Changing water has a beneficial effect on water quality in a pond. In a pond with static water, accumulation of waste products or depletion of trace metals or organic compounds can have a harmful effect on shrimp. Such occurrences do not always result in mass mortality which would be easily recognized. They can exert small effects on growth which pass by unnoticed. The end result is the same, however, poor production (see Section 3).
Frequent water exchange is also beneficial in introducing new food organisms to a pond. In a pond where water is not changed for a long period of time, all the desirable food organisms may be eaten. Or a species not well suited as a food organism may become dominant, suppressing growth of more desirable species. If heavy rains dilute the pond water, species dominant in the pond might not be well suited for growth at the lower salinity. These will die off or grow slowly.
Following are recommendations for water change in different types of culture.
Water should be changed as often as possible. Ordinarily, this would be on every high tide. This procedure ensures entry of the maximum number of young shrimp and brings in food organisms.
Two schedules for changing water are presented below. Both have been used successfully.
Water is changed every 12 to 14 days. When changing water, one-third of the water in the pond is drained and replenished each day for two or three days. Fertilizer is applied after the water change and then again after six to seven days.
One-half to one-third of the pond water is exchanged once a week. Fertilizer is applied after every change of water.
This type of management requires frequent water change to dilute the waste products formed by the decomposition of unused food and also to ensure that adequate oxygen levels are maintained in the pond water. Decomposing food can easily use up all the dissolved oxygen in the water near the bottom. For this reason, water should be discharged from the bottom of the pond. Two types of water exchange which have been used successfully in ponds are described below.
A one-third change of water daily by draining and refilling is used in Thailand. Refilling is by pumping.
In Panama, it was reported that when the level of dissolved oxygen in a pond is 3 ppm or above, water is flowed through the pond at a rate sufficient to change 3 percent of the water daily. If the level of dissolved oxygen in a pond decreases below 3 ppm, the flow of water is increased.
Drying the pond bottom periodically is an accepted practice in brackishwater farming within the region. The main reason given for doing this is to mineralize the organic material which builds up in the soil. This makes more nutrients available for plant growth. It also reduces the production of H2S and other harmful substances that would be produced during anaerobic reduction of the organic material when the pond is full of water. If a pond is completely dried, all unwanted predators and competitors are killed and there is no need to treat with chemicals to get rid of them. Drying the soil is especially useful in ponds where lab-lab is grown for food. The firm soil provides a good surface for the algae to attach to.
A word of caution is needed concerning drying pond bottom in areas which have soils of high potential acidity. During the drying process pyrites can be oxidized. When the pond is filled, acids are formed and pH of the water is lowered. This type of pond should be flushed thoroughly after drying.
Some participants advised that for lab-lab production in ponds with hard bottoms, the soil should be tilled after drying. This turning over of the soil not only helps loosen it, but it also helps in the mineralization of organic matter. However, others commented that in their experience, the benefits from loosening the pond bottom never justified the expense. More research is needed in this area. There is recent evidence that oxidizing acid sulfate soils may actually be harmful. This is particularly true where recent digging or tilling has exposed fresh soil to the air. So tilling is never recommended for acid sulfate soils. We would not recommend tilling of non-acid pond bottom soil on a regular basis. Tilling should be done only when the bottom is hard, and production during the previous culture period was low.
There is no agreement on the drying procedure itself. There is no standard procedure, either for the length of time the soil must be dried, or for how often the drying should be done. Excessive drying seems to be harmful, and over-drying which results in crumbling and reduction of the thin surface crust to powder is to be avoided. Following are some procedures recommended by the participants.
In ponds with internal canals, the accumulated mud and organic debris must be removed periodically. Cleaning and deepening the internal canals should not be done while shrimp are in the pond. The large amount of H2S released by digging could cause some shrimp to die. It should be done while the pond is being dried. The excavated sediment is usually thrown on the dike by hand. It is important that the dikes be covered with grass so that the material removed is not washed back into the pond with the first heavy rain.
In ponds where water is not changed frequently, soil pH should be at least 6.5 for proper management. Ponds with a soil pH lower than 6.5 can be managed only as long as frequent water changes can be made. A change of water is required at least every three days. Methods for sampling and determining soil pH are given in Sections 3.2 and 3.2.2.
One way of improving ponds with acid sulfate soil is to repeatedly dry the pond and then flush it by repeatedly filling and draining. Acids formed by pyrite oxidation will gradually be removed by this process. After a pond is dug in an acid soil area, it should be flushed well until no, or only a little, red coloured scum from oxidized iron is observed. Lime should be added only after the pond is flushed.
Lime can be used to control soil and water acidity. Application rates for brackishwater ponds can be determined by following the procedures given in Section 12.6. If it is not possible to perform the soil test to determine the correct amount of lime to be added, a soil pH can be taken and the following guidelines followed. Due to the high cost of treatment, applying agricultural lime may not be advantageous when soil pH is very low, less than 2.5. For soil with a pH of 5, treatment with 3 tons per ha of agricultural lime has been effective. When lime tailings are used (from hydrate of lime processing), only one-half of the recommended dose of agricultural lime is used. The lime should be worked into the soil. This can be done with a hand pulled harrow. Another method is especially recommended for old fishponds. The pond bottoms are treated by broadcasting 1.5 tons of agricultural lime per hectare. The bottom is then levelled and another 1.5 tons is worked into the soil.
Lime of calcium carbonate (calcite) is not soluble at the pH of seawater and is not an effective buffer in seawater. Thus, while agricultural lime will raise the soil pH, it will not have much of an effect on maintaining pH of the pond water. Natural carbonates which contain a small percentage, a minimum of 4 percent magnesium (e.g. dolomite, mollusc shells or coral), are more soluble at the pH of seawater and will aid in maintaining optimum alkalinity and pH levels of the pond water (King, 1973). So it would be useful to place some of these materials in a pond to protect against reduced water pH.
If the dikes are constructed of acid sulfate soils, careful water management can reduce the problems associated with them. “By maintaining water levels in adjacent ponds equal and keeping this level higher than water levels in the canal system, the transfer of acids and active aluminum and iron into ponds by seepage through dikes can be limited. Proper control of the water table in drained pond soils can be used to limit the proper depth of soil drying, thereby limiting pyrite oxidation and acid formation” (Potter, 1976).
Controlling erosion to prevent acid runoff into the pond is especially important when the dike soils are acid sulfate or when material from internal canals is thrown on the dikes during cleaning and deepening. Acid tolerant African Star Grass (Cynadon plectostachus) provides good vegetative cover. Other Cynadon species also are worth trying. The following procedure is recommended to establish grass on acid soil (Anonymous, 1977). Planting should begin at the start of the rainy season. First, the soil should be tilled to a depth of 5 cm. Then agricultural lime is added in the amount determined by soil acidity. Fertilizer is added next; 5 tons per ha of chicken manure and 35 kg per ha of 14-14-14. The prepared area is covered with a 5 cm thickness of rice straw. Cuttings are then planted at 30 cm intervals.
Before shrimp are stocked, eggs and larvae of competitors such as noxious fish, crabs, and fish should be killed by poisons.
“Lab-lab” is characterized mostly by benthic blue-green algae and diatoms, but many other forms of plants and animals are associated with it and contribute to its nutritional value. For good growth, “lab-lab” requires low water levels from 5 to 40 cm. Best growth is reported to be at salinities of 25 ppt or higher.
The requirement of “lab-lab” for high salinity is not compatible with optimum growing conditions for P. monodon which is reported to grow best at slightly lower salinities (10–25 ppt). It is well suited for P. indicus/ merguiensis. However, the shallow water requirement for “lab-lab” means the pond water will become too hot for almost all species of shrimp. This is especially true for large adults.
Two suggestions were made for utilizing “lab-lab” in shrimp culture.
“Lab-lab” can be used for shrimp culture during the first two months of culture or up to a point when the shrimp grow to a size of 10 cm. Experience has shown that after shrimp reach this size, survival is reduced greatly in ponds managed for “lab-lab”. The survivors grow to a large size. From this, it can be assumed that “lab-lab” might be a suitable food to grow in a nursery pond.
Ponds can be constructed with a large number of interior canals at least 1.5 m deep to provide shrimp with shelter against high temperature during the day. As shrimp feed almost exclusively at night, the shallow portions with “lab-lab” would serve as feeding platforms on which the shrimp could graze during the cool of the night.
“Lumut” is composed primarily of filamentous grass-green algae. Many other forms of life are associated with these algae and contribute to the nutritive value of “lumut”.
“Lumut” grows best at low to medium ranges of salinity, 25 ppt and below. The most favourable water depth is from 40 to 60 cm. These growing conditions are considered to be satisfactory for P. monodon and other species of shrimp.
“Lumut” should not be grown in a nursery pond because the postlarvae become tangled in it and die. Heavy growths of “lumut” can even be harmful to adult shrimp, and it is recommended that some fish be stocked in the pond to eat the “lumut” and keep its growth down. In fact, “lumut” is best used for polyculture and not monoculture. Milkfish, mullet, rabbit fish, and scad are all suitable. Tilapia can be used, but this is recommended only when the growing period is short since this fish propagates so fast.
Phytoplankton is composed of small plants which float in the water. A pond in which phytoplankton is grown has a lot of small animals (zooplankton) as well as pieces of organic material which also serve as food. Shrimp do not feed directly on the phytoplankton. They feed on the small animals that eat the phytoplankton or on bacteria that grow on the dead phytoplankton cells which accumulate on the bottom.
Phytoplankton production is better in ponds with a water level of 70 cm or more, but it has been grown in shallower ponds. One must keep in mind that phytoplankton is composed of living organisms which have environmental tolerances. Most types of phytoplankton are normally found in deeper water where temperature does not get as high as it does in shallow ponds. The high temperature might restrict their growth. Some people have had difficulty in maintaining plankton growth in low salinity water. Others report that plankton can be grown at low salinity. This difference is probably due to the management system and type of fertilizer used and it should be assumed that phytoplankton will grow in almost any salinity. Types of phytoplankton which give the water a yellow-green or yellow-brown colour are good. Heavy mortality of shrimp has occurred in ponds when the water had a bright green or reddish colour.
The conditions suitable for growing phytoplankton are well suited for shrimp growth at all life stages.
It was mentioned that heavy application of organic fertilizers encourages the growth of chironomid larvae which provide for good growth of shrimp. Yang (personal communication) reports that P. merguiensis likes to feed on chironomid larvae and a 0.06 g juvenile consumed 23 Chironomid larvae in 24 hours. Dense populations of chironomids are often associated with low levels of dissolved oxygen, however, and care should be taken in encouraging their growth in this manner. Heavy populations of chironomids graze down lab-lab.
Start with phytoplankton or “lab-lab” for no more than the first two months after postlarvae are stocked.
After this, the shrimp should be held in ponds managed for production of phytoplankton (all salinities) or “lumut” (low salinity).
This can be accomplished by growing the shrimp in a nursery pond for the first two months and then transfering them to another pond. A second method is to keep the water level in a pond low for the initial two months and then raising the water level sufficiently to encourage the growth of other types of plants.
Soils with a high clay content support the best growth of “lab-lab”. The relationship between soil texture and algal growth can be seen in the accompanying table from Villaluz (1953).
|Sample||Percent sand||Percent silt||Percent clay||Soil texture||Growth of benthic algae|
|2||15||44||42||Silty clay loam||Abundant|
|3||63||14||23||Sandy clay loam||Few|
|4||79||10||11||Sandy loam||Very few|
Preparation of the pond soil is very important in growing “lab-lab”. To assure a uniform growth of algae, the pond bottom should be levelled so that there are no high points or depressions. The pond bottom must be firm enough to serve as a hold fast for the algae, but not hard. Firming the pond bottom is done by drying. The bottom should not be bone dry. It is best to dry it just until a man can walk on it without sinking in. It usually takes 7 to 10 days drying to reach this point.
Growth of “lab-lab” is also directly related to the amount of organic matter present in the soil. Villaluz (1953) reported the following relationship of organic matter to the growth of algae.
|Organic matter (percent)||Growth of algae|
|Above 16||Very abundant|
To increase the amount of organic matter in the soil, fertilizer, chicken or other manure is applied to the dry pond bottom at the rate of 350 kg/ha. The chicken manure should be dried and not treated with insecticide. If no manure is available, inorganic fertilizer can be used: one or two 50-kg bags of 18-46-0 (N-P-K) or two or three 50-kg bags of 16-20-0 per ha.
Immediately after fertilization, 3 to 5 cm of water is let into the pond. After one week, the same amount of fertilizer is applied and the water level is raised to 10 to 15 cm. The fertilization is repeated after the second week and the water level is raised to 20 to 25 cm. Additional water is added as needed to make up for that lost by evaporation. Some farmers recommend refertilization every seven days during the culture period.
Soft mud bottoms with pH of 6.8 to 7.5 favour rapid growth of “lumut”. Bottom with a pH lower than 6.5 should be “washed” or treated with lime. The degree of success of the liming will depend to a great extent on how well the lime is incorporated in the soil. If possible, it should be mixed in to the soil.
The pond bottom must be dried for “lumut” culture also, but only for three days. After the bottom has been dried, sufficient water is let in to moisten the soil and the pond bottom is seeded. This is done by sticking a portion of the filaments of very young plants, or light green ends of older plants, into the mud. It usually takes two to four weeks from the time of planting until the pond is ready for stocking. After the seeding is completed, the pond is flooded to a depth of 20 cm. Three to seven days after planting, the pond is fertilized with 16-20-0 at a rate of 18 to 20 grams/cubic metre (m3) of water. The inorganic fertilizer can be applied by broadcasting or by dissolving from a platform placed 10 cm below the water level. After one week, the water level is raised to 40 cm. Starting with the second week, weekly application of fertilizer at the rate of 9 to 10 grams/m3 of water is continued until six weeks before the crop is to be harvested.
In unfertilized or underfertilized ponds, starting growths of “lumut” are yellowish-green. As growth continues the colour turns to grass-green. When the plants have reached the surface and spread out, only the fringes and those directly over the bottom continue to have this healthy colour. Those on the top, especially at the centre of the floating mass, become yellowish. During dry season, this colour changes to dirty-brown. The portion of the algae near the bottom turns the same yellowish or brownish colour after the initial growth subsides. In contrast, algae in ponds correctly fertilized retain the healthy grass-green colour. A clear indication that the amount of fertilizer is too little is slow growth and yellowing of the algae. A slight overdose of fertilizer causes the algae to become dark-green. Growth is stopped and the algae may settle to the bottom and disintegrate. Sometimes a dense growth of phytoplankton occurs. In this case, the pond water should be changed immediately to prevent complete loss of the “lumut”.
The effects of adding fertilizer are not confined to the “lumut”. Organisms such as bacteria, protozoans, diatoms, nematodes, small crustaceans, etc. which attach to the algae increase in number and serve as food for the shrimp. Examination of “lumut” from fertilized ponds showed layers of organisms twice as thick as the algal filaments (Padlan, undated).
Rows of twigs and small branches should be placed in the pond to keep wind waves from dislodging the “lumut”. Twigs placed closely in lines 6 to 15 m apart and perpendicular to the direction of the prevailing winds will minimize wave action and catch stray algae that have been broken loose. With adequate wind breaks the water can be maintained at a depth of 60 cm.
In shrimp culture the benefits of fertilization are indirect. That is, fertilization causes a good growth of phytoplankton, various micro-organisms feed on the phytoplankton and the shrimp feed on the microorganisms. There is little information available concerning fertilization of brackishwater ponds to grow phytoplankton. It has been observed that growth of shrimp is better in ponds in which the most common types of algae are true diatoms. Poor growth has been observed in ponds in which the predominant algae were phytoflagellates. These two types of phytoplankton have different nutrient requirements. In laboratory and tank culture nitrogen (N) to phosphorus (P) ratios of 20 or 30 to 1 have been found most suitable for diatoms and ratios close to 1:1 most suitable for phytoflagellates. The same nutrient requirements should also hold true for algae growing in ponds. To aid in calculating how much of each element to add, the following table gives suggested amounts of nitrogen and phosphorus to use at various levels.
|ppm Nitrogen||ppm Phosphorus|
One of the most important factors to consider in a programme of fertilization is that both nitrogen and phosphorus do not remain in solution for very long after they are added to the pond water. They become incorporated in living organisms or in the bottom soil. This is especially important for N as larger amounts are added. Mandal (1962) reports that following the application of ammonium-bearing fertilizers, most of the added nitrogen was absorbed by colloids in the bottom soil within a few days and remained strongly bound there. The amount of nitrogen absorbed in the bottom soils was quite small when a nitrate fertilizer was added, and a higher level of available nitrogen was maintained in the water. He points out that in selecting the form of nitrogenous fertilizer, ammonium or nitrate, to use in salt water pond, consideration should be given to the type of organisms to be cultured as food. If phytoplankton is to be grown, nitrate fertilizers would be better. If bottom growing organisms such as blue-green algae are to be cultured, ammonium-based fertilizers would be better.
As a great portion of the nutrients added to a pond became bound up in the soil after a short time, frequent applications of small amounts of fertilizer give the best results. About every 7 to 10 days is recommended.
The nutrient composition of seawater varies both from location to location as well as seasonally. Consequently, a programme of fertilization that works successfully in one location might not be good in another area. Also, it might be necessary to vary the rate of fertilizer used at different times of the year.
The best way to develop a suitable method for fertilizing pond water is to apply a moderate amount and observe what effect it has on phytoplankton growth. Then adjust the rate of application up or down as necessary. To judge the density of phytoplankton growing in the pond, a Secchi disc can be used (see Section 12.5). When the Secchi disc reading is about 30 cm, phytoplankton density is good. If the Secchi disc disappears from sight at less than 25 cm, the phytoplankton is too dense and the pond water should be changed. The next application of fertilizer should be reduced. If the Secchi disc disappears from sight at more than 35 cm, phytoplankton growth is not enough and more fertilizer should be added during the next application. Eventually a farmer will learn how much fertilizer is required to maintain a good growth of phytoplankton in his pond.
A level of 0.95 ppm nitrogen and 0.11 ppm phosphorus should be suitable as a starting dose. The following method can be used to calculate the amount of nutrient required to achieve these levels. First, estimate the volume of water in the pond. For example, a one-hectare pond has a surface area of 10 000 m2. If it has an average water depth of 60 cm, the volume of water in the pond would be 10 000 m2 × 0.6 m = 6 000 m3. One ppm is equal to 1 gram per m3 of water. So to find the amount of nitrogen which should be added to the one-hectare pond to get a level of 0.95 ppm, the volume of water is multiplied by 0.95 g, thus:
6 000 × 0.95 g = 5 700 g or 5.7 kg N
The quantity of phosphorus to be added is found in the same manner.
6 000 × 0.11 g = 660 g or 0.7 kg P
Once the amount of nutrient required is determined, the amount of fertilizer which contains the desired amount of nutrient can be determined, as follows:
If the pond is to be fertilized with ammonium sulfate which contains 21 percent nitrogen, then the quantity of ammonium sulfate required is as follows:
Triple superphosphate contains 39 percent phosphorus. So following the same procedure, the amount of triple phosphate required would be:
The percent nitrogen (N) in some common fertilizers are:
|Urea - CO (NH2)2||=||46.6%|
|Ammonium sulfate - (NH4)2 SO4||=||21%|
|Ammonium chloride - NH4C1||=||25%|
|Ammonium nitrate - NH4NO3||=||37%|
|Calcium nitrate - Ca (NO3)2||=||17%|
The percent phosphorus (P) in superphosphate is:
|Double superphosphate - Ca (H2PO4)||=||26%|
|Triple superphosphate - P2O5||=||39%|
Many fertilizers contain more than one primary nutrient. In these, the primary nutrients are designated by a numbering system indicating percentages in each nutrient. The numbering system is always listed in the following order: N (nitrogen), P (available phosphoric acid P2O5), and K (potash K2O). K is usually present in sufficient quantity in brackishwater and it is not necessary to add any.
By referring to the numbers printed on a fertilizer bag, one can tell which nutrients and how much of each are contained in each bag of fertilizer. For example:
12-24-12 contains 12% N, 24% available P2O5 and 12 K.
16-20-0 contains 12% N, 20% available P2O5 and 12 K.
45-0-0 contains 45% N, no available P2O5 or K.
0-0-60 contains no nitrogen or available P2O5, but has 60% K.
Since these numbers are percentages, a 50 kg of 12-24-12 would contain 6.0 kg N, 12.0 kg available P2O5 and 6.0 kg of K2O. As P2O5 contains only 44 percent P, the weight of P is 4.7 kg (Davide).
It is not compulsory to use only inorganic fertilizers, organic fertilizers can be used as well. The percent of N and P in some types of organic fertilizers is listed in Table 6. Frequently, additional N or P is required to obtain maximum benefits from the organic fertilizer. This is well illustrated by the results of pond culture experiments with milkfish reported by Camacho, 1977. The following table gives total weight harvested (milkfish plus wild species) in kg per ha from forty six 500 m2 earthen ponds. The ponds were stocked at a rate of 3 000 milkfish per ha. The rearing period was six months. The ponds were under different fertilization and water management schemes, i.e. lab-lab and plankton.
|Chicken manure + urea*||514.0||826.7|
|Chicken manure + ammonia phosphate**||424.0||721.3|
|Chicken manure + phosphate***||878.3||341.7|
It can be seen that highest production in those ponds managed for plankton was obtained when additional N was supplied. Conversely, highest production in the ponds managed for lab-lab was obtained when additional P was added. Forgetting the species being cultured, the important point is that plankton produced more when the level of N was high in relation to P.
The platform method is an effective way to apply inorganic fertilizers to ponds for producing and maintaining good growths of phytoplankton. It is good because the nutrients from the fertilizer on the platform are released into solution slowly and distributed through the pond by water movement. A typical platform is shown in Figure 16. The platform should be positioned so that its top surface is about 15 to 20 cm below the water surface, and located near the end of the pond from which the prevailing wind comes. A single platform is sufficient for pond up to 7 ha when plankton is grown. Suggested platform top sizes for ponds of different sizes are:
|Pond area (ha)||Platform top dimensions (m)|
|1||0.85 × 0.85|
|2||1.25 × 1.25|
|3||1.50 × 1.50|
|4||1.70 × 1.70|
|5||1.90 × 1.90|
|6||2.10 × 2.10|
|7||2.25 × 2.25|
An application of fertilizer is simply piled onto the platform and left alone (Anonymous, 1976b).
Supplementary feeding of shrimp is still in the early stages of development in the region. Most feeding is done to supplement natural productivity, or as an emergency measure when growths of natural food in a pond become depleted. Numerous feeds have been used with varying degrees of success. Types of feeds used are:
Only a limited amount of information is available about intensive culture of shrimp. Much experimentation has been done especially for P. monodon. SEAFDEC is testing many types of pelletized food, some of which have been developed in other countries. Results of this research should be available soon.
Very encouraging results have been obtained experimentally for P. monodon in Tahiti (Aquacop, 1977). There it was found that a diet containing 40 percent protein and 3.3 Kcal/g supported the best growth. An artificial dry pellet was developed and its ingredients are listed in the accompanying table.
Diets used in growth experiments
(After: Aquacop, 1977)
|Ingredients||For production||For postlarvae|
|Cod liver oil||4||4|
S.F.P.C. 80 - Soluble Fish Protein Concentrate 80%
F.P.C. - Fish Protein Concentrate
Three-gram shrimp were stocked in an earthen pond at a density of 10/m2. After seven months, they had grown to a mean weight of 25 g. Survival was 90 percent. The food conversion rate was 3:1. Overall growth was good in the series of three experiments. Survival varied from 80 to 96 percent and food conversion from 3.0:1 to 4.1:1.
Two experiments were conducted growing postlarvae in nursery ponds. Shrimp weighing 0.003 g were stocked in earthen ponds at densities of 20 and 55/m2. After 60 days, survival in both ponds was 100 percent. The shrimp had grown to 0.9 g in the low density pond and to 0.8 g in the high density pond. The food conversion rate was 1:1.
The Aquacop workers computed theoretical growth curves showing optimum and medium growth (Fig. 9c). They calculate that it appears possible to grow 25 g shrimp from 0.05 g juveniles in 140 days. They estimated that 20 tons/ha/year could be produced in intensive systems.
Commercial production of P. monodon has been achieved in one farm in Thailand. The earthen pond with an effective culture area of 4 840 m2 was stocked with hatchery-produced fry. During the first growing period, 300 000 postlarvae were stocked. Seven-and-one-half months later, 2 544 kg of shrimp with an average weight of 38 g were harvested. This is equivalent to 5 100 kg/ha. Mortality was 79 percent. In a second growing period, 100 000 postlarvae were stocked. After growing for 5-½ months, 1 222 kg of shrimp with an average weight of 33 g were harvested for a production rate of 2 500 kg/ha. Mortality was 62 percent. Production per year would be 7 600 kg/ha. For feed, trash fish, mussel, rice bran and crab were ground in a grinder and fed twice daily, at early morning and evening. The trash fish was of mixed composition containing about 5 percent shrimp and even shellfish. The pond bottom was disturbed once a day by dragging a chain through the pond. This was followed by an immediate change of water. The feeding rate was regulated by observing whether or not food was left uneaten; if it was, the ration is reduced. Workers dived to observe the presence of leftover fish. Feeding was done by putting the food on an earthen platform extending along the edge of the long, canal-like pond.
Interest in the intensive culture of shrimp is picking up and hopefully, suitable processed foods will soon be generally available. Already one company within the region (Universal Robina, Philippines), is making a shrimp feed. It is a processed pellet containing 26 percent protein, 7 percent fiber and 3 percent fat. The price is 86.001 (approximately US$11.50) per 50 kg bag.
Liao (personal communication) reports that excellent results have been obtained with a food producedin Taiwan by the President Company. Food conversion rates as low as 1.8:1 were attained. The feed is made into columniform pellets with a diameter of about 23 mm. Analysis of the feed indicates it comprises 7.99 percent moisture, 36.58 percent crude protein, 3.8 percent crude fat, 0.38 percent crude fiber, 0.75 percent ash and 41.43 percent others.
There is no optimum stocking rate. The stocking rate must be calculated for each pond depending on the farmer's management capability, type of management, cost of inputs and marketing strategy. A farmer has to decide what size of shrimp he wants to harvest and estimate how many kilograms per hectare he can produce per crop. The number of postlarvae he must stock can then be calculated from Table 12. Mortality must then be estimated and added to this figure.
1 (Philippine pesos) 7.478 = US$1
A farmer thinks he can produce 350 kg of shrimp with a size of 40/kg. From Table 12, it can be seen this requires a stocking rate of 14 000 postlarvae per hectare. He then estimates mortality will be 30 percent. As 30 percent of 14 000 is 4200, this is added to 14 000 to obtain a stocking rate of 18 200 postlarvae per hectare. If his estimate of mortality was 50 percent, he would have to stock at a rate of 21 000 per hectare in order to harvest the desired final weight of 350 kg of shrimp.