A STUDY OF THE POTENTIAL:
market for the product and careful selection of suitable sites for prawn culture, whether it be for the larval (hatchery) or grow-out phases, is an essential prerequisite for successful farming.
Failure to realize this before any project is commenced is likely to cause the ultimate downfall of the enterprise, which not only has unfortunate consequences for the farmer and investor(s) involved but may also cause serious damage to the image of prawn farming, both nationally and even internationally.
The current section of the manual contains a brief description of the essential characteristics of good sites for freshwater prawn farming. More detailed information is available in a review by Muir and Lombardi (2000).
You are also strongly recommended to obtain and study the FAO manuals on topography (FAO 1988, 1989b), soils (FAO 1985), and water (FAO 1981), as well as the section on site selection in FAO (1995)1 .
Hatcheries and indoor nurseries:
The site requirements for hatcheries and indoor nurseries, which are normally associated with each other, are similar. In this section of the manual, reference to hatcheries therefore includes indoor nurseries.
Needs For Good Quality Water:
Although the larval stages of freshwater prawns require brackishwater for growth and survival, hatcheries do not have to be located on coastal sites. Prawn hatcheries can be sited on inland sites. There, the necessary brackishwater can be obtained by mixing locally available freshwater with seawater or brine (and sometimes artificial seawater) which has been transported to the site.
Two decades ago, when the original FAO manual was written, most hatcheries operated on flow-through systems. Many still do so but the establishment of inland hatcheries, the costs of obtaining and transporting seawater or brine, and increasing concerns about the discharge of saline water in inland areas have encouraged some operators to minimize water consumption through partial or full recirculation systems.
Inland hatcheries have the advantage that they can be sited wherever suitable freshwater is available and their market (namely outdoor nurseries and grow-out facilities) is close by. Where to site a hatchery is therefore not only a technical but also an economic consideration.
This involves balancing the costs of transporting seawater and brine, or using recirculation, against the advantages of an inland site. Prawn hatcheries, regardless of type, require an abundant source of freshwater as well as seawater or brine.
The quality of intake water, whether it be saline or fresh, is of paramount importance for efficient hatchery operation. Water quality is thus a critical factor in site selection. Hatchery sites should preferably be far from cities, harbours and industrial centres, or other activities which may pollute the water supply.
Due to the extra problems and dangers involved, it is generally recommended that freshwater prawn hatcheries should not be sited where the only source of water is surface water. However, this guidance has not always been observed.
The minimum requirement during site evaluation should be to carry out watershed surveys and water analyses, especially for pesticides and oil spill residues. In coastal areas, it may be possible to draw good quality water from sub-surface layers, usually with freshwater overlying more saline water.
The ideal site, where wells sunk to different depths provide both freshwater and seawater, is rare, although it is sometimes possible to make good use of groundwater sources, which are usually cleaner and less liable to become contaminated. The quality of water depends on the soil materials. In coastal areas with underlying coral rock, hatcheries can often get good quality seawater, free of pollution or harmful protozoa and bacteria.
If sites with borehole seawater are not available, direct access to a sandy beach with mixed sand particle size can be selected. On this type of site a shallow beach filter of the type described in Annex 2 can be utilized. Muddy areas are not so suitable, but a larger filter may be used, provided it can be cleaned out periodically.
Many freshwater prawn hatcheries utilize surface supplies for both freshwater and seawater. Often, seawater can be drawn from areas where the salinity is 30 to 35 ppt, usually through a rigid pier off-take in the sea or a flexible buoyed system. Crude screening can be used to prevent the entry of the larger flora and fauna but this alone is not sufficient to protect the larvae from disease and parasitical problems. The use of unfiltered water will almost certainly result in disaster, so additional filtration is essential.
Brine, sometimes used instead of seawater for inland hatcheries to minimize transport costs, can be obtained from salt evaporation pans. The brine, which is often between 80-100 ppt salinity but can be as high as 180 ppt, can be diluted with freshwater to form brackishwater (in theory, the higher the salinity of the brine used, the better; this is because the sudden osmotic shock which occurs when brine and freshwater are mixed together may reduce the numbers of bacteria and parasites present in the original supplies).
Some hatcheries obtain freshwater pumped or fed by gravity from surface supplies such as rivers or irrigation canals. This practice exposes the hatchery to severe variations in water quality and particularly to water contamination from agricultural chemicals.
In all cases, water supplies need careful analysis during site selection, to determine their physical, chemical, and biological characteristics, and the extent to which these may vary daily, seasonally, or through other cycles.
Special care is needed where hatcheries are situated in or near areas where the use of pesticides, herbicides, and fertilizers is intensive. Ideally, freshwater should be obtained from underground sources, though some of these may be unsuitable because of high levels of iron and manganese, which are lethal to prawn larvae. Methods of reducing the levels of these ions are provided later in this section of the manual.
City tap water is also normally suitable, provided it is vigorously aerated for 24-48 hours before use to remove residual chlorine, but may be too expensive to use. Well water should also be aerated, by cascading for example, to bring its dissolved oxygen level up to, or near to saturation point.
The brackishwater derived from the mixture of seawater, brine or artificial sea salts with freshwater for use in M. rosenbergii hatcheries should be 12-16 ppt, have a pH of 7.0 to 8.5, and contain a minimum dissolved oxygen level of 5 ppm. Water of various levels of salinity is also required for hatching Artemia as a larval food (Annex 4); the ideal hatching salinity depends on the source of cysts.
The use of estuarine water, which would theoretically limit the need to balance freshwater and seawater to obtain the optimum salinity, is possible. However, the salinity of estuarine water varies, both diurnally and seasonally, making management difficult. In addition, although estuarine water can be utilized if its salinity is above the hatchery operating salinity, its use is not recommended because the levels of micro-organisms and potential pollution may be high.
Both freshwater and seawater must be free from heavy metals (from industrial sources), marine pollution, and herbicide and insecticide residues (from agricultural sources), as well as biological contamination (e.g. as indicated by the presence of faecal coliforms, which can be common in residential and agricultural areas). The analyses of water found suitable for use in freshwater prawn hatcheries are given in Table 2.
Not much is known about the tolerance of larvae to toxic materials but it can be assumed that larvae are at least as (probably more) susceptible to pollution and toxicity as juveniles. As safe and lethal levels of specific substances are not yet fully understood, it is inappropriate to
provide a summary of current research in this manual. Those who wish to know more about this topic are recommended to consult Boyd and Zimmermann (2000), Correia, Suwannatous and New (2000) and Daniels, Cavalli and Smullen (2000). If seawater or freshwater is drawn from surface supplies, some form of treatment is essential, as discussed later in this manual.
Both freshwater and seawater used for hatchery purposes should have a pH and a temperature as close as possible to the optimum range. Hydrogen sulphide and chlorine (e.g. from tap water) must be absent.
High levels of nitrite and nitrate nitrogen must be avoided. Seawater should have as little diurnal or seasonal variation as possible. Very hard (reported as CaCO3 level) freshwater should be
avoided. The levels of iron (Fe) and manganese (Mn) should be low; copper (Cu) toxicity may also be a problem, especially after larval stage VI. However, some iron and manganese can be precipitated from well water by aeration; the resultant floc can be removed by sand filtration, or by biofiltration and settling (Box 1).
High levels of heavy metals, such as mercury (Hg), lead (Pb) and zinc (Zn), should also be avoided - these are most likely to be caused by industrial pollution. In general, especially where surface water is used, hatcheries should not be sited where their water supplies are endangered by pollution from tanker discharge, oil refineries, tanning, agricultural pesticides and herbicides, or chemical factories. In practice, an ‘ideal’ water supply might be difficult to define, but a summary of the characteristics of water found suitable for use in freshwater prawn hatcheries is provided in Table 2.This table can be found in article deciding how much water is needed for freshwater prawn culture .
Artificial seawater has been used in some recirculation systems, especially in research. The stimulus for such work is that its use may reduce the problems caused by water pollution, parasites, and the presence of prawn competitors and predators in larval rearing tanks.
Many formulations for artificial seawater exist and commercial preparations are sold in the aquarium trade. However, not all have been found suitable for fresh- water prawns and many are complex and expensive. The exact and specific ionic composition that is optimum for freshwater prawns is not yet known. The formula for a simple preparation which has been used in Macrobrachium rosenbergii hatcheries is given in Table 3.
This contains the essential ions sodium, potassium, chloride, bromide, carbonate and sulphate, together with the correct ratio of calcium and magnesium. This preparation may not be complete, and there is some evidence that its use increases oxygen consumption after larval stage V, but it (and variations of the formula) have been used in research and a few commercial cycles in Brazil. The unit cost, even for such a simple formula, is high (e.g. US$ 75/m 3 in Brazil in 2000).
However, not much is required because evaporative losses can be made up with freshwater alone and, if properly handled and processed, the same brackishwater can be used for two consecutive larval cycles without affecting production.
The productivity of systems using artificial seawater is reported to be as high as 40 PL/L but the larval cycle may take about 10% longer than when natural seawater is used. Due to its cost and the uncertainty about its effectiveness, the use of artificial seawater is not recommended in this manual. Whenever possible, the use of natural seawater or brine is recommended.
Author:
Michael B. New
