Living in a Turbulent World—A New Conceptual Framework for the Interactions of Fish and Eddies

The natural habitats of fishes are characterized by movements of water driven by a multitude of physical processes of either natural or human origin. The resultant unsteadiness is exacerbated when flow interacts with surfaces, such as the bottom and banks, and protruding objects, such as corals, boulders, and woody debris. There is growing interest in the impacts on performance and behavior of fishes swimming in "turbulent flows". The ability of fishes to stabilize their postures and their swimming trajectories is thought to be important in determining species' distributions and densities, and hence the resultant assemblages in various habitats. A theoretical framework is proposed to quantify the interactions of fish and flows. Dimensionless parameters are derived based on a physical description of the flow structures and different regimes are predicted describing fishes' responses to a wide range of physical perturbations. We found the ratio of eddy size to fish size, the "momentum ratio" (ratio between momentum of the eddy and the momentum of the fish), as well as the time of interaction between eddy and fish to be especially important in determining thresholds for the fish's posture and trajectory.