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What is role of hydrodynamic interactions in a fish school ?
Fish schools offer a fascinating example of collective behavior in which the group coordinates without any leader. But what is the role played by the fluid flow in this coordination ? To address this question, we have developed a model that simulates the hydrodynamic interactions between hundreds of fish in a far-field approximation. In this model, each fish change its heading as a response to the position and orientation of its neighbors according to behavioral rules based on tracking experiments on real fish. But this heading is also affected by the flow generated by the other fish.
This model has allowed us to evidence a surprising role of hydrodynamic interactions : fish spend less energy when swimming in school because they benefit, on average, from a "hydrodynamic drafting" effet from their neighbors. Hydrodynamic interactions also induces a new phase in which fish align in the same direction, but produce a curve trajectory with a large radius at the scale of the group. Finally, fluid plays the role of a thermodynamic noise on the orientation of fish.