section of the river and its lakes during migration 

 (Mills et al., 1966). In the 1960's fingernail clams 

 formed 50.2% (volume) of the food items taken by carp 

 collected in the river between Beardstown and its mouth 

 (Starrett and Paloumpis, unpublished). Only one finger- 

 nail clam was found in the food contents of the carp ex- 

 amined from the remainder of the river. Tubificidae 

 worms comprised only 4.3% (volume) of the food ingested 

 by carp taken from the section of the river between 

 Beardstown and the mouth, whereas in the upper river 

 (source to Starved Rock dam) , where there was a virtual 

 absence of fingernail clams, carp fed heavily upon 

 Tubificidae worms (30.5% volume). Carp collected during 

 the early 1960's from the lower section of the river, 

 where fingernail clams formed an important part of 

 their diet, were deeper bodied than those taken from 

 the remainder of the river (Mills et al. , 1966) ." 



Paloumpis and Starrett (1960) reported that the population of 

 fingernail clams in lower Quiver Lake (mile 122) dropped from 1,115 

 individuals per square foot in 1952 to 54 per square foot in 1953 and 

 to in 1954. Snail populations in lower Quiver Lake also declined 

 during this period. 



As recently as the spring of 1973, no living fingernail clams were 

 collected at several stations in Quiver Lake (mile 122-124) where dead 

 shells indicated they had formerly been abundant. 



Although the reason for the die-off is unknown, it appears that 

 the Sangamon River, which enters the Illinois at Beardstown, may be di- 

 luting some material in the river which is toxic to fingernail clams. 

 Fingernail clams may have died out in some bottomland lakes because of 

 low oxygen levels due to sediment oxygen demand (mentioned above) or 

 because flocculent sediments make it impossible for the clams to main- 

 tain themselves at the mud — water interface, or because the sediments 

 interfere with normal feeding activities. 

 Aquatic vegetation die-off s (1920 's and 1950 's) 



The backwaters and bottomland lakes of the Illinois River were 



once veritable aquatic gardens, as described by Kofoid (1903): 



"The aquatic environment at Havana impresses the 

 visiting biologist who for the first time traverses 

 its river, lakes, and marshes, as one of exceedingly 



54 



