Size-dependent interactions between piscivorous perch Perca fluviatilis (age ≥1 year) and their fish prey age 0 year perch, pikeperch Sander lucioperca and roach Rutilus rutilus in the biomanipulated Bautzen Reservoir indicated that the highest ratio of prey total length (LT) to predator LT was 59%. Perch LT and prey fish LT were positively and linearly related. Perch LT was strongly related with both gape width and gape height. Within the range 80–110 mm LT, the gape height of perch exceeded gape width, while beginning at 120 mm LT the gape width exceeded gape height. The minimum, maximum and mean prey LT and prey body depths of all three prey species increased with increasing predator size, but the increases in mean sizes of perch and pikeperch as prey were less than that of roach. The low limit of the ‘predation window’ observed in this study coupled with results of previous studies on perch in the Bautzen Reservoir indicated that perch had a major impact on the population dynamics of both perch and pikeperch.