Distribution and Population Characteristics of Cercopagis pengoi in Lake Ontario

Abstract

The spatial and vertical distribution of a recent exotic species, the predatory cladoceran Cercopagis pengoi, was studied in Lake Ontario in September 1999. Only typical forms of the species C. pengoi, characterized by a relatively long tail with an S-bend and claws with straight or backwardly bent tips, were found. Structure of the Cercopagis population was rather uniform over the lake, consisting mainly (over 90%) of parthenogenetic females. Median epilimnetic abundance and biomass was 295/m 3 (max. = 2,544/m 3) and 13.4 mg DW/m 3 (max. = 113.3 mg DW/m 3), respectively. Cercopagis contributed a median of 15.8%, and at maximum 73.8%, of the total crustacean zooplanktonic biomass (exclusive of nauplii). Abundances showed a significant positive relationship with distance from shore (r 2 = 0.34, p < 0.01), but distribution was independent of the depth and temperature of the epilimnion. Cercopagis did not exhibit any diurnal vertical migration patterns: over 90% of the individuals stayed either in the epilimnion or within the metalimnion during the day and night. The proportion of individuals, both live specimens and carcasses of dead individuals, in cooler layers was negligible (< 3%). The following weight (W) – body length (L) relationship was developed during the study: ln(W) = 2.98*ln(L) - 6.42 (r 2 = 0.85, p < 0.001).