Fish that school together save energy, study finds

You'll catch it during the Olympics this weekend: scores of cyclists banding dangerously close together on a flat road during a race. The formation, known as a peloton, allows riders in the middle to maintain the same speed as those on the periphery, but by using less power.

In June, a group of researchers showed that a similar effect occurs with fish in turbulent water. Fish swimming in schools, the team realized, expend less energy than those traveling solo. The team's study, published in the journal PLOS Biology, is one of the first to directly measure how schools of fish are affected by turbulence.

"To some degree, this makes sense," said Rui Ni, an engineer at Johns Hopkins University and an author of the new study. "When an environment gets tougher, you group together."

The findings could lead to a better understanding of how external factors that cause water turbulence can affect fish populations. It may also someday inspire new technologies, like underwater vehicles or flying drones, that are designed to move in groups as a way to reduce their energy use.

Many animals participate in what scientists refer to as collective movement. Insects swarm to mate more effectively; birds flock for better navigation and defense against predation. But scientists have been torn over whether acting as a group reduces the amount of energy each individual expends, or increases it.

The researchers of the new study hypothesized that fish inside schools might be sheltered from the small whirlpools, or eddies, that create aquatic turbulence, and with that protection be able to maintain pace with less effort.

To test this idea, the team built a "water treadmill" -- creating a sealed loop of water equipped with a propeller to control the speed of the water's flow, cameras to record the position of fish in the loop and a probe to measure oxygen consumption as the fish swam.

The experimental setup was not unlike the way researchers measure the energy expenditure of an athlete. "If you want to measure the oxygen consumption of a human running on a treadmill, then you put a mask on the human's face," said Yangfan Zhang, a zoologist at Harvard University who led the study. "But it's really hard to put a mask on a fish."

Zhang's team studied the behavior of giant danio, a small fish species commonly found in freshwater creeks, both swimming alone and in a school. They found that, in highly turbulent conditions, schooling fish pulled closer together, allowing individual fish to reduce the amount of energy expended by as much as 79 per cent compared with the energy use of a lone swimmer.

They also discovered that solitary fish spent 22 per cent more effort swimming in a turbulent flow than in a smooth one with no eddies. By comparison, fish swimming in schools exerted the same amount of effort in either conditions.

That individual fish have a harder time swimming against eddies "is not exactly shocking," said Ty Hedrick, a biologist at the University of North Carolina at Chapel Hill who was not involved in the work. "After all, getting knocked around by turbulence in the water is probably going to require some extra energy."

What was surprising, he said, was that the turbulence didn't affect the fish in a school at all, a result that suggested the formation was somehow changing the water's flow. But, at least for now, this finding holds only for giant danio swimming in strictly controlled lab conditions. "This is great," Hedrick said, adding, "It's a start, not a finish."

Many similarities exist between the collective behavior of schooling fish and cyclists riding in a peloton. Both groups travel more efficiently by moving closer together. And members in the middle get an energetic advantage over those on the outskirts.

But it's not a perfect analogy, Ni noted. Although wind can be turbulent, air eddies aren't the biggest concern for road cyclists. Rather, it's the presence of crosswinds, arriving from a direction different from the way riders are moving.

Moreover, he added, "the evolutionary penalty is much more severe than the sports penalty" -- for fish, the decision to school can mean the difference between life and death.

Still, "there is something really unifying about a principle that happens across biology," Zhang said. "We can learn a lot from nature."

In the future, the researchers hope to hone in on the finer details of the energetic benefit that schooling affords, including how different sizes and species of fish can change the results, and how the turbulence inside a school of fish compares with that of the outside environment.

Perhaps, Hedrick joked, these results might someday inspire a new event at the Olympics: team swimming, in which competitors use the wake of a person beside them to move more efficiently.

"We don't do that," he said. "We have nice lanes. But maybe science can pave a way for that in the future."