TRENDS IN LARGEMOUTH BASS AND BLUEGILL POPULATIONS AMONG THE UPPER AND LOWER 
ILLINOIS RIVER, 1957-2006

Michael A. McClelland and Greg G. Sass

Illinois River Biological Station, Illinois Natural History Survey, 704 North 
Schrader Avenue, Havana, Illinois 62644

      Largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochirus) 
are popular sportfish in Illinois waters.  Previous research and anecdotal 
evidence from the Long-Term Illinois River Fish Population Monitoring Program 
(LTEF) suggest a potential decline in largemouth bass and bluegill populations 
in the lower reaches of the Illinois River.  We examined collections from the 
monitoring program to test for spatial and temporal differences in largemouth 
bass and bluegill populations among the six reaches of the Illinois River.  
Prior analyses have indicated spatial differences among upper (Starved Rock, 
Marseilles, and Dresden) and lower (Peoria, La Grange, and Alton) river reaches 
and temporal changes between early (1957-1981) and late (1982-2006) years of the 
program.  We used these spatial and temporal breaks to compare abundance trends, 
growth (length/weight relationships), and recruitment of largemouth bass and 
bluegill.  Electrofishing catch per unit effort (CPUE) showed an increasing 
trend over all years for bluegill and largemouth bass in the upper river. 
Bluegill catches increased in the lower river, while largemouth bass catches 
consistently declined.  No differences were observed in bluegill and largemouth 
bass growth among reaches and time periods.  Stock assessments of bluegill show 
increasing numbers of fish at and above stock length in both the upper and lower 
river, while largemouth bass collections show increasing numbers of fish at and 
above stock length in the upper river only.  Largemouth bass stock assessments 
revealed a decreasing trend of fish above stock length with little increase in 
fish of stock length in the lower river.  The similarity observed in growth 
rates between the upper and lower river for the early and late time periods may 
indicate a similar level of productivity in the Illinois River through time or a 
lack of density-dependence.  Increases in largemouth bass and bluegill 
abundances in the upper river may be due to higher water clarity, the presence 
of aquatic vegetation, less variable hydrologic conditions, and improvements in 
wastewater treatment.  

Keywords: bluegill, electrofishing, Illinois River, largemouth bass, Long-Term 
Illinois River Fish Population Monitoring Program (LTEF)