Electrofishing survey 30 May 1991 Page 15 



could form ammonia under anaerobic conditions. Again, more information is needed to 

 determine whether this correlation is coincidental or causal. Virtually any pollutant that 

 occurs in sewage or industrial effluent will occur in higher concentrations in the upper 

 river, near the sources, than farther downstream, so ammonia is only one suspect. 

 In order to determine whether the slight declines we observed in Wr values 

 between 1989 and 1990 might be attributable to different length ranges of fish captured 

 in the two years, the weight-length values were plotted and visually examined (Figures 4- 

 8). With the exception of smallmouth bass, the lengths in the two years overlap and are 

 in the same range. Too few smallmouth bass were captured in the Upper River in 1990 

 to make a valid comparison of Wr values (Figure 8). 



Saueer. Although the sampling design of the long-term electrofishing program is biased 

 against sauger and walleye (no tailwaters are sampled and no night electrofishing is 

 done), it is gratifying to note that the increased catch of sauger, starting in 1979, is 

 verified independently by the IDOC sampling program and by the development of a sport . 

 fishery for sauger in the Upper Illinois River (Figure 9). Fish sampling begun in 1990 as 

 part of the Long Term Resource Monitoring Program (LTRM) likewise shows that a 

 substantial population of sauger occur in the La Grange Pool of the Illinois River, just 

 below the navigation dam at Peoria (Figure 1). Although the increased catch of sauger, 

 like the Wr values, show»an improvement in the fish populations that may be related to 

 improved water quality in the last decade, the recent decline in numbers of sauger and in 

 Wr since 1985 may be an early warning of impending problems, and an incentive to 

 determine and correct the causes. 



