The success of shrimp farming is measured by its rate of return on investment which mainly depends upon the yield, capital investment, market price and the production cost. This in turn is affected by a number of factors the most important of which are farm operation and management.
Fry mortality often occurs during the initial period of stocking in ponds. Stress by handling, transportation and acclimation are among the main causes. Hence, young fry of size 0.5–1.0 g are first stocking in nursery ponds for about one month to 45 days before being transferred to grow-out ponds.
Fig. 18. Transportion of fry in tanks.
Fig. 19. Fry transportation in polyethylene bags
Nursing of fry can be done either in concrete tank, nursery pond or in net cages installed in deeper ponds or in rivers.
8.1.1 Concrete tank
The tanks are first filled with filtered sea water. The water is aerated. Pure culture of diatoms is then added to maintain water quality and for reducing water transparency. Suggested stocking density is about 5000 fry/m3 of water for P. monodon, 10,000/m3 for P. indicus and P. merguiensis. It is advisable to use artificial substrates such as polyethelene netting to increase surface area in the nursery tank since post-larvae habitually cling to wall surfaces and tank bottom.
The fry are fed with finely chopped mussel or cockle meat. Newly stocked fry are often provided with Artemia nauplii to minimize cannibalism. Young and adult Artemia may also be used to supplement the diet. 50% of the water is changed daily. Flow-through system is used to ensure good water quality for growth and survival.
8.1.2 Nursery pond
The size of nursery pond ranges from 500 to 2,000 m2 with water depth of 40–70 cm. Each nursery pond has at least one gate installed with a fine screen (1 mm mesh size) to prevent undesirable organisms from entering the pond as well as to prevent the escape of stocks (Fig. 20).
Stocking density in nursery pond is about 50–150 fry/m3 depending on the size of the fry.
The nursery pond should be adequately prepared prior to stocking. The pond is completely drained of water and dried until bottom soil cracks. Derris root at 4 g/m3 will be applied when the pond cannot be completely drained. Lime at 500 to 2,000 kg/ha, chicken manure at 500 to 2,000 kg and inorganic fertilizer (16–20-0) at 25 to 100 kg/ha are then applied.
Fig. 20. Nursery pond
About 30% of the water is changed daily. If the pond is not heavily stocked i.e., less than 5/m2, water exchange during rising tide may be sufficient. Chopped mussel or cockled meat are fed to the larvae at the rate of 20% total biomass. The nursing period is 30–45 days or when the larvae reached 0.2–1.0 g body weight.
8.1.3 Nursery cages
Synthetic net cages (0.5–1.0 mm mesh size) installed within bamboo or wooden frames are kept afloat by bamboo raft or synthetic floats. The cages can be used to nurse shrimp larvae. Nursery cages are mostly employed in calm water such as river, lagoon or fishpond. The cage (3 m3) is usually stocked at 1000–2000 fry/m3 of water. The feeding scheme is similar to that being used in earthen pond nursery (Fig. 21).
Optimization of pond yield largely depends on how the ponds are stocked and the operational management thereafter. Stocking of fry is an important operational function. Optimum stocking density depends on the stocking size, natural mortality, pond productivity and culture systems used.
8.2.1 Acclimation
Shrimp fry are very sensitive to abrupt change in environmental conditions such as temperature and salinity. Long duration of transportation always increase water temperature of the containers with shrimp fry. Fry should therefore be acclimated to the pond conditions before being released into the rearing ponds.
Shrimp fry are acclimated to pond water condition by gradually mixing container water with water from the pond. Alternatively, plastic bags containing the shrimp fry are allowed to float in the pond until water temperature stabilized and then release to the pond slowly.
8.2.2 Stocking time
The fry should be stocked preferably in the early morning (0700–1000 hours) or late in the evening (2100–2400 hours) when the pond water temperature is low.
Fig. 21. Nursery cage
8.2.3 Stocking rate
The decision on how many shrimp fry a pond should be stocked depends on the carrying capacity of the pond as well as the required harvesting size. The latter is a decision based on economic returns. The basic principle is that pond capacity should be optimized to produce the best economic returns. Optimal stocking density can be determined based on field experiments.
Example optimal stocking density for Penaeus monodon culture using extensive method of culture operation.
| Stocking density (ind.m2) | Production (kg/ha) | Harvesting size (g) | Growth rate (g/day) | Yield (kg/ha/day) |
| 1 | 305 | 35 | 0.29 | 2.9 |
| 2 | 405 | 30 | 0.25 | 5.0 |
| 4 | 560 | 15 | 0.116 | 4.6 |
It is apparent that stocking density at 2/m2 gives maximum yield as well as best economic returns considering the preferred harvesting size (30–35 g) for export as well as domestic markets.
However, in intensive method of culture operation which depends entirely on artificial feeds and intensive water management, a higher stocking density can be achieved within the limit of pond carrying capacity. The general formula for calculating stocking density is as follows:
STOCKING - EXPECTED YIELD (G) EXPECTED
(NUMBER) INDIVIDUAL GROWTH RATE MORTALITY
Example. If a shrimp farm for P. monodon has a target yield of 1200 kg/ha/crop, expected average body weight is 30 g, and expected survival rate of about 70%, the stocking density is calculated as follows:
The normal stocking density used in difference culture operation systems in shown in Table 3.
Table 3. Stocking density of shrimp in different culture
| Species | Extensive | Semi-Intensive | Intensive |
| (× 103 individuals/ha) | |||
| P. monodon | 5–10 | 20–50 | 50–300 |
| P. indicus | |||
| P. merguiensis | 10–50 | 80–100 | 100–500 |
| P. japonicus | 30–100 | 100–300 | 300–2,500 |
After initial stocking, routine checking of pond conditions should be done every morning to monitor for physico-chemical parameters, physical condition of embankment, water supply and drainage canal, occurence of undesirable species as well as the behavior of cultured stocks. The early morning hours are critical for monitoring oxygen regime of pond water. It is also important to inspect the feeding trays to verify that the feed given the previous day were consumed. If the feeds were not fully consumed, the possible explanation should be found and feeding rate adjusted accordingly.
8.3.1 Water management
Frequent change of pond water is needed to maintain water quality. The process also helps to introduce new food organisms into the pond and stimulate molting of shrimp. In stagnant water, decomposition of accumulated organic wastes or depletion of trace metals may affect shrimp growth.
The water in the pond can be changed through tidal flows or by means of a mechanical pump.
Tidal exchange of pond water is normally practiced in traditional shrimp farm. Water in the pond is drained to one half of the pond level during low tide and is replenished during rising tide. The exchange is only effected within 5– 7 days during spring tide. Pond fertilization has to take place after the last day of the water exchange cycle.
Water pumps of various capacities are used to replenish pond water in semi-intensive and intensive culture operations.
For semi-intensive culture method, the pump is used only during neap tide while tidal water facilitates exchange during spring tide. About 50% of pond water is changed during each replenishment. For intensive culture methods, frequent changes are essential to remove or reduce water contamination due to decomposition of uneaten food and also to maintain the pond oxygen levels in pond water. hence, water at the pond bottom should be replace during each change of water. Quality of water can be effectively maintained in intensive culture method by changing ⅓ of the pond water daily or adopting a flow-through system.
Water analysis kits are field equipment commonly used for monitoring the physico-chemical parameters of pond water. These kits give relatively accurate results, comparatively inexpensive, compact and handy. The kits are easy to use and permit rapid reliable analysis for aquaculture purposes.
The most important variables requiring constant monitoring are dissolved oxygen, pH, ammonia and nitrate. Various methods for measurements are presented in Table 4. Temperature and turbidity can be measured with an inexpensive thermometer and a secchi disc, respectively.
8.3.2 Sampling of stocks
Regular sampling of shrimp stocks in pond is highly necessary to monitor, their growth performance. This is especially important for adjust the amount of feed used according to shrimp size and standing crop. This is done through periodic sampling of the shrimp to determine their body weight.
In the case of Penaeid shrimps, seining is not possible since these species are usually burrowing or wandering types. It is therefore difficult to obtain a representative sample. However, since these are hardy species, a cast net has proven to be an excellent sampling gear. Usually ten casts is sufficient to get a representative sample of one hectare pond. The dimension of a cast net should be measured to determine the area covered in order to calculate the total stock in the pond. The following formula is used to determine the pond stock:
where X = stock in pond
Sampling in early hours of the morning or at night is preferable as the shrimps are more active during these periods and are more evenly distributed that during hot sunny day. For large pond, 15–20 casts may be necessary to obtain a more accurate average.
