Brackishwater fishfarms are properly designed and laid out if all the pond compartments, canals and the water control structures mutually complement each other, thereby resulting in an optimum efficiency particularly in water management. Prawn earth ponds require among other things that:
- the ponds can be drained dry after harvest. This is to enable the fishfarmer to prepare/recondition the pond soil, level the pond bottom and eliminate the predators.
- water exchange in the required amount in percent of the pond water volume done at specified times during the culture period, is the basic requirement in management.
The lack or absence of a proper plan, details and a workable design for a coastal prawn pond for a site type has often resulted in poor production.
There is a need to modify the layout of farms now under construction in view of improved and accepted methods and techniques of prawn culture, in addition to improving the structural stability of the farm components and of water management by use of water control structures.
For instance, it is suggested that construction of the Polekkuru and of the other farms now under construction be stopped until revisions on plans and on designs have been done and that work can continue only when a qualified engineer will have been assigned to supervise the work of the contractor in accordance with material and work specifications.
- The pond compartments of fishfarms for prawn pond culture
shall preferably be rectangular in shape with the
long to short dimensions of the rectangle to be in a
ratio of 2 to 4:1, as this shape provides effective water
exchange. To accomplish removal of old pond water
and replenishment with new water, the pond shall have
gates at each of the opposite short sides.
The inlet and outlet gates shall preferably be designed
to handle filling of the ponds to the desired depth and
evacuation of all pond water in the shortest possible
time, respectively.
- Production ponds, particularly the grow-out ponds would have filling and drainage systems permitting them to fill and drain independently of the other ponds. To be able to accomplish this requirement, there shall be independent supply and drainage channels that serve the ponds. Complete and rapid pond drainage is an essential requirement. This is preferably accomplished by gravity flow.
- For the ponds which have permeable bottoms, if they have to be remodelled and improved, means may be provided to reduce seepage losses. The pond bottoms lack the proper amount of clay to form a seal.
The pond bottoms are sealed by laying a clay blanket of suitable clayey material, such as bentonite, said to be available, that may be hauled from an identified borrow area and from which the material can be (economically) hauled to the farmsite. The clay blanket also provides seal for the dikes.
- Old pond water removed and drained through a drain channel must not mix with (new) water supplied to the ponds. This feasibility, in the case of the Narakkal farms, may have to be looked into further. For instance, the Appangad creek will supply water to the ponds and the Nagambalan creek to serve as pond drainage outfall.
- To be able to provide effective water management, the use of water control structures is necessary. Main, secondary and tertiary gate structures are a must in farm design.
- Collar plates of wood or equivalent material placed at the sides of wooden gates prevent streaming of water through the gate-bund interface. Necessary works to prevent seepage and piping at wooden gate bottoms shall be included in design of wooden gates.
- In the case of the Narakkal farms, a central and narrow canal may be constructed separating the farm block into two pond subgroups. This canal will provide water holding facility for water pumped from the Appangad creek. A main gate is provided and the pump and pumping facility is integrated into the gate design.
- There has been no information as to how polluted are the backwaters of Cochin Taluk. Pending availability of this information, it is suggested that areas at places north of the Kerala coast be looked into and the site evaluated for suitability for development into a pilot fishfarm for Type V.
- The nursery ponds should be arranged to be in the vicinity of the grow-out ponds and having earth dikes, preferably but not necessarily, in a manner to permit direct transfer of the fry to the grow-out ponds with the water from the nursery ponds. In general, no flow through is necessary but provisions must be made to compensate for water loss.
- All ponds must be filled to the desired pond water depth of 1.00m. At this depth the adverse effects of temperature and of salinity are stabilized. The gates must have total water seal and water loss due to evaporation must be replenished.
- In the case of tide-fed farms, the local pattern of the tide curve upon which fishpond design is based can be obtained from an analysis of the tide curve or the design tide curve at the farmsite.
The bottom elevation of the ponds is the primary consideration in the design and is based on the design tide curve but without sacrificing drainage by gravity flow. On the economic side of construction particularly at areas where tidal amplitude is high but such is limited at the farmsite, there is the need to strike a balance between excavation or filling work and the tidal range. For a site with a relatively high ground elevation, it is possible in a specific instance that pumping water to the ponds can prove to be more economical than excavating the soil to the desired elevation.
- There shall be, in the design of a farm, a main water control gate with properly designed components as the floor, apron, cut-off walls, side or breast walls, wing wall, catwalks, flashboards, screens, and, if a pump facility is integrated, shall at least have two doors and to have a sump and facility for screens installed at and before the pump.
- The contractors doing the Assangao and the Polekkuru farms should be advised of the need to build the dikes in three horizontal stages and that the next and upper layer shall not be placed until the lower layer shall have been properly compacted and inspected. To accomplish compaction to maximum density, the soil layer shall be at “optimum moisture content”4.
- There shall be strict conformance to materials and work specifications drawn as part of the contract documents. The supervising engineer shall be held responsible in the carrying out of the work according to the specifications.
- The dike side slopes shall be hand compacted while wet and shaped to the specified side slope.
- The Ratnagiri farms (Type III) may be remodelled to improve management and increase production. The area, being small (4.5 ha) may not meet requirements for a pilot model, however.
Mangrove areas in the vicinity north of the farm at Satherter village were seen to have potentials for development into aquaculture ponds for prawn. Inquiries made revealed about 80 ha, of approximately 100 m width west of the Sague creek is available for development. Tidal amplitude at the site is 1.2 m; the salinity ranges from 20 to 40 ppt and the area is relatively free from pollution. Tide-fed farms could be designed for pond development in this area.
- Due to conditions under which coastal soil forms, iron pyrites (laterite soils) accumulate. As long as these pyrite containing soils remain submerged, they are subject to little change. The case of the Ratnagiri farms is a case in point.
The pond bottoms must be drained dry to oxidize the pyrites and successively be flushed to leach. The Ratnagiri pond dikes were built from soils of these types, the dike soil are subject to oxidation, and during the monsoons sulfuric acid, active aluminum and iron are washed into the pond with the eroded soil creating water quality problems.
A detailed survey on soil conditions is well advised before construction in acid soil areas.
- For new construction, percolation rate and permeability measurements are a must (Refer to Annexures E & F). The extent of water loss through the pond bottoms and through the dikes affect both design and management. Soil amendments may be resorted to and or a clay blanket of suitable material may have to be hauled (if economically) as a practical solution to coastal sites ot Types III, IV and V.
- In the case of farms designed with an earthen reservoir for water supply, freshwater or slightly brackishwater is pumped from shallow or deep wells located on or near the site, and freshwater is pumped into the reservoir to obtain proper salinity.
