A School of Fish is Quieter than Just One
A School of Fish is Quieter than Just One
It may not be sociability that makes fish swim together. Turns out that moving in unison can help them avoid detection.
A stampede of wild horses thunders in comparison to the trot of a lone stallion. The same is true of packs of wolves and flocks of geese as they travel together: More is noisier.
While that connection makes intuitive sense, it turns out that it is not a universal rule. A group of engineering researchers at Johns Hopkins University in Baltimore has found that in fish, a school is quieter than an individual swimming alone.
“If there are multiple fishes—let’s say there are two fish—we knew that the sound may not be doubled because there are some interaction effects,” said Jung-Hee Seo, a professor of mechanical engineering at the university’s Whiting School of Engineering and a member of the Flow Physics and Computation Laboratory there. “But we never thought it would actually be lower than the single fish. That was actually really surprising.”
When fish swim out of sync, the acoustic waves trailing off their tails interact chaotically, making them noisy. Image: Johns Hopkins University
The lab had spent decades studying the dynamics, hydro and otherwise, of butterflies, mosquitoes, fish, and other creatures that move through fluids. “We realized that hydroacoustics can also be a very interesting topic and there isn’t much research about it,” said Ji Zhou, a mechanical engineering graduate student at the university and lead author of the paper “Effect of schooling on flow generated sounds from carangiform swimmers,” which describes the findings and came out in Bioinspiration & Biomimetics in April 2024.
The lab, and others like it, had long used computation models that mimicked thousands of schooling fish. But to look at how the sound waves coming off individual fish interact as they swim together, Zhou and his colleagues built a 3D model of a mackerel. “We got the geometries from real fish,” he said. “We have collaborators with fish in tanks and we watched the videos they provided.”
The team put its virtual fish in a variety of configurations and let them swim. The researchers were wowed by how quiet they were.
When swimming in unison, the waves tend to align perfectly and cancel each other out, reducing noise by as much as 90 percent.
Image: Johns Hopkins University
“The main cancellation comes from the tail beat,” Zhou said. “They match the phase difference between the leading fish and the trailing fish so they can make their waves cancel each other.”
When the phase of the waves rolling off the tips of the fish tails match perfectly, nine swimming mackerel were quieter than one mackerel swimming on its own.
Of course, real fish aren’t synchronized swimmers and they don’t likely match their phases with such perfection. But however randomly the beat of the fish tails, the trailing waves interfere with each other, and the group is still quieter swimming together than the sum of the volume of that the same fish would make swimming on their own.
Does this discovery imply that fish evolved to swim in schools so that they would be quieter to better evade predators? Perhaps. As it happens, the same wave interactions that dampen their volume also make them swim faster when together.
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Just how well the virtual fish mimic the sound levels of real fish is yet to be known.
“It’s really hard to get very valid measurements, because in the lab, the tank is too small,” Zhou said, “and in the wild there’s a lot of other noise that can make it really hard to get a very valid result.”
Working with its biologist colleagues to get an answer to that question is one of the next steps for the team. It is also looking at possible applications that would put its findings to use for stealthier vessels. “That’s one of the objectives of this research,” Seo said. “Propellers may not be able to cancel out the sound, but if we employ a fish fin-like propulsion system, then we could do that.”
Michael Abrams is a technology writer in Westfield, N.J.
While that connection makes intuitive sense, it turns out that it is not a universal rule. A group of engineering researchers at Johns Hopkins University in Baltimore has found that in fish, a school is quieter than an individual swimming alone.
“If there are multiple fishes—let’s say there are two fish—we knew that the sound may not be doubled because there are some interaction effects,” said Jung-Hee Seo, a professor of mechanical engineering at the university’s Whiting School of Engineering and a member of the Flow Physics and Computation Laboratory there. “But we never thought it would actually be lower than the single fish. That was actually really surprising.”
When fish swim out of sync, the acoustic waves trailing off their tails interact chaotically, making them noisy. Image: Johns Hopkins University
The lab, and others like it, had long used computation models that mimicked thousands of schooling fish. But to look at how the sound waves coming off individual fish interact as they swim together, Zhou and his colleagues built a 3D model of a mackerel. “We got the geometries from real fish,” he said. “We have collaborators with fish in tanks and we watched the videos they provided.”
The team put its virtual fish in a variety of configurations and let them swim. The researchers were wowed by how quiet they were.
When swimming in unison, the waves tend to align perfectly and cancel each other out, reducing noise by as much as 90 percent.
Image: Johns Hopkins University
When the phase of the waves rolling off the tips of the fish tails match perfectly, nine swimming mackerel were quieter than one mackerel swimming on its own.
Of course, real fish aren’t synchronized swimmers and they don’t likely match their phases with such perfection. But however randomly the beat of the fish tails, the trailing waves interfere with each other, and the group is still quieter swimming together than the sum of the volume of that the same fish would make swimming on their own.
Does this discovery imply that fish evolved to swim in schools so that they would be quieter to better evade predators? Perhaps. As it happens, the same wave interactions that dampen their volume also make them swim faster when together.
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Just how well the virtual fish mimic the sound levels of real fish is yet to be known.
“It’s really hard to get very valid measurements, because in the lab, the tank is too small,” Zhou said, “and in the wild there’s a lot of other noise that can make it really hard to get a very valid result.”
Working with its biologist colleagues to get an answer to that question is one of the next steps for the team. It is also looking at possible applications that would put its findings to use for stealthier vessels. “That’s one of the objectives of this research,” Seo said. “Propellers may not be able to cancel out the sound, but if we employ a fish fin-like propulsion system, then we could do that.”
Michael Abrams is a technology writer in Westfield, N.J.
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