Hornbach, D.J. 1992. An examination of the population structure, community relationships and habitat characteristics for the winged mapleleaf mussel (Quadrula fragosa) at Interstate park, Saint Croix river, Wisconsin and Minnesota. Final report submitted to the Minnesota Department of Natural Resources. 17+ pp.
Freshwater mussels (Family Unionidae) are widely distributed throughout the United States. There are 44 species of freshwater mussels currently on the federally endangered species list (Fish and Wildlife Service, 1991). Despite this fact, there is little known concerning the factors which control the distribution of these organisms, especially in flowing water systems. Certain factors such as surface geology, stream size, water quality, substrate type, water flow, and food availability, among others, are important in determining the community structure and population dynamics of freshwater mussels.
In the past there have been studies on the unionids of the St. Croix River. Baker (1928) cited 15 species from the St. Croix River although he classifies some species as statewide. Dawley (1947) reported 29 species of unionids from the St. Croix River (in addition to 4 species found in tributaries to the St. Croix but not in the river proper). Fuller (1978) recorded 23 species of unionids from the St. Croix River at Hudson, WI. Stern (1983) reported 14 species of unionids from a single site on the St. Croix. Doolittle (1988) has conducted the most extensive study to date on the distribution of unionids in the St. Croix River. Thirty-seven species of unionids (including 2 only represented by dead shells) were reported by Doolittle in the river proper. Quantitative studies by Hornbach (1992) at Franconia, MN have indicated densities of 12 mussels/m2 at that site. Semi-quantitative estimates by Doolittle (1988) gave ranges of 0.1 to 16.3 mussels/m2 in established beds in the St. Croix.
Of particular interest in the St. Croix River is the presence of two species of endangered mussels, Lampsilis higginsi and Quadrula fragosa. Lampsilis higginsi, while found in the St. Croix is also found throughout the Upper Mississippi River, albeit at low densities. The winged mapleleaf, Quadrula fragosa, previously distributed in 11 other states, is presently restricted to the St. Croix River.
Due to the highly restricted nature of Q. fragosa, the major thrust of this research project was to investigate the factors which may influence the distribution and abundance of this endangered species. In addition, efforts were made to characterize the mussel community associated with the presence or absence of Q. fragosa.