cannot explain the decline in the condition of commercially important 

 species such as carp. 



A decline in the size and condition of commercial fish indicates a 

 problem in the food supply. Starrett (1972: 151) studied the food 

 habits of carp in the Illinois River in the 1960's and found that in 

 the study reach of the Illinois River fingernail clams comprised 50% of 

 the volume of food items in carp stomachs, whereas in the middle and 

 upper reaches of the river only one fingernail clam was found in all 

 the stomachs examined. Figure 3 shows that in 1963 the condition 

 factor of carp was considerably better in the study reach of the Illinois 

 River than in the middle and upper reaches. More recent data used 

 in Figure 4 show that there was a good correlation between condition 

 factor of carp and the total abundance of bottom fauna in the Illinois 

 River in 1975. Figure 3 demonstrates that there has been a general 

 decline in the condition factor of carp in the whole Illinois River 

 since 1963. 



Prior to the 1950' s the greatest harvest of commercial fish gen- 

 erally occurred along the middle reach of the Illinois River (mile 80.2 

 to mile 210), in areas where food organisms, such as fingernail clams, 

 were most abundant (Richardson, 1921b: 462-465). Paloumpis and Starrett 

 (1960) documented a die-off of fish food organisms in this reach of the 

 Illinois River in the mid-1950' s (see the section of this report on 

 Illinois River benthos) . Some unidentified factor apparently eliminated 

 the fingernail clam and other benthic organisms from the middle reach of 

 the Illinois River, with a consequent effect on bottom-feeding species 

 of fish, many of which are commercially important, such as carp, buffalo, 

 and drum. In contrast to the middle reach of the river, the commercial 

 harvest from the study reach of the river has been relatively constant 

 since 1950 (Table 11). Starting in 1962, the study reach has consistently 

 ranked second in production among the four pools with commercial fisheries 

 (Table 11). 



An annual electrof ishing survey of the Illinois River by the 

 Illinois Natural History Survey has show, that the number of channel 

 catfish taken from the Alton Pool of the Illinois River consistently 



61 



