Examining collective mechanical properties of fish schools using projected light fields

Social animals including insects, fish, birds, and even humans exhibit self-organized collective behavior. Models have shown that simple local interactions between individuals gives rise to the emergent self-organized macroscopic states such as flocks, swarms, or schools. We investigate the material properties of laboratory fish schools by exploiting the negative phototaxicity of Rummy-Nose Tetra (Hemigrammus blehri). To do this, we use an overhead high speed camera to record individual fish trajectories in a quasi-two-dimensional tank. By projecting two dark regions moving in opposite directions, the school of fish is strained as individuals use both social and environmental information to determine their behavior. The school undergoes a large deformation before snapping back into one of the dark regions. Our results show that the school exhibits a stress-stain relationship similar to Hooke's law.