Every year, billions of capelin embark on a remarkable migration to the Norwegian coast to spawn, attracting a hungry horde of predators ready to feast. Scientists analyzing one such migration witnessed the most dramatic predation event ever recorded, as millions of cod descended upon a colossal capelin shoal over just four hours. This observation provided invaluable insights into thePopulation dynamics of these two crucial fish species.
Capelin ( Mallotus villosus ), small fish that sustain themselves on plankton and krill, are a cornerstone of the North Atlantic’s food web, playing a role akin to anchovies in warmer waters. While their numbers have suffered crashes in the past, their ability to produce vast numbers of eggs allows for rapid recovery, making them a resilient species.
Capelin are particularly vulnerable to predators during their spawning migration. While their North American counterparts have found alternative spawning grounds, European capelin gather on gravel beds 2-100 meters (6-300 feet) below the surface.
Cod, keenly attuned to this migration pattern, time their own journey to their spawning areas to coincide with the capelin’s arrival, transforming the event into a massive feeding frenzy.
In 2014, using advanced sonar technology called Ocean Acoustic Waveguide Remote Sensing (OAWRS), MIT’s Nicholas Makris and his team tracked the movements of these fish over a vast expanse.
On February 27th, the OAWRS revealed capelin swimming in loose groupings. As dawn broke, they headed towards the seafloor, coalescing into a massive shoal spanning tens of kilometers. This colossal gathering, estimated to contain 23 million fish weighing 414 tons, moved as a coordinated unit.
Makris explained, "Capelin display a critical density, a phenomenon predicted by physical theory that we now observe in the wild. If they are close enough, they can synchronize their speeds and directions, forming a colossal, coherent shoal." While shoaling is a common behavior in many fish species, it had never been observed in capelin before.
This tightly packed shoal served a dual purpose: conserving energy and confusing predators. However, their concentrated presence made them an irresistible target for cod. As soon as the capelin shoaled, the cod formed their own formidable aggregation – estimated at 2.5 million individuals – and launched their attack. An estimated 10.5 million capelin perished before the shoal dispersed a few hours later.
Density waves rippled through both populations, starting at the same location and traveling faster than any individual fish could swim, indicating a fascinating interplay between predator and prey.
Predators also benefit from shoaling, as a collective attack can overwhelm prey defenses, dispersing individuals into easier targets.
Documentaries like Blue Planet have showcased breathtaking feeding frenzies of small fish, offering glimpses of nature’s abundance. However, it’s through advanced technology like OAWRS that we can truly grasp the immense scale of these events.
Makris described the scene, "It’s like a wave of capelin zooming in, like a wave around a sports stadium, gathering together to form a defense. At the same time, the predators are coming together, coherently attacking."
Though the team believes this event doesn’t pose a threat to Regional capelin numbers due to the sheer magnitude of their annual migration, they are concerned about the impact of rising global temperatures. Climate change could make some of the capelin’s spawning grounds unsuitable, driving the population into fewer and fewer hotspots.
Makris cautions, "Such catastrophic predation events of a keystone species could have far-reaching consequences, both for the species itself and for the many species that rely on it.” Capelin are critical food sources for seabirds and marine mammals, as well as for fish like cod. A decline in capelin populations would have a domino effect on the entire ecosystem.
Interestingly, the scientists observed that capelin in the densest parts of the shoal were most likely to survive, suggesting an evolutionary advantage to tight schooling behavior.
The analysis of the OAWRS data required intricate processing due to the initial inability to distinguish between the acoustic signatures of cod and capelin. However, advances in analysis revealed that it’s possible to differentiate fish based on the unique resonant frequencies of their swim bladders.
Makris explains, "Cod have large swim bladders that resonate like a Big Ben bell, while capelin have tiny swim bladders that resonate like the highest notes on a piano, allowing us to identify them individually.
This groundbreaking study, published in Communications Biology, sheds light on the complex interactions between predator and prey in a rapidly changing world.