CHANGES IN ABUNDANCE OF AQUATIC VEGETATION, BLUEGILLS, AND
LARGEMOUTH BASS IN LAKE ONALASKA (POOL 7, UPPER MISSISSIPPI RIVER)
FROM 1991 TO 1998.

Steven J. Zigler1, Robert J. Kennedy1, Heidi Langrehr2, and Michael R. Dewey1.
1U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, Wisconsin
54603  2Wisconsin Department of Natural Resources, Onalaska, Wisconsin, 54650. 

Many backwaters in the Upper Mississippi River historically contained large beds of aquatic
vegetation.  However, vegetation abundance in the Upper Mississippi River precipitously
declined in the late 1980's.   To investigate the ecological relations between aquatic vegetation
and fish, we sampled bluegills and largemouth bass by electrofishing at 21 stations in Lake
Onalaska, a large backwater lake (3155 ha) in Pool 7 of the Upper Mississippi River, each fall
from 1991 through 1998.  Captured fish were subsampled to estimate age and growth from
scales.  Aerial photographs of Lake Onalaska were interpreted and digitized into GIS coverages
to quantify the spatial extent of aquatic vegetation beds (submersed, emergent, mixed) during
each year.   Areal coverage of aquatic vegetation beds increased from 465.8 ha in 1991 to 1442.6
ha in 1998 (>300% increase).  Expansion of submersed vegetation beds from 101.4 ha in 1991 to
1118.1 ha in 1998 accounted for the largest proportion of the increase.  Although age structure of
bluegills and largemouth bass substantially varied, biomass (CPUE) of both species significantly
increased during the study and may be related to the increases in aquatic vegetation.  Future
analyses will evaluate growth and other population vital rate responses of fish to the changes in
vegetation abundance.

Keywords: aquatic vegetation, bluegill, largemouth bass