Lake herring (Coregonus artedi) can drastically alter the zooplankton community of a lake through size-selective predation. I studied the first step in the predation sequence, reaction to prey, for different zooplankton taxa by lake herring. Reactive distance was significantly shorter for smaller zooplankton taxa (p < 0.0001). Reactive volume was calculated from measured distances and angles of elevation and bearing. This volume augments the reactive distance data and showed a three dimensional geometry, roughly analogous to a deflated basketball, that is amenable to a cruising style of planktivory. Reactive volume is dynamic with respect to variations in prey size (p < 0.001). Reactive volume is a refined measure of a ‘reaction' by fish to a prey item than are simple, linear distances or other, assumed shapes. Thus, for a cruising planktivore, three dimensions need to be considered.