Most species of shrimp can be harvested by draining pond water and catching them in a net placed in the water outlet sluice gate. This means that the excavated, smooth bottom ponds needed for some types of fish culture are not required for shrimp culture. A simple canal and levee type of construction is all that is needed. This results in considerable savings in initial capital outlay and, consequently interest on capital.
Many people recommend that ponds be built only in areas where there is sufficient tidal fluctuation to permit filling ponds by tidal exchange. In most cases, this calls for excavation of the total pond bottom. From a purely economic point of view, it would often result in substantial savings to construct ditch and levee (dike) style ponds and pump water into the ponds. In areas where a great deal of excavation is required to construct tidal ponds, the annual savings of interest on the capital that would be required for total excavation would pay for pumping costs. A recent estimate for earthwork in mangrove swamps in Malaysia was M$0.80/yd3 (M$0.875/m3).* Using this figure, the cost of excavating a 10 ha pond to a depth of 1 m can be estimated at M$87 500. If a perimeter canal 10 m wide was excavated to obtain fill for the levee, estimated cost for the 10 ha pond would be only
M$12 250. The difference in cost for the two types of construction amounting to M$75 250. This is admittedly very rough estimate, but it gives an idea of the savings that could be realized. The large water gates usually constructed for tidal ponds would not be required, small wooden gates could be used instead. The savings here should more than pay for the cost of a pump. For instance, at one farm in Malaysia, large 6 lane concrete sluice gates cost M$16 000 and an 8 lane gate cost M$20 000. The 200 ha farm had 4 sluice gates, this would give a cost of approximately M$85 000. If pumps were used, the cost of wood sluice gates should not exceed M$15 000.
With a levee-canal type of construction, overall costs are not only reduced, but a portion of capital cost is transferred to operating cost. A reduction in capital costs will make financing of shrimp farms much more attractive to lending institutions, and permit people with limited capital assets to start shrimp farms.
The levee-ditch type of pond has several benefits for management:
Most species of shrimp cannot withstand the high termperatures encountered in shallow ponds. The deeper water in the peripheral ditch (sheltering canal) serves as a refuge from high temperature during the heat of the day.
The extensive shallow portion of the pond will not get too hot in large ponds if the minimum depth is kept over 40 cm. This allows sunlight to penetrate to the bottom and increases productivity.
Aged top soil forms the major portion of the pond bottom and this leads to increased production.
The large flat portion of the pond can be drained and dried easily.
The interior canals can serve as a holding area when pond water level is lowered for chemical treatment, resulting in considerable savings in the cost of treating for predators.
Pumping gives added flexibility to water management. For instance, the farmer does not have to wait for a high tide to exchange water if a pond has low oxygen.
If pumps are used, many more sites can be utilized for pond culture.
It is not necessary to totally clear the land to construct ponds for shrimp culture. Brush and tree should be cleared and stumps cut short so they do not become landing places for birds. If stumps are not cleared, it becomes essential that puddle trenches should be incorporated in the levees to prevent water seepage under the levee. During levee construction large stumps and roots should be removed and care should be taken to ensure that all smaller roots are broken up so they do not extend through the levee and cause seepage.
Ponds should have separate inlet and discharge sluice gates and water canals. If possible, sluice gates should be on opposite sides of the pond to facilitate water circulation. Using the same canal for both intake and discharge for a series of ponds frequently causes problems, because of the ease with which water from one pond enters another. This encourages the spread of disease and pollution.
To minimize erosion of levees by wind waves, the levees should not rise in a continuous slope from the bottom. A narrow platform (berm) should be made below the water level to break the waves before they hit the levee. In large ponds, low secondary levees can be incorporated to act as wave breaks (Figure 9). Cut mangrove branches placed on the levee platform help prevent erosion and keep out wading birds.
Design criteria - A sluice gate should be able to perform several functions besides simply holding water. It should be designed so that bottom water can be drawn off when low levels of dissolved oxygen occur in a pond and allow surface water to be drawn off during periods of heavy rain. Provision must be made for screening of incoming water and for harvesting.
Wood is an acceptable construction material for small sluice gates, a well constructed gate made of treated wood lasting 8 to 10 years in many areas. Concrete is, of course, preferred, but in some places, construction with concrete is difficult and costly. Frequently, sand, gravel and even fresh water have to be transported to the construction site. In a goodly number of areas, local workers have little experience in working with concrete and gates are made that either do not hold water or fall apart easily. Care should be taken during planning to find out what the conditions of supply and labour are at the construction site.
An example of a simple wood sluice gate is shown in Figure 10(a,b). The main points are:
The brace boards should be internal so the pressure from the earth presses the side boards into the braces.
Anti-seep boards on the sides and bottom are essential to prevent water seepage and resultant washouts. The anti-seep boards should be placed adjacent to the first set of closure boards.
There should be two sets of closure boards to give management flexibility: both sets can be removed during harvesting or to let water into the pond; the top board of both gates can be removed to let surface water drain off; to drain off bottom water, or to circulate water through the pond, the top board of the second set of boards and the bottom board of the first set are removed.
Pilings under the gate prevent settling.
Frequently, problems are encountered in sealing the closure boards tight enough to prevent water seepage. One common procedure is to pack the space between the two sets of closure boards with earth. This works well, but it is time consuming to remove and replace the earth if the boards must be removed often. Modification of the traditional groove to permit the use of wedge fasteners (Figure 11) should prove useful on small gates.
It can be assumed that fertilization of brackishwater ponds will have the same beneficial effect that enrichment of freshwater ponds has. There is, however, very little information on fertilization of brackishwater ponds and no recommendations for application of fertilizers can be given with assurance that they would work at any specific location. It must be remembered that seawater is a much more complex medium than freshwater and a great deal of technology used in freshwater is not directly applicable. A few general points can be discussed.
One of the main considerations is that the primary nutrients (N and P) required do not remain in solution very long, both phosphate and ammonium based nitrogen sources such as urea and ammonium sulfate become incorporated in the bottom soil by precipitation, absorption or adsorption. They can sometimes become available again if N or P concentration in the water is low. Nitrate based fertilizers on the other hand, remain in solution for extended periods of time (Mandal, 1962). This has management implications, for if one wants to encourage the growth of benthic algae, ammonium (NH3, NH4) based fertilizers would be better while nitrate based (NO3) fertilizers would be more suited for phytoplankton. The fact that ammonium based fertilizers become incorporated in the bottom after only a few days is of importance in planning for water exchange. If urea was used as a source of nitrogen, water could be changed a week after the addition of fertilizer without as great a loss of nutrients as would occur if a nitrate based fertilizer was used.
For shrimp culture, it is generally assumed that the benefits of fertilization are indirect. That is, fertilization causes a bloom of phytoplankton, various micro-organisms feed on the phytoplankton and the shrimp feed on the micro-organisms. A bloom of true diatoms, which generally color the water yellowish brown, are much more desirable than blooms of other types of algae such as dinoflagellates, which usually color the water bright green. Large-scale mortalities of shrimp have occurred in ponds colored bright green. These two groups of algae have different nutrient requirements. In laboratory culture of algae, nitrogen to phosphorus ratios in the order of 30:1 have been found most suitable for diatoms, and ratios approaching 1:1 for dinoflagellates. With this background information, attempts can be made to fertilize ponds to encourage growth of diatoms by adding a greater amount of nitrogen based fertilizers.
One must keep in mind that the nutrient composition of seawater varies both from location to location as well as seasonally and, consequently, a fertilization programme that is used successfully in one location will not necessarily work in another. Water in shrimp ponds at Samut Sakorn, Thailand had low levels of nutrients, nitrogen was usually less than 0.1 ppm (nitrate, nitrite) and phosphate less than 0.7 ppm. These ponds can benefit from a balanced fertilizer containing both nitrogen and phosphorus, and in one experiment, the added income derived from fertilizing a 1.4 ha pond with both ammonium sulfate and superphosphate was over US$50.00 in a 60-day growing period. At Surat Thani, Thailand, on the other hand, nitrogen values ranged from 7 to 9 ppm (nitrate, nitrite) and phosphate was less than 0.2 ppm. It is likely that at Surat Thani, greatest benefits would be obtained with the use of superphosphate alone (Cook, 1973).
At this stage of development, every shrimp pond operator must develop his own procedure for his own local conditions. It would be helpful if water analyses are conducted to measure levels of nitrogen and phosphorus in the water to determine the amount of each that should be added to encourage the growth of favourable types of algae. Simply adding inorganic fertilizers without knowing the level of nutrients present in pond water can be disastrous. In Indonesia, for instance, the author tried to use the same fertilization scheme used in Samut Sakorn, Thailand referred to above, 62.7 kg/ha of ammonium sulfate and 3.8 kg/ha superphosphate in a pond used for milkfish/ shrimp polyculture, and a fishkill resulted. Evidently, nutrient levels in the pond were already high and the addition of fertilizer was harmful.
That a programme of fertilization can be advantageous if properly carried out is illustrated by another example from Thailand. Average shrimp production in Thailand is low, only 109 kg/ha (Dept. of Fisheries, 1973), but with fertilization, a private shrimp farmer obtained a yield of 885 kg/ha/ year valued at US$ 1 328. The method of grow-out used in this farm is essentially that described by Cook (1973). Pond layout is similar to that in Figure 12. Water containing wild fry is pumped into the nursery pond daily for 30 days. At that time, the pond is treated with tea seed cake to kill predaceous fish and the shrimp are transferred to one of the grow-out ponds by gravity flow of water. The process is then repeated, except that the shrimp are put into the second grow-out pond. By the end of the third 30-day period of pumping into the nursery pond, shrimp in first grow-out pond have been in that pond for 60 days and are ready for harvest. After the shrimp are harvested, small shrimp from the nursery pond are stocked in that pond again. The process is repeated, providing alternate harvests from each pond every month. The total growing time varies from 60 to 90 days, some shrimp entering the nursery pond on the first day of pumping and some the last day before the stock is transferred.
The following data on harvest for the 14 ha farm is for 1972-73 (Potaros, personal communication, 1974). Stocking by pumping water was started in 1 November and the first harvest was in February.
| Month | Weight harvested |
| (kg) | |
| February | 973.8 |
| March | 1 321.5 |
| April | 1 217.5 |
| May | 1 528.1 |
| June | 1 129.3 |
| July | 1 391.7 |
| August | 1 279.5 |
| September | 1 416.3 |
| October | 1 354.6 |
| November | 781.7 |
| TOTAL | 12 394.0 |
The harvest equalled 875 kg/ha/year valued at US$ 1 312/ha/year. Operating costs were estimated at US$750 per month for 14 ha or US$643/ha/year. Gross profit then amounted to US$562/ha/year.
The recommended formulation for fertilizing shrimp ponds is as follows (Potaros, personnal communication, 1976):
| - | Mix: | 100 kg dry duck or cow manure |
| 1 kg superphosphate | ||
| 2 kg urea | ||
| - | Add mixture to the pond at a rate of 18.75 kg/ha | |
| every 10 days. | ||
There are no processed foods especially prepared for shrimp available in the region. Consequently, feeding operations have to be done with natural foods. An example of a successful shrimp farm with feeding is that of a fish farmer whose 1.3 ha farm is located near Chantiburi, Thailand. The pond is excavated and water is changed daily by tidal flush. The average water depth is 1.25 to 1.5 m. The ponds have a unique bottom contour as shown in Figure 13, which depicts one canal of the two-canal system. The two similar canals are separated by a walk-way. A mixture of ground squid, trash fish, crab scraps, and green mussel is fed twice a day. Once a day, a chain is dragged through the pond to stir up the bottom.
Each crop is grown 6 to 7 months. After each harvest, the bottom mud is removed and rotenone is used to kill the fish. the pond bottom is not dried because water drainage is not complete. Expenses were given as follows for one crop:*
| Food (1.1 baht/kg) | Bht | 64 814 |
| Labour | 11 200 | |
| Seed stock (transport from Phuket hatchery) | 20 000 | |
| Nets | 1 000 | |
| Fuel | 53 062 | |
| Other | 9 924 | |
| TOTAL COSTS | Bht | 110 000 |
Income for the same crop was Bht 240 906. Composition of the harvest was as follows:
| Crabs with eggs | 500 kg |
| Crabs | 500 kg |
| Sea bass (100 fish) | 40 kg |
| Thread fish (300 fish) | 40 kg |
| Milkfish (300 fish) | 100 kg |
| Penaeus mondon | 2 544 kg |
| P. merguiensis | 182.5 kg |
| Metapenaeus monoceros | 332 kg |
* 20 baht (Bht) is equal to US$1.00.
The operator pays no tax or interest. The land is rented from the government for Bht 1 200 for 10 years. Total investment is Bht 1.5 million.
In the Philippines, Metapenaeus ensis has been profitably cultured with milkfish (Chanos chanos). A 1.75 ha pond was stocked whth an unspecified number of wild caught juvenile shrimp (2-5 cm TL) at a cost of 5 pesos* for 3 liters. After forty-five days, the shrimp were harvested by trapping and 350 kg (200 kg/ha) of shrimp were obtained. Milkfish production remained at the same level as before the practice of stocking M. ensis juveniles was started. Though of small size, the M. ensis bring 13 pesos per kg from October through January and 8 pesos per kg during the remainder of the year. (Jaranilla, personal communication, 1976). This added income would make a substantial supplement to milkfish farmers income if this form of polyculture could be put into general practice.
A refinement of this procedure would be to stock repeatedly at set intervals and harvest with traps having a mesh size regulated to permit the escape of small shrimp so that only the larger shrimp would be harvested. The small shrimp would then be free to grow to a larger size. A note regarding species suitability for culture, is that the named pond operator observed that survival of white shrimp, presumably P. indicus, was poor so that she no longer bothers to stock them.
All the successful shrimp farms visited by the writer practice frequent water changes. There are no set procedures, but in a pond using fertilizer, changing water every 10–14 days with a reapplication of fertilizer immediately following the change is recommended. In a pond receiving supplemental food, water should be exchanged as often as possible. An exchange of water has the following beneficial effects: reduces metabolites; replenishes flora and fauna; replenishes trace elements and organic compounds.
* In January 1976, 7.5 pesos (
) = US$1.00.
All water entering a pond should be filtered to prevent the entry of unwanted pests. It is important that the water be screened as fine as possible to prevent entry of fish eggs and larvae. This is often a problem as fine mesh screens clog rapidly. Two methods of setting up a series of different size screens to minimize the effects of clogging are illustrated in Figure 14. Placement of the fine mesh screens before the pump gives more net surface area, thereby reducing water velocity through the netting, and the high water pressure from the pump is eliminated. The fine mesh screens are fitted with a collection box as in Figure 1. The nets should be checked and cleaned regularly and the catch removed. The postlarval shrimp caught can be stocked in the pond.
Fish are one of the main problems in shrimp ponds, both as predators and competitors. In Thailand, the average amount of fish harvested from shrimp farms is about 32 percent of the total weight, but only one percent of the value (Anon, 1971). If, as in this case, the shrimp are more valued than the fish, it is necessary to eliminate fish from the ponds, as the fish either eat shrimp or compete with them for food. The easiest method is to prevent their entry using screens as described in the previous section.
If fish do enter a pond, they can be removed with the use of chemicals. Saponin is well known for its capacity to kill fish while not harming shrimp. It is traditionally applied as tea seed cake which is a residue of a wild tea seed (Camelia sp.) after extraction of its oil. To rid ponds of fish, it can be applied at rates of from 10 to 25 ppm.
Swimming crabs are predators of shrimp and if they are observed in a pond, traps should be used to remove them. Burrowing crabs are one of the major causes of water leakage through pond levees. If water leakage is excessive, it becomes almost impossible to maintain an adequate level of water in a pond, nutrients are lost, postlarval and juvenile shrimp escape through the holes, and predators enter the pond on high tides when water flow through the hole reverses. There is no really effective method to eliminate these crabs, but continuous maintenance should be conducted to block up the holes and keep damage to a minimum.
Predation by wading birds can be a problem. Birds can be shot, but that is not always successful. Most wading birds need a place to land and in ponds with a sheltering canal extending around the internal perimeter, they cannot land on the levee and wade across the canal to get to the main portion of the pond. If water over the flat main portion of the pond is kept deep enough and with a bloom of algae as the birds cannot see the bottom, they will not land. Mangrove branches or other brush placed at the edge of the pond (Figure 9) will prevent wading birds from walking along the pond shore.
Other types of predaceous birds might be kept away by the bird scaring device shown in Figure 15. It is used at the Jitra Fisheries Station in Malaysia to scare eagles. It is simply a windmill with mirrors that revolve and flash brilliantly, scaring the birds. Since installation of the device, birds are no longer a problem at that station (Masekarm, personal communication, 1976).
There have been relatively few reports of disease from ponds in this region. One condition that has been observed several times is “black gill” disease. Black gills can be caused by several things; fungus, bacteria and detritus. Farmers should be encouraged to preserve diseased specimens so the causitive agent could be determined.
Another condition observed, is shrimp which have thin papery exoskeletons and soft flesh. This condition is likely to be caused by poor nutrition and/or pond conditions. Farmers observing shrimp with this condition in their ponds are advised to change pond water if possible and add supplemental food. Again, specimens should be preserved so the presence or absence of disease can be determined.
Fungal diseases occur in shrimp hatcheries and it is possible that infected postlarvae are occasionally stocked unknowingly in ponds. Once the postlarvae are stocked in a pond, sampling to determine if they are healthy is difficult. A farmer usually stocks a pond and then waits hopefully until the shrimp are large enough to sample effectively, sometimes as long as four weeks. With hatchery produced postlarvae this is not a good practice, as the shrimp might be diseased and die soon after their introduction to the pond. A good practice is to keep a few postlarvae in an aquarium containing pond water for observation. If these postlarvae die an operator knows to check his pond carefully and arrange for restocking if necessary.
Most people consider a large shrimp for the export market when they think of shrimp culture. This is caused by the fact that large shrimp are much more valuable than small ones. It is generally accepted that in a pond shrimp growth rate slows down as size increases, and a farmer must determine the break even point between growth and value to obtain the greatest profit. From a management point of view, elimination of risk is very important and producing large size shrimp may not be the most important consideration. There are several factors concerning risk which make the growing of small to medium size shrimp attractive. Most important of these is that we know how to grow small shrimp at low cost, with only fertilization to increase the supply of food in a pond. Since larger shrimp are less adaptable to extremes of environment, most mortality in ponds comes when the shrimp are held for a long time to grow them to a large size. The longer the growing period the more susceptible the crop is to natural calamities like typhoons or excessive rains. It follows, therefore, that it is safer to harvest the shrimp at a small size before they are exposed to conditions that might cause them to die. Risk is considerably less when a culture programme calls for multiple crops of small size shrimp as opposed to one or two crops of large shrimp. For instance, if a farmer plans to grow four crops of 11 cm TL shrimp in a year, and one crop is lost, seventy-five percent of projected income would still be realized. If, on the other hand, it is planned to grow two crops of 14 cm TL shrimp, and one crop is lost, only fifty percent of projected income would be realized.
A second factor is economics. By harvesting more often an artisanal farmer can realize more frequent cash income. This will reduce the need for credit and reliance on middlemen who furnish cash for food between harvests.
A third factor is the reduced level of management skill needed to grow small size shrimp. It should be possible to grow up to 400 kg of shrimps/ha/crop with only fertilization, water level control, and predator control as management. To grow increased weights of large size shrimp it will be necessary to feed supplemental foods. Feeding again raises the risk factor. The addition of supplemental foods upsets the biological balance in ponds and requires more sophisticated management, mostly exchanging water and preparing the pond bottom. Because of the high level of management needed the danger of mass mortality is increased. Perhaps more important, a farmer who supplies supplemental food has more capital invested in his crop. If the crop fails he loses not only his labour, but also capital. In addition, shrimp feeds will require protein that could be used for human needs.
A fourth factor is that shrimp such as Metapenaeus, which do not grow to a large size, can be cultured. There are several examples of the adaptability of Metapenaeus to pond conditions. In the Philippines M. ensis is abundant and frequently is stocked naturally in shrimp ponds. One pond produced the equivalent of 1 000 kg/ha of 6–10 shrimp. In Thailand Metapenaeus sp. got into milkfish ponds through natural stocking with water change. When they were harvested by trapping the yield was 10 kg/200 m2, or 500 kg/ha.
For the above reasons, the writer believes that the most favourable entry into shrimp culture for indigenous farmers with little or no risk capital, or experience in shrimp farming, is the frequent harvest of small size shrimp. Once an industry of this type is established it forms a base to incorporate more advanced techniques as they are developed.
That a lack of rudimentary skills can hinder the development of large scale projects was demonstrated to the author in Malaysia. There he visited a shrimp culture project with an investment of over M$200 000.* The manager of this farm was having trouble with his workers, who could not understand the need for opening and closing sluice gates on a set schedule. The manager thought this was responsible for limiting his production by affecting both recruitment of postlarvae and maintenance of good water quality.
The income producing potential of shrimp farming makes it attractive to investors, and a number of large scale shrimp grow-out facilities have been constructed. The author is aware of several that have experienced difficulty for one reason or another.
In Malaysia several large shrimp trapping ponds modeled after the practice in Singapore have been constructed. One 80-ha farm had already spent M$200 000 on construction and the operator estimated it would cost another M$60-70 thousand to construct additional sluice gates. The entire 80 ha had been enclosed by a 2.4 m high dike, yet the water area in the excavated internal canals amounted to only 6-6.5 ha. The rest of the area was high and dry. The operator of this farm was not satisfied with production and it was plain that the system used in Singapore should have been modified for use in the new area.
In Thailand another large, extensive type farm was built with a substantial amount of capital invested in large perimeter levees. Before production even started the manager decided to change to a feed type operation which required deep canals instead of extensive shallow areas. Modifications were made to the already constructed ponds, leaving large areas of unused land. The modifications were not adequate to ensure good water exchange and initial grow-out trials were disappointing.
In the Philippines a substantial private shrimp hatchery was constructed, but problems were encountered in its operation and it has not attained commercial production.
* Approximately 2.5 Malaysian dollars (M$) = 1 US$.
The difficulties encountered in the shrimp farms described above could have been avoided if small scale pilot projects had been started initially. Problems could have been worked out and changes in design made at a fraction of the cost involved in modifying the large scale facilities. Investors should be wary of entrepreneurs who advocate initial large scale production. Anyone, especially those inexperienced in shrimp culture, contemplating starting a shrimp farm should start off with a small pilot operation, and expansion should not be undertaken until the pilot project demonstrates profitability.
