FINGERNAIL CLAM (SPHAERIIDAE) DENSITIES AND DIVING DUCK USE IN
THE UPPER MISSISSIPPI RIVER LOWER POOL 8.  

Randy Burkhardt 1, Jennifer S. Sauer 1, Lara Hill 2, and Shawn Weick 1.   1
U.S. Geological Survey, Upper Midwest Environmental Science
Center, Onalaska, WI 54650; 2 U.S. Fish and Wildlife Service,
Onalaska, WI 54650.  

Fingernail clam densities in Mississippi River Lower Pool 8 have
been relatively low from 1992-1997 (0-211.5 m-ý; data collected
by the Long Term Resource Monitoring Program).  However, data
collected from fall/winter sampling in lower Pool 8 indicated
that fingernail clam densities significantly (P<0.05) increased
during 1998, reaching densities as high as 8,519 m-ý.  More
importantly for diving ducks, these clams were available as a
food source during the 1998 fall migration.  In mid-November of
1997, diving duck counts on Pool 8 were low (total=710; data
collected by the U.S. Fish and Wildlife Service).  With the
increase of fingernail clams, diving duck total counts increased
to 45,420 in Pool 8 during mid-November of 1998.  The direct
cause of the increase in fingernail clam densities is not
completely understood.  Recent findings however, suggest that
flow and water depth may play an important role in determining
fingernail clam location and densities, but we need to further
verify the causal relationships.  

Keywords:  fingernail clam, diving duck, Mississippi River,
macroinvertebrate, Musculium transversum