Fish that gather in large groups may seem safer from predators, but new research shows that, in some cases, the opposite is true. A team of oceanographers from MIT and Norway discovered that when fish such as capelin form large shoals, they may become more prominent targets for predators. This was observed during the spawning season for capelin, small Arctic fish that travel from the edge of the Arctic ice sheet to the Norwegian coast each February. While spawning, they face danger from their primary predator, the Atlantic cod. The researchers used new imaging techniques to observe this predator-prey interaction in detail.

Using a sonic-based imaging method called Ocean Acoustic Waveguide Remote Sensing (OAWRS), the scientists tracked the movements of the capelin and their predator, the cod, over a vast area. The team noticed that the capelin, which initially moved in loose clusters, eventually grouped to form a massive shoal. In response, the cod formed their group and swarmed the capelin. Over a few hours, the cod consumed more than 10 million capelin—about half of the shoal. This event, which took place over a wide area of the Barents Sea, is the largest predator-prey interaction ever recorded.

The study found that while the capelin population is large, the concentration of fish in one area makes them more vulnerable to coordinated attacks from predators. The research highlights how large-scale predator-prey interactions can suddenly shift the balance in ocean ecosystems. The team’s observations are critical for understanding the dynamics of marine food webs, especially as the effects of climate change and human activity continue to impact ocean life.

The researchers hope to continue using OAWRS to monitor large-scale events in other fish species. By understanding how critical changes in fish behavior can lead to massive predator-prey shifts, scientists can better predict how marine ecosystems will respond to environmental changes. This research, published in Communications Biology, could inform conservation efforts and protect vulnerable species that play vital roles in maintaining healthy ocean ecosystems.

References

• Pednekar, S., Jain, A., Godø, O. R., & Makris, N. C. (2024). Rapid predator-prey balance shift follows critical-population-density transmission between cod (Gadus morhua) and capelin (Mallotus villosus). Communications Biology, 7(1), 1–10. https://doi.org/10.1038/s42003-024-06952-6
• Chu, J. & Massachusetts Institute of Technology. (2024, October 29). Oceanographers record the largest predation event ever observed in the ocean. Phys.Org; Massachusetts Institute of Technology. https://phys.org/news/2024-10-oceanographers-largest-predation-event-ocean.html