Selective predation on an introduced zooplankter ( Bythotrephes cederstroemi ) by lake herring ( Coregonus artedii ) in Harp Lake, Ontario

1. Bythotrephes cederstroemi (Crustacea: Onychopoda: Cercopagidae) invaded Harp Lake, Ontario in 1993, since when the zooplankton community has shifted from dominance of small‐bodied to large‐bodied species. During 1995 diets of adult lake herring (Coregonus artedii), Harp Lake’s primary planktivorous fish, were examined to determine the extent to which this conspicuous zooplankter has become integrated into the lake’s foodweb and whether fish condition has been affected in consequence. 2. Bythotrephes and Daphnia galeata mendotae were strongly preferred prey, whilst Holopedium gibberum and calanoid and cyclopoid copepods were negatively selected by lake herring. Predation on Bythotrephes and Holopedium was not size‐selective, although D. galeata mendotae and calanoid copepods (Leptodiaptomus minutus and L. sicilis) consumed by herring were significantly larger than co‐occurring conspecifics in the lake. 3. Caudal spines of Bythotrephes may form boluses in lake herring stomachs. However, the number of caudal spines in fish digestive tracts did not differ significantly from the number of Bythotrephes’ mandible pairs, indicating that the former were not differentially retained. 4. Lake herring weight‐at‐length relationships in lakes in Muskoka, Ontario, invaded by Bythotrephes did not differ from those in adjacent non‐invaded lakes, indicating that Bythotrephes invasion of lakes apparently has not affected condition of lake herring.