2.1 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.
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-

BOX 1
Removal of iron and manganese

WELL OR BOREHOLE

water is often high in iron and manganese but low in dissolved oxygen (DO2).
Aeration provides a source of DO2, which will convert iron and manganese from their ferrous and manganous forms to their insoluble oxidized ferric and manganic forms. 1
ppm iron (Fe) needs 0.14 ppm DO2 for oxidation; 1 ppm of manganese (Mn) requires 0.27 ppm DO2. Thus, aeration provides a means of removing iron and managanese from water, since the insoluble precipitates formed by converting them to their insoluble forms can be settled or filtered out. Additionally, aeration also helps to strip out the volatile organic compounds and the hydrogen sulphide (H2S) also found in this type of water source.
DO2 should be supplied in an aeration tank, using fine bubble air diffusers. The water must spend at least 10 minutes under aeration (10 minutes residence time). The water should then be circulated through another tank containing biofiltration media. Once this filter has been developed (i.e. run for some time), the iron and manganese particles will tend to fall out of solution and accumulate on the surface of the biofiltration media. In large-scale systems the water is then passed through a pressure filter. However, passing it into a third (settling) tank, where most of the rest of the Fe and Mn precipitates will settle out, should provide water sufficiently low in Fe and Mn for use in your hatchery. It is suggested that the water be allowed to remain in the settling tank for 24 hours before the water is pumped (without disturbing the sediment) into the hatchery for use.
Obviously, the biofiltration media will have to be regularly washed; placing the plastic media within stainless steel or plastic cages makes it easy to remove it from the filtration tank for this purpose. The settlement tank will also need to be cleaned out. The dimensions of the equipment you use depend on the quantity of water you need to treat.

SOURCE: FURTHER DETAILS ON FLOW-THROUGH SYSTEMS FOR STRIPPING WELL WATER AND OTHER TYPES OF WATER TREATMENT ARE AVAILABLE FROM WATER INDUSTRY SUPPLIERS. THIS BOX WAS DERIVED FROM A WWW.GOOGLE.COM LINK TO THE WEBSITE OF DRYDEN AQUA (WWW.DRYDENAQUA.COM), WHICH IS GRATEFULLY ACKNOWLEDGED

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/m3 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.

DECIDING HOW MUCH WATER IS NEEDED

The quantity of freshwater and seawater required for a freshwater prawn hatchery depends not only on the proposed scale of operation but also on the type of management utilized (flow-through, recirculation, use of brine). Flow-through systems obviously require the maximum quantities of water. All other systems will either require less seawater or, in the case of those which utilize brine or artificial seawater, none. It is therefore not possible in this manual to define the exact quantities of water needed, as these are scale, site and management system dependent. An example of the water requirements for a flow-through system using seawater that includes ten 5 m3 larval tanks, each capable of producing 50 000 postlarval prawns (total 500 000 per larval cycle) within a maximum of 35 days, is provided in Box 2.

TABLE 2
Characteristics of water suitable for freshwater prawn hatcheries

 

NOTE: THE SIGN ‘-’ MEANS ‘NOT KNOWN’ OR ‘NO SPECIFIC RECOMMENDATION’.
SOURCE: DERIVED FROM NEW AND SINGHOLKA (1982), CORREIA, SUWANNATOUS AND NEW (2000) AND VALENTI AND DANIELS (2000)

 

TABLE 3
Artificial brackishwater (12 ppt) for M. rosenbergii hatcheries

SALT QUANTITY (G/M3)
Sodium chloride (NaCl) 9 200
Magnesium sulphate (MgSO4.7H2O) 2 300
Magnesium chloride (MgCl2.6H2O) 1 800
Calcium chloride (CaCl2.H2O) 467
Potassium chloride (KCl) 200
Sodium bicarbonate (NaHCO3) 67
Potassium bromide (KBr) 9

NOTE: WEIGH AND DILUTE THE SALTS INDIVIDUALLY WITH PREVIOUSLY FILTERED FRESHWATER. ADD THE RESULTING SOLUTIONS TO A TANK IN THE ORDER SHOWN ABOVE, AND MIX THOROUGHLY USING A PVC STIRRER. THEN ADD FRESHWATER UNTIL THE SALINITY IS REDUCED TO 12 PPT. MAINTAIN THE FINAL SOLUTION UNDER STRONG AERATION FOR 24 HOURS AND ADJUST THE SALINITY AGAIN TO 12 PPT, IF NECESSARY, BEFORE TRANSFER TO THE RECIRCULATION SYSTEM.

SOURCE: VALENTI AND DANIELS (2000)

OTHER REQUIREMENTS FOR HATCHERY SITES

In addition to having sufficient supplies of good quality water, a good hatchery site should also:

have a secure power supply which is not subject to lengthy power failures. An on-site emergency generator is essential for any hatchery - this should be sized so that it has the output necessary to ensure that the most critical components of the hatchery (e.g. aeration, water flow), can continue to function;

have good all-weather road access for incoming materials and outgoing PL;

be on a plot of land with an area appropriate to the scale of the hatchery, that has access to the quantity of seawater and freshwater supplies required without excessive pumping. The cost of pumping water to a site elevated high above sea level, for example, may be an important factor in the economics of the project;

not be close to cities, harbours, mines and industrial centres, or to other activities that may pollute the water supply;

be situated in a climate which will maintain water in the optimum range of 28-31°C, without costly environmental manipulation;

have access to food supplies for larvae;

employ a high level of technical and managerial skills;

have access to professional biological assistance from government or other sources;

have its own indoor/outdoor nursery facilities, or be close to other nursery facilities;
and
be as close as possible to the market for its PL. In the extreme case, it should not more than 16 hours total transport time from the furthest farm it will be supplying.