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0/16/12 Fish culture in undrainable ponds A manual for extension

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Title: Fish Culture in undrainable ponds - A manual for extension...

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6. CONSTRUCTION OF NEW PONDS AND FARMS

Village ponds, homestead or backyard kitchen ponds, garden or farm ponds, irrigation pondsand occasional ponds such as brick mine pits and quarries, etc., occupy enormous freshwaterareas in the tropics and are used for fish culture with minor improvements. However, pondsdesigned and constructed for fish culture are easier to manage and are expected to give higherproduction.

Although certain well-defined guidelines do exist for the construction of fish ponds, it is mainlythe 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 themethod of pond construction.

6.1 Site selection

Selection of suitable sites for fish farm construction is very important. The following threeessential conditions guide the proper site selection:

Topography

Source of water and its quality

Soil type

6.1.1 Topography

It is economical and convenient to construct ponds in waterlogged areas, irrigation commandareas or in marginal lands. In such areas construction cost is relatively low mainly due to limitedearth 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 ofearth may be obtained only from 1.1 m depth ofcutting. This limited depth of cutting reduces the construction cost considerably. However, fullconsideration should also be given to the possible effects of flood. The surface features of thearea proposed for the pond or the farm is also equally important. A saucer-shaped area may bean ideal site for a large dug-out pond, because it may hold appreciable quantity of water with asmall amount of earthwork.

For smaller and flat areas eye estimation is enough, but for a big area proposed for farmconstruction with a number of ponds for different purposes and of different sizes, it is essentialto conduct contour survey for determining the topography and land configuration. The siteshould 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 forconstruction and operation should also be locally available as far as possible. From an efficientmanagement point of view the pond site should, if possible, be within the sight of the farmer'shouse. It also reduces the risk of poaching. Siting fish ponds near the farmer's other agriculturalor livestock farming activities makes it easier to integrate all the farming activities.
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6.1.2 Source of water and its quality

A dependable source of water supply must be available within or near the site, even forundrainable ponds. However, unlike drainable ponds, undrainable ponds require just sufficientwater to fill the ponds and to compensate the water loss through seepage and surfaceevaporation thereafter. Equally important is the need for avoiding excess water and hence theremust 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 adeep tube well or irrigation canal or from perennial sources like spring, stream, river, etc. Pondsshould be on the lower lands to allow accumulation of surface runoff from a larger catchmentarea. However, care should be taken to provide proper bypass or spillway to avoid flooding. Ahigher subsoil water table due to irrigation in surrounding fields and percolation from artificial ornatural channels, in addition to absorption from rain water, also helps in maintaining water levelin undrainable ponds (Sahoo, 1984).

The quality of the available water is also equally important for fish culture. Pond fish productionis influenced by the physical and chemical properties of the water. Water should be clear as faras 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 gillfilaments and cause breathing problems. Water temperature also significantly influences thefeeding and growth of fish. Prevailing water temperature, ranging between 15C and 35C intropical areas, is most suitable for carps. The chemical quality of water depends on its contentof dissolved salts. Rain water does not carry any dissolved salts. However, it collects nutrientsalts from the ground surface of the catchment area. The water should be neither too acid nortoo 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 notproper for fish culture. The farmer will need huge quantity of lime to neutralize it while highlyalkaline water may cause the precipitation of both phosphate and iron, and if it remains

continuously above pH 9, it may be harmful to fish.

6.1.3 Soil type

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 arethus the impermeable clay which can be easily compacted and made leak proof.

Table 5

Infiltration rates of different types of soil (Stern, 1979)

Soil type Infiltration rate (mm/ha)

Clay 15

Clay loam 510

Silty loam 1020

Sandy loam 2030

Sand 30100

Loamy soils can also be used, but they need well compacting, and may leak slightly in the earlystages, although they tend to seal themselves with time. Sandy and gravelly soils should beavoided, but if they are the only ones available they must be made impermeable with a thickcoating 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 + 15% cement or soil +1020% cowdung. Treated areas should be kept moist for 23 days by gently sprinkling water toavoid 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

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sufficient liming, while the organic matter decomposition may lead to dissolved oxygendeficiency. Soils rich in limestone also create special problems, since the excessive limecontent tends to precipitate phosphate and iron. Such ponds would then have little planktonpopulation and macrophytes and would be relatively sterile. This can be overcome by addingsufficient 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 thetest 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 pondconstruction. Accurate determination of the composition of the soil and its water-holdingcharacter is possible by hydrometer method. Several test holes may be made across the siteand soil samples may be collected vertically from every 0.5 m of depth reaching up to a level of34 m in a test hole. Using the results of the soil tests, a soil profile chart for the proposed sitemay be drawn. An arbitrary soil profile chart is presented (Fig. 12) showing the presence ofclayey soil up to a depth of 3.5 m.

6.2 Designing

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 beconsidered.

6.2.1 Water area ratio among pond types

The production or stocking ponds are stocked with large size fingerlings of about 1015 cm sizein the case of composite fish culture. To attain this size, the hatchlings are reared in muchsmaller and shallower ponds called nursery and rearing ponds for about 23 months. In thenursery ponds the hatchlings are reared up to fry stage and in the rearing ponds the fry arereared till fingerling stage. The ratio of water area among nursery, rearing and stocking ponds ina 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 beused for raising the brood fish, while in the case of fish production farm only stocking ponds areto be constructed for producing table size fish from fingerlings. The layout of a complete farm isgiven in Figure 13.

There is no hard and fast rule regarding the size of a pond. However, nursery ponds should besmall and shallow. Ponds having 0.020.06 ha water area and 11.5 m depth are most suitableas nurseries. Rearing ponds are relatively larger, preferably between 0.06 to 0.10 ha in size and1.5 to 2.0 m in depth. The sizes of stocking ponds vary tremendously. For newly constructedundrainable ponds, total water area of 0.25 to 1.0 ha is recommended (Table 6).

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Figure 12. Soil Profile

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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 bottomlayer. Dead plankton and faecal matter from fishes may fall on the bottom layer where thenutrients may be locked up. However, in case of rain-fed areas where the water table goesdown during the dry season, the depth should be kept around 3.0 3.5 m to store more waterduring the rainy season.

Although a square pond is economical to construct for its minimum length of dyke, a rectangularshape of the pond (length:width in proportion of 3:1) is considered to be ideal. In any case thepond width should not exceed 30 to 40 m as it is difficult to operate a fishing net in broaderponds. The nursery and rearing ponds may be square, since they are too small to pose anyproblem 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 14Aand 14B.

Table 6

Practical size and depth of nursery, rearing and stocking ponds (Sahoo, 1984)*
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Pond typeSize

(ha)

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

6.2.2 Dyke

The dyke should be properly designed so that it can hold maximum water in the pond andwithstand 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.
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Figure 14a. Design of Nursery, Rearing and Stocking Ponds

Figure 14b. Cross Section Details of Ponds

Table 7

Suitable slopes for different soils

(Sahoo, pers.comm.)Soil type Soil (horizontal:vertical)

Clay 1:1 to 2:1

Clay loam 1.5:1 to 2:1

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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 widerberm 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 intoaccount its usage. Usually the minimum top width of the dyke should be 1.5 m. The wider crestrequires 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 earthexpected 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 upto the above standard, provision may be made for a clay core to make the dyke watertight.While designing, about 1012% allowance may be given for settling of earthwork (Fig. 15).

6.3 Construction

Before initiating the construction work, proper estimates have to be prepared based upon thedesign details, which will include the cost of all the materials and the labour. Strict supervision isrequired at every step of construction to ensure the adherence to specifications laid down in the

design.

6.3.1 Time of construction

If the construction work is taken up at the most appropriate time or season of the year, the workbecomes easier and economical. The best time of the year for constructing ponds in clayey soilis post-rainy period and winter when the soil is soft rather than at the end of the dry seasonwhen it is very hard. For swampy and waterlogged areas the most desirable time is the latesummer when the area becomes completely dry. However, if a pond is built during winter orearly summer and is not filled immediately, weeds may grow and cover the bottom. In suchcases deweeding is needed before filling the pond.

Figure 15. Design of a Dyke with Core Well and Key Trench

6.3.2 Preparation of site

The site should be thoroughly cleared of all the trees, bushes, etc. Even the roots of treesshould be removed. No woody material should be left because the same will eventually rot andcause leaks. Some tree trunks rot very slowly and may cause problems during netting.

6.3.3 Marking the outlines

This operation involves laying out the features of ponds on the ground in order to mark out theareas 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 fixingstakes or posts. Strings are stretched between the tops of pegs and posts to mark thecomplete profile of the dyke with its correct height, width and slopes (Fig. 16).

6.3.4 Pre-excavation work

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Prior to pond excavation and dyke construction, all loose surface soil should be removed fromabout 20 cm depth within the total outlined area of the dyke and the surface should beroughened by ploughing or digging. In order to unite the body of the dyke to subsoil, it isdesirable to dig a small V shaped key trench (Fig.15). When the dyke is to be made on asandy, gravelly or marshy soil base, the construction of a key trench becomes essential and insuch cases digging should be done until watertight foundations are reached. The key trench is asmall ditch or furrow dug along the line of the centre of the walls about 0.5 m 1.0 m wide and0.5 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 purposeof the trench is to stop seepage of water underneath the walls.

6.3.5 Pond excavation and construction of dykes

The excavation work can be carried out within the area marked for the pond bottom eithermanually or mechanically. However, the final levelling of the pond bottom and sides should bedone manually with proper ramming and finishing as per the original design. The construction ofthe pond becomes economical if earthen dykes are made around the pond using the excavatedearth from the pond bed. All dykes should be raised, dumping the earth layer by layer stretchingright 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 bywatering and ramming. The sides and top of the dykes should be properly dressed and finishedwith 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 tomake 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 1015 cm thick layers. Each layershould be adequately moistened before the next layer is laid and precaution should be taken toprevent the puddle from becoming dry and cracking.

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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 thatsmall vehicles can also run along when needed. Short creeping grass is recommended to begrown on the top and sides of the dyke. Trees are not desirable since their dense shade inhibitsthe productivity of the pond.

6.3.6 Water inlet structure

Since we are concerned here with static and undrainable ponds, a feeder stream runningdirectly 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 inletstructure should be provided through which water can be let into the pond. A proper inletenables the quantity of water flowing into the pond, to be regulated, preventing the entry ofundesirable fish and other aquatic animals and the escape of stocked fish. For small ponds thebest inlet structure is a galvanized iron pipe of about 10 cm diameter with a control tap and ascreen basket (Fig. 17 A). The downstream end of the pipe should be 3040 cm above the

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water level. A sluice is also suitable for this purpose, especially for larger ponds. A screen isalso fixed to check the entry of undesirable fishes and other animals (Fig. 17 B). To avoidscouring when the pond is being filled, a concrete apron can be built at the sluice, or morecheaply, a layer of gravel laid down. Similarly, if water is let in with a pipe there should be agravel 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 evendirectly into the pond; but, in that case, the intake should be securely wrapped by a firm net toprevent undesirable fish and other animals from entering into the pond along with the water.

6.4 Maintenance

Proper maintenance of the pond and pond structure is most essential. Most of the earthenstructures, especially the dykes, are susceptible to weathering action and hence they needperiodical checks. Attending to minor damages regularly avoids the chances of more costlyrepairs later. The grass turfing needs special attention. Proper and timely mowing prevents theformation of weedy growth and tends to develop a root system more resistant to runoff. Erosionfrom the top during heavy rains causes grooving out of small channels and it is an indicationthat the top has not been properly consolidated. The area should be levelled with more soil andthoroughly 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 dueto frequent wave action, particularly if the grass at the edge has been grazed by grass carp, cancause undercutting of banks and subsequent collapse of dykes. Some methods used to provideprotection against such erosion are earth berms, stone or brick pitching, stakes/bamboo piling(Fig. 18).

Figure 17A. View of an Inlet Structure

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Figure 17B. Additional Detail

Surface washings and organic additions cause siltation which reduces the pond depth and pondfertility. The undrainable ponds should therefore be dewatered in the summer months at theinterval of 57 years. This has already been described under Section 4.
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