100 |- 



o 



E 

 8 



a> 



> 

 o 



E 



4> 

 « 



£ 



CONTROL o 



0.01 ppm A 



0.1 ppm A 



1.0 ppm ■ 



60 90 120 



TIME (minutes) 



180 



Figure 9-4. 24-Hour Recovery: Comparison of Filtering 

 Activity of Mytilus edulis After 24 Hours of Recovery from 

 Two Weeks Exposure to W.A.F. No. 2 Fuel Oil. 



Gradual improvement of all groups was observed as the animals remained in 

 clean water. After two weeks, recovery was almost complete (Figure 9-5). The 

 control and 0.1 ppm groups both filtered at an accelerated rate of 30.9 ml/min, 

 and the 1.0 ppm group filtered at a rate of 17.2 ml/min, characteristic of 

 pre-exposure rates. The higher filtration rates observed may reflect increasing 

 ambient water temperatures at the time of testing. Temperatures increased 

 from 11 °C at the time of pre-exposure testing, to 19°C during this latter 

 testing period. Due to a laboratory failure resulting in reduced water flow, and 

 subsequent anaerobic conditions in the recovery tank which held the 0.01 ppm 

 exposure group, it was necessary to discard these mussels without 

 demonstrating their complete recovery. However, since mussels at higher oil 

 concentrations did recover, it seems reasonable to assume recovery for the 0.01 

 ppm exposure group as well. There remained a significant difference between 

 the 1 ppm exposure group and the controls after two weeks in clean water; 

 however, after one month of recovery, they actually fed slightly better than 

 the controls. 



Based on the current investigation, it appears that the adverse effect of oil 

 on filtration rate of mussels is reversible, provided the stressed animals are 

 returned to unpolluted conditions. However, the data also strongly suggest that 

 recovery does not occur under conditions of continued exposure. Further 

 investigation is currently in progress to determine the implications of reduced 



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