FISH ABUNDANCES OF BACKWATER LAKES WITH CONNECTIVITY GRADIENTS IN THE LAGRANGE 
REACH, ILLINOIS RIVER.

Michael A. McClelland, Mark A. Pegg, Kevin S. Irons, and T. Matt O’Hara
Illinois Natural History Survey, Illinois River Biological Station, 704 N. 
Schrader Ave., Havana, Illinois 62644.

Flood plain productivity in large river ecosystems can be greatly influenced by 
the level of connection of backwater habitats to the river’s main stem.  
Backwater habitats of the LaGrange Reach of the Illinois River were examined for 
differences in fish abundances according to their main channel connectivity.  
Two backwater categories were assessed, one with continual connection to the 
main channel, and one with seasonal connection to the main channel.  Data 
obtained from the Long Term Resource Monitoring Program (LTRMP) were analyzed 
from multiple gear collections (electrofishing, fyke netting, mini-fyke netting, 
and seining) occurring over a 12-year period (1993-2004).  Seasonal variability 
was also analyzed to determine catch differences throughout a given hydrological 
event, within three sample periods occurring from June 15-October 31.  Mean 
catches for electrofishing (198.0 fish per run) and mini-fyke netting (179.1 
fish per net) were significantly higher (P<0.005) in seasonally connected 
backwaters.  Mean electrofishing catches were also significantly higher 
(P<0.005) during the two later sample periods (284.0 and 265.3 fish per run, 
respectively) in seasonally connected backwaters.  Catches based on maximum 
annual river stage from the LTRMP data set were also compared to those observed 
by the Long Term Illinois River Fish Population Monitoring Program (LTEF).  In 
general, fish catches had a tendency to increase as maximum annual flood levels 
increased.  These results indicate that although flood events are important for 
all backwaters, lakes exhibiting seasonal connection to the main channel can 
have a positive affect on fish abundances.

Key words: Illinois River, connectivity, backwaters, fish, Long Term Resource 
                    Monitoring Program