FISH RESPONSE TO EMERGENT VEGETATION PRODUCTION IN BACKWATERS OF MISSISSIPPI 
RIVER POOL 25: AN EVALUATION OF ENVIRONMENTAL POOL MANAGEMENT.

S. Reid Adams1,2, James E. Garvey1,2, Michael B. Flinn2, Brooks M. Burr2, Matt 
R. Whiles2, and Robert J. Sheehan1,2, 1Fisheries and Illinois Aquaculture 
Center, Southern Illinois University, Carbondale, IL 62901, 2Department of 
Zoology, Southern Illinois University, Carbondale, IL 62901

The St. Louis District of the U.S. Army Corps of Engineers implemented 
Environmental Pool Management (EPM) in 1994 on Mississippi River Pools 24, 25, 
and 26.  The operational goal of EPM is to maintain relatively low, stable water 
levels in the lower portion of the pools, following maximum drawdown in the 
spring, in order to better simulate a natural hydrograph without inhibiting 
commercial navigation.  When possible, water levels are held up to 2.0 feet 
below the target pool elevation at the lock and dam for at least 30 days.  
Previous investigations of mudflats exposed via EPM showed substantial 
production of emergent vegetation consisting primarily of millet, chufa, and 
smartweeds; however, little data exist evaluating the response of other biota to 
EPM.  From 1999-2002, we studied the fish response to EPM by examining fish use 
of recently flooded vegetation in the fall.  Paired study plots (400 m2), one 
vegetated and one experimentally devegetated prior to reflood, were established 
at four sites in lower Pool 25.  Sites represented a range of contiguous 
backwater types (e.g., island slough, island tip, and shallow bay).  Fish were 
collected on multiple trips each fall with 1-m2 pop nets (first year only) and 
with a 3.7-m seine (1.6-mm mesh).  

Data collection corresponded to three very different drawdown regimes, resulting 
in variable vegetation responses:  1999 (high vegetation production), 2000 
(little to no vegetation production), and 2001 (moderate vegetation production).  
A total of 41,065 fish, including 24 species, were collected in the backwater 
habitats.  Four species had higher (p<0.05) or marginally higher (p<0.10) 
abundance in vegetated plots (spotfin shiner, common carp, bullhead minnow, and 
western mosquitofish), and only the emerald shiner was more abundant in 
devegetated plots.   Significantly (p<0.05) more individuals of spotfin shiner, 
channel shiner, and western mosquitofish < 20 mm were collected in vegetated 
plots in 1999, indicating the vegetation was providing nursery habitat for these 
late season spawners.  In 1999, some vegetated plots had very low dissolved 
oxygen values (< 3.0 mg/l), and were dominated numerically by western 
mosquitofish and common carp.  Not surprisingly, fish assemblages between study 
plots in 2000 were very similar (Percent Similarity Index = 0.75), as there was 
no structural difference between plots.  Higher, stable water levels in the 
summer of 2000 resulted in a dramatic (9-fold) increase in abundance of YOY 
orangespotted sunfish and bluegill.  In 2001, sunfish abundance remained high, 
the vegetation provided nursery habitat for late season spawning fishes, and 
chronically low dissolved oxygen was not found.  Our data suggest the fish 
response to EPM will vary depending on vegetation characteristics, summer 
hydrology, and location.  We observed positive responses by fish in all three 
years, however, the drawdown regime resulting in moderate vegetation production 
(2001) tended to provide the most benefits. 

Key Words:  Mississippi River, Environmental Pool Management, fish, vegetation, 
nursery habitat