The commercial catch of carp in the Alton Pool has 

 changed little since 1950.... two factors — loss of 

 fingernail clams plus low dissolved oxygen — could 

 explain the dearth of commercial-size carp in the 

 middle and upper reaches of the Illinois River." 



Mills, et al., (1966) also used carp as an indicator of the effects 

 of pollution: 



"These are two noticeable effects of pollution on this 

 species. First, the length-depth ratio of individuals 

 goes up with increasing pollution. By dividing the 

 depth into the standard length, an index is obtained 

 which, if 3 or greater, indicates that the fish is too 

 thin for commercial uses. Any index under 3 would in- 

 dicate a satisfactory commercial fish. Second, carp 

 exhibit a rachitic bone malformation (an abnormality 

 characterized by malformed heads and gill covers) 

 known as a "knothead" condition. This becomes more 

 conspicuous with increased pollution." 



The study suggested a relationship between the die-off of finger- 

 nail clams that occurred in the middle section of the Illinois River 

 in 1955, and the decline on carp, which like other bottom-feeding 

 fishes, find fingernail clams a nutritious food. Mills, et al. , (1966) 

 found that the length-depth ratios of carp in the 1963 electrof ishing 

 collection indicated a marked difference between fish taken above and 

 below Beardstown (mile 88.5). Carp caught below Beardstown had ratios 

 less than 3, while above Beardstown the ratios were 3 or more. The 

 difference appeared to be attributable to the loss of fingernail clams 

 above Beardstown. The Sangamon River enters the Illinois River at 

 Beardstown, and may dilute some type of pollution which is responsible 

 for the absence or paucity of fingernail clams in the middle and upper 

 sections of the river. In 1973 and 1974, the length-depth ratios of 

 carp appeared to show the same trend of relatively thinner fish above 

 Beardstown as in 1963. More will be said in Part IV about the impact 

 of the die-off of fingernail clams in 1955. 



In 1973 and 1974, the incidence of knothead among carp was re- 

 markably reduced in the upper river compared to 1926-27 and 1963. In 

 the older collections, more than 50 percent of the carp taken above 



2 5 



