Predator-prey encounters in turbulent waters

Hans Pécseli
Institute of Physics, University of Oslo, Norway

With reference to studies of predator-prey encounters in turbulent waters, we demonstrate the feasibility of an experimental method for investigations of particle fluxes to an absorbing surface in turbulent flows. The analysis is based on data from a laboratory experiment, where an approximately homogeneous and isotropic turbulent flow is generated by two moving grids. The simultaneous trajectories of many small neutrally buoyant polystyrene particles are followed in time. Selecting one of these to represent a predator, while the others are considered as prey, we obtain estimates for the time variation of the statistical average of the prey flux into a suitably defined ``sphere of interception''. The variation of this flux with the radius in the sphere of interception, as well as the variation with basic flow parameters is well described by a simple model, in particular for radii smaller than the integral length scale. The study is extended by introducing a simple, yet realistic, model for the capture probability of prey. Simple scaling laws with the turbulence parameters and radii in the sphere of interception result from the model. These have been tested experimentally, using the same data. The prey flux actually captured by a predator is a result of the prey flux determined by the turbulent mixing together with the probability of capturing prey. For large turbulence levels, the results are rather surprising, and maybe counter-intuitive, by predicting a captured prey flux to be independent of the turbulence level.