(CN) — Researchers found hundreds of thousands of fish move together in repeated wave-like motions as a way of protecting themselves from the attacks of predatory birds.
According to a study published Wednesday in the journal Current Biology, researchers found evidence small fish known as sulphur mollies do the wave like a stadium of spectators at a sporting event. The waves of fish substantially reduced the frequency of predatory bird attacks, helping to double the amount of time the birds waited before swooping down for the fish.
“It’s one of the first studies that really shows the benefits of collective behavior in the wild with a lot of data on predator-prey interactions,” said the study’s co-lead author David Bierbach of the Leibniz Institute of Freshwater Ecology and Inland Fisheries, noting that studies observing actual wild predator attacks are rare.
The mollies pack together by the hundreds of thousands in sulfur springs that are toxic to most other fish. Bierbach said the fish survive in the oxygen-starved conditions of the springs by dwelling close to the surface. Fish-eating birds flock around hoping to feast on the densely grouped fish.
The fish collectively manage to evade or reduce the attacks of the birds through a coordinated, cascading movement. A press release accompanying the research refers to the movement as “surface waves that were highly conspicuous, repetitive and rhythmic.”
According to Bierbach, the fish can complete a wave every three to four seconds and repeat the waves for up to two minutes. They estimate there are up to 4,000 fish per square meter and that sometimes hundreds of thousands of fish will participate in a single wave.
“When [most] fish are in danger, they dive or escape or swim away from the area of danger,” Bierbach said. He contrasted this with the sulphur mollies the researchers studied, which use their repeated waves to successfully stave off attacks from predators. “This unique collective behavior has an effect on the predator. Most interestingly, it could be a signal from the fish to the predator that the predator is detected and not to attack because [the predator] will lose a lot of time and energy to try to catch up.”
Researchers hope to investigate further what specific effect the waves have on the predatory birds to deter their attacks and how the fish communicate with each other to facilitate the unique visuals.
“We don’t really understand what the fish do underwater,” Bierbach said, noting the scientists are interested in comparing the response of the fish to attacks from birds of varying levels of dangerousness. “The next step will be to look underwater. Why really do the fish dive? Can they communicate somehow through the water about the danger that stems from the attacks?”
Bierbach said they hope to go to Mexico next year to conduct further observations and learn more about how the fish coordinate their collective behavior and the effects it has on predators.
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