Perch or plankton: top-down control of Daphnia by yellow perch (Perca flavescens) or Bythotrephes cederstroemi in an inland lake?

1. Seasonal termination of the vernal clear‐water phase in Long Lake, Grand Traverse Co., Michigan coincided with severe size‐selective predation on juvenile Daphnia pulicaria from 0.8 to 1.8 mm in length. This could be caused by predation by age‐0 yellow perch (Perca flavescens) or by the exotic predatory zooplankter Bythotrephes cederstroemi. 2. During the initial decline of Daphnia, Ivlev’s electivity coefficient for yellow perch from 15.0 to 20.0 mm in length was 0.50 for copepods and −0.75 for D. pulicaria. 3. Bioenergetics modelling of both yellow perch and Bythotrephes demonstrates that, during the initial Daphnia decline, Bythotrephes consumed 1.5–5 times greater total mass than yellow perch. Furthermore, models in which Bythotrephes consumed juvenile Daphnia were more consistent with the timing of the Daphnia decline than those in which yellow perch consumed juvenile Daphnia. 4. The invasion of Bythotrephes into Long Lake seems to be a significant perturbation, introducing effects that propagate throughout the food chain. Bythotrephes created a possible bottleneck for age‐0 yellow perch in late June by suppressing Daphnia.