Perch or plankton: top-down control of Daphnia by yellow perch (Perca ¯avescens) or Bythotrephes cederstroemi in an inland lake? JOEL C. HOFFMAN, MEGHAN E. SMITH and JOHN T. LEHMAN Department of Biology, Natural Sciences Building, University of Michigan, Ann Arbor, MI, U.S.A. SUMMARY 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 ¯avescens) or by the exotic predatory zooplankter Bythotrephes cederstroemi. 2. During the initial decline of Daphnia, Ivlev's electivity coef®cient 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 signi®cant 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. Keywords: bioenergetics, Bythotrephes, Daphnia, perch, trophic cascades Introduction The seasonal clear water phase typical of many inland lakes has been attributed to intense grazing by herbivorous zooplankton (Lampert, 1978; Edmondson & Litt, 1982; Lampert et al., 1986). An extended period of unusual water clarity has been associated with a trophic cascade (Hrba Âcek, 1962; Carpenter, Kitchell & Hodgson, 1985; Carpenter et al., 1987; Brett & Gold- man, 1997), in which piscivores prey on planktivorous ®sh, thereby releasing grazers from predation and permitting a clear water phase. Subsequently, intense feeding by planktivorous ®sh reduces grazer num- bers, truncating the clear water phase. A sudden decline in grazing zooplankton, especially Daphnia, has usually been attributed to predation by plankti- vorous ®sh such as minnows (Carpenter & Kitchell, 1988) and yellow perch (Perca ¯avescens (Mitchill)) (Mills, Forney & Wagner, 1987), but also to predation by Cyclops (Lampert, 1978) and a combination of starvation and ®sh predation (Lampert et al., 1986). A clear water phase has been a regular feature in Long Lake, Grand Traverse County, Michigan (B. Lishawa, unpublished). At such times, Secchi disk transparency ranges from 10 to 19 m. Daphnia pulicaria Forbes is dominant during the clear water phase, reaching a density exceeding 5800 m )3 . Recent surveys of yellow perch in Long Lake indicate that most age classes are growing faster than the average for Michigan lakes (Michigan Department of Natural Resources, 1997). Perch data seem consistent with a trophic cascade interpretation, wherein D. pulicaria is the primary food, and the clear water phase ends when perch reduce the density of D. pulicaria. This could be similar to ®ndings in Oneida Lake, New York, where control of Daphnia populations by age-0 Correspondence: John T. Lehman, Department of Biology, Natural Sciences Building, University of Michigan, Ann Arbor, MI 48109±1048, U.S.A. E-mail: jtlehman@umich.edu Freshwater Biology (2001) 46, 759±775 Ó 2001 Blackwell Science Ltd 759