Why Do Fish School? One Reason Is to Help Each Other Through Turbulent Waters

Schools of fish can resemble a single organism as they speed through the ocean in search of food or gather to defend their territory.

A new study has found that it is also easier for fish to swim through turbulent water if they are in a group compared with swimming alone.

“The ecological and evolutionary benefits of energy-saving in collective behaviors are rooted in the physical principles and physiological mechanisms underpinning animal locomotion,” the researchers wrote in the study. “We discovered that, when swimming at high speeds and high turbulence levels, fish schools reduced their total energy expenditure (TEE, both aerobic and anaerobic energy) by 63% to 79% compared to solitary fish.”

Many facets of animal behavior are linked with locomotion — from migration to feeding and reproduction — so many animals have adapted to improve their efficiency of movement.

The researchers proposed a “turbulent sheltering hypothesis,” which asserted that fish moving in schools are able to shield one another from rough water currents, making it easier to travel.

“Moving in turbulence is particularly challenging and energetically expensive for solitary fish. Solitary creek chub (Semotilus atromaculatus) swimming in turbulence reduced maximum sustained swimming speed (by 22%) because large turbulent eddies (approximately 76% of body length) disrupt the movement trajectories of fish,” the authors wrote. “Also, the cost of locomotion by solitary Atlantic salmon (Salmo salar) can increase by approximately 150% in turbulence.”

To test their hypothesis, the researchers conducted trials with giant danios (Devario aeqipinnatus). They observed the fish swimming in groups of eight or alone in smooth and turbulent water. High-speed cameras were used to observe the fishes’ movements as they swam, while a respirometer measured their energy expenditure and respiration rates.

The experiments showed that fish traveling in schools gathered together more closely in turbulent water as compared with steady water. Solitary fish, however, had to more vigorously beat their tails to keep up the same speed in rougher currents.

“What is the function of schooling behavior in fishes? We show that being in a school substantially reduces the energetic cost for fish swimming in a turbulent environment, compared to swimming alone, providing support for the hypothesis that schooling behavior protects individual fish from the increased energetic cost associated with swimming in turbulence,” the authors wrote in the study, as Phys.org reported.

The results indicated that efficiency of locomotion may be a major component behind the evolution of fish schools. This is useful information for understanding the fundamentals of hydrodynamics, fish ecology and could also be applied to habitat maintenance and design in harboring protected species or hindering invasive ones.

Studies like this one have broader implications as well.

“Moreover, studies on animal locomotion and turbulence have profound implications for a better understanding of the planetary ecosystem, e.g., turbulence generated by groups of fish can contribute to vertical mixing of the ocean,” the authors wrote.

The researchers noted that the study could also inform future research into the group movement energy dynamics of other aquatic or aerial animals.

The study, “Collective movement of schooling fish reduces the costs of locomotion in turbulent conditions,” was published in the journal PLOS Biology.