A team of engineers at Johns Hopkins University has found that a school of fish moving together in just the right way is stunningly effective at noise reduction: a school of seven fish sounds like a single fish.
Computational models are used to examine the effect of schooling on flow generated noise of fish swimming using their caudal fins. Zhou et al. simulated the flow as well as the far-field hydrodynamic sound generated by the time-varying pressure loading on these carangiform swimmers. Image credit: Zhou et al., doi: 10.1088/1748-3190/ad3a4e.
“It’s widely known that swimming in groups provides fish with added protection from predators, but we questioned whether it also contributes to reducing their noise,” said Dr. Rajat Mittal, senior author of the study.
“Our results suggest that the substantial decrease in their acoustic signature when swimming in groups, compared to solo swimming, may indeed be another factor driving the formation of fish schools.”
Dr. Mittal and colleagues created a 3D model based on the common mackerel (Scomber scombrus) to simulate different numbers of fish swimming, changing up their formations, how close they swam to one another, and the degrees to which their movements synched.
The model, which applies to many fish species, simulates one to nine mackerel being propelled forward by their tail fins.
The authors found that a school of fish moving together in just the right way was stunningly effective at noise reduction: a school of seven fish sounded like a single fish.
“A predator, such as a shark, may perceive it as hearing a lone fish instead of a group. This could have significant implications for prey fish,” Dr. Mittal said.
The single biggest key to sound reduction, the team found, was the synchronization of the school’s tail flapping — or actually the lack thereof.
If fish moved in unison, flapping their tail fins at the same time, the sound added up and there was no reduction in total sound.
But if they alternated tail flaps, the fish canceled out each other’s sound.
“Sound is a wave. Two waves can either add up if they are exactly in phase or they can cancel each other if they are exactly out of phase. That’s kind of what’s happening here though we’re talking about faint sounds that would barely be audible to a human,” Dr. Mittal said.
“The tail fin movements that reduce sound also generate flow interaction between the fish that allow the fish to swim faster while using less energy,” added Ji Zhou, first author of the study.
“We find that reduction in flow-generated noise does not have to come at the expense of performance.”
“We found cases where significant reductions in noise are accompanied by noticeable increases in per capita thrust, due to the hydrodynamic interactions between the swimmers.”
The researchers were surprized to find that the sound reduction benefits kick in as soon as one swimming fish joins another.
Noise reduction grows as more fish join a school, but the team expects the benefits to cap off at some point.
“Simply being together and swimming in any manner contributes to reducing the sound signature. No coordination between the fish is required,” Dr. Mittal said.
The study was published April 3 in the jounral Bioinspiration & Biomimetics.
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Ji Zhou et al. Effect of schooling on flow generated sounds from carangiform swimmers. Bioinspiration & Biomimetics, published online April 3, 2024; doi: 10.1088/1748-3190/ad3a4e