Collective movements are a fascinating phenomenon, from human crowds to gigantic schools of sardines. In our team, we are interested in schools of fish, seeking to take into account the complexity of the interactions involved: social interactions and hydrodynamic interactions, which are still poorly understood. We know that fish can sense hydrodynamic signals thanks to a specific organ, the lateral line. But while several studies have shown that it is possible to measure fluid flow around a single fish, there is currently a lack of experimental hydrodynamic data to characterize flow around a group of fish. The project consists in carrying out an experimental study of the hydrodynamic wake of a group of fish, identifying and analyzing the dynamics of the vortices created by an individual fish at first, then by several fish, and following them as they interact with the flow produced by the other swimmers. Obtaining a time-resolved mapping of the flow within a small group of fish would represent not only a methodological achievement in the field of hydrodynamics, but also a breakthrough in understanding the role of hydrodynamics in school organization.