ASSESSING THE SPREAD OF ZEBRA MUSSELS IN THE ST. CROIX RIVER USING DENSITY MEASUREMENTS AND NATIVE MUSSELS

Byron N. Karns, National Park Service, St. Croix National Scenic Riverway Dan Kelner, U.S. Army Corps of Engineers, St. Paul District

The St. Croix National Scenic Riverway was the first wild and scenic river unit of the National Park System, in 1968. The Riverway is considered a nationally significant resource for its richness and abundance of freshwater mussels (~40 species, the greatest in the Upper Mississippi watershed) and is recognized for its outstanding recreational and biological assets. The diversity of unionids within the Riverway is well documented and many threats to that diversity have been identified. This faunal group will be severely impacted by a zebra mussel infestation and from other exotic invasions. Freshwater mollusks are a keystone faunal group of freshwater systems and their potential loss is unacceptable.

In order to understand the invasion of zebra mussels into the St. Croix, measurements of density were taken within the known infestation zone (the last 21 miles of river). Anecdotal evidence from the upper Mississippi River suggests zebra mussel colonization predominates on native mussel beds, especially when substrates are less favorable for recruitment (e.g., sand, silt, etc.). Therefore, sample locations were chosen based on native mussel bed survey work previously conducted by the second author. Six locations were identified from Stillwater, MN, to Prescott, WI, reflecting the range of habitats and hydrology found in the infestation zone. Thirty 1/8-meter quadrate samples were collected by divers at each of the locations. These samples were processed off river, frozen and examined under magnification. Data collected will aid managers who are creating policy based on the spread and intensity of the invasion.

The presentation will showcase the methods established to determine zebra mussel densities on the lower river, and present results of not only this invasive, but of Asian Clams and snails found during the sampling events. It will also highlight management decisions resulting from this information.