Fish swimming together fare better in turbulent waters

Schooling fish including zebrafish, rainbowfish, and opahs/moonfish like to stick together in the big blue. Scientists believe that traveling in schools has numerous evolutionary benefits, but it also could be more beneficial to their general movements. They also may have an easier time swimming through the ocean’s more turbulent waters than those that go it alone. The findings are described in a study published June 6 in the open-access journal PLOS Biology.

Do the locomotion

Locomotion–the way animals move from one place to another–is critical to several aspects of their behavior. Various movements are needed during their migration periods, reproduction, and when feeding. Many species have different adaptations to make moving around more efficient to compensate. Fish have sleek and streamlined bodies that create little resistance in the water, scales that allow for flexible movements and physical protection, and gills that extract oxygen.

“Generating movement through the environment is one of the defining features of animals. Yet the environment can be very challenging with obstacles and animals often face difficult conditions that increase the cost of movement,” study co-authors and evolutionary biologists Yangfan Zhang and George Lauder of Harvard University, tell Popular Science.

Scientists in different fields have proposed several hypotheses for why the collective movements of animals are beneficial. It could enhance mating success, help them avoid predators, or make it easier for animals to communicate when finding food.

In this study, the team proposes a new hypothesis for navigating more challenging waters. Their turbulent sheltering hypothesis suggests that traveling in schools allows fish to shield each other from more disruptive water currents.

“The turbulence sheltering hypothesis has not been previously proposed so this study is the first to both propose and test it,” says Zhang. “For many years now, there have not been any substantially new ideas on why fish in particular might school and move as a collective group. The turbulence sheltering hypothesis provides a new idea for why fishes might gain an energetic advantage by moving as a school.”

Testing the turbulence sheltering hypothesis

To put this hypothesis to the test, the team ran trials with giant danios (Devario aeqipinnatus). These types of carp regularly swim in schools and are only about one to two inches long, despite the superlative in their name. The team observed the danios swimming alone or in groups of eight in both turbulent and more steadily flowing water. High-speed cameras captured the animals’ movements as they swam and the team simultaneously measured the fishes’ respiration rate and energy expenditure.

They found that the schooling fish spent up to 79 percent less energy when swimming in turbulent water compared to swimming alone. Schooling fish also clustered more closely together in the turbulent water compared to the more steady streams. Solitary fish had to beat their tails more vigorously to keep their same speed in more turbulent currents.

This activity is somewhat similar to competitive cyclists drafting off of one another during a triathlon to reduce drag.

“A significant difference between humans in a triathlon, or cyclists moving behind each other and benefiting from the reduced flow (called a drag wake) is that fish are accelerating the flow behind them,” says Zhang. “Previous research has shown that fish can even benefit from thrust wake (an increased fluid velocity). Fish move a bit more elegantly through the fluid than humans.”

‘Dramatic benefits’

The results add some support to the turbulence sheltering hypothesis, indicating that locomotion efficiency might be a driving factor behind whey schooling behavior evolved.

“The most surprising part of the study is the dramatic benefits that moving in a group confers when fish swim in turbulence,” says Zhang. “It’s much, much, better to swim in a school if the environmental flows are turbulent and challenging than it is to swim as a solitary individual.

In future experiments, the team plans to conduct experiments to understand what specific mechanisms enable energy saving in individual animals when they move within a group. This data is valuable in general for understanding fish ecology and the fundamentals of hydrodynamics. It could also potentially be applied to the design and maintenance of habitats that are meant to harbor protected fish species or to hinder more invasive ones.