Hydrodynamics, Sensing, and Control in Schooling Fish

Fish schools are among the most spectacular examples of collective behavior in nature. Individual fish coordinate their motion with neighbors primarily through vision and flow sensing. Each fish generates flow disturbances that can influence the motion and energetics of nearby swimmers. Yet fundamental questions remain unresolved: To what extent can schooling be explained by passive hydrodynamic interactions, and where are individual-level sensing and control required to maintain group cohesion? This review examines collective swimming through the lens of fluid mechanics. By synthesizing experimental, theoretical, and computational studies spanning pairs of fish to large schools, I discuss how hydrodynamic interactions affect coordination, stability, and energetic cost. Flow interactions can yield robust pairwise coordination and locomotor benefits, but they can also drive collective instabilities and unequal benefit distributions in larger groups. These findings point to a fundamental interplay between hydrodynamics and sensory–motor feedback in the formation and maintenance of fish schools. More broadly, they suggest that hydrodynamics shapes collective swimming not only through energetic costs and benefits but also through the physical constraints it imposes on coordination, stability, and information flow within groups.