of 1967 (Gale, 1969: 149). Populations of fingernail clams in Pool 



19 



v). 



have reached over 100,000/m 2 (Gale, 1969 



While the study reach does not receive the intensive duck use that 

 Pool 19 receives, significant populations of diving ducks do appear on 

 Pool 26 in winters (December-January) when Pool 19 freezes over (per- 

 sonal communication, Frank C. Bellrose, Wildlife Specialist, Illinois 

 Natural History Survey, Havana, Illinois, 1978). Colbert et al. 

 (1975: 613) found maximum populations of fingernail clams were only 

 268/m in Pool 24 but they may not have sampled areas where diving 

 ducks are known to congregate and feed. 



Recent Changes in the Benthos. Thompson and Sparks have documented 

 a significant decline in the fingernail clam populations of Pool 19 during 

 1976 and 1977 (1978). While clam numbers during the fall migrations 

 of 1973-1975 reached pool means (all stations) of 34, 000-47, 000/m 2 , the 

 1976 figure fell to 16, 000/m 2 and populations continued to be depressed 

 through September, 1977 (Thompson and Sparks, 1978). The cause of the 

 population decline is unknown but may be related to the low river dis- 

 charge of 1976-1977 and a deterioration in water quality (Thompson and 

 Sparks, 1978). The fingernail clam populations in the study reach of 

 the river may have been similarly affected. 



32 



