Cercopagis pengoi in Southwestern Lake Michigan in four years following invasion

In 1999, a long-term monitoring program in southwestern Lake Michigan captured the invasion of Cercopagis pengoi (fish hook flea), a predatory zooplankton native to the Ponto-Caspian region. This invasion provided an opportunity to examine both the responses of a population following establishment in a new area and the immediate response of the native community. We compared seasonal dynamics and several morphological and life history traits of C. pengoi both among and within the 4 years following its invasion into Lake Michigan. Samples collected from five nearshore sites in southwestern Lake Michigan from 1999-2002 indicated that the average density doubled from 75 individuals/m³ in 1999 to 150 individuals/m³ in 2002. We found no evidence for rapid changes in body size or average clutch size as the population established. We did, however observe a shift in seasonal phenology, with the population occurring in the water column earlier each year. By 2002, Cercopagis pengoi appeared in the water column by July and persisted until early fall. We also compared the average monthly densities of several potential prey items in August in the years before (1998-1999) and after (2000-2003) establishment. Although we found a significant reduction in the average number of rotifers, the general trend was a decline in all zooplankton species. Given the number of previously established exotic species in this system, it may prove difficult to quantify the impact of this most recent addition on what is left of the native community.