One of the important pre-requisite for undertaking brackishwater aquaculture is the selection of suitable site. The site has to be selected by taking into consideration topography, soil type, tidal fluctuation, wave action, flood drainage, cyclonic hazards, seed abundance and other ecological, biological and environmental factors. Tidal mud flats, swamps, marshes and other low lying estuarine areas are generally suitable for brackishwater farm construction. The topography of the site should be flat or gently sloping towards the outlet. For construction of fish ponds, a site with slight depression may be ideal. For the entire farm, earth work i’n excavation should, as far as possible, balance with the earth work in filling for raising the peripheral and internal dykes. Water supply and drainage should preferably b accomplished by gravity.
Soil types of salt affected areas occuring in the coastal tracts show variation in different maritime states. The soil of the prospective site must be water retentive. Porous soil should be avoided as such soils will neither retain water nor permit compaction. However, the coastal saline soils, in general, are of moderate to heavy texture which are likely to retain water for all practical purposes.
Primarily, a constant or atleast periodic supply of brackishwater in spring tides, must be available to the ponds throughout the year. This factor facilitates water exchange in fish ponds to replenish water loss through seepage and evaporation and also to maintain the hygiene of the pond bottom. The water flow pattern and direction,rise and fall of tidal amplitude at the selected site should be ascertained before designing a farm. Where heavy evaporation takes place, freshwater from ground source or any other perennial source, is required to be added in the fish pond for preventing ecalation of salinity. High rate of precipitation and freshwater run-off, on the other hand, may cause sudden fall in salinity in brackish water pond leading to heavy mortality of penaeid prawns which needs due consideration in designing a brackishwater fish farm.
Silt load of the water source to be used for farming, should be kept in mind before site selection as heavy silt laden water are detrimental to brackishwater fish nfarming, both from physical and biological point of view. When silt laden tidal water enters into the confined area of the pond, the silt gets deposited at the pond. If a site is found otherwise suitable, except for siltation problem, it is recommended to provide a silt trap of special design inside the farm near the main sluice gate and the silt thus trapped may be removed periodically. If cyclone and flood prone areas are selected for farm construction, nece.ssary precaution has to be taken by raising stronger peripheral dykes and providing bypass channels for excess flood discharge.
- Central Inland Fisheries Research Institute, Barrackpore