turbidity and silt-laden water interfere with the feeding of mussels 

 and in silty water mussels may remain closed 75-90 percent of the 

 time (Ellis, 1936: 40). 



The passing of barges can produce water level drawdowns which 

 expose bottom-dwelling organisms such as insects, snails, and clams 

 (Thomas and Sparks, unpublished). Following a protracted period of 

 unusually high water levels in 1972 and 1973, an Illinois Natural His- 

 tory Survey crew examined a bed in the fall of 1974 at river mile 

 106.5 (upstream from the project area), where mussels were being 

 regularly exposed as towboats passed, and where some mussels had 

 recently died (Sparks, 1975a: 3). 



Clams will close their shells and snails will withdraw into their 

 shells when exposed this way, thus disrupting their normal activities 

 such as feeding and respiration. If the animals do not open regularly 

 to feed and respire, they will eventually die. Growth and reproduction 

 are probably slowed by levels of disturbance that do not result in 

 death. Some species of mollusks respond to gradually falling water 

 levels by burrowing into the mud or retreating to deeper water. Based 

 on our observations at mile 106.6, it seems that some mussels do not 

 exhibit this adaptive response to repeated short-term exposures. 



In conclusion, the mussel fishery in the study area of the Illinois 

 River declined due to pollution and overharvesting prior to the com- 

 pletion of the nine-foot navigation system. The dams associated with 

 the nine-foot channel and the reduction of diversion from Lake Michigan 

 probably also affected the mussel fauna by reducing the current velocity 

 and increasing sedimentation in some areas. Dredging and spoiling opera- 

 tions to maintain the navigation channel can destroy mussel beds. The 

 increase in boat traffic which resulted from construction of the nine-foot 

 channel has probably affected mussels. Barges resuspend bottom sediments, 

 temporarily draw water away from shallow areas as they. pass, and produce 

 wave wash along the shores. Large pleasure boats also cause pronounced 

 wave wash along the shore. All these disturbances can adversely affect 

 mussels. 



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