The majority of freshwater fish ponds in the Indian subcontinent are the dugout ponds of an undrainable nature which at times lack proper embankments. During the course of culture operations, such ponds receive huge amounts of feed, fertilizers and manures as critical inputs, and sediment particles carried down by rain water from the catchment area. A portion of the organic production in the pond also undergoes death and decay and gradually adds to the pond bottom sediment. Thus, with the advancement of time, a thick sediment layer is formed reducing the depth of the pond. They are quite rich in organic and inorganic nutrients, but due to slow bacterial action under prevailing anaerobic conditions the nutrients are almost locked up in the sediment and are not available for primary production. Further, the anaerobic decomposition of the organic matter accumulated in the sediment releases harmful gases and depletes dissolved oxygen level in the water. Thus, it becomes necessary to renovate the existing ponds periodically every 4–6 years by removing sediment from the pond bottom, redressing and repairing the dykes, etc., in order to make the ponds more suitable and to regain their fertility. For this, the following practical measures are recommended.
As soon as the water table of the area surrounding the pond goes down the renovation work can be initiated. Summer is the most suitable period for this Purpose as complete drying of the water body is possible. In this period pond renovation can be carried out efficiently and economically. Removal of slushy silt from the partially dried pond bottom is difficult, laborious and expensive.
It has been observed that most of the rural ponds are not properly managed and become weed-infested in course of time. Before dewatering the pond, large floating weeds such as water hyacinth should be eradicated by pulling them out manually or mechanically. Otherwise, collection and removal of such weeds will require more labour and time. Other rooted emergent or submerged weeds can be taken care of only after draining the pond.
Dewatering of the existing pond is possible either by draining the water after cutting a portion of the embankment or by pumping out. If the water table in the surrounding area is high, there is considerable inflow of water from the pond bottom. This phenomenon of sub-surface secretion is called percolation. In case the rate of percolation is high, several furrows or ditches may be made towards the lowest contour point where a pit may be dug out to drain all the percolated water (Fig. 11). Periodical pumping of water from the pit facilitates keeping the bed dry.
Where the bed is found to be uneven, contouring is necessary to estimate the amount of silt to be removed. It is done by taking the level measurements at certain spots on the pond bed. It will also help in redesigning the pond taking into consideration the highest flood level and maximum rain water level.
Figure 11. Drained-out pond with furrows
After complete dewatering, the pond bed is allowed to dry and develop cracks in the silt mass. The texture of silt is different from that of the bottom hard soil and cracks quickly. At this stage dried silt is cut and removed manually or mechanically and heaped at a suitable place for its utilization in agricultural fields. Where complete drying is not possible due to high rate of percolation, walking platforms made up of bamboos or wooden planks may be put on the slushy bed to facilitate desilting work. In some larger ponds it becomes difficult to dry the central portion of the pond bottom as it is nearer to underground water table. In such cases the slushy and loose silt should be scrapped and spread to the sides with the help of wooden planks tied with ropes for pulling. This helps in drying the silt and easy removal thereafter.
Derelict waters in millions of hectares, lying unutilized, are common sights in most of the South Asian countries. Such untapped water bodies with potential for aquacultural production may be reclaimed and made suitable for fish culture by adopting more or less similar procedures. In case of larger water areas, it would be better if they are connected temporarily to nearby natural or man-made drainage systems having relatively lower bed level for complete dewatering by gravity and making the entire area completely dry. However, if such topographic facilities are not available, heavy duty water pumps may be put into use for quicker dewatering.
In extensively large areas dewatering by draining or by pumping is not feasible. Moreover, the dry period of the year also may not last long enough to permit the work to be completed. It has been experienced that such areas can also be successfully reclaimed and renovated by partitioning into smaller units by raising cross bundhs, farm roads, etc. Each newly formed unit then can be dewatered, dried and desilted.
In general, rural ponds lack proper embankments. During high rainfall or peak irrigation periods in canal-irrigated areas such ponds get inundated with water from the neighbouring agricultural fields causing stocked fish to escape, predators and unwanted species to enter and at times results in mass fish kills due to pesticide pollution. Hence, provision of proper dykes is a must. Existing pond dykes should be repaired every year after the monsoon. Rats and crabs cause great harm to pond dykes by making holes. Such holes allow serious leakage and if not checked immediately, may endanger the stability of the dykes. Periodically, and especially at the time of renovation, such spots should be properly repaired by stuffing binding clay, claylime mixture or any other locally cheap cementing material. Due to poor consolidation, erosion from the top of the dyke during heavy rains usually results in grooving out of small channels. These areas should be covered with earth, levelled, thoroughly rammed and grass turfed. In relatively larger ponds, wave action due to wind also causes large-scale dyke erosion. By putting large floating aquatic plants such as water hyacinth along the sides of the dykes exposed to wave action during the windy season such erosion can be checked. Frequent erosion in steep dykes during heavy rain or wind can be avoided by strengthening the inner sides of the dykes with poles or bamboos or corrugated cement planks.
Most of the traditional pond dykes are below the required height; as a result, overflow of water occurs during heavy rains or flood. These dykes should be properly raised and the height may be kept at a minimum of one meter above the maximum water level recorded in that area. While raising the dykes, the top width may be kept at a minimum of 1.5 m with 2:1 slope (horizontal: vertical). Cutting the dyke to allow water into the pond from the surrounding area without any secured screening is a normal practice, which however creates many management problems. It is necessary to provide permanent inlet structures wherever is possible. Details about inlet and spillway structures are described in Section 6.
The silt mass is very rich in organic and inorganic nutrients making it most suitable for application in agriculture and horticulture. Being non-cohesive and unstable, it is unsuitable for making dykes as it may be washed back in the pond.
