A school of cod eats 10 million fish eggs in a few hours

Researchers using sonar technology detected the largest predation event ever recorded in terms of both the number of individuals involved and the area over which the activity occurred.

A recent study by Massachusetts Institute of Technology and Norwegian oceanographers sheds light on the interactions between predators and prey in the ocean . A feeding frenzy took place off the coast of Norway, in which a school of cod consumed more than 10 million roe fish in a matter of hours. The study found that while swarming can provide benefits such as increased foraging efficiency and protection against predators, the behavior can also make some fish more vulnerable to attack, especially when the swarm is large and dense. The results of the study were published in Communications Biology .

The number of fish eggs eaten is up to more than 10 million. (Photo: iStock).

This is the first time they've seen predator-prey interactions on such a massive scale, says Nicholas Makris, a professor of mechanical and marine engineering at MIT. It's a struggle for survival.

Roe are small Arctic fish about the size of anchovies. Every February, billions of roe migrate from the Arctic ice sheet to the Norwegian coast to spawn. However, this event also coincides with the southward migration of Atlantic cod, a species that preys on roe. Although scientists have long known about the relationship between the two species, they had never measured predation on such a large scale.

Using advanced sonar imaging technology, the team observed predator-prey interactions in the Norwegian Sea. Cod swarmed together, overwhelming the prey and devouring more than 10 million roe in a short period of time. This was the largest predation event ever recorded, both in terms of the number of individuals involved and the area it took place.

To conduct the study, the researchers reviewed data collected during a 2014 expedition to the Barents Sea using the Ocean Acoustic Waveguide Remote Sensing (OAWRS) system . OAWRS is an imaging technique that uses a vertically positioned beam of sound sources to send sound waves into the ocean. These sound waves travel through the water, bouncing off objects like fish and other underwater structures.

Another boat, equipped with a sonar receiver, continued to collect echoes from dozens of kilometers away. Scientists then analyzed the data to create a real-time map of the ocean. "Fish have bladders that echo like bells. Cod have large bladders with low resonance, while sardines have tiny bladders that echo like the highest notes on a piano ," Makris said.

The researchers stress that this mass predation event is unlikely to have a major impact on overall cod numbers. However, it does highlight the potential risks facing the species due to climate change. As Arctic sea ice shrinks, cod will have to travel longer distances, making them more vulnerable to attack. Continuous monitoring of cod numbers and their interactions with predators is vital to understanding and mitigating threats and ensuring the health of marine ecosystems.