Thomas W. Hermanson, Leda A. Cunningham, Katie G. Esse, Jensen C. Hegg, Mark C. Hove, Jennifer L. Mann, and Daniel J. Hornbach. Macalester College, Biology Department, 1600 Grand Avenue, St. Paul, Minnesota 55105

Population dynamics of freshwater mussels (Unionidae) were observed since 1991 at three locations in the St. Croix River. Mussel communities were assessed quantitatively and qualitatively to calculate density, species richness, and age structure to identify long-term trends. Mussels and substrate were collected from at least 100 0.25 m² quadrats at each location. Substrate was separated into 5 size classes and all mussels were identified and measured. From these measurements, population density and community diversity were calculated at each location and compared to past sampling years. At Franconia mussel density decreased from 10.44 mussels/m² in 1991 to 9.76 mussels/m² in 1995, and 4.52 mussels/m² in 1999. Species richness decreased from 26 species in 1991 to 19 species in 1995, and 15 species in 1999. One endangered winged mapleleaf mussel was found in quantitative samples in 1995. At Wild River State Park, mussel density decreased from 37.36 mussels/m² in 1993 to 29.56 mussels/m² in 1996 continuing to decline to 21.08 mussels/m² in 1999. Species richness decreased from 21 species in 1993 to 18 species in 1996, and remained the same in 1999. At Prescott, Wisconsin mussel density decreased from 7.8 mussels/m² in 1994 to 5.64 mussels/m² in 1999, while species richness declined from 21 to 18 species. One Higgins’ Eye (Lampsilis higginsi) was found in both 1994 and 1999. In addition we found two invasive bivalve species at Prescott: one live zebra mussel attached to an threeridge (Amblema plicata) as well as one Asian clam (Corbicula fluminea.). We will statistically analyze these data to determine if these trends are significant and warrant management action.
 

Key words: unionid, demographics, Saint Croix River