The distance at which Lepomis gibbosus reacts to zooplankton is shown to be a linear fuilction of prey size. If fishes sweep out a cylindrical path as they forage the frequency of prey encountered is then proportional to the square of this reaction distance. With the inclusion of the probability of capture success after a fish pursues a prey, a model of prey selection can estimate the effect of predacious zooplankton on the energy ingestion of planktivorous fish. Since predacious zooplankton are much larger and more visible to the fish than their own prey, this more than compensates for the energy lost by the additional trophic level and the modcratc ability of predacious zooplankton to elude capture.Carnivorous and omnivorous zooplankton are abundant. By their predation they alter the abundance of other zooplankton in a lake (Confer 1971; Dodson 1974), thus ahering the food supply available to planktivorous fish. Predacious zooplankton niay decrease the food supply of fish by eating prey that the fish might otherwise use or increase it by eating small prey gencrally ignored by the fish and, in turn, being eaten by the fish. We here estimate the effect of predacious zooplankton on the energy gain of planktivorous fish.To do this, it was necessary first to develop a model of the rekitive capture frequency of prey by planktivorous fish. Basic elements of this model are the distances at which different species and sizes of zooplankton elicit fish pursuit and the success of capture after pursuit is started. Lepomis gibbosus, the pumpkinseed, was used to determine the reaction distances and capture success with various zooplankton. Capture frequency may also be influenced if fish show selectivity by ignoring one type of prey or choosing one type when two are in view simultaneously. Ivlev (1961) and Werner and Hall (1974) have shown that selectivity decreases with deercasing prey densities. Our model predicts the relative capture frequency of prey by fish at low plankton densities when selectivity is minimal.Capture frcquencics predicted by the model are compared to
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