SPATIAL VARIATION IN FISH-TISSUE MERCURY CONCENTRATION IN THE ST. CROIX RIVER BASIN IN MINNESOTA AND WISCONSIN, 2004

Victoria G. Christensen, U.S. Geological Survey Water Science Center of Minnesota
Stephen P. Wente, U.S. Geological Survey Water Science Center of Minnesota
Mark E. Brigham, U.S. Geological Survey Water Science Center of Minnesota
Mark B. Sandheinrich, University of Wisconsin, LaCrosse, River Studies Center

Parts of the St. Croix River are under fish consumption advisories because of elevated mercury concentrations measured in fish collected from the river. The St. Croix River is one of several National Scenic Riverways managed by the National Park Service. The U.S. Geological Survey, National Park Service, and Minnesota Department of Natural Resources cooperated on a study to determine the spatial variation of mercury in fish in the St. Croix River and selected tributaries.

Game and non-game fish species were collected and identified at 6 St. Croix River mainstem sites and 8 tributary sites between July and September 2004. One hundred ninety-three (193) composite tissue samples were analyzed for total mercury as whole fish, skin-on fillet, or skin-off fillet. No longitudinal trend in fish-tissue mercury concentration was identified in St. Croix River mainstem sites. Tributaries identified as having high median fish-tissue mercury concentrations were Rush Creek near Rush City, Minnesota and Apple River near Balsam Lake, Wisconsin. Previous studies identified Rush Creek as having high concentrations of methylmercury in water and high concentrations of mercury in sediment when compared to other sites in the St. Croix River Basin.

Data collected during this study were appended to a national data set and evaluated using a national descriptive model for mercury in fish tissue. The national model (http://emmma.usgs.gov) standardized mercury concentrations in fish tissue to specific fish species, tissues sampled, and fish length. This predictive model will allow the National Park Service to more reliably predict fish-mercury concentrations in the St. Croix River Basin and better assess health effects of fish consumption by humans and wildlife.

SUGGESTED READING

Wente, S.P., 2004, A statistical model and national data set for partitioning fish-tissue mercury concentration variation between spatiotemporal and sample characteristic effects: U.S. Geological Survey Scientific Investigations Report 2004-5199, available on the Internet at http://pubs.water.usgs.gov/sir20045199/, 15 p.