First occurrence of Cercopagis pengoi in Lake Michigan

Adults and juveniles of the predaceous cladoceran Cercopagis pengoi, native to the Ponto-Caspian region, were found in nearshore waters of southern La ke Michigan during August and September 1999. This invasive cladoceran was previously reported from Lake Ontario, but this is the first report of C. p engoi from Lake Michigan. In Lake Michigan, densities of C. pengoi peaked a t about 75/m(3) on 22 September when water temperature was 16 degreesC, and first and second instars comprised up to 90% of the samples. Of the third and fourth instar reproductive forms. parthenogenetic females were most abu ndant (up to 33% of all individuals sampled) and were present on all sampli ng dates. Males and females with resting eggs appeared in low abundance on the last two sampling dares perhaps in response to decreasing temperatures. The impact of this invader on the Lake Michigan food web is yet undetermin ed, but C. pengoi individuals were found in the stomachs of alewife on 22 S eptember indicating that fishes will feed on C. pengoi.