B. Data Analysis 



At each site, number of individual fish and total weight (pounds) were tallied 

 for each species. Fish catch rates were calculated as the number of individuals 

 collected per hour of electrofishing (CPUEn) and as weight in pounds collected per 

 hour of electrofishing (CPUEw). For 1 994-1 998, data from sites were grouped into 

 reaches defined by navigation dams (Figure 1) as follows: Alton Reach, river mile 

 (RM) 0-80; La Grange Reach, RM 80-158; Peoria Reach, RM 158-231; Starved 

 Rock Reach, RM 231-247; Marseilles Reach, RM 247-271.5; and Dresden Reach, 

 RM 271.5-286 on the Des Plaines River. Data from reaches were combined further 

 into three groups (lower and middle Illinois River segments, and the upper Illinois 

 Waterway segment) defined by their location along the river and by the amount of 

 off-channel habitat accessible to fish per unit length of river (Lerczak et al. 1994:5 

 and Figure 1 ). Lerczak et al. (1 994, 1 995, and 1 996) showed that river fish 

 communities of the three segments differed substantially enough to give segment 

 designations biological meaning. Separate tables were constructed listing only those 

 species that accounted for at least 95% of the total catch rates. This percentage 

 was arbitrarily chosen to emphasize species of most importance in analyzing fish 

 community composition. 



10 



