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James C. Liao, a graduate student in the Department of Organismic and Evolutionary Biology, reports in the journal Science how fish hitch a ride. (Photo by Laura Farrell)

Scientists show how fish save energy by swimming in schools

Fish use ingenious technique to exploit flow of turbulent waters

By Steve Bradt
FAS Communications

Using a tank designed to mimic a turbulent waterway, scientists have found that fish employ a unique and clever swimming motion to harness the energy of eddies in flowing water. By essentially hitching a ride and letting these vortices propel them along, the scientists say, fish can swim against a current with considerably less exertion than is required in calmer settings.

The work provides new insights into the hydrodynamic benefits fish reap by swimming in schools. Researchers at Harvard University and the Massachusetts Institute of Technology (M.I.T.) report the results in the Nov. 28 issue of the journal Science.

"The annual upstream voyage of fish to spawn has long been viewed as one of the classic struggles of the natural world, but our work suggests that this journey may not be nearly as exhausting and heroic as it appears," says author James C. Liao, a graduate student in Harvard's Department of Organismic and Evolutionary Biology. "Rather than swimming blindly upstream through turbulence, swimming fish use specific body motions to yield to natural eddy formations, using energy in the environment to direct their bodies upstream without much muscular investment."

Liao likens the technique fish use to swim against a turbulent current to tacking, the back-and-forth motion that allows a sailboat to move forward passively, if somewhat indirectly, against a breeze.

In their study, Liao and his colleagues at Harvard and M.I.T. looked at both animal behavior and the physics of water flow. They placed an ordinary cylinder in a flow tank to create eddies similar to those generated in the wild by the flow of water past submerged branches or coral reef formations, or the propulsive movements of other fish, and then recorded the muscle activity of trout as they swam among the vortices generated downstream from the cylinder.

fish seen from below
Liao likens the technique fish use to swim against a turbulent current to tacking, the back-and-forth motion that allows a sailboat to move forward passively, if somewhat indirectly, against a breeze. (Photo by James C. Liao)

The scientists discovered a previously unknown swimming motion, dubbed the "Karman gait," that exploits the energy of eddies to minimize fish muscle activity. Viewed from below, the bodies of fish swimming in turbulent waters flutter gently, like flags flapping in the breeze, a motion that entails far less muscle activity than swimming through still waters.

"This new link between the use of eddies and decreased muscle activity supports a hydrodynamic explanation for fish distributions in schools and in current-swept habitats," Liao says. "Our work is the first to establish a causal mechanism to explain the hydrodynamic benefits of fish schooling, and resolves the paradox of how wild fish can move upstream against currents faster than they have been reported to swim in the laboratory."

Fish schooling has long been viewed as an energy-saving tactic. However, the study by Liao and his colleagues provides the first direct view of the technique used by fish to exploit the energy of eddies generated by the swimming movements of other members of a school. The findings could help advance the design of fish ladders and passageways at hydroelectric dams, and could aid in the design of autonomous underwater vehicles capable of negotiating turbulent flows in natural habitats.

Liao was joined in this research by George V. Lauder, professor of biology and Alexander Agassiz Professor of Zoology in the Museum of Comparative Zoology at Harvard, and by David N. Beal and Michael S. Triantafyllou at M.I.T. The work was supported by grants from Sigma Xi, the American Museum of Natural History, the Robert A. Chapman Memorial Scholarship at Harvard, and the National Science Foundation, as well as an M.I.T. Sea Grant.

Copyright 2007 by the President and Fellows of Harvard College