Modelling habitat of freshwater mussels (Bivalvia: Unionidae) in the lower Great Lakes 25 years after the Dreissena invasion

Finding remnant populations of species that are of conservation concern can often be difficult, particularly in aquatic habitats. Ecological niche models can aid in the discovery of refuges. Remnant populations of native freshwater mussels (unionids) have been found in lakes Erie and St. Clair. Our goal was to predict undiscovered potential refuges in Lake Ontario and to conduct surveys to test those predictions. We used large- and small-scale habitat characteristics of locations where unionids occurred in Lake Erie to predict locations of suitable habitat in Lake Ontario. The model was built on a suite of environmental variables including attributes of the benthic zone and shore-line characterization. Model predictions were tested by sampling 34 locations on the United States shores of Lake Ontario, allowing to discover previously unknown unionid refuges. The model predicted that 0.8% of the near-shore area of Lake Ontario is potential habitat for unionids. Surveys of Lake Ontario found 1800 unionids of 11 species and we found a positive relationship between model predictions and the number of species present (p = 0.004, adj. r2 = 0.14). Our prediction was successful even though the mussel assemblages of Lake Erie and Lake Ontario were very different, signifying importance of our approach for conservation and management. The variables that had the highest contribution to the model were bathymetry, fetch and shoreline geomorphology. These large-scale parameters defined local-scale environmental variables important for unionid survival, including presence of vegetation and substrate composition that explained app. 22% of the variance in presence, abundance and richness. While some of these small-scale parameters were described previously, we were the first to detect the most important large-scale parameters that determined local habitat important for unionid persistence. Our study demonstrated that ecological habitat modeling provides a tool for locating local concentrations of mussels across extensive areas.