The Clearing of Water 191 



Another advantage is that fish and aquatic 

 insects that prey on the larvas have no difficulty 

 in capturing them and under these conditions 

 are extremely valuable in eradicating them. In 

 large ponds and at the edges of lakes where larvae 

 are frequently found it is often necessary to remove 

 all vegetation in the water, and along the shore, 

 if it wiU ultimately reach over into the water. 

 Once this is accomplished the fish will keep down 

 mosquito propagation until the vegetation again 

 becomes rank, or until algae gives the required 

 protection. The application of small quantities of 

 copper sulphate along the shore or periodical appli- 

 cations of larvacide to the small area infested with 

 algse will destroy it. Lagoons and many streams 

 of slow current contain water surface vegetation, 

 such as leaves of lilies, etc., which affords excellent 

 protection to the larvae of both Culex and A nopheles. 

 Culex are seen in large patches under these condi- 

 tions and may be so close together as to form a 

 black mass of thirty or more square feet. This 

 takes place even when there are fish. When all 

 vegetation is removed, additional fish arrive and 

 the larvae soon disappear. To sum up the advan- 

 tages to be gained, the clearing of water may in- 

 crease the stream velocity, destroy the food supply, 

 remove the hiding-places, enable fish to become 



