Modelling the attack success of planktonic predators: patterns and mechanisms of prey size selectivity

A mathematical model of the attack success of planktonic predators (fish la rvae and carnivorous copepods) is proposed. Based on a geometric representa tion of attack events, the model considers how the escape reaction characte ristics (speed and direction) of copepod prey affect their probability of b eing captured. By combining the attack success model with previously publis hed hydrodynamic models of predator and prey perception, we examine how pre dator foraging behaviour and prey perceptive ability affect the size spectr a of encountered and captured copepod prey. We examine food size spectra of (i) a rheotactic cruising predator, (ii) a suspension-feeding hovering cop epod and (iii) a larval fish. For rheotactic predators such as carnivorous copepods, a central assumption of the model is that attack is triggered by prey escape reaction, which in turn depends on the deformation rate of the fluid created by the predator. The model demonstrates that within a species of copepod prey, the ability of larger stages to react at a greater distan ce from the predator results in increased strike distance and, hence, lower capture probability. For hovering copepods, the vorticity field associated with the feeding current also acts in modifying the prey escape direction. The model demonstrates that the reorientation of the prey escape path towa rds the centre of the feeding current's flow field results in increased att ack success of the predator. Finally, the model examines how variability in the kinetics of approach affects the strike distance of larval fish. In ca ses where observational data are available, model predictions closely fit o bservations.