Investigating material properties of fish schools with dynamic light fields

Many social animals, such as birds, fish and insects, exhibit complex group 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 strain the school using projected dynamic light field. To do this, we use an overhead high speed camera to record individual fish trajectories in a quasi-two-dimensional tank. The strain is generated by projecting two dark regions moving in opposite directions, where fish use both social and environmental information to determine their behavior. We find that schools can undergo large deformations before collapsing back to one of the regions. We show that the school exhibits a linear stress-strain relationship, analogous to the Hooke’s law.