ASSESSING THE THREAT AND PREDATOR CONTROL OF A NON-NATIVE, AQUATIC INVADER (ZEBRA MUSSEL DREISSENA POLYMORPHA) IMPACTING ENDANGERED NATIVE MUSSEL RESOURCES IN THE ST. CROIX NATIONAL SCENIC RIVERWAY (SACN): AN UPDATE

Lynn Bartsch, U.S. Geological Survey (USGS), Upper Midwest Environmental Sciences Center
Michelle Bartsch, U.S. Geological Survey (USGS), Upper Midwest Environmental Sciences Center
William Richardson, U.S. Geological Survey (USGS), Upper Midwest Environmental Sciences Center
Steve Zigler, U.S. Geological Survey (USGS), Upper Midwest Environmental Sciences Center
Byron Karns, National Park Service, St. Croix Falls
Brenda Moraska Lafrancois, National Park Service, St. Croix Watershed Research Station

Invasions of aquatic non-native species are increasing and can result in catastrophic changes in community composition and ecosystem function. The St. Croix National Scenic Riverway (SACN) is one of the last best refuges for rapidly declining populations of native unionid mussels in the United States. National Park Service (NPS) monitoring in the SACN shows that zebra mussel (ZM: Dreissena polymorpha) populations are rapidly increasing in lower Lake St. Croix, yet there is evidence that benthivorous fish are using ZM as a food source, potentially decreasing the rate of ZM colonization within this system. However, the benefit of ZM consumption by these benthivorous fish might be partly offset by an increase in predation pressure on juvenile unionid mussels. Our research objectives are to quantify the effects of the ZM invasion in the SACN by assessing 1) the mechanisms of impact on native adults and juveniles (e.g., direct impact via encrustation, indirect via competition for seston food resources); 2) the extent of ingestion of ZM by local populations of benthivorous fish; and 3) efficacy of mitigation efforts such as mechanical removal of ZM by hand-scrubbing native mussels. We will quantify unionid condition using sensitive biochemical metrics of tissue glycogen (quantity) and lipids (quality and quantity), including essential fatty acids. We will also use less sensitive but more easily measured indices of condition — body mass change, shell growth, and individual survival. Analysis of lipids in the seston, as well as the concentration of chlorophyll a, volatile and total suspended solids, will quantify the quality of food available for mussels; differences in quality and quantity of seston related to variation in ZM density will indicate the effect of ZM filtering on food resources. Results of this study should provide critically needed information to guide and justify management actions necessary for the protection and restoration of imperiled populations of native mussels in the SACN and other park units experiencing invasion of ZMs.