Info

a Average water depth 0.9 m.

b For filling ponds initially and subsequently and for rapid flushing in emergency. Assumes (1) that the unit pond size is 0.2 ha and that a pond can be filled within 12 hours, and (2) that it will never be necessary to fill more than one pond (or 10% of the farm surface area, whichever is the greater) at one time. Local experience will tell if this is either insufficient or excessive. c Continuous water demand, based on 140L/ha/min minimum and 560L/ha/min maximum. Actual requirement depends on whether 'continuous' culture is practised or not. The minimum value should cope with average seepage and evaporation losses. d This consists of the maximum maintenance rate, assuming 'continuous' culture for the whole farm, plus the quantity necessary to fill all ponds four times per year, averaged out to a volume per minute consumption basis.

conditions, the principal factors being the incidence and strength of solar input, relative humidity, rainfall, soil permeability and the degree of pond compaction. Generally, an average 0.5m3/min is sufficient to supply 1ha of surface water area, the equivalent of 13.8 days average residence time for aim pond depth, or a water exchange of 7.2%/day. New & Singholka (1985) suggested flow rates of 0.14 to 0.28 m3/min/ha (2—4%/day) to replace evaporation and seepage losses, and 0.56m3/min/ha (8%/day) to keep continuous flow in the ponds. Rodrigues et al. (1991) and Correia &Cavalcanti (1998) suggested flow rates of0.12 to 0.18 m3/min/ha and 0.13 to 0.26 m3/min/ha, respectively. According to Lombardi (1996), the ideal flow rate for semiintensive M. rosenbergii grow-out should have the capacity to replace 10% ofthe pond volume daily. New (2002) states that farms must be designed with a water distribution system that will allow the filling of one pond (or 10% of the pond surface area, whichever is the greater) at any time without starving the other ponds of replacement and flow-through water. Table 8.1 shows water requirements related to the total area surface water occupied by the ponds.

To estimate water availability at a specific site, a hydrolog-ical assessment should be carried out as far as is practicable. For smaller local catchments, an annual and seasonal water balance can be defined, taking into account the area from which water can be collected for the site, patterns of rainfall, soils and vegetation over the area, the associated evapotranspiration, and the extent to which water is distributed to surface and sub-surface levels. For larger systems, general data on hydrological conditions may be available, from which estimates of local features maybe made. Other uses of water and the effects of abstraction and discharge will also need to be considered. For lakes and reservoirs, the classical shape and productivity features, seasonal turnovers and other conditions of instability (e.g. wind effects), the size and position ofinlets and outlets, and the balance ofevapo-ration from the water surface should be taken into account. For rivers and streams, seasonal patterns affecting flow rate, water levels and water quality should be measured, particularly noting extreme (i.e. flood and drought) conditions. Similar considerations apply to irrigation supplies, though the interaction of the irrigation management system with climatic and environmental conditions will also be an important factor. In coastal areas, wind, barometric and tide conditions, and the interaction between fresh and tidal waters through seasonal and tidal variation may all affect the availability and quality ofwater. For groundwaters, the characteristics of the aquifer (water conducting zone), its depth, permeability and potential yield, need to be determined.

A range of techniques is available for water resource measurement, and can be found in standard sources on hydrology and agricultural water assessment (ILACO 1981). In many cases, local information may be available and can be adapted for pond culture use. Rodrigues et al. (1991) described simple techniques for estimating supply from river or stream flow speed, depth, width and length data. For more accurate assessment, flow meters or measurement weirs should be used. For groundwaters, local information may be available, test wells dug, or assessments done by specialist contractors. Water supply for freshwater fish farming, whose techniques are very similar for freshwater prawn farming, is also discussed in detail in FAO (1981).

8.1.5 Water quality

Good water quality is essential for all successful aquaculture enterprises. The relation of water quality to the grow-out of M. rosenbergii is discussed in Chapter 13. It will be necessary to ensure that these quality requirements can be met and maintained at the site, without complex or expensive treatment or management techniques. The ability to change water easily when required is also very important.

8.1.6 Soils

Good knowledge of physical and chemical properties of available soils is fundamental in determining overall site suitability and the most appropriate methods of pond development. Soils develop from bedrock materials through a range of erosive and depositional geomorphic processes, and may be classified according to constituent particle sizes and their distribution, for which there are established standards (FAO 1985; Boyd 1995). With the exception of larger particles such as pebbles, cobbles and boulders, which are excluded from analyses, particle sizes range from <2 ^m (clays) through to 2 ^m to 2 mm (fine to coarse silts) to 2 mm to 2 cm (fine to coarse sands) according to the International System (Atterberg Classification). The shape of the particles, as defined by their angularity, and the primary soil origin (e.g. soft calcareous or hard metamorphic rocks) may also be important. Though a range of soil quality parameters can be defined, four primary features need to be considered:

• Physical strength: determining the load-bearing capacity of the soil, the ease of carrying out construction work, and the potential for supporting buildings, roadways and other installations. In basic terms, a well-graded soil, with a range of particle sizes, tending to larger, more angular particles, provides greater strength.

• Permeability: defining the ease by which water flows through the soil, and hence whether ponds or retention dams will hold water or be subject to seepage. Finer soils are generally less permeable and so a suitable clay and silt content is important. Note that for aquifers, the converse property is required, and larger particles, with higher permeability, are preferable.

• Plasticity: determining how well soils will compact to their optimum strength and permeability conditions, and how this relates to the moisture content of the soil (see later). This is related to particle sizes and pore spaces between particles. The stability of the soil under varying moisture conditions is also affected by this property. • Physico-chemical interactions: determined by the surface properties of the soils, the pore sizes and the conditions under which the soil has formed. Finer particles are more reactive, with a higher potential for ionic binding. Larger particles may contain more organic matter and may provide a more open structure for aerobic and anaerobic biological action. The bedrock origin will also determine factors such as pH, and metal content.

If the site has diverse soil conditions, their relative distribution also needs to be considered; this will determine choices of layout, pond location and the most effective means of development. The evaluation of soils will depend on the site type and conditions; in some cases background information may be available (e.g. from agricultural soils analysis or river basin management studies) and should be consulted for primary assessments. Geological and contour maps can also be used to determine whether soils in the area are vertically or horizontally homogeneous, formed through major riverine, glacial or aeolian (wind formed) processes, or are likely to be more diverse. From this information the number, location and depth of the samples required can be considered. In basic terms, flat, lower-lying river floodplains in major depositional areas tend to be more homogeneous, and will normally require fewer samples. Preliminary assessments of particle sizes, permeability and plasticity can be done using simple field techniques, using trial pits or auger samples down to the depths at which it is anticipated soils will be moved (FAO 1985), and from this can be determined the number of samples required for detailed analysis. As an approximate guide, at least three 1 kg samples should be collected for every 1000 m2 of land with, if necessary, a range of depths (typically every 20 cm down to 20 to 30 cm below the intended excavation level) and sent to a laboratory for physico-chemical analysis.

The most appropriate soils for pond construction are composed of silt and clay, together with a small percentage of sand, adequate to provide strength and structure to the soil (FAO 1985; Correia & Cavalcanti 1998). According to Malecha (1983), clay-silt soils offer better water retention capacity (~85% of water retained over a production period). Ideally, however, clay content should not be higher than 60% in the total soil composition (New & Singholka 1985), or the soils will be too weak, and will swell and crack under high and low moisture conditions, respectively. The cracking of clay soils in particular can cause problems of leakage, although the soils are themselves relatively impermeable. Sandy soils cause excessive seepage, but water loss can be reduced by constructing an inner key and core in the middle of bunds (Fig. 8.2), filled and compacted with impervious material such as clay (or soil/cement mix). Other

Fig. 8.2 Examples of keys filled with impervious material to avoid seepage.

techniques to prevent seepage are described in detail by Wheaton (1977).

Rocky soils should be avoided because they make excavation and earth-moving work difficult, and rock outcrops can create major expense in construction. Flooded lands and saturated soils are difficult to work, with reduced excavation yield, or require specialised equipment, and should also be avoided. However, some sites may only be seasonally flooded, and construction can be done outside these periods. In some cases, the soil can be drained and the site protected by a temporary bund to facilitate construction. In coastal areas, soils with a high salt content may be difficult to use, particularly for placing concrete, and care will be required.

There is a lack of specific studies to determine the chemical properties of the most suitable soils for M. rosenbergii culture. However, it is known that sulphate and peaty soils are not suitable, as they characteristically have low pH (<4.5), and levels of soluble iron, manganese and aluminium that are harmful to M. rosenbergii (New & Singholka 1985; Wickins & Lee (2002); Correia &Cavalcanti 1998). Chapter 13 discusses the management of soils and sediments in the grow-out culture of M. rosenbergii.

8.2 Site development

The development of a site for freshwater prawn culture depends on a number of factors, which inter-relate the production objectives and site characteristics. In many cases, the features of the site and the cost and technical feasibility of changing it in certain ways may dictate the options for scale and type of production and the potential layout and dimensions of key features. It may also be necessary to adjust production objectives if the site does not permit the original aims or methods. By this stage, however, it should be possible to establish that strategic issues are satisfactorily resolved, that topographical, soils and landscape features are acceptable, and that access to suitable water supplies and other services is feasible (FAO 1992). Subject also to the costs of acquiring the site and decisions to finance the project, it may then be possible to plan its construction and development. Key issues at this stage include:

• the overall co-ordination of the scale of the project and its different components, and the confirmation that these will be workable and effective within the site;

• considering future options to expand and develop, and if intended, ensuring that this could be done easily from initial production stages, with minimal disruption;

• decisions about the methods of construction, and how and when this would proceed, considering access to equipment, skills of local staff or external contractors;

• the timing of the different elements of the development, and how this will link with available resources of staff and equipment, materials and components, and with seasonal or other variable factors;

• the administrative issues of developing designs and specifications, obtaining necessary permits, selection of contractors and suppliers, establishing contracts for work to be done;

• proceeding with the development, whether directly or via contractors, ensuring it is carried out to the appropriate standard and in a timely manner, and monitoring and controlling costs; and

• completing the development, cleaning up the site, testing the quality of the completed construction works, commissioning key equipment, obtaining certification if required, commencing production and making final settlements with contractors.

Decisions about these factors, and about whether or not this is to be handled by outside specialists and contractors, depends on the scale and complexity of the project and on the skills and knowledge of the developers. The following sections (8.2.1-8.2.5) outline some of the important practical points.

8.2.1 General considerations

As noted earlier, linkages between the site topography and its water supply are key determining factors in defining the suitability of a site, its potential for development, and the specific locations of individual features. The general objective is normally to develop the site at least cost, and to arrange for simple, low maintenance and trouble-free operation. At a more detailed level, these factors will be particularly influenced by relative site levels, soils and their distribution, and excavation and construction costs. Various regulations may also have to be considered, in terms of zoning permissions, protection of conservation areas, water abstraction and discharge, environmental management, and general construction, building and safety standards. Not all of these may apply, but their potential impact needs to be considered. The timing of development may also be important; in some sites, seasonal variations in rainfall, and the presence of surface water on construction areas and roads, may be critical factors for the ease and cost of construction.

The issue of site development varies greatly with the scale and complexity of the project. At one extreme this may only concern building a simple pond and arranging a convenient inlet and outlet from/to a nearby stream. At the other, it may involve a major production complex, with key elements including ponds, large-scale water and drainage works, buildings of various types and specifications, possibly including offices, sales and other public areas, management and staff accommodation and facilities, ambient or temperature-controlled stores, packing/processing facilities, workshops, garages, pump and generator housings, together with roads, parking areas, power distribution, telecommunications, and potable water supply and sewage treatment facilities.

In all cases, and regardless of the scale of construction, it pays to take a well-organised and methodical approach, in terms of planning the sequences of development, ensuring that work and materials are of the appropriate quality and are used well, and managing the costs, payments for work and regulatory and other permits.

8.2.2 Water supply development

Once the availability and quality of water is determined for the site, its means ofaccess has to be considered in more detail. The aim is to develop systems which are simple, reliable, easy to access (but difficult to tamper with) and inexpensive to develop. For practical reasons of cost, for everything up to very large-scale projects, it is not usually feasible to draw water supplies more than approximately 1 km from the pond site. The water supply system typically includes an abstraction point, often including various screening or filtering devices, a distribution system of channels or pipes, the various devices for controlling and directing water in the system, and the distribution system for conducting waste or discharge waters away from the ponds. Other components may include pumping installations, storage reservoirs and treatment lagoons, and specialised control and treatment devices. However, as the objective is usually to control costs, these are used only when specifically required.

Surface water development is probably the simplest and most common arrangement for most forms of pond farming, including those for Macrobrachium. Sources include rivers, streams, reservoirs, lakes, or various water supply or land drainage canals, and water can be abstracted by gravity or pumping. Seasonal patterns of supply (and water level) and water quality are key determinants of design, defining the relative elevations of structures, the need to protect against flooding or ensure sufficient access in low flow periods, and the need to screen or filter the water prior to use. The intake structure should therefore be capable of abstracting water at the lowest expected water levels in the supply source, protect the downstream system from flooding (and be strong and secure enough to withstand flood conditions) and be able to screen out undesired materials.

It is more common to screen out larger particles (e.g. plant and wood debris, waste materials) at the intake, but to allow finer materials to pass into the distribution system to a point where they can be removed. This avoids the risk of blocking the intake, or the need for very frequent cleaning of fine materials. Large particles are typically removed using coarse vertical or inclined screens, preferably steel or plastic mesh, though basketwork screens can also be used, as long as they are well maintained. If the risk of blockage is not too great, finer particles can be removed at the intake using fine mesh screens or filter boxes. In some cases, carefully designed sand and gravel filters can be constructed in the bed or the side of the water source. In other cases, it may be more convenient to use a water supply reservoir, to allow particles to settle, creating a settling area at some point on the intake supply, or to use filters or screens at the ponds themselves (FAO 1992).

For gravity supplies, relative elevations during all periods of operation will determine the dimensions of supply systems - pipes or channels - delivering water to the site. For larger flows, open earth or lined channels are usually the cheapest option, but pipes may be convenient for smaller flows, and can be safely dug in to reduce visual disturbance and risks of damage or contamination. In some cases it may be useful to combine sections of pipe and channel, for example if supplies are to be run under roads. The transects of supply routes need to be considered carefully to provide a steady fall, and to ensure that velocities keep conduits clear without eroding them. Where multiple ponds are to be fed, it is important to make sure that water will reach all of the ponds during normal operations, as slight changes in level can create unequal flows. Standard texts on hydraulics or irrigation water supply (Hansen et al. 1979; Yoo & Boyd 1994) may be consulted for typical design procedures. If it is feasible to do so, and water is available, it may be worth making supply lines bigger than initially suggested, as it can be expensive and disruptive to enlarge these later.

Pumped systems for surface waters can use a gravity flow type intake, usually with a small sump or pump chamber set behind the intake structure, from which the pump is supplied. Alternatively, the pump may simply be mounted with its own screened intake directly into the supply water. Surface-mounted or submersible pumps may be used; for the former it is important to ensure that the mounting is secure and accessible, and that the intake is submerged below a sufficient level ofwater at all times (usually a minimum of twice the intake pipe diameter). Many submersible pumps are capable of running at least partly dry, but they should also normally be covered. Provision must be made to ensure that they can be raised for inspection and maintenance. For ponds set up in low-lying flat land, adjacent to irrigation or drainage canals, pump installations may be particularly simple, and in some cases, light, locally manufactured low-head tubular lift or propeller pumps may be used, lifting water 2 to 3 m at most, from the canal and over the pond or supply channel embankment. Overall, it is important to ensure that pump costs are controlled, and so capacity and operating duty have to be well planned.

Groundwater supplies may be abstracted from a number of different sources and in different conditions, ranging from a simple well (often dug adjacent to a surface water supply, providing a simple 'ground filtering' effect) to a tubewell or borehole system. It is advisable to seek local advice concerning the site and type of intake, and to establish the potential flow and quality, and the pumping depth (and hence capacity) required to develop the supply. The same considerations apply to pump installations as they do for surface waters, though specialised systems may be required for deeper boreholes. Though groundwaters are unlikely to require screening or filtration, they may be deficient in oxygen, and may contain higher than desirable levels of toxic dissolved gases or dissolved metals, particularly iron, which may have to be treated.

Where estuarine or coastal water sources are to be used, water systems have to be planned carefully with respect to the position of saline waters and their diurnal, seasonal, and use-related variation to ensure that water of acceptable (low) salinity is available. For open water intakes, water may be taken from the surface or in some cases from a sub-surface intake point where preferred salinity and water quality can be derived. Groundwater intakes have to be considered carefully with respect to the effects of saline intrusion, where excessive pumping, by the farm or others, may raise the saline water table, resulting in gradually increased intake salinities. The risks, and advice on siting such wells, should be discussed locally.

Depending on need, storage and treatment systems may be incorporated into the water supply. Though they entail additional cost and complexity, these can serve a number of purposes, including:

• storage of water for intermittent supply conditions (e.g. access to lower salinity water during tidal fluctuations, seasonal rainfall). Reservoir ponds are normally used, or widened and deepened supply channels. Their size depends on the storage required, and on conditions such as evaporation and seepage loss;

• deposition of fine solids and, in some cases, removal of pathogen particulates. Sizing depends on the removal levels required and is usually carried out by the use of reservoir ponds. A minimum of 30 minute residence time is normally required for removing medium-fine particles and much longer for smaller sizes;

• aeration and oxygenation, either through atmospheric contact and photosynthesis, or through mechanical stirring or oxygen injection. This may also help to precipitate iron, and may reduce dissolved organic matter loadings;

• chemical dosing, e.g. for pH control, sterilisation, floccu-lation, etc., in tanks or ponds. This is used only in special circumstances and has to be designed specifically for the purpose concerned; and

• collection, mixing and distribution from different water sources. This may take place in mixing tanks or ponds and may concern surface and groundwater. In some cases water may be partially recycled from the farm outlets to conserve and reuse waters.

In most cases, these functions can be satisfied in a single unit, typically an intake/water supply pond, which may be further partitioned for specific functions.

Water distribution and control systems need to be planned to ensure they are capable of delivering the specific flows required, under all operating conditions, to the component parts of the farm. M. rosenbergii ponds should be planned with individual water supply systems. The waste water from one pond should not be used to supply another (Fig. 8.3), in order to avoid the accumulation of toxic substances in downstream ponds (New & Singholka 1985; Valenti 1991; Lombardi 1996; Correia & Cavalcanti 1998; New 2002). Layouts should be simple, direct and easy to manage, with convenient access and the means to adjust

Fig. 8.3 Schematic of parallel derivation for pond water supply.

water flows as required. Small brick, concrete, wood or plastic-lined channels can be used, or pipes with simple gate structures, weirs or valves. Simple means of estimating water flows such as 'v' notch weirs may also be incorporated. In some cases, small pumps may be needed for occasional transfers of water.

The final component ofthe hydraulic system is the waste water collection structure, which has to be designed to be able to accommodate discharge flows from all parts of the farm, including normal operations, drainage and flushing ofponds, and the receipt of intake overflows and local runoff during high rainfall periods. Where discharge is simple and direct to receiving waters, the correct dimensioning of the drainage pipes and canals is the only issue. Otherwise, it may be necessary to reduce solids loads and improve oxygen levels. If additional settling ponds or lagoons are required, it may then be convenient to reuse at least some of the water.

Finally, as water supplies are the most critical features in the successful operation of a Macrobrachium farm, it is important to remain aware of the safety factors. Reasonable provisions against drought, floods, silt levels and other water quality changes, power or pump failure, blockages of channels, pipes and flow controls, need to be made.

8.2.3 Pond layouts

The number, size and layout of ponds interlink with site features, water and power supplies, road access and with other construction features. The primary determinant is usually the production planning exercise, which defines the biomass targets, the timing of stocking and harvesting, the crop sizes, and the intended transfer plans (if any) between earlier and later growth periods. In the special case of integrated systems such as gher and otherponds, factors such as agricultural crop planning and the size of crop areas versus deeper water areas may need to be considered. Once the primary issues have been defined, the relative size and number of ponds can then be determined, and these can be located appropriately within the site. An important construction objective is to minimise the costs of earth moving, and thus arrangements which balance the volumes of soil 'cut' and 'fill', allowing also for soil balances for other components such as roads and canals, are to be preferred. The need for soils for other purposes, such as flood protection bunds, landscaping, etc., also needs to be considered.

Ideally, the aim should be to provide a logical layout with respect to the flow of water through the site, the movements of stocks from stocking to harvesting, the provision of production inputs, the need for access, and the need for security. For integrated systems, the different components need to be arranged so that they work effectively together. Particularly for larger and more intensive farms, it may be appropriate to define and group sub-systems, representing different production stages, management areas or site zones. Some of the key points are as follows:

• pond size generally increases with the production stage, with small ponds for nursery or early rearing and larger ponds for grow-out. These may be grouped together, or arranged in separate sections. Final pond size may be linked with intended harvest batch size;

• large ponds are generally cheaper than small ponds, per volume enclosed, but are more difficult to manage. As ponds increase in size, they require less excavation depth to produce enough material for their bunds. Consequently they are easier to build on lower lying, flatter ground. The converse applies to smaller ponds;

• where site contours are steep, it is easier to build ponds along the contours rather than across them. However, this may require long supply and drainage systems;

• according to New & Singholka (1985), ponds should be oriented to align with prevailing wind directions to increase natural aeration. Wind action at drainage points can also facilitate water replacement. However, Correia & Cavalcanti (1998) recommended that the ponds should be placed contrary to wind direction to avoid mud accumulation at drainage points. In large ponds, excessive wave action may also increase erosion of pond bunds; the choice of alignment depends on whether aeration or erosion is the more important issue;

• it is common to arrange for nursery or early rearing stages to be nearer the main centres of activity and infrastructure access, to ensure that they are well protected and managed. They also commonly have the best water supplies;

• in a sloping site, it is usually most convenient to have main access, infrastructure, buildings and early rearing stages at the top of the site, and grow-out ponds in the lower areas, serviced with a harvest access road; and

• access should be available to all functional parts of the farm, and all ponds should be visible from one or more convenient vantage point. The farm layout may also be screened visually from outside.

8.2.4 Infrastructure

The importance of the availability of external road access, distance to shipping and air ports, and local services has already been noted. At the planning stage it is important to consider the potential infrastructure demands, their timing, and the means by which infrastructure can be made accessible to the site. Local roadworks are usually fairly straightforward, and may be incorporated into site construction activities, though repair and upgrading of public roads may be necessary if the project will generate significant additional traffic. Within the site, it will be important to provide suitable working, turning and parking space for vehicles and equipment.

Power supplies may be more problematic and, if available, will usually require access to the distribution system, the potential need for further transformer capacity, and arrangements for internal distribution within the site. Energy consumption is associated mainly with mechanical aerators, pumps, refrigerators and cold storage facilities, feed manufacture, lighting, office equipment and generalpurpose machinery, and will vary with the size and sophistication of the farm. A three-phase power supply is preferable for heavier-duty equipment, but is not essential. In some cases, a direct power drive (e.g. from a diesel or gasoline engine) may be cheaper and more effective. The reliability and stability of the power supply is also important. Protective and control devices may be needed to avoid problems with voltage surges and phase cut-outs. Power may be generated on site, usually with a diesel generator, though solar, wind and hydro-power sources can sometimes be tapped. Where the farm is to have significant dependence on electrical power, a back-up power source - usually a diesel generatoris essential to cover key items.

A range of buildings may be required, as earlier noted. Project offices, work areas, laboratories, housing for managers and employees, processing facilities, stores for feeds, harvesting seines, machines and vehicles, garages, workshops and other units may all be required. Specifications, key dimensions and locations of these facilities need to be considered, together with their service requirements. In some cases it may be convenient and effective to house all functions within a single building; in other circumstances, separate and more specialised units may be advised. Where conventional phone systems are not available, radio-telephone or satellite communications may be considered. Radio-telephone and other systems are also useful for communicating around the site, for management and safety purposes, particularly for large projects. Protective fences and alarm systems may be needed to limit access to installations, and to improve farm security.

Main water and waste disposal facilities are an added advantage but are not essential, as provision can be made on site, though this needs to be considered in site selection. Batch supplies of drinking water can be obtained, or borehole or filtered rainwater collected on site. Water for washing and other purposes may be obtained locally. If water is needed for processing and packing, or for ice production, it needs to be of good quality, normally the equivalent of drinking water. Aqueous waste disposal can be carried out either using a septic tank, waste lagoon or simple soakaway. In some cases a package treatment unit may be considered, though these tend to be expensive for their capacity. Solid wastes may be incinerated, buried in suitably isolated and protected locations, or collected for disposal elsewhere.

8.2.5 Planning pond construction and development

Features of sites and their water supply andpondlayout have already been noted, and it is clear that local topography and soils, associated with factors such as access and cost/ease of construction, are important determinants in pond location. Once the overall features of the site have been identified and primary decisions made about relative positions and locations, a more detailed topographic study is necessary in order to plan the distribution of the ponds within the site. This study should provide measurements every 10 to 50 m in unlevelled terrain, at vertical intervals of 20 cm or less. On a very uneven site more detailed measurements may be required. Together with the overall layout ideas, which may be adjusted according to detailed levels, this should determine the best sites and means of development to avoid excessive earth moving during pond excavation. Further information is also provided in section 8.3. Detailed topographical information can also be used to determine the appropriate sequence of development for large sites.

Once these data are established and final decisions about layout have been made, arrangements can be made to set out and mark the main construction features, organise construction plans, obtain planning or other regulatory permits, and finalise agreements for equipment supply, construction contractors, additional workforce, etc.

8.3 Pond system construction

8.3.1 Pond dimensions

The choice of pond size is determined by the technology intended for the project, together with topography and local climate. In suitable site conditions, larger ponds are generally less expensive to construct per m2 than smaller ponds, as the bund volume per area enclosed is smaller and as the costs of inlets and outlets do not vary directly with size. However, much less soil needs to be taken from the floor of a large pond to generate sufficient material for its bunds, and so they may be less suitable when site levels require that the ponds are dug deeper into the ground, as too much excavated earth will be produced. Because of the smaller bund area relative to surface area, large ponds also use site space better and, because of their greater water mass and thermal inertia, they are less susceptible to temperature oscillations. However, environmental and other conditions may be more variable across the area of larger ponds, and management may be more difficult. It is common for yields per unit area to be lower in larger ponds under equivalent production conditions.

The individual area of M. rosenbergii grow-out ponds generally varies from 0.2 to 0.6ha (Ling & Costello 1979; Valenti 1991, 1996; Correia & Cavalcanti 1998; New 2002).

Gher and other small-scale integrated ponds may have a smaller area, from 0.05 ha upwards, but may reach 0.5 ha or more. As noted earlier, larger ponds make management more difficult, particularly with respect to feed distribution, sampling and harvesting. However, some farms have ponds larger than 1.0 ha (Sandifer et al. 1983; New & Singholka 1985; Hsieh et al. 1989).

Valenti (1985) noted that freshwater prawn ponds can be round, square or rectangular, though artisanal gher and similar pond types may be irregular in shape, depending on individual land holdings. However, rectangular ponds usually offer better and more convenient management conditions. Their length to width ratio typically varies from 2.5:1 to 4:1 (Ling & Costello 1979; Sandifer et al. 1983; New & Singholka 1985; Correia & Cavalcanti 1998). Seine harvesting is not very efficient in ponds larger or wider than 30 m (Lam & Wang 1986; New 2002). Feed distribution at the centre of the pond also becomes more difficult. Generally, the length of large commercial ponds varies between 100 and 125 m, and the width from 30 to 50 m. It becomes more feasible to use larger ponds when feed can be broadcast mechanically, and seine harvesting can be assisted by tractors or mechanical winches.

The pond base is usually slightly sloped, varying between 0.5 and 1.0% (i.e. 5 to 10 cm per 10 m length) towards the drain, thus allowing the pond to be completely emptied by gravity. The nominal depth of a pond is given by the average between the shallowest (supply end) and deepest (drainage end) point. Thus a pond with 1 m of water at the intake end and 1.5 m at the outlet, has an average depth of 1.25 m. The water depth of freshwater prawn ponds recommended by various authors has differed. New & Singholka (1985), Valenti (1996) and New (2002) suggested an average water depth of 0.9m. Rodrigues et al. (1991) and Lombardi (1996), respectively, suggested 1.4 and 1.25 m as a suitable average depth for sub-tropical regions. Mendes (1992) stated that M. rosenbergii can be produced in ponds with depths varying from 10 to 40 cm in steady temperature regions. Shallow ponds are more utilised in tropical regions. However, this can also result in overheating of pond water and excessive growth of macrophytes on the pond bottom. According to Hsieh etal. (1989) and Wickins & Lee (2002), deeperponds keep more stable temperatures. However, they need to be partially drained to make seine harvesting easier, which may represent an undesirable waste of water in some circumstances.

Pond bunds should normally have a freeboard of 0.30 to 0.60 m above water level to avoid overflow or overtopping through wave action (Wheaton 1977). Therefore, for a pond of 1.25 m average depth with 0.3 m freeboard, the bunds would average 1.55 m in height from the pond base (Fig. 8.4). As noted earlier, for small-scale integrated ponds built in seasonally flooded land, flood levels are another

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