Annoying a fish school to understand the coupling between cognitive and physical interactions

Collective motions on a macroscopic scale involve not only physical but also cognitive interactions (intention, memory, decision-making). This aspect has mainly been addressed by numerical or theoretical modeling, essentially by introducing an arbitrary social force (see works by Vicsek, Helbing, Chaté, Sire, ...). In fish, the role of hydrodynamics has been investigated focusing on the energetic efficiency of swimming (see works by Godoy-Diana, Triantafyllou, Koumoutsakos, Argentina,...). So far, little is known about the coupling and relative weight between social and physical forces. To address this general question, we develop original experiments in which the collective behavior of a group of fish is perturbed by physical obstacles, thus testing the stability of the collective effect. These experiments reveal the existence of a characteristic length, purely related to cognition. I will also briefly mention work in progress where hydrodynamics comes into play.