In this chapter, we describe a range of practical site selection, design and construction issues which affect both the small artisanal pond systems that characterise a large part of the present-day Macrobrachium sector, and large commercial farms. Whatever the format, the planning, design and development of ponds for grow-out is clearly an important aspect of any Macrobrachium project, in terms of the costs of development, the ease and effectiveness of management, productivity, economic performance, longer-term environmental security, and overall sustainability. The aim of this chapter is to provide the necessary background to enable an intending producer to identify and develop a site and system effectively, and thereby to improve opportunities for profitable production. Though stock, feed and labour costs are usually the most critical elements in overall production costs, secure and stable systems, capable of providing the appropriate rearing environments and permitting effective husbandry of the stocks, are fundamentally important in ensuring that these inputs are used to best effect. This chapter should be read in conjunction with the other chapters in this book, which define the optimum operational grow-out parameters, particularly Chapters 9 and 13.
The grow-out of freshwater prawns is generally carried out in earthen ponds. These structures are usually cheap and simple to construct and operate, and with suitable management and simple inputs allow for the development of natural foods such as plankton and benthos, while providing relatively stable water conditions throughout the production cycle. Such features favour the growth and survival of the on-growing stock, and will normally allow producers to minimise the costs of production. Ponds for freshwater prawn culture are broadly similar to those used for fish culture, and can either be built by means of an embankment across a watercourse or by excavating and building up a structure into which water can be conducted by various means. In some special cases, for example the ghers of southwest Bangladesh, ponds are formed in rice paddy or other low-lying land, and typically contain an elevated central platform on which rice is grown, either simultaneously or consecutively, and high exterior bunds to protect the pond from flooding.
Embankment or interception ponds are made by constructing a dam across a watercourse, thus using the natural topography of the valley. The objective is normally to create a substantial volume of water with a relatively small and simple embankment, without making the pond too deep and the embankment correspondingly more massive to withstand the water pressure. In some cases, a series of such ponds can be laid out down a valley floor (Huet 1986). Though this pond type can be relatively inexpensive for the volume of water held, it has critical disadvantages. As water is drawn directly from the river, pond management and the maintenance of fertility is poor due to the lack of control over incoming water flow. The irregularity of the pond bottom makes seine harvesting very difficult and the dams can be susceptible to overtopping or damage in floods. Ifponds are laid out in series, it is almost impossible to drain, fill or otherwise manage one pond without affecting the others in the series. For such reasons, this pond type is normally used only for extensive M. rosenbergii culture.
The other major pond types, diversion or derivation ponds, are normally built along a watercourse or other water supply, and (subject to the local topography) are laid out to provide a regular structure. The water is provided independently to each pond or group of ponds, and the water source can be fully or partially used. The bottom regularity makes seine harvesting easier. Though these arrangements may be more expensive per pond volume created, the advantages to management and productivity in most cases easily outweigh this. These ponds are therefore widely used for semi-intensive M. rosenbergii culture, the most appropriate approach for commercial farms (New & Singholka 1985; Lombardi 1996; Correia &Cavalcanti 1998; Valenti 1998; New 2002). A further variation on this system occurs when ponds are used as raceways, with a high water exchange, high feeding rates and intensive production.
However, though technically viable, these systems tend to have uncompetitively high production costs, and are therefore not widely developed. For this reason, this chapter focuses specifically on the design and development of semi-intensive diversion ponds. Further details of the construction of these and other types of ponds and raceways can be found in Bard et al. (1974), Wheaton (1977), Huet (1986) and a series of FAO manuals providing technical information for building ponds for freshwater fish farming (FAO 1981, 1992, 1994, 1995, 1996) that are also available via the FAO website (www.fao.org) These resources can be very useful for planning freshwater prawn farming, as the main structures and processes used for both activities are similar.
Semi-intensive ponds are closely dependent on local site features and on the disposition of land, water and other productive elements, therefore good site selection is a fundamental requirement for successful freshwater prawn production. Local features determine the technological level to be employed, the potential production, and the construction and operation costs. The criteria for site selection for M. rosenbergii production have been presented by New & Singholka (1985), Valenti (1985), Cavalcanti et al. (1986), Correia & Cavalcanti (1998) and New (2002). Wickins & Lee (2002) described in detail the main features to be analysed when setting up crustacean farms. Perhaps unsurprisingly, there is general agreement regarding the aspects to be considered, such as market access, local social and economic conditions, infrastructure, topography, climatic conditions, water and soils. However, while it may be relatively straightforward to describe the ideal or preferred site conditions, it is unusual for sites to provide optimal requirements in all factors. The practical question then becomes a matter of balancing good and less favourable features to determine whether the overall mix of conditions is suitable. In such practical conditions, Geographical Information Systems (GIS), which have useful application for selecting sites for marine shrimp farming (Giap & Yakupitiyage, 2005), may also be considered for freshwater prawn farming.
It may broadly be assumed that potential developers of a Macrobrachium project will have established their overall objectives and will have a concept of their intended output, production plans, approach to management and intended markets. It is outside the scope of this chapter to review the process of identifying and developing the commercial decisions. However, in practice, several situations might commonly be encountered:
• A project concept is broadly in place, and a suitable site has to be located, without pre-definition, to match its objectives.
• It is the aim to develop profitable aquaculture in a certain area or region, Macrobrachium culture is a potential activity, and one or more local sites need to be found; or
• A site is available for use, either unused or to be converted from other forms of activity (possibly including other kinds of aquaculture) for the purposes of the project, and requires to be developed as effectively as possible.
The strategic issues of site selection concern all the background, non-biotechnical factors which will have a bearing on the suitability of the site. Wickins & Lee (2002) described these as including markets, roads, labour and legislation. While all of these may be important, their relative impact will depend on the development situation as outlined above, and on the specific project objectives. Underlying all of these factors will be the basic political, social and economic conditions, i.e. the commercial environment in which the project is to be located. As with other forms of aquaculture, there is a basic requirement for physical and financial safety and security, and for a suitable degree of institutional support. For projects with local social or economic development objectives, local political, institutional and community conditions have to be capable of engaging effectively with aquaculture, and prepared to commit to the project.
According to New & Singholka (1985), a market assessment is a primary requirement. This would establish the farm capacity and technology to be applied. In practical terms, assessment would include location and capacity of markets, and consumer preferences for size and product form. If the objective is to meet international or larger urban markets, product requirements are usually clearly defined, and quality and consistency of supply are key issues. Good quality, cost-effective processing and transport links are therefore critical. For local and smaller urban markets, using local market chains, options for product form and distribution may be more flexible, but local transport and market capacity may be significant. It is important to base market assessments on the intended product (an evaluation based on marine shrimp is unlikely to be valid), and to consider potential competitors supplying the same products, or possible substitutes. Market and commercialisation strategy is discussed further in Chapter 19.
Whether for small farms or large commercial projects, the availability and skill level of local people need to be considered. At the artisanal level, farmers and their families need to have sufficient time available to be involved in constructing and maintaining the ponds and to manage production, and may need to have access to advice and extension support. For commercial projects, a suitably skilled labour resource would be required. The number and skill level of employees is related to farm size and technological level. For larger farms (>5 ha water surface area), staff would typically include a technical manager, plus field technicians and unskilled labour. Regardless of farm size, there would usually be at least one technically skilled person, responsible for planning, co-ordinating inputs and outputs, making strategic decisions, and guiding the workforce, who would be responsible for daily field tasks, such as feeding, collecting water and stock data, and general maintenance. Correia & Cavalcanti (1998) suggested that the ratio of the number of labourers to water surface area (in hectares) is 1:1 and 1:2 for large and small farms, respectively; small farms normally hire short-term help during harvest and other peak work periods. New & Singholka (1985) cited a 40 ha M. rosenbergii farm run by only 2 senior staff and 6 labourers. Valenti (1985) suggested that though skill levels need to be high for most routine aspects of freshwater prawn farming, it is essential that farms should have specialised technical orientation. The nearby availability ofresearch or training institutions also makes it easier to seek technical advice.
The political and legal framework within the area concerned is important, and relates to both support and control issues. Clearly defined and legally protectable resource rights are essential for any longer-term aquaculture project, whether for small farmers or for corporate producers. The legal establishment of financial systems and company structures makes it possible to finance projects and manage financial matters reliably. Access to credit for small farmers, and general development support provisions, whether in the form of grants, loans, credit guarantees or partnership or joint-venture support, can also be important. In some cases, taxation incentives may be available whether in setting up a farm or in supporting product development and export promotion. In many countries there is legislation to regulate land use, water abstraction and discharge, as well as the clearing of vegetation, coastal or floodplain considerations, and the disruption or destruction of sensitive habitats. Environmental and conservation legislation varies from country to country, and within individual countries. In an increasing number of cases, environmental impact assessments (EIAs) may be required before development can proceed. Finally, particularly for larger projects, open access to international markets is essential, and support to ensure that products meet international quality and environmental standards is increasingly important. General aquaculture policy and legislative issues are not discussed in this book but an example ofthe approach to these issues, albeit in a country where Macrobrachium culture is unlikely, has been provided by New (1999).
A range of physical infrastructure factors needs to be considered. The site should have easy access for its major supplies, its workforce and its markets. Nearby postlarvae (PL) suppliers, food and other necessities are advantageous. Freshwater prawns are perishable, and transport to the consumer should be rapid and reliable, so main and secondary roads should allow good access conditions in all weather conditions. Convenient access to shipping and air services is important for export markets.
A main power supply is desirable, and may be preferred for intensive systems which depend on aeration (Correia & Cavalcanti 1998; New 2002). Simple production systems can be operated without electrical power, and generators may be used when power is required for intermittent use. The practical options are discussed later. It is also advantageous if main water and waste disposal facilities are available, but they may not be essential as alternatives may be found. Communications with key suppliers and markets are also important; landline or mobile telephone, fax and internet access are the simplest options. Radio or satellite communication networks can be installed when there is no local phone system, or if its quality or capacity is limited.
The main objectives concerning the physical aspects of site selection are to identify a suitable configuration of land and water resources, in which ponds can be easily and inexpensively constructed and water supplied and carried away in an effective manner. For pond construction and site development, a key issue is to minimise the quantities of earth to be moved. Flat or slightly sloped lands are generally the most appropriate for pond construction, and slopes close to 2% (2 in 100 m) usually offer good savings and low earth removal (Wolf 1994). Steeper slopes (>5%) should normally be avoided, as earth moving expenditure may be excessive. In most cases suitable gradients correspond to the middle and lower reaches of river plains, though local geological conditions may determine other possibilities. Sites subject to flooding, land-slips or erosion should be avoided. However, the Bangladesh gher ponds, and many of the Macrobrachium ponds of Thailand, are built by excavating into flat, usually seasonally flooded land, relying on their high bunds to protect against normal flood levels.
Ideally, water should be drawn from above pond level, and discharged below the ponds, allowing water distribution by gravity, thus saving expense. When the water supply is below the pond level, dams may be used to elevate the water level if the topography allows (Valenti 1985), though care has to be taken that these continue to function during low water flow periods. Alternatively, pumps may be used, either to move water into the system, or to drain the ponds. In many cases, a single water source may be available, though sites may also be developed with multiple sources. More simply, gher ponds are generally rain-fed, though water is also transferred in more advanced systems using simple field pumps, as the lack of water exchange limits stock loading and productivity.
The interaction between site and climate is a primary factor in determining the suitability for pond production of M. rosenbergii. Key issues include temperature, rainfall, sunlight and wind exposure, together with dependent factors such as evaporation.
Though modified slightly by pond characteristics and methods of management, temperature is the key determinant of the number of yearly harvests and the most appropriate period for culture. According to Valenti (1996), a site can be used for M. rosenbergii culture wherever the monthly average is higher than 20°C for a period of at least 7 months, though periods as short as 4 months are utilised in temperate regions. Temperatures are optimum when the average varies between 25 and 30°C year-round. The importance of temperature for the optimum grow-out of M. rosenbergii is discussed fully in Chapters 9 and 13, while grow-out under temperate conditions is discussed in Chapter 10. Sites with large diurnal and seasonal temperature fluctuations should be avoided, in order to reduce stress and maintain optimal production conditions. Inland areas are generally subject to larger temperature variation than coastal sites (Wickins & Lee (2002), but other local factors may also be involved.
The water temperature in the pond system is primarily controlled by the effect of air and ground temperature on incoming water supplies and pond water, by solar warming, and by the cooling effects of wind and evaporation. If water is exchanged rapidly, pond water temperature will be controlled by that of the incoming water; otherwise pond heating and cooling effects will dominate, and will be affected by factors such as pond depth and wind exposure. The temperature ofwater supplies will depend on their origin. Surface waters exposed to conditions similar to those of the site are likely to have similar temperature ranges, while those affected by factors such as seasonal snowmelt may be notably different. Groundwaters may be more affected by ground temperatures, whose average will be similar to that of the air temperature, but whose range will be less. Generally, the deeper the groundwater source, the closer its water temperature will be to the annual average air temperature. These factors apart, when no data are available regarding water temperature for a site, this can be partially estimated from monthly average air temperatures (Valenti
1996). These values can often be obtained at local meteorological stations for multi-annual periods, from which the corresponding monthly average and ranges can be calculated.
Rainfall, evaporation, air relative humidity, and wind speed and direction should also be considered. Ideally, evaporation losses should be equal to or slightly lower than rainfall input, to maintain an approximate water balance. However, in some locations this balance changes seasonally, with cooler high rainfall periods in which water can be stored in deeper ponds, and hotter high evaporation periods in which water supplies diminish. By adjusting production plans, it is still possible to produce one or more crops. Mild winds favour gas transfer between water and the atmosphere, helping to oxygenate the water. However, strong winds can increase water losses by evaporation and may also generate wave action, provoking bund erosion. Malecha (1983) also recommended that constantly cloudy areas should be avoided, as this can make it hard to maintain a steady water temperature because it interferes with solar penetration. Periods of cloud cover of several days' duration may also be problematic in causing algal blooms to crash, thereby upsetting the oxygen balance in ponds. Highly unstable meteorological regions should be avoided. Strong storms and winds increase the risks of flood and erosion damage, and may lead to problems with transport access and power supply.
Prawns (M. rosenbergii) are normally reared in ponds supplied with freshwater, although the use of slightly saline water is not precluded (Chapter 13). Supplies of freshwater can be obtained from surface water, as represented by rivers, lakes, reservoirs and irrigation channels, or from sub-surface (ground) water. Surface water is the most commonly used. According to Wheaton (1977), groundwaters usually have alkalinity and hardness levels that are acceptable for culturing aquatic organisms, and are free of pollution, predators and competitors. However, they are likely to have low dissolved oxygen (DO2) levels and high levels of dissolved carbon dioxide and in some cases may contain toxic gases such as hydrogen sulphide and methane. Such waters would normally have to be well aerated before use. Though DO2 levels are generally higher in surface water, they may contain predators and competitor species, suspended particulate material (silt), and pollutants from agriculture, industry or urban sources.
In coastal and estuarine areas, brackishwater may be more easily available for M. rosenbergii grow- out, abstracted either from surface or sub-surface sources. In many coastal locations, the salinities of local water supplies may vary tidally and seasonally, in association with major rainfall and dry periods. As saline water is denser than freshwater, and unless actively mixed is physically separated at the halo-dine, it lies below the freshwater level, and extends upwards into the lower water column of estuaries, or inwards below the freshwater table in sub-surface zones. Here, excessive drawdown from the water table may encourage the ingress of saline coastal waters; during wet seasons, when freshwater levels are higher, this process may be reversed. As with freshwater, brackishwater quality depends on the source of supply; estuary waters are typically highly mixed, with high silt loads, organic matter and biological activity. In developed areas, urban and industrial wastes may be significant sources of contamination. Groundwaters, by contrast, may be relatively clean and free of biological or other risk factors.
Surface water can be abstracted through a suitable system and directed by gravity (Fig. 8.1) or pumped to the pond supply point. If water quality is good and it can be supplied
by gravity, this is usually the cheapest and simplest water supply arrangement. Natural (resurgent or artesian) springs can provide excellent water quality, but usually have limited flow rates, and several might be needed to meet the water requirements of a commercial farm. Shallower groundwater can be exploited by means of open cistern-type wells and low head pumps, and can be relatively cheap to develop and operate. In suitably porous strata, flow yields can be good, but in many other cases these may be restricted. Deep underground water can be exploited by means of wells, with water quality generally suitable for commercial projects. However, costs may be relatively high. Pumping systems may be economically feasible, but maintenance and operation costs need to be considered carefully in the project feasibility study (Correia &Cavalcanti 1998). If water tables drop and energy costs rise, such systems can very quickly become unviable.
As noted earlier, surface water temperatures are generally closer to air temperatures, while groundwaters tend to have more stable temperatures, equivalent to mean averages. In some cases it maybe feasible to mix sources to obtain desired temperatures, or to use different water supplies at different stages or production periods.
Water supply is the key limiting factor in calculating the potential for pond water exchange, and reflects directly on prawn productivity (New & Singholka 1985; New 2002). For semi-intensive M. rosenbergii culture, a moderate but continuous degree of water replenishment flow system is recommended if possible, in order to:
• maintain the pond water level to compensate for evaporation and seepage losses;
• replace organic and inorganic nutrients, essential to support the trophic web of the pond;
• eliminate toxic soluble materials (such as ammonia and nitrite nitrogen) resulting from the metabolic activity of stock and other organisms, and gases (sulphides, methane, etc.) arising from the anaerobic decomposition of organic matter;
• prevent the depletion of DO2 levels; and
• flush out excessive solids.
In practice, as not all sites or operating conditions allow a more continuous exchange, partial and periodic water replacement is often used to flush ponds to eliminate or reduce algal blooms, avoid DO2 depletion or counteract excessive increases in toxic substances or other stress factors. Estimates of water requirements should also include those needed to fill ponds at the beginning of each cycle, together with consumption for use in any supporting facilities such as the laboratory, processing areas, employees' housing, etc. According to Valenti (1985), water requirements for freshwater prawn ponds vary according to local
Table 8.1 Water requirements for rearing M. rosenbergii in freshwater ponds. (Reproduced from New & Singholka 1985, copyright 1985 with permission of FAO.)
Maintenance flowc (m3/min) d Total farm water Maximum pond __Average consumption0
surface areaa (ha) fillingb (m3/min) Minimum Maximum (m3/min)
Table 8.1 Water requirements for rearing M. rosenbergii in freshwater ponds. (Reproduced from New & Singholka 1985, copyright 1985 with permission of FAO.)
surface areaa (ha) fillingb (m3/min) Minimum Maximum (m3/min)
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