The invasive New Zealand mud snail, Potamopyrgus antipodarum, is a world-wide invasive species currently found in Europe, Australia, Japan, and, most recently, North America. It was first discovered in Lake Ontario in 1991. The purposes of this study were to update the current known geographic distribution of the snail, determine the relationship between depth and population densities, and examine the relationship between Potamopyrgus and dreissenid mussel densities in Lake Ontario. We sampled several locations in Lake Ontario and determined that the range of Potamopyrgus has expanded. However, densities appear to have moderated in the past ten years. In one location (Wilson, NY), the densities of the snail are dependent upon depth with highest densities occurring between 15 and 25 m. At this location, no snails were found at depths of less than 15 m. We found no correlation between the densities of Potamopyrgus and invasive mussels. The reasons for apparent reduction in densities over time and the apparent lack of Potamopyrgus in shallow water are discussed.

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