No. 6 (1917) A NOTE ON TREATMENT OF SWAMPS, ETC. I65 



form splendid feeding grounds for the small Haplochilus and 

 these can be found in large numbers where not interfered with by 

 basket traps, etc. In the Tanjore district and up the West Coast 

 where fish food is plentiful and the paddy fields are not trapped, 

 they simply swarm with small larvae eating fish, especially 

 Haplochilus. 



ESTUARIES, BACKWATERS, SALT AND BRACKISH 

 POOLS NEAR THE COAST. 



A careful search should be made along the shores of the 

 estuaries and backwaters and all isolated pools within the malarial 

 area should either be filled in (if small) or an open trench dug into 

 the estuary or backwater to enable fish to have clear access. All 

 sea-weed and decomposing debris should be removed from these 

 pools. Fish will soon discover the larvse and rapidly destroy 

 them. At Ennore where most of the breeding places inside a 

 prescribed area were filled in by Major Ross, I discovered a salt 

 water pool on the shores of the backwater with one living mass of 

 larvae. As their breeding areas had been restricted the mosquitos 

 had evidently bred in this small pool in thousands. It was 

 shallow, not more than 6 to 8 inches at the deepest part, but was 

 cut off from the backwater by a small sandbank. I divided this 

 pond into sections and cut a channel from one into the backwater 

 deep enough to allow small fish a clear passage. I removed all 

 sea-weed and debris from both sections and examined the follow- 

 ing morning. Fish were found in the one section and most of the 

 larvae were destroyed, only a few remaining in the shallowest 

 portion ; whereas in the other isolated section they were teeming. 

 The sides of the former were sloped to make it deeper round the 

 margins and the same evening an examination proved the fish 

 had cleared the lot of larv^ out of the section opened to the back- 

 water. Backwaters as a rule when open to the sea contain an 

 abundant supply of larvicides in the way of small fish, and an 

 examination along the shores where fish have free access will 

 prove that there are very few larvae ; but, on the other hand, when 

 the bars of the backwaters are closed and netting is carried on 

 along the margins by men using small mesh nets, you will find 

 numerous mosquito larvae. The supply of fish larvicides is 

 depleted and no fresh supply can come in from the sea. In the 

 case of brackish isolated pools where it is either too expensive or 

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