VIRTUAL-REALITY OF PLANKTIVORES - A FISHS PERSPECTIVE OF PREY SIZE SELECTION

Traditionally, field studies of size selective predation by visual-fee ding planktivorous fish compare the size distribution of prey in stoma chs to that of zooplankton measured in the field. However, the size fr equency of prey perceived by a fish may differ from that measured by r esearchers using integrative plankton nets. In this study, a mechanist ic spatial foraging model was developed to test the hypothesis that si ze-selective predation by visual-feeding planktivorous fish can arise simply by random encounters with prey. Our model was based on movement of a single predator and a size structured prey population distribute d in 3-dimensional space. The model assumed that a predator's encounte rs with prey was a function of prey size and predator swimming speed. Upon encounter, the predator either selected prey by random choice or the largest apparent size. The size frequency and distance distributio n of prey encountered, selected, and captured were estimated using Mon te Carlo techniques. The model performance was initially evaluated wit h hypothetical prey size frequencies, and different light attenuation coefficients, capture efficiencies, visual distances and predator swim ming speeds. Simulation results showed that the size frequency of prey encountered randomly by a predator is very different from the size fr equency of the ambient prey in the environment. The size frequency of prey selected by random choice only differed from that selected by app arent size choice when visual distance was greater than reactive dista nce. Prey distance distribution, which was defined as frequency of pre y encountered at different distances, showed that as prey density incr eased predators selected prey at closer distances by the largest appar ent size choice than by random choice. We also tested the model specif ically for the bay anchovy Anchoa mitchilli. Ambient zooplankton size frequencies found in mid-Chesapeake Bay were used to predict the size frequencies of zooplankton consumed by fish. Predicted prey size frequ ency in the diet matched the size frequency of zooplankton found in ba y anchovy stomachs. We conclude that prey size selection by fish can b e described mechanistically by differential random encounter from a fi sh's perspective, and that behavioral choice plays a minor role in pre y size selection.