Greenwater Part 3: First Moves

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They can be either up-flowing Figure 30 or down-flowing but need to be easy to clean and should be designed to minimize water losses in the system. Sand or bead filters may be adequate for freshwater prawn hatcheries. However, drum screen filters do not clog so much and have an automatic backwash. The volume of these filters needs to be calculated so that they can cope with the volume and flow rate of the specific recirculation system you propose to run.

Physical filters are typically placed in the system before the UV units if used and the biofilters, for maximum efficiency. They need to be flushed on a regular basis at least once a day to prevent them becoming blocked up with organic material and becoming potential sources of pathogenic bacteria.

Sand filters can be backwashed with freshwater and air to save on brackishwater this is especially important when artificial seawater is used. If water is going to be passed through UV units, substantial particle reduction is required to reduce the amount of suspended matter, thus improving the efficiency of this form of water treatment. UV treatment is uncommon in freshwater prawn hatcheries but future research may demonstrate whether its use would be advantageous. Biological filters are essential in recirculation systems Figure There are several types of these filters Figure Submerged biofilters are efficient, simple, and cheap.

The type that is horizontally divided into chambers as shown in Figure 31 seems to be the most efficient. Biofilters require aeration to maintain enough dissolved oxygen to supply the nitrifying bacteria. Crushed oyster shell, dolomite [CaMg CO3 2], or coral 5 mm particles is suggested as the filter medium this provides the surface area where the nitrifying bacteria live. There is a tendency for the water in recirculation systems to become acidic the pH value falls but calcareous media contain an inexhaustible source of buffer material carbonate and bicarbonate ions , which slowly dissolves into the water.

However, plastic filter media, which have no buffering capacity, are often used in biological filters. This is because they are easy to handle and are supplied in shapes and particle sizes which mazimize the surface area available to the nitrifying bacteria. Recirculation systems using plastic media may eventually need buffering by the addition of sodium bicarbonate NaHCO3 or sodium carbonate Na2CO3 to the water to maintain its pH at 7. Using a calcareous filter media avoids this problem.

Placing the filter medium in plastic or nylon bags makes handling easier. It has been estimated that a system rearing 2 million larvae would require about kg of crushed coral within the biofilters once the larvae reach a maximum biomass. This can be modified according to the specific scale of hatchery operations. This means an initial bacterial inoculum needs to be added to the larval rearing system to reduce start-up time; the bacteria will then multiply to cope with the nitrogenous load in the water of the system.

The bacterial inoculum can come from another operational filter or from a separate pre-conditioning tank, which is run at the same temperature and salinity as the larval culture tank.

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Further details on biofilter activation, derived from Valenti and Daniels are given in Box 5. The filtration technology used in marine fish and shrimp hatcheries is generally more sophisticated than the methods described above. The application of these techniques to freshwater prawn hatcheries may prove beneficial in future. Detailed information on these systems is available in Van Wyk et al. Many items of small equipment are essential in every hatchery. These include, for example, buckets, epoxy-resin paint, weighing scales, fibreglass repair kits, nets, tools, nylon and cloth mesh, brushes, flexible tubing, postlarval transport equipment bags, tanks, portable air supply, etc.

All equipment needs to be suitable for use in seawater and free from potential contamination from the leaching of metals such as copper, brass, or zinc. FIGURE 31 Close-up of a biological filter shared between two larval tanks in Brazil, showing the water entering the mechanical filters foreground , from where it passes through the biological filter and exits back to the two tanks by means of simple airlift pumps. BOX 5 Activating biofilters. When this amount is consumed by the bacteria as evidenced by the reduction in total ammonia in water samples , add the same amount of ammonia again.

Repeat the process until the bacteria are able to convert all the ammonia into nitrate within a 24 hour period.

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Then add double the initial amount of ammonia and repeat the process. Keep adding ammonia, monitoring the removal of ammonia and doubling the amount of ammonia added until your biofilter can cope, within a 24 hour period, with the maximum amount of ammonia expected to be generated when there are larvae in the tanks.

Once that maximum bacterial load is achieved, the production cycle can begin. The bacterial population on the media needs to be maintained at the maximum level of ammonia and nitrite consumption. Addition of media to the biofilter should coincide with the increase of NH3-N produced by an increase in the larval biomass. The bacterial population provided through daily addition of media should always be sufficient to remove all ammonia and nitrite.

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When the larval cycle is complete, remove all the biofilter media, thoroughly rinse it, and either store it dry or return it to the pre-conditioning tank to re-establish and maintain the bacterial colony. Alternatively, the substrate can be chlorinated to kill all bacteria, de-chlorinated, and then re-seeded with stock bacteria from another pre-conditioning tank.

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Each cycle of operations in a freshwater prawn hatchery takes up to 40 days, including the time necessary to prepare for the next cycle. Careful attention to all aspects of hatchery management is essential to achieve success the production of the maximum number of healthy postlarvae at the cheapest cost. Water needs to be treated before it can be used in the hatchery.

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If the incoming water is not filtered, or is still turbid, it may be necessary to allow the solids to settle in one tank before transferring it to another tank for treatment. Mix the seawater or brine with freshwater to form 12 ppt brackishwater see Table 4. Then treat the water as in Box 6. Water drawn from underground sources may not need to be settled However, the removal of protozoa and bacteria by chlorination, as indicated in Box 6, is still essential. Other forms of water treatment may be helpful.

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  5. Some add 10 ppm of the chelating agent EDTA to larval rearing water to improve performance. Others use water which has a potentially unsuitable iron content see Box 1. BOX 6 Treatment of brackishwater. It is essential to remove as much of the suspended solids as possible, otherwise the chlorination that follows may be partially or totally ineffective.

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    Chlorinate the brackishwater with 5 ppm of active chlorine 7. Remove the residual chlorine by vigorous aeration for 6 hours before use note : sodium thiosulphate can be used to remove the residual chlorine more quickly but its use is not recommended because it also may prove toxic to larvae.

    Vigorous aeration for 6 hours is adequate. Water quality remains important, not only in the incoming supply but also within the hatchery itself. Monitor the water in your larval tanks frequently to see that its quality is being maintained Box 7.

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    Simple field kits can be purchased to check the parameters listed in Box 7 but they are not specified in this manual because they are standard water quality items. For small flow-through hatcheries it is not practical to install facilities for the other types of analytical work, especially where they involve the analysis of seawater or brackishwater. Samples of water for the analysis of other parameters, such as hardness, metals, pesticide residues, etc.

    Further reading on water quality and analysis is available in Boyd This salinity allows larvae to be stocked directly from the hatching tank without acclimation. The salinity in the culture tank should then be increased to 12 ppt. Neither artificial brackishwater nor freshwater should be added through the biofilter tank once the biofilter substrate has been added. Its bacteria are sensitive to sudden changes in temperature and salinity. New water should be disinfected and filtered prior to introduction into the system. The use of ultraviolet light UV to treat the water in recirculation systems has been recorded in experimental hatcheries for M.

    The pH of the water used in recirculation systems does not usually vary much during the larval cycle but occasionally, especially if the biofilter medium is artificial non-calcareous , periodical buffering with sodium bicarbonate NaHCO3 or sodium carbonate Na2CO3 may be necessary, as noted earlier. Temperature should be kept steady, not only for the sake of the larvae but also because the biofilters do not operate efficiently if it fluctuates.

    The biofilters should maintain levels of unionized ammonia, nitrites and nitrates at acceptable levels. BOX 7 Regular monitoring of larval water quality. NOTE: dissolved oxygen meters are very expensive in some countries. Monitoring this parameter is ideal but can be omitted if you are sure that your aeration system is working perfectly. The following additional parameters should be measured in recirculation systems:. Captive broodstocks are not normally maintained in tropical zones where a ready supply of berried females is available from the wild or from grow-out farms, even though there may be advantages in doing so, as discussed earlier in this manual.

    Slight salinity results in better egg hatchability and recent research Law, Wong and Abol-Munafi indicates that careful control of pH markedly enhances the hatching rate hatchability. Temperatures below the optimum also cause some eggs to drop and increase the time for egg development.

    Light does not seem to affect egg hatchability, although direct sunlight should be avoided. There is no need for you to feed females when they are only being held for a few days simply for larval collection. You can hatch your larvae in a special broodstock holding system see Figure 12 and then transfer them to larval rearing tanks in 12 ppt water. In hatcheries operating recirculation systems, newly hatched larvae Stage I are often harvested from the broodstock holding tank using a collecting device. If you are operating a simple flow-through hatchery you can place females with brown to grey eggs directly into the larval tanks. Then, remove the females with a coarse dip-net after their eggs hatch. Some hatcheries put the females into coarse-meshed cages within the larval tanks, which makes them easier to remove after their eggs have hatched.

    When females are put into the larval rearing tank the water level should be about 30 cm and, as noted above, the salinity should be about 5 ppt with a pH of 7. Egg hatching, which occurs predominantly at night, can be observed by the presence of larvae in the tank and the absence of eggs on the underside of the abdomens of the females. Use a white board Figure 33 to make it easier to observe larvae.