1984 were damaged. All the damaged mussels (14 of 75 - 18.7%) 

 collected at the plot were found in fall after use of the area by 

 recreational boaters. 



The next highest damage rate was in the grounded plot where 

 107 of 952 (11.2%) mussels collected in 1984 were damaged. The 

 spring rate was 4.5% (17 of 3 74) and the fall rate was 15.6% (90 

 Of 578) . 



The spring damage rate for the plot where barges were 

 attached to pilings was 2.7% (7 of 264), the fall rate was 6.1% (2 

 of 33), and the total rate was 3.0% (9 of 297). We presume that 

 rates were relatively low in the piling plot because barges were 

 usually in water deep enough to keep them from contacting the 

 bottom and the mussels. 



The lowest overall damage rate of 2.7% (3 of 110) was in the 

 downstream control. The spring rate there was 1.8% (1 of 46) and 

 the fall rate was 3.7% (2 of 54). 



In every plot, the percentage of damaged mussels was greater 

 in the fall 1984 collection than in the spring 1984 collection, 

 indicating that a detectable amount of additional damage was 

 being done in a period as short as 3.5 months. 



Data for Amblema plicata , the most abundant species, showed 

 a similar trend (Figure 5) . Highest damage rates were in the 

 upstream control (20.8%) and grounded plots (15.0%). The lowest 

 damage rate (1.2%) was in the downstream control plot. We 

 collected no unmarked Amblema plicata in the piling plot. 



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