink of collapse, there often exists that final substantial shoal. Once that last dense group disappears, collapse follows,” he warns. “It’s crucial to understand what exists in these ecosystems before it’s too late, as they face substantial pressures.”
This research received support from the U.S. Office of Naval Research and the Institute of Marine Research in Norway.
Photo credit & article inspired by: Massachusetts Institute of Technology
