In the selected sites, pond, areas from 0.5 ha and above are to be excavated to hold a water depth of 80-180 em. Peripheral dykes are to be constructed with compact soil to withstand external tidal pressure during spring tide period.
Basically, there is the difference in tidal amplitudes in higher latitudes compared to lower latitudes, showing a declining trend from north to south. In the gradient zone of the Hooghly estuary, the high water tide reaches more than 5 m during spring tides whereas, at Kakinada coastline, the tide reaches up to 1.43 m height only. Therefore, the height and width of the dyke should be finalized after taking into consideration the tidal amplitude at the farm site.
The sluice is an integral part of the dyke system of a brackish water farm. In the dynamic tidal system with diurnal changes in tides and current pattern, a master sluice of strong design and construction is necessary. This would be the controlling centre for the water management of all the farm ponds fed through feeder channels. The volume of water required to pass through the sluice in a specified time (i. e. duration of the tide) will determine the width of the sluice. Material and type of the sluice will depend mainly upon the nature of the soil.
In view of low consistency of the soil, light wooden structures are preferred. The wooden sluice with an open top is preferred over the closed box as a heavy column of silt which usually gets deposited by intake of tidal water, can easily be cleared from the open top type. The subsidiary sluice of the individual fish pond of the farm should be provided with adjustable shutters so that by adjusting the shutters, water from different levels can be taken into the fish pond and drained out as well to prevent excessive siltation. The sluice should be placed over the mud bottom strengthened by wooden pins.
1.Culture pond preparation
Newly excavated brackishwater pond bottom is generally hard and low in organic matter. To such bottom initially a heavy dose of decomposed cow dung manure ( @ 5000 kg ha ) is to be applied and ploughed thoroughly to make the bottom suitable for culture of prawns and fishes. Ponds already under culture are prepared by draining out water after total harvest and drying the bed to eradicate unwanted and harmful organisms. The drying also helps in enriching the pond soil by oxidation of metabolic wastes and decaying organic matters accumulated in the bottom mud. When complete drying is not practicable, the water level is kept as low as possible to apply either mohua oil cake @ 200-250 ppm or tobacco waste @ 100-150 kg ha or tea seed cake @ 500-1000 kg ha for the eradication of predatory fishes and other unwanted organisms.
Repeated raking of the bottom and application of lime @ 300 kg or more ha, depending on the soil pH, will help to remove obnoxious gases and accumulated metabolites. After initial fertilization, tidal water is drawn into the pond and allowed to stand at a depth of 10-15 cm for a few days till the formation of a thick layer of benthic algae and associated microorganisms. The water level is then increased to 60-80 em by intake of fresh tidal water and stocking is accomplished.
2.Fertilization and manuring schedule
Benthos being the chief food constituents of cultured brackishwater fin-fish and shell-fish, soil fertilization instead of water fertilization is more effective in coastal aquaculture. Initial analysis of the pond soil reflects the nutrient deficiencies in the soil. Application of deficient nutrients correct the soil for better productivity. However, it is often safe to apply both organic manure and inorganic fertilizer at the same site since the combined action generally produce advantageous results. In saline soils response to phosphatic fertilizer is generally not well pronounced.
However, in alkaline pH, frequent application of super phosphate at low doses and in acidic soils, rock phosphate at 600 kgfhafyr is likely to enhance the productivity. When nitrogenous fertilizers are to be applied, ammonium forms should be used to ‘increase productivity of fish ponds. As organic manure, well decomposed chicken manure is preferable because of its potential value. If chicken manure is not available, well decomposed cattle dung at a dose 3 times more than the chicken manure may be used. The following fertilization schedule may be followed for optimum results. However, depending on the basic properties of vergin soil of a particular farming site, the fertilization schedule prescribed hereunder may be modified particularly in respect to inorganic fertilizers.
Quantity Initial dose
kg I ha Subsequent dose monthly
|Cowdung||750||150||As a substitute when poultry
manure is not available.
|Rock Phosphate||120||50||As a substitute for super
phosphate in acidic soil.
- Central Inland Fisheries Research Institute, Barrackpore