282 THE MOSQUITOES OF NEW JERSEY 



merit whatever of the sheet piling seems likely to initiate the process of 

 washing out. Not only must the piling fit but they must be very firmly 

 bound together in single straight lines or they do not stand the strain. 



The problem of preparing a proper dike, sluice boxes, and tide gates 

 for draining a given area is an engineering one. Suffice it to say that the 

 trenching, diking, sluicing, and tide gating must be so planned as to 

 keep out all but the most extraordinary high tides and to free the sur- 

 face from water within five days after a heavy rainfall. 



The last step in the initial treatment of this type of marsh to prevent 

 maturing of mosquitoes is the arrangement of the drainage system and 

 the manipulation of the tide gates in such a fashion that tide water with 

 its supply of killifish can be brought into the area and caused to circu- 

 late (without overflowing the ditches) throughout the system and again 

 escape from the area. Experience has clearly shown that failure to pro- 

 vide for this circulation is followed by breeding in stagnant water in the 

 ditches. This requires the extensive use of oil or larvicide which cannot 

 usually be made sufficiently complete to prevent the emergence of all 

 of the mosquitoes. 



At this point the danger which marsh enclosure involves should be 

 emphasized. If the brackish water is withdrawn from the marshes and 

 kept off too continuously the salt grasses will die, their deep, penetrat- 

 ing roots will rot, the particles of soil will come closer together, and the 

 surface of the marsh will lower to such an extent that removal of water 

 by pumping may become necessary. If the gates are kept completely 

 open from the first of November until the end of the following March, 

 especially if the circulatory system above described is inaugurated, the 

 strength of the salt grasses sod is not likely to be impaired. 



The second type of salt marsh, enclosed, shrunken, but non-polluted 

 basins, must either be covered with tidal water and schools of killifish, 

 or the water removed by sluices and tide gates supplemented by pumps. 

 Numerous examples of this type of enclosed marsh exist along the coast. 

 The examples are usually where the marsh, which has been diked, 

 sluiced and cultivated, has been allowed to fall into neglect. The dikes 

 have been breached and the sluices have rotted away, and ordinary tide 

 covers the land regularly. In such places no breeding occurs except 

 along the edges of the highland where the mosquito breeding water is 

 screened by aquatic vegetation from the fish. 



When dikes have been erected to keep the sea out, sluices and tide 

 gates will be found useful to remove the surplus water, lowering the 

 water level to about one foot above mean low tide. Pumps must be in- 

 stalled to remove the balance because the surface is likely to be covered 



