Village ponds, homestead or backyard kitchen ponds, garden or farm ponds, irrigation ponds and occasional ponds such as brick mine pits and quarries, etc., occupy enormous freshwater areas in the tropics and are used for fish culture with minor improvements. However, ponds designed and constructed for fish culture are easier to manage and are expected to give higher production.
Although certain well-defined guidelines do exist for the construction of fish ponds, it is mainly the topography of the site which determines the basic design of the pond/farm. There are, however, certain basic principles to be considered when choosing a site and deciding the method of pond construction.
Selection of suitable sites for fish farm construction is very important. The following three essential conditions guide the proper site selection:
Topography
Source of water and its quality
Soil type
It is economical and convenient to construct ponds in waterlogged areas, irrigation command areas or in marginal lands. In such areas construction cost is relatively low mainly due to limited earth cutting. For example, a pond of 100 m × 40 m (0.4 ha) of water area requires only 3 234 m3 of earth to construct around a dyke of 2 m high above ground level (GL) with side slope ratio of 2:1 and top width of 1.5 m. This quantity of earth may be obtained only from 1.1 m depth of cutting. This limited depth of cutting reduces the construction cost considerably. However, full consideration should also be given to the possible effects of flood. The surface features of the area proposed for the pond or the farm is also equally important. A saucer-shaped area may be an ideal site for a large dug-out pond, because it may hold appreciable quantity of water with a small amount of earthwork.
For smaller and flat areas eye estimation is enough, but for a big area proposed for farm construction with a number of ponds for different purposes and of different sizes, it is essential to conduct contour survey for determining the topography and land configuration. The site should be easily approachable so that there may not be any difficulty in the transportation of input materials and in the marketing of the produce. The labour and materials required for construction and operation should also be locally available as far as possible. From an efficient management point of view the pond site should, if possible, be within the sight of the farmer's house. It also reduces the risk of poaching. Siting fish ponds near the farmer's other agricultural or livestock farming activities makes it easier to integrate all the farming activities.
A dependable source of water supply must be available within or near the site, even for undrainable ponds. However, unlike drainable ponds, undrainable ponds require just sufficient water to fill the ponds and to compensate the water loss through seepage and surface evaporation thereafter. Equally important is the need for avoiding excess water and hence there must be arrangement for the excess water to escape through a bypass channel or a spillway. The water supply to the pond should as far as possible be natural, preferably rain water. However, alternative arrangements of water supply should be made for dry season either from a deep tube well or irrigation canal or from perennial sources like spring, stream, river, etc. Ponds should be on the lower lands to allow accumulation of surface runoff from a larger catchment area. However, care should be taken to provide proper bypass or spillway to avoid flooding. A higher subsoil water table due to irrigation in surrounding fields and percolation from artificial or natural channels, in addition to absorption from rain water, also helps in maintaining water level in undrainable ponds (Sahoo, 1984).
The quality of the available water is also equally important for fish culture. Pond fish production is influenced by the physical and chemical properties of the water. Water should be clear as far as possible. Turbid waters which carry suspended solids cut the light penetration, thus reducing primary productivity of the pond. Excess of suspended solids also adhere closely to the gill filaments and cause breathing problems. Water temperature also significantly influences the feeding and growth of fish. Prevailing water temperature, ranging between 15°C and 35°C in tropical areas, is most suitable for carps. The chemical quality of water depends on its content of dissolved salts. Rain water does not carry any dissolved salts. However, it collects nutrient salts from the ground surface of the catchment area. The water should be neither too acid nor too alkaline; neutral or slightly alkaline waters are most suitable for fish culture and hence acid water should be limed to make it neutral. Waters with pH values below 5.5 or over 8.5 are not proper for fish culture. The farmer will need huge quantity of lime to neutralize it while highly alkaline water may cause the precipitation of both phosphate and iron, and if it remains continuously above pH 9, it may be harmful to fish.
Pond soil must retain water. Soils with a low infiltration rate are most suitable for fish pond. Table 5 shows the filtration rate of different types of soils. The best soils for our purpose are thus the impermeable clay which can be easily compacted and made leak proof.
| Soil type | Infiltration rate (mm/ha) |
|---|---|
| Clay | 1–5 |
| Clay loam | 5–10 |
| Silty loam | 10–20 |
| Sandy loam | 20–30 |
| Sand | 30–100 |
Loamy soils can also be used, but they need well compacting, and may leak slightly in the early stages, although they tend to seal themselves with time. Sandy and gravelly soils should be avoided, but if they are the only ones available they must be made impermeable with a thick coating of clay or with polythene sheeting. Soil impermeability can also be achieved by soil compaction at the pond bottom and dyke with either a mixture of soil + 1–5% cement or soil + 10–20% cowdung. Treated areas should be kept moist for 2–3 days by gently sprinkling water to avoid cracking and finally the pond is filled with water (Sahoo, pers.comm.).
Peat soils have special problems, since they are usually very acidic in nature and need sufficient liming, while the organic matter decomposition may lead to dissolved oxygen deficiency. Soils rich in limestone also create special problems, since the excessive lime content tends to precipitate phosphate and iron. Such ponds would then have little plankton population and macrophytes and would be relatively sterile. This can be overcome by adding sufficient organic matter such as cowdung, poultry manure, etc.
A general and convenient field test for the soil quality is to take a handful of moist soil from the test holes made at the proposed site and to compress it into a firm ball. If the ball does not crumble after a little handling, it indicates that it contains sufficient clay for the purpose of pond construction. Accurate determination of the composition of the soil and its water-holding character is possible by hydrometer method. Several test holes may be made across the site and soil samples may be collected vertically from every 0.5 m of depth reaching up to a level of 3–4 m in a test hole. Using the results of the soil tests, a soil profile chart for the proposed site may be drawn. An arbitrary soil profile chart is presented (Fig. 12) showing the presence of clayey soil up to a depth of 3.5 m.
Based upon the survey on topography, soil type, water supply, etc., the detailed designing and layout of the ponds/farm are done. However, the following additional points are also to be considered.
The production or stocking ponds are stocked with large size fingerlings of about 10–15 cm size in the case of composite fish culture. To attain this size, the hatchlings are reared in much smaller and shallower ponds called nursery and rearing ponds for about 2–3 months. In the nursery ponds the hatchlings are reared up to fry stage and in the rearing ponds the fry are reared till fingerling stage. The ratio of water area among nursery, rearing and stocking ponds in a fish farm depend upon the basic objective of the farm. In case of a fish seed farm, only nursery and rearing ponds are to be constructed with a small area for few stocking ponds to be used for raising the brood fish, while in the case of fish production farm only stocking ponds are to be constructed for producing table size fish from fingerlings. The layout of a complete farm is given in Figure 13.
There is no hard and fast rule regarding the size of a pond. However, nursery ponds should be small and shallow. Ponds having 0.02–0.06 ha water area and 1–1.5 m depth are most suitable as nurseries. Rearing ponds are relatively larger, preferably between 0.06 to 0.10 ha in size and 1.5 to 2.0 m in depth. The sizes of stocking ponds vary tremendously. For newly constructed undrainable ponds, total water area of 0.25 to 1.0 ha is recommended (Table 6).
Figure 12. Soil Profile
Figure 13. Layout of a Fish Farm (Land area 3.6 ha)
In shallow ponds the water becomes heated easily. In deeper ponds light cannot reach the bottom. In very deep ponds thermal stratification may occur with colder deoxygenated bottom layer. Dead plankton and faecal matter from fishes may fall on the bottom layer where the nutrients may be locked up. However, in case of rain-fed areas where the water table goes down during the dry season, the depth should be kept around 3.0 – 3.5 m to store more water during the rainy season.
Although a square pond is economical to construct for its minimum length of dyke, a rectangular shape of the pond (length:width in proportion of 3:1) is considered to be ideal. In any case the pond width should not exceed 30 to 40 m as it is difficult to operate a fishing net in broader ponds. The nursery and rearing ponds may be square, since they are too small to pose any problem for netting. The corners must be curved to avoid fish escaping the net during harvesting. The layout plans of nursery, rearing and stocking ponds are given in Figures 14A and 14B.
| Pond type | Size (ha) | Depth* (m) | |
|---|---|---|---|
| Irrigated command/water logged areas | Rainfed+/non-irrigated areas | ||
| Nursery pond | 0.02 – 0.06 | 1.0 – 1.5 | 1.5 – 2.0 |
| Rearing pond | 0.06 – 0.10 | 1.5 – 2.0 | 2.0 – 2.5 |
| Stocking pond | 0.25 – 1.0 | 2.0 – 2.5 | 2.5 – 3.5 |
* Excluding the freeboard
+ May vary depending on impermeable strata at pond bottom
The dyke should be properly designed so that it can hold maximum water in the pond and withstand the hydraulic pressure. The slope of the dyke usually depends on the type of soil. Suitable side slopes for different soil types are given in Table 7.
Figure 14a. Design of Nursery, Rearing and Stocking Ponds
Figure 14b. Cross Section Details of Ponds
| Soil type | Soil (horizontal:vertical) |
|---|---|
| Clay | 1:1 to 2:1 |
| Clay loam | 1.5:1 to 2:1 |
| Sandy loam | 2:1 to 2.5:1 |
| Sandy | 3:1 |
Provision for a berm of sufficient width may also be provided for stabilizing the slopes. A wider berm also helps in operating the net in the pond. The berm should be 1 m or more in width (Saha and Gopalakrishnan, 1974). The top width of the dyke should be decided taking into account its usage. Usually the minimum top width of the dyke should be 1.5 m. The wider crest requires not only a larger area for dykes, but also an increased amount of earth material involving heavy expenditure. It is always wise to design the dyke as per the quantity of earth expected to be available from excavation work. A soil-type containing approximately 25% silt, 35% sand and 40% clay is most suitable for dykes. However, if excavated soil quality is not up to the above standard, provision may be made for a clay core to make the dyke watertight. While designing, about 10–12% allowance may be given for settling of earthwork (Fig. 15).
Before initiating the construction work, proper estimates have to be prepared based upon the design details, which will include the cost of all the materials and the labour. Strict supervision is required at every step of construction to ensure the adherence to specifications laid down in the design.
If the construction work is taken up at the most appropriate time or season of the year, the work becomes easier and economical. The best time of the year for constructing ponds in clayey soil is post-rainy period and winter when the soil is soft rather than at the end of the dry season when it is very hard. For swampy and waterlogged areas the most desirable time is the late summer when the area becomes completely dry. However, if a pond is built during winter or early summer and is not filled immediately, weeds may grow and cover the bottom. In such cases deweeding is needed before filling the pond.
Figure 15. Design of a Dyke with Core Well and Key Trench
The site should be thoroughly cleared of all the trees, bushes, etc. Even the roots of trees should be removed. No woody material should be left because the same will eventually rot and cause leaks. Some tree trunks rot very slowly and may cause problems during netting.
This operation involves laying out the features of ponds on the ground in order to mark out the areas from where the earth will have to be cut and removed and also where earth will have to be embanked. Initially, lines are drawn according to the layout, followed by pegging and fixing stakes or posts. Strings are stretched between the tops of pegs and posts to mark the complete profile of the dyke with its correct height, width and slopes (Fig. 16).
Prior to pond excavation and dyke construction, all loose surface soil should be removed from about 20 cm depth within the total outlined area of the dyke and the surface should be roughened by ploughing or digging. In order to unite the body of the dyke to subsoil, it is desirable to dig a small “V” shaped key trench (Fig.15). When the dyke is to be made on a sandy, gravelly or marshy soil base, the construction of a key trench becomes essential and in such cases digging should be done until watertight foundations are reached. The key trench is a small ditch or furrow dug along the line of the centre of the walls about 0.5 m – 1.0 m wide and 0.5 m deep. This trench is filled in with a good clayey soil and is well rammed. If good clayey soil is not available in the area, ordinary soil should be well compacted into the trench. The purpose of the trench is to stop seepage of water underneath the walls.
The excavation work can be carried out within the area marked for the pond bottom either manually or mechanically. However, the final levelling of the pond bottom and sides should be done manually with proper ramming and finishing as per the original design. The construction of the pond becomes economical if earthen dykes are made around the pond using the excavated earth from the pond bed. All dykes should be raised, dumping the earth layer by layer stretching right across the whole section, and in such cases each layer should not exceed 20 cm in thickness. All large clods should be broken and each layer should be thoroughly consolidated by watering and ramming. The sides and top of the dykes should be properly dressed and finished with wooden thappies (wooden block with handle for ramming).
In case the soil quality is not suitable for making dykes, a clay core is provided in the dyke to make it watertight (Fig.15). A mixture of 1:2 of sand and clay is used to make the clay puddle. This should be consolidated, compacted and deposited in 10–15 cm thick layers. Each layer should be adequately moistened before the next layer is laid and precaution should be taken to prevent the puddle from becoming dry and cracking.
Figure 16. Layout and Pegging before Pond Construction (Corner View)
Dykes must be well compacted to render them stable and the top should be rammed flat so that small vehicles can also run along when needed. Short creeping grass is recommended to be grown on the top and sides of the dyke. Trees are not desirable since their dense shade inhibits the productivity of the pond.
Since we are concerned here with static and undrainable ponds, a feeder stream running directly into the pond should be avoided. The feeder stream must therefore be diverted along the side of the pond and from a suitable point water is channeled to the pond when required. An inlet structure should be provided through which water can be let into the pond. A proper inlet enables the quantity of water flowing into the pond, to be regulated, preventing the entry of undesirable fish and other aquatic animals and the escape of stocked fish. For small ponds the best inlet structure is a galvanized iron pipe of about 10 cm diameter with a control tap and a screen basket (Fig. 17 A). The downstream end of the pipe should be 30–40 cm above the water level. A sluice is also suitable for this purpose, especially for larger ponds. A screen is also fixed to check the entry of undesirable fishes and other animals (Fig. 17 B). To avoid scouring when the pond is being filled, a concrete apron can be built at the sluice, or more cheaply, a layer of gravel laid down. Similarly, if water is let in with a pipe there should be a gravel bed laid down where the water stream falls into the pond. If gravity feed is not possible, water must be pumped from the supply source into the channel leading to the pond or even directly into the pond; but, in that case, the intake should be securely wrapped by a firm net to prevent undesirable fish and other animals from entering into the pond along with the water.
Proper maintenance of the pond and pond structure is most essential. Most of the earthen structures, especially the dykes, are susceptible to weathering action and hence they need periodical checks. Attending to minor damages regularly avoids the chances of more costly repairs later. The grass turfing needs special attention. Proper and timely mowing prevents the formation of weedy growth and tends to develop a root system more resistant to runoff. Erosion from the top during heavy rains causes grooving out of small channels and it is an indication that the top has not been properly consolidated. The area should be levelled with more soil and thoroughly rammed and then grass should be planted to bind it. Side erosion at the dyke bottom may be due to a number of reasons. The worst damage is done by common carp. Erosion due to frequent wave action, particularly if the grass at the edge has been grazed by grass carp, can cause undercutting of banks and subsequent collapse of dykes. Some methods used to provide protection against such erosion are earth berms, stone or brick pitching, stakes/bamboo piling (Fig. 18).
Figure 17A. View of an Inlet Structure
Figure 17B. Additional Detail
Surface washings and organic additions cause siltation which reduces the pond depth and pond fertility. The undrainable ponds should therefore be dewatered in the summer months at the interval of 5–7 years. This has already been described under Section 4.
