4. DESIGN CRITERIA REQUIRED BY SHRIMP

Most concepts of pond engineering and layout have been developed for culturing fish. The physiological requirements and behaviour of shrimp are, in some cases, quite different from fish. By examining these factors, it should be possible to gain an insight into how to construct ponds suitable for shrimp culture.

4.1 Temperature

Both growth and survival are affected by temperature. Generally, the rate of growth increases with temperature, but at higher temperatures mortality increases. While each species has its own optimum temperature range, temperature between 26 to 30°C are generally considered best in terms of maximum yield. That is, growth is relatively fast and survival is high. Temperatures above 32°C should be cause for concern. In postlarval stages of P. aztecus, the rate of growth was observed to increase with temperature, up to 32.2°C. Survival for one month was greatly reduced at 32.5°C, and no shrimp survived at 35°C (Zein-Eldin and Aldrich, 1965). The following table gives results of an experiment to determine the effect of high temperature on survival of P. merguiensis 8.5 cm in length (Piyakarnchana et al, 1975).

Liao (personal communication) supplied similar data for P. monodon

The best way to ensure that the temperature of pond water does not become too hot is to provide a greater depth of water. This can be done by deepening the total pond area, or by excavating deep channels within the pond for the shrimp to seek shelter in. No accurate information is available on the minimum depth of water required. It is, however, suggested that the minimum water depth be at least 0.5 m. If interior canals are used, a water depth of 1.5 m should be provided if the water is turbid. If the water is clear a depth of 2 m is required.

The effect of wind action on water movement and mixing is not as great in deep ponds as it is in shallow ponds. Consequently, stratification of water layers as in the case of heavy rains can become dangerous to shrimp. So, while ponds should be designed so that relatively deep water levels can be maintained during the hot dry season, added precautions have to be taken to ensure adequate mixing of the water (see Sections 3.2 and 3.3).

Shading portions of the pond with floating material such as coconut leaves has been found beneficial.

4.2 Salinity

Young shrimp can tolerate wide fluctuations of salinity. In most species salinity has little effect on either survival or growth of postlarvae, except at extremes. The ability to withstand extremely low salinities varies from species to species. The period of acclimation is important in determining the lowest salinity at which a shrimp can survive. Changes in salinity should be as gradual as possible because abrupt exposure to very low salinity can cause death. Very little is known of the salinity tolerance of sub-adult and adult shrimp. Of the important species cultured in this area, it is generally considered that P. monodon and most Metapenaeus spp. can grow in almost freshwater. P. merguiensis and P. indicus require more saline water, probably above 10 ppt. Piyakarnchana, et al (1975) report that optimal growth of P. merguiensis was obtained at 27 ppt but that growth was good over the range from 20 to 30 ppt. P. semisulcatus seems to require very saline water. All species of Penaeus require almost marine seawater for sexual maturation and spawning.

Even less is known about the tolerance of shrimp to the extremely high salinities which can occur in some shallow ponds when it is not possible to exchange water regularly.

Prevention of low salinity is best achieved by locating ponds in areas where the normal range of salinity is within that tolerance of the species to be cultured. Accordingly, culture ponds for P. merguiensis and P. indicus should be located fairly near the coast while those for P. monodon and Metapenaeus spp. can be further away from the shore.

To protect against abrupt changes in salinity, the following criteria should be met:

1. There must be a capacity to change pond water rapidly, and whenever it is required. Since the latter requirement is often a problem with tidal ponds pumps might be useful.

2. Sluice gates must be designed to permit rapid draining of surface water during and after heavy rains.

3. Sluice gates should be designed to permit the inflowing water for replenishment from the bottom at times when the surface water is of low salinity in the adjacent natural waters.

4. Pond water should be at least 50 cm deep for temperature control. This also aids in control of salinity as the greater water volume provides more protection against dilution. For example, if a pond 10 cm deep receives 10 cm of rainfall, salinity will drop by 50 percent. If the pond water is being maintained at a depth of 50 cm, however, the same 10 cm of rain will only reduce the salinity by only 17 percent.

5. Diversion canals should be provided to divert rain water runoff from adjacent land away from the pond to prevent destruction of dikes and flooding of the pond.

6. To prevent high salinity resulting from evaporation, windbreaks such as trees or high dikes may be useful. Trees with more or less evergreen leaves should be used because if a lot of leaves fall into a pond they may cause problems when they decompose.

4.3 Oxygen

Maintenance of adequate levels of dissolved oxygen in the pond water is very important for shrimp. Many workers have suggested that the minimul level of oxygen needed for good shrimp growth is 2 ppm, but there are inadequate data to support this conclusion. Two studies have been conducted to investigate the short-term effects of low oxygen levels. Egusa (1961) reported that for P. japonicus, stress is signalled at 1.4 ppm when burrowing occurs. MacKay (1974) observed that in P. schmitti the majority of shrimp began swimming at the water surface when the level of dissolved oxygen was reduced to 1.2 ppm. Ten minutes later the shrimp began jumping out of the water. They then fell to the bottom and became immobile. When the immobile shrimp were placed in well-aerated tanks, about 50 percent recovered. Considering the above, perhaps a dissolved oxygen level of 1.2 ppm should be considered as a base at which shrimp start to die with even a short exposure.

Even less is known of the long-term effects of sublethal dissolved oxygen levels. Rigdon and Baxter (1970) found that white areas of degenerated tissue in the tail muscles of P. aztecus were associated with low levels of dissolved oxygen and high temperature. Shrimp with this condition frequently died. When the affected shrimp were placed in well-aerated tanks, however, the white areas dissipated within 24 hours and the shrimp became active. This same condition has been observed with P. merguiensis in culture ponds.

Fishery biologists feel that when dissolved oxygen levels reach 3 ppm or below in fishponds, remedial action is necessary. The same is probably true for shrimp. So in formulating guidelines based on the small amount of laboratory information available, we can perhaps state that growth should be best at dissolved oxygen levels above 3 ppm, and that mortalities will occur after short-term exposure at dissolved oxygen levels below 1.2 ppm. However, this may not always hold true in a pond where several factors interact as Shigueno (1975) recorded a die-off in a pond when the oxygen level reached a low of only 2.7 ppm during the night (see Section 4.6). Mortality can be reduced in shrimp suffering from a lack of dissolved oxygen if the oxygen level is raised quickly.

A common method of expressing the concentration of dissolved oxygen in ponds is to give the percent solubility. Tables 1 through 4 give the percent solubility of oxygen at saturation and the critical levels for shrimp at different levels of temperature and salinity. It can be seen that water with a high temperature and salinity holds less dissolved oxygen than does water with low temperature and salinity. Consequently a deeper pond would be beneficial in maintaining reduced temperature and providing for increased oxygen solubility which with proper management could result in increased levels of dissolved oxygen.

The shrimp being cultured are probably not the main consumers of oxygen in a pond with a low level of dissolved oxygen. Shigueno (1975) reported the estimated percentages of oxygen consumed in one night in a “polluted” shrimp pond as follows:

The water which includes algae, bacteria and detritus was the main consumer of oxygen. The most effective way to correct low dissolved oxygen levels in such a pond is to reduce the amount of algae, bacteria and detritus in the water. This can be done by draining a portion of the pond water and refilling it with clean water.

Heavy rains can cause stratification of water layers, especially if the pond is deep and there is not much wind. The lighter freshwater floats on top of the more dense salt water. Such stratification can result in oxygen depletion in the lower salt water layer. Provision should be made to promote mixing of water after heavy rains.

Increased water movement provides more aeration and can be used to help keep dissolved oxygen levels from falling to a critical point. It can also be used to raise critically low levels. This can be provided by:

1. Water change, especially letting new water into a pond. Sometimes pumping is the only way to do this at the time it is needed. All the shrimp in a pond could die if one had to wait several hours for a high tide to let new water in.

2. Installation of aeration equipment

   • Air blower or compressor with air stones in the pond
   • Electric water mill

   • Wind mill

3. Orientation of the long axis of the pond with the prevailing wind during the construction stage. Caution must be exercised here, as in areas with strong winds, wave action might cause excessive dike erosion, especially in large, deep ponds, and it might be necessary to provide wind wave breaks near the dike. In such areas it may be more advantageous to orientate the short axis of the pond with the prevailing wind and rely on other means of providing aeration.

4. Construction of large ponds which allow a greater sweep of wind across the pond.

5. Lowering the water depth to accentuate the effect of wind action. (Care must be taken that sufficient depth is maintained to prevent high water temperature.)

6. Not constructing dikes excessively high so that they block the wind.

7. Not planting trees on dikes.

All the above factors can have an effect on some other aspects of pond management and each factor must be evaluated to assess its effects on the overall scheme in each locality.

4.4 pH

A low water pH can affect the shrimp directly. Wickins (1976) found that even though P. monodon grew without suffering mortalities in water with a pH of 6.4 in the presence of inorganic carbon, growth was reduced 60 percent. However, a drop in pH that is associated with a loss or rapid reduction of inorganic carbon, such as occurs with the addition of a strong acid, can be lethal. In water with a pH of 6.4, and less than 10 to 12 mg/l of inorganic carbon, P. merguiensis and P. aztecus exhibited greatly reduced growth and lower survival. When pH fell below 5.0, heavy mortalities occurred. A fall in pH may have indirect effects also, for instance, resistance of the shrimp to pathogens might be reduced.

One of the most important causes of low water pH is acid soil (see Section 3.2.2). Acid and potential acid sulfate soils are commonly overlaid by good soil which lies above the mean high water level. If the land is excavated to make the pond bottom at a level where the pond can be filled and drained using tidal fluctuation, acid sulfate conditions develop when the subsoil is exposed. This will result in low pH of the pond water unless the soil is improved. Considering the cost and difficulty required to improve an acid sulfate subsoil, it is suggested that in areas where there is a non-acid topsoil, it may be more economically favourable to use bar ditch type construction and fill the ponds by pumping.

If ponds must be excavated, the surface layer of good soil can be set aside and replaced as a surface layer on the pond bottom and dikes. If the amount of good soil is limited, it should be used to surface dikes of small ponds such as nursery ponds. This will prevent rains from washing acid from the dikes into the pond water and killing the fry. This is much more critical in small ponds than in large ponds. Pond bottom can be leached or limed to reduce or eliminate the acid condition. In areas where there is not enough good topsoil to surface dikes, the dikes can be made with a berm, and a ditch can be cut in the berm to catch acid water runoff and prevent it from contaminating the pond water (Potter, 1976).

High pH has an effect on ammonia toxicity because it increases the ratio of toxic unionized ammonia in solution to the total ammonia present. This is discussed in the following section.

4.5 Nitrogen compounds

The following discussion of three forms of nitrogen and the effects of sublethal levels on shrimp growth is based on data presented by Wickins (1976).

Nitrate. Two tests with nitrate showed that the growth of P. monodon was not affected by a concentration of 200 mg NO₃-N/l after three to five weeks exposure.

Nitrite. In a test with P. indicus, growth was reduced by nearly 50 percent over a period of 34 days when nitrite concentration was 6.4 mg NO₂-N/l.

Ammonia. Chronic toxicity tests for ammonia were conducted with five species of penaeid shrimp: P. japonicus, P. occidentalis, P. schmitti, P. semisulcatus and P. setiferus. The tests showed that a mean concentration of 0.45 mg NH₃-N/l reduced growth by 50 percent of that of controls. Wickins estimated that a “maximum acceptable level” at which growth would be reduced by only 1 to 2 percent is 0.10 mg NH₃-N/l.

As it is more convenient to measure ammonia in terms of total ammonia nitrogen (not free NH₃ or unionized ammonia), Wickins compiled the following table to give values of total ammonia nitrogen which correspond to the value 0.10 mg unionized ammonia (NH₃-N) per liter at selected temperatures, salinity and pH.

The concentration of total ammonia nitrogen (in mg/l) that corresponds to a calculated level of 0.1 mg/l unionized ammonia nitrogen in water at a constant pressure of 1 atmosphere at different values of temperature, salinity and pH (from Wickins, 1976)

From the table it can be seen that pH has a major effect, with the percentage of toxic unionized ammonia being much greater at high pH than at low pH. In water with a temperature of 28°C, salinity of 24 ppt and pH of 6.8, the critical level of 0.1 mg/l unionized ammonia occurs when the total ammonia level is 26.1 mg/l. In water with a temperature of 28°C, salinity of 24 ppt and pH of 8.4, a level of 0.1 mg/l unionized ammonia occurs when the total ammonia level is only 0.8 mg/l.

The normal pH of brackishwater is 8.0 to 8.3 and in ponds with a good growth of phytoplankton, pH values of 9 and above are common in the late afternoon. As there is not much that can be done to modify this and still keep pond production high, efforts should be concentrated on keeping ammonia levels low. Most of the ammonia in a pond is formed as waste products of the organisms which are living in the pond. The higher the density of both the species being cultured and the organisms cultured for food, the greater the production of ammonia. Ammonia will eventually be converted to nitrate, but there is a danger that ammonia production will exceed the capacity of the pond to convert the ammonia rapidly enough to prevent it from exceeding toxic levels. Some species of planktonic algae such as Chlorella sp. can utilize ammonia and nitrate directly. If these are present the danger of ammonia build-up will be reduced. However, it is very difficult to control the species of algae growing in a pond.

An additional factor is that when dissolved oxygen levels are low, nitrates are reduced to ammonia, thus increasing the level of ammonia in the water. A decrease in dissolved oxygen also increases the toxicity of unionized ammonia. Conversely, an increased level of dissolved oxygen reduces toxicity (Spotte, 1970).

The simplest way to prevent the build-up of ammonia and other harmful substances is by changing water on a regular basis.

4.6 Hydrogen sulfide

Hydrogen sulfide (H₂S) in a pond is produced by the chemical reduction of organic matter which accumulates on, or in, the pond bottom. The bottom soil turns black and sometimes a rotten smell is discharged. As shrimp live primarily on, or in, the bottom, a build-up of H₂S in the bottom soil, or in water near the bottom, is important. It has been determined experimentally by Shigueno (1975) that shrimp (P. japonicus) lost equilibrium when exposed to a level of 0.1 to 2.0 ppm hydrogen sulfide (H₂S) in water. The shrimp died instantly at a concentration of 4 ppm.

Studies in one pond showed that the concentration of sulfide-sulfur (mostly H₂S) in interstitial water 2 cm deep in the pond bottom reached as high as 10 ppm. In the pond water it exceeded 0.09 ppm, varying from 0.037 to 0.093 ppm. A die-off of shrimp occurred in this pond. Shrimp burried in the bottom draw in water from above the pond cottom, so it is not likely that the level of H₂S in the water was lethal. Dissolved oxygen level in the water did not fall below 2.7 ppm, again above the lethal limit.

To determine the longer term effects of H₂S on shrimp, soil on the bottom of 3.3 m³ tanks were treated with an application of iron oxide (70% ferrous oxide [FeO]) at the rate of 1 kg per m². This prevents the formation of H₂S and FeO is formed instead. The shrimp in the tank with the treated bottom grew significantly better than those in the untreated tank. After a 68-day growing period, shrimp in the treated tanks had an average weight gain of 204 percent while suffering only 4.4 percent mortality. Shrimp in the tank without FeO had an average weight gain of only 150 percent and a mortality rate of 20.8 percent.

It might not be practical to treat bottoms of large ponds with FeO, but frequent changes of water would prevent the build-up of H₂S in the pond water. If ponds are constructed with peripheral canals, treatment of only the canals with FeO might be practical, as most of the organic debris is deposited in these canals and H₂S production should be greater in them.

4.7 Bottom contour

As the behaviour of shrimp permits harvesting by other than the collection methods traditionally used for fish, pond designs calling for either a sloping or flat pond bottom need not be followed. The bottom can be left uneven or contoured in any way that permits complete drainage of pond water. This can have several benefits.

1. The bottom can be left flat. There is no need for a harvest basin.

2. Portions can be excavated so that the deeper water provides shelter for the shrimp. These areas also serve as holding areas when water level is reduced for economy of chemical application.

3. A pond can be built with the traditional ditch type construction using manual labour. Construction costs are much less than for total excavation.

4. Some food organisms grow best in shallow water. Their growth can be encouraged by leaving a portion of the pond with a shallow depth.

5. Internal dikes can be built to break up wind waves and prevent erosion.

4.8 Structures for attachment

P. Monodon like to cling to some surface at all life stages, but especially during the postlarval and early juvenile stages. Clusters of branches placed around the pond provide protection from predators, help prevent cannibalism during moulting, provide protection from poachers, and provide a place for food organism to grow. A floating board or piece of bamboo raft with branches attached to the bottom is suitable and it also provides shade.