Natural Enemies i8i 



In ditches, streams, ponds, and at the edges of 

 lakes and rivers the less the amount of debris, 

 grass, algae, or other obstructions, the more useful 

 the fish become. It follows that in cotmtries 

 where rank vegetation and algae are produced 

 rapidly, fish are less reliable as destroyers of 

 mosquito larvse than in more northern climates. 



During the first American anti-malaria cam- 

 paign at Havana, fish were of greater assistance, 

 and reduced the mosquito propagation more than 

 in the Canal Zone. Large amounts of finely 

 divided debris with bits of twigs and leaves are 

 washed down the streams during heavy downpours 

 of rain and collect in the lakes and quiet parts of 

 rivers. The constant winds collected this material 

 and concentrated it. At these places small fish 

 were nearly always to be seen, and darted about 

 catching the larvae as soon as the sheet of debris 

 was stirred up or disturbed. By dipping out and 

 stirring a small portion of it in a white enamel 

 pail Anopheles larvffi in all stages of development 

 were seen as weU as large pupae; it was evident 

 that the fish caught but few of the larvae so hidden. 



When portions of green algae are detached from 

 the stream bank, fish invariably follow the float- 

 ing mass and work hard for the few larvae they 

 catch. They cannot penetrate the mass, nor 



