Predators may affect their prey both directly by eating them and indirectly by affecting their behaviors. The behavioral, non-numerical, effects of predator presence on the relative performances of juvenile perch (Perca fluviatilis) and juvenile roach (Rutilus rutilus) were studied in a field enclosure experiment. Each enclosure consisted of a predator-safe part with complex physical structure (artificial vegetation) and an open water with predators present or absent. In half of the enclosures, the bottom of the prey refuge was sealed with a plastic sheet which prevented the juvenile fish from bottom feeding and also reduced the densities of invertebrates attached to the artificial vegetation. In the absence of predators (piscivorous perch), juvenile roach fed mainly on Bosmina whereas they fed mainly on detritus/algae in the presence of predators
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Lennart Persson (1993) studied this question.