New school of thought on how fish swim

Do fish save energy by swimming in schools? New technology suggests the answer to this old question is yes.

German and Chinese scientists used biomimetic fish-like robots and hydrodynamic models to show, they say, that fish take advantage of the swirls of water generated by those in front by applying a simple but previously unknown behavioural rule.

By adjusting their tail beat relative to near neighbours – a strategy called vortex phase matching – the robots were shown to benefit hydrodynamically from a near neighbour no matter where they were positioned with respect to that neighbour.

AI-assisted analysis of body posture of goldfish swimming together then revealed that the same strategy is used by free swimming fish.

“Fish schools are highly dynamic, social systems,” says Iain Couzin from Max Planck Institute of Animal Behaviour (MPIAB), senior author of a paper in the journal Nature Communications.

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

The robots had soft tail fins and moved with an undulating motion real fish, with the added advantage that it was possible to measure their energy output.

The researchers – from MPIAB, the University of Konstanz and Peking University – studied them in pairs, running more than 10,000 trials with follower fish in every possible position relative to leaders.

The results, they say, showed a clear difference in energy consumption for robots that swam alone versus those that swam in pairs. And the secret is in synchronisation.

Follower fish must match their tail beat to that of the leader with a specific time lag based on spatial position. When followers are beside leader fish, the most energetically effective thing to do is to synchronise 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.

To visualise the hydrodynamics, Couzin and colleagues emitted tiny hydrogen bubbles into the water and imaged them with a laser, making the vortices created by the swimming motion of the robots visible.

“It’s not just about saving energy. By changing the way they synchronise, followers can also use the vortices shed by other fish to generate thrust and help them accelerate,” says co-author Mate Nagy.