MIT filmed the largest inter-species massacre in the ocean. Millions of creatures died in four hours.

Unprecedented death

In ecological terms, nature almost always progresses with two main players: the hunted and the hunter. This game of cat and mouse, with a mixed end for everyone’s interests, is what we call predation. However, no hunting scene resembles that observed in the North Atlantic. Scientists have captured an impressive and unusual massacre on the high seas.

Every year, millions of capelin (anchovy-like fish) migrate to the Norwegian coast to spawn, attracting large numbers of predators along the way. During one of these migrations, MIT scientists witnessed the largest predation activity ever recorded: millions of cod attacked a huge school of capelin, consuming an estimated 10.5 million of these fish in just four hours .

This event, fascinating for various reasons, was the subject of a study published in
Nature

, allowing researchers to observe the population dynamics of the two species, fundamental activities for the North Atlantic marine ecosystem. The capelin, the reluctant protagonist of this story (Mallotus villosus), is a small fish that feeds on plankton and krill in the cold waters of the North Atlantic. The species plays a vital role in the oceanic food chain, similar to that of anchovies in warmer waters.

Although its population is abundant, it has experienced declines in the past. However, the large quantity of eggs it produces allows it to recover quickly compared to other species. This abundance also makes them very vulnerable to predation, especially during this migration to spawning areas in the gravel of the ocean floor.

As explained by MIT, the researchers used advanced sonar technology, called Ocean Acoustic Waveguide Remote Sensing (OAWRS)to follow the movements of these fish over a wide area in real time. On February 27, 2014, the OAWRS revealed how capelins, initially dispersed, regrouped into a gigantic school of 23 million individuals. This aggregation phenomenon allowed scientists to observe natural behavior of capelins, which manage to coordinate their movements when they reach a critical density.

Schools and predators

Although forming schools allows capelin to conserve energy and, in some cases, avoid predators, this concentration also makes them a very attractive target. So, as soon as the massive school of capelin formed, a group of cod, estimated at 2.5 million fish, began the “hunt.” The scientists observed that the density waves moving through the two populations appeared to be synchronizedshowing dynamic interaction between hunters and prey on an unprecedented scale.

Although this type of massive predation does not pose a direct threat to capelin populations, given that their annual migration in the region includes billions of individuals, scientists express some concern about the impact of climate change. The loss of spawning grounds due to increasing global temperatures could limit safe spaces for this type of species, which could have serious consequences for the entire North Atlantic marine ecosystem.

The sonar technology used, OAWRS, is able to distinguish species by their different resonant frequencies, allowing scientists to accurately identify the signals of each type of fish. As Nicholas Makris of MIT explained, fish swim bladders emit resonances similar to bells. They thus discovered that those of cod are low-pitched, while those of capelin produce high-pitched sounds. This distinction has been crucial for the detailed study of these massive predation phenomena and opens the way to a better understanding of marine ecology.