The invasive species Cercopagis pengoi in Lake Ontario I. Position in the food web II. Impact on mirex concentrations in the biota III. Temporal changes in mirex concentrations in salmon

Abstract

This study investigates the position of invasive zooplankton species Cercopagis pengoi in the Lake Ontario food web as well as its impact on mirex concentrations in the biota. Seasonal Cercopagis samples were collected weekly from May through November 2000 for abundance measurements. Seasonal zooplankton samples were also collected weekly from May through September 2001 for abundance measurements using a Wisconsin net (63 µm mesh size, 50-cm diameter). Each catch was washed into buckets, transferred to bottles, and preserved with 10% buffered formalin. In the lab, zooplankton were identified and enumerated to the species level following methods of Gannon. Zooplankters were subsampled and measured from the anterior margin of the helmet to either the base of the tail spine or caudal setae. Zooplankton samples were also collected during the summers of 2000 and 2001 for pesticide analysis. Samples were visually inspected for relative composition, then placed in solvent rinsed glass jars, kept in ice and transported back to the lab, where they were frozen until analysis. Salmonids were collected during their spawning run in the fall of 1999 via electroshocking and gill netting. Alewives were collected monthly from May to November 2000 by gill netting. Fish length, weight, sex, and age were determined by standard procedures. Salmonines subsamples were prepared for chemical analysis by taking a fillet from the fish and homogenizing it before freezing it in solvent rinsed glass jars. Whole alewives were gutted and homogenized. The stomachs of salmonine and alewife sub-samples were removed and their diet was analyzed. Mirex and photomirex were quantified by electron capture (63­Ni) gas chromatography. All samples were analyzed for stable isotopes of carbon and nitrogen. Results indicate that the invasion of Cercopagis pengoi into Lake Ontario does not represent an “extra step” of significance in the pelagic food web, but it is a link. While nutrients flow from Cercopagis to alewives, it only represents a small portion of the alewife diet. As a result, no increases were observed in the monthly mirex concentrations in alewives.