FISH CONTROL 



185 



aquatic stages of the mosquito, unless protection was provided by plants 

 and debris. From the knowledge which has thus been gained, we may 

 conclude that, if a pond furnishes little or no protection for mosquito - 

 larvae, a small number of top minnows is sufficient, but, if it does 

 furnish protection, a much larger number is desirable. Anti-mosquito 

 work, however, may be started with a very small number of Gambusia, 

 for this fish multiplies rapidly. There appears to be no danger of over- 

 stocking, as observations indicate that the more fish a pond supports the 

 more certain are the practical results." 



Precisely similar considerations apply to control of streams and 

 ditches, swamps and marshes. 



Fig. 129. — A pool in which fish control cannot be entirely effective on account of 



vegetation. 



Underground concrete or brick cisterns should be stocked 

 with anywhere from 5 to 50 fish, depending upon the degree of 

 infestation, size of the cistern and other conditions. Fish, 

 particularly Gambusia, cannot be employed in metallic, surface 

 cisterns, since the water gets too hot and kills the fish. 



Low-lying culverts that hold water for any considerable time 

 should also be stocked liberally with the fish. The writer has 

 found this to be the cheapest way of handling such culverts. 



Shallow wells, where they are abundant, are frequently trouble- 

 some breeding-places. The writer has found that satisfactory 

 results follow, if from three to five male Gambusia are lowered 

 into each of them. Such wells should be inspected frequently, 

 however, as it often happens that users of the wells haul out the 

 fish with the water and, not knowing their purpose, do not put 



