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Rainbow trout use only their anterior muscles when swimming between vortices in water flow (revealed here with bubbles).
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Science

Need some inspiration for staying calm amidst the stress and chaos of daily life? Perhaps it’s time to set aside the yoga videos and consider our aquatic friends, the fish. New research indicates that these animals negotiate turbulent waters using a special energy-conserving method that allows them to go with the flow — literally. The findings have implications beyond stress management: They may ultimately help researchers design robotic underwater vehicles or improve passageways that allow fish to bypass dams.

Beneath its surface, a river is a chaotic and turbulent place. Objects such as branches, rocks or moving fish produce strings of small whirlpools, or “vortices” in the water flowing past. Somehow, fish manage to make their way through this turbulence without exhausting themselves. In Friday’s issue of the journal Science, published by AAAS, the nonprofit science society, James Liao of Harvard University and his colleagues explain how the fish do it.

One of the reasons that scientists can make a living studying fish swimming is because the Navy and other agencies want to build automated vehicles that can maneuver through turbulent waters as well as living creatures do. These fish-inspired robots may someday be able to use the energy from vortices generated by waves near the shore or the wakes of boats, for example.

“Our goal is to shed light into the fundamental principles that govern aquatic locomotion by observing how fishes swim,” said Liao.

“A decade ago, people were thinking about moving through the water by brute force, trying to produce more thrust. Now we’re understanding how you can get through water more efficiently by manipulating vortices,” he said.

A fish’s body can be thought of as a “foil,” similar to a sail on a boat or an airplane wing. When a stream of water or air moves around the foil, it produces thrust, propelling the foil forward.

Engineers have figured out that foils capture the most energy from a vortex if they intercept it directly, but this action requires a relatively large amount of energy to begin with. Liao and his colleagues found that swimming fish take quite a different approach.

They set up a tank with water flowing in one direction and a cylinder poking down into the water to produce a consistent series of vortices spinning off in alternating directions. Then the researchers recorded the motions of trout swimming against this turbulent flow with a high speed digital video camera.

Wu wei for fish
Liao’s team found that the fish were actually slaloming in between the vortices rather than intercepting the centers of each one. The fish adopted a swimming style that resembled the motion of a flag flapping slowly in the breeze, letting the spinning water push their body forward.

“So the motions of the fish reflect what flow is doing; it’s very ‘wu-wei,’ meaning ‘to do without doing,’” Liao said.

The swimming style was akin to a sailboat tacking back and forth upwind in order to move forward using the energy of the wind. Just as the sailboat only demands energy to flip the sail back and forth, the fish only need to contract their body muscles near the head to change position among the eddies. They don’t use all their body muscles to propel themselves forwards, as they would in smooth currents.

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The slaloming fish have to move side-to-side quite a bit in order to move forward just a little, but by doing so they can remain stationary, or “hold station,” against the moving current.

Liao thinks that in nature, where vortices may not occur as predictably as they do in the tank, the fish probably hold station when they encounter turbulence and then resume normal swimming once they are out of the rough patch.

River dam rest spots
A second experiment revealed that the trout’s slaloming swim truly was taking the easy way.

The researchers attached electrodes along the fish’s bodies to measure their muscle activity during swimming. They found that when the fish did their vortex slalom, they used only their anterior axial muscles, which are just behind the head. That represented a drastic reduction in muscle activity compared to normal swimming.

Other fishes with a variety of body shapes and ecological niches also perform this unusual gait, according to the authors.

“This reduced muscle activity is also the first direct evidence we have for the energy-saving benefits of swimming in a wake. It hints at energetic benefits of fish schooling and gives valuable pointers for the design of fish ladders,” writes Ulrike Müller of Wageningen University in the Netherlands, in a related commentary that accompanies the Science study.

Liao also thinks these findings could be important for making better fish ladders. These structures are passageways alongside dams that fish can travel along in order to continue their river journeys. Effective fish ladders are a key way to lessen some of the ecological damage that dams cause, by helping fish such as salmon return upstream to spawn.

Traveling up a fish ladder is pretty hard work, especially if the dam is large and steep. The idea is that adding some vortex-creating poles along the way might provide the fish some “rest stops” along the way, where the vortices would allow the fish to hold station for a while and conserve energy.

“Fisheries biologists would benefit greatly by knowing precisely how fish will deal with turbulence,” Liao said.

So, perhaps, would shoppers during this holiday season. If the turbulence of the shopping mall becomes overwhelming, think of the trout, and just “go with the flow.”

© 2013 American Association for the Advancement of Science

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