Doolittle, T.C.J. 1987. The qualitative analysis, relative abundance, and distribution of freshwater unionid mussels in the St. Croix and Namekagon rivers. Draft final report submitted to the Minnesota Department of Natural Resources. 21 pp.
Eighty-four 60m2 belt line transects using SCUBA and 35 midden pile searches conducted over the contiguous portions of the St. Croix and Namekagon Rivers (405 km) proved effective as a qualitative survey technique but was inadequate quantitatively in the assessment of rare species. Thirty-eight species of freshwater unionid mussels live and/or dead were identified in the St. Croix River. 16 species were identified live in the Namekagon River. Anodontoides ferussacianus was the only species that was unique to the Namekagon River and a total of 39 species were documented in the complete river survey. Range extensions were documented for Cumberlandia monodonta, Simpsonaias ambigua, Quadrula fragrosa, Truncilla truncata, Epioblasma triquetra, Quadrula quadrula, Truncilla donaciformis, Tritigonia verrucosa, Quadrula metanevra, Lampsilis higginsi, Elliptio crassidens, and Ellipsaria lineolata. Live shell and dead shell means indicate that the mussel populations in the riverway are generally healthy. Island distributions and species clustering were exhibited throughout the river system. Cluster analysis by species indicates that less common species were more likely to be found with other uncommon species. Locations with the greatest species diversity were usually the locations where the least common species occurred. Ninety-six percent of all the mussels were collected when river depths were 0.5-3.5 m. Sixty-four percent of all the mussels collected occurred in sand, gravel, and rock bottom substrates in varying combinations.