Fish schools offer a fascinating example of collective behavior in which the group coordinates without the need for a leader. But what is the role played by the fluid and flow in this coordination ? In order to answer this question, a group of researchers from CNRS (including the LPT), Centrale Marseille, Aix-Marseille University, Loughborough University (UK), and University of Southern California (USA) has developed an original model to simulate hydrodynamic interactions between hundreds of fish. In this model, each fish changes its trajectory depending on the position and orientation of its neighbors according to realistic cognitive rules already studied at the CRCA and LPT, but this trajectory is also modified by the flow that the other fish generate. This model has demonstrated a surprising effect of the interaction between fish and the fluid : fish spend less energy when they swim in a school, because they benefit on average of the flow generated by their neighbors. The interaction with the fluid also induces a new state of the school in which the fish swim in the same direction, collectively producing a large-scale curved trajectory.