do fish school reduce energy cost of swimming?

Do fish save energy by swimming in schools? Now, scientists from the Max Planck Institute of Animal Behavior (MPI-AB), the University of Konstanz, and Peking University have provided an answer that has long been suspected but never conclusively supported by experiments: yes.

Using biomimetic fish-like robots, the researchers show that fish could take advantage of the swirls of water generated by those in front by applying a simple behavioral rule. By adjusting their tail beat relative to near neighbors—a strategy called vortex phase matching—robots were shown to benefit hydrodynamically from a near neighbor no matter where they are positioned with respect to that neighbor.

"Our results provide an explanation for how fish can profit from the vortices generated by near neighbors without having to keep fixed distances from each other."

Answering the question of whether or not fish can save energy by swimming with others requires measuring their energy expenditure. Accurately doing so in free swimming fish has so far not been possible. the robots allow for direct measurement of the power consumption associated with swimming together versus alone.

Running over 10,000 trials, they tested follower fish in every possible position relative to leaders—and then compared energy use with solo swimming.

they discovered, is the way that fish in front influence the hydrodynamics of fish behind. The energy consumed by a follower fish is determined by two factors: its distance behind the leader and the relative timing of the tail beats of the follower with respect to that of the leader. In other words, it matters whether the follower fish is positioned close to the front or far behind the leader and how the follower adjusts its tail beats to exploit the vortices created by the leader.

That is, follower fish must match their tail beat to that of the leader with a specific time lag based on the spatial position—a strategy the researchers called "vortex phase matching." When followers are beside leader fish, the most energetically effective thing to do is to synchronize tail beats with the leader. But as followers fall behind, they should go out of synch having more and more lag as compared to the tail beat of the leader.

They used AI-assisted analysis of body posture of goldfish swimming together and found, indeed, that the strategy is being used in nature.

Theory

 

detected real fish movement by body posture tracking

reference

Vortex phase matching as a strategy for schooling in robots and in fish. Liang Li, Máté Nagy, Jacob M. Graving, Joseph Bak-Coleman, Guangming Xie & Iain D. Couzin. Nature Communications (2020). DOI: 10.1038/s41467-020-19086-0