Fish Population Adjustment 153 



poisoning. In such a combined operation the section of lake receiving 

 complete treatment should represent 20 to 80 per cent of the total lake 

 area (Figure 6.8). In calculating the dosage of rotenone to be used, the 

 part of the lake to receive complete treatment should be dosed at 1 ppm— 

 the remainder being given a marginal treatment with an amoimt required 

 to treat it at the rate of 0.25 ppm. This marginal amount is sprayed within 



Figure 6.7. Canvas "fence" used in partial poisoning of a lake. Water to 

 right of canvas was treated with sufficient rotenone to kill all of its fish. Several 

 fish in the "live box" on the left side of the canvas were unaffected. 



a 20-foot strip parallel to the shore and completely encircling the lake 

 exclusive of the section receiving the heavier dosage ( Figure 6.8 ) . 



When a sectional poisoning is combined with marginal treatment, the 

 fish collected from the sectional part may be considered a representative 

 sample of the entire population of the lake. From the kinds and sizes 

 of fish in this sample it may be possible to estimate the extent of artificial 

 cropping in the current treatment and what further measures may be 

 required to thin out the more severely stunted components of the pop- 

 ulation. 



In planning the partial poisoning it is well to calculate the dosage, order 



