Jg. Luo et al., VIRTUAL-REALITY OF PLANKTIVORES - A FISHS PERSPECTIVE OF PREY SIZE SELECTION, Marine ecology. Progress series, 140(1-3), 1996, pp. 271-283
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.