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CHANGING AGE STRUCTURES IN POPULATIONS OF ZEBRA MUSSELS IN THE ST. CROIX NATIONAL SCENIC RIVERWAY
Byron Karns, National Park Service, St. Croix National Scenic Riverway
Zebra mussels have been a threat to the St. Croix watershed since the early 1990. In 1992, the first mussels were discovered in the Mississippi above the confluence with the St. Croix River. The first boat discovered with attached zebra mussels was in 1994 and reproduction was pinpointed by 2000. There is a critical need to understand the implications of an ever expanding and increasing number of zebra mussels in the river. This is a high priority for the NPS and ACoE and other natural resource management agencies. The NPS will gather information about the age structure of these populations to determine recruitment, growth rates, and mortality. This will aid in determining the affects of this animal on native fauna, including freshwater mussels. Anecdotal accounts of periodic, but substantial zebra mussel die-offs in large river systems in the Midwestern U.S. have been noted in the last several years. Details from the Illinois and Upper Mississippi rivers, suggest an early season recruitment followed by a late season population crash. However, these observations have been casual and not systematic or well documented. In order to predict impacts to river biota, an organized assessment of seasonal population dynamics of zebra mussels in a large river system is necessary. The St. Croix River is a 6th order system with moderate zebra mussel infestations within the downstream most 22 miles, and especially in the lower 6 miles below the Kinnickinnic Narrows. In 2006, densities of this invasive animal reached over 700m2 within this last pool. The affects of large numbers of zebra mussels in freshwater systems in North America have been well documented. Particularly, native mussels have been severely impacted by direct food and oxygen competition and indirectly by shell colonization. If, however, condition in certain river systems allow for veliger settlement and establishment, but limit growth through maturity, implication for management are numerous.
Keywords: Zebra Mussels, Population Dynamics, Lake St. Croix, Water Quality