Factors influencing the distribution and abundance of the endangered winged mapleleaf mussel Quadrula fragosa in the St. Croix River, Minnesota and Wisconsin

We examined physical and biological factors that may influence the distribution of the endangered winged mapleleaf mussel Quadrula fragosa (Conrad, 1835). Quantitative sampling of the mussel community was undertaken at two sites in the St. Croix River known to harbor Q. fragosa. Additional searches were conducted specifically for Q. fragosa individuals. For each quantitative sample of mussels, substrate composition, water velocity and depth were assessed and mussels were identified and measured. In general, Q. fragosa does not have habitat requirements different from the rest of the mussel community, although Q. fragosa occurred in shallower areas with lower bottom current velocity compared to the overall mussel community. There was no difference in substrate composition in areas with and without Q. fragosa. Mussel community density and richness were higher in areas where Q. fragosa was found. The mussel community associated with Q. fragosa was not significantly different from the general mussel community in the area. However three species (Truncilla truncata, Truncilla donaciformis and Quadrula metaneura) were significantly associated with Q. fragosa. Due to its association with dense and diverse mussel communities, management that benefits the entire mussel community should be effective in protecting this endangered species.