In any earthen pond culture system, the bottom soil play a major role in pond yield. High organic matter content in neutral soil often promotes higher primary productivity and hence higher fish yield. Natural food organisms are one of the most important food sources in ponds. It is rich in protein, vitamins, minerals and other essential growth elements that simple supplementary feed cannot complete. Fish yield in pond can also be affected by the presence of predators, deteriorating water quality and improper pond management. Hence, pond preparation is a first step towards ensuring a better pond production.
Shrimp culture operation in Asia can be grouped into three major categories:
Irrespective of the culture operation used, it is always advisable to prepare the ponds in sustaining high natural productivity throughout the culture period.
Prior to pond preparation, soil samples are collected from the pond bottom and the dikes for pH and organic matter contents analysis. Soil pH analysis is generally conducted to determine lime requirement. This procedure is important for newly developed ponds where the occurence of acid sulphate soils is common. When acidic soil condition is detected, corrective measures can be incorporated in the pond preparation activities. Sampling of soil is not necessary for well conditioned ponds.
Collection of samples must be done on wet or moist soil using soil augur or an improvised sampler made of bamboo or PVC pipe. About 12 subsamples in an S-shaped pattern should be taken in a one hectare pond. Only the top soil (0–15 cm) is sampled. Stones, rubbish and coarse particles should be removed before taking the soil samples. The subsamples are mixed thoroughly and a representative portion is taken and labelled accordingly. This representative portion is then air-dried by spreading thinly on plastic sheet and protected from direct sunlight, wind and dust. The dried soil is then packed in labelled plastic bag and send for analysis to the nearest soil laboratory. Request for result interpretation may be included when submitting samples for analysis.
Fig. 12. Section of gate showing grooves for slabs and screen
In most soil laboratories, it usually takes one to two weeks before the result can be given. Thus for routine soil pH determination, the shrimp farmer can take upon himself to do the analysis using a pH meter. To prepare for the analysis, the air-dried soil sample must be pulverized using a wooden mallet or mortar and pestle. It is then passed through a 2-mm mesh sieve and stored in a properly labelled bag or glass jar. The sample is ready for pH determination (see Section 6.6.3).
When the pond soil is found to be acidic, it is normally “leached. This is done by flushing and washing the pond bottom with water to leach away undesirable metallic compounds like aluminum, iron and excess sulfur ions.
The drying of the pond bottom is the most practicl cheap and effective method of eliminating undesirable species in pond prior to the culture period. Drying oxidizes harmful chemical substances especially sulfides and facilitates mineralization of organic matter. The pond is dried until the soil cracks or when it is firm enough to hold one's weight without sinking more than 5 cm on walking over the surface.
During the process of drying the ponds, other activities must be undertaken. These include repair of dikes and gates, reconditioning of pond bottom trench, levelling, installation of screens and substrates. Substrate installation such as twigs or coconut fronds at the pond bottom are very important in the first few weeks of culture because the juveniles have the habit of clinging on vertical surfaces. Aside from that, substrates also serve other purposes: (i) provide additional surface area for some benthic food organisms, (ii) provide shelter against direct sunlight, and (iii) reduce mortality of shrimp juveniles from predators.
Tilling or ploughing of bottom soil improves soil quality by exposing subsoil to the atmosphere thereby speeding up the oxidation process and the release of nutrients that are locked in the soil.
Undersirable species which are pests, competitors and predators consist of finfishes, crustaceans, molluscs, reptiles, amphibians, birds and mammals. Pests are species that generally do not have direct harmful effects on the cultured stock. In most cases, however, pests are also competitors. Some pests for example are certain species of crabs that burrow into the dikes. These can destroy the dikes and cause leakages which may allow the entrance of undesirable species or the escape of cultured stock especially in nursery ponds. Others are wood borers and barnacles.
Competitors are species that compete for space, food, oxygen, etc. with the cultured stock. Generally, these are of different species. Competition arises out of the similarity in environmental demands which can pose limitations in the culturd species development. Both intraspecific and interspecific competitions prevail in any shrimp pond. It is essential to minimize such competition by adequate management procedures in stocking of shrimp fry and prevention of undesirable species from entering the pond.
Predatory species on the other hand, are species that prey on the culturd stock. These species include snakes, birds, finfishes, amphibians, crustaceans and mammals. The presence of predators is a serious problem for shrimp growers especially in nursery ponds. Predators are generally, but not always, larger than the culture animals. In some predatory finfishes for instance, there are a number of distinct structural features such as the head is about 30% of the body length and the horizontal width of the mouth opening is about 40–50% of the head length against 16–24% in non-predatory fishes. The capability of the predators to capture their preys is affected by size, density, general behavior (eg., escape instincts), color and structure of the preys.
6.5.1 Important undesirable species in shrimp pond
Predatory finfishes are especially destructive to juveniles. The more voracious ones are seabass (Lates calcarifer) and ten pounder (Elops hawaiiensis). Other common predatory species include tilapia, thread fin bream (Polynemus sp.) and Therapon sp. which are very harmful to shrimp larvae.
Finfishes that compete for food and space with the cultured stock are mullet (Mugil sp.) mud skipper (Periophthalmus sp.), clupeiods sp. and Leiognathus sp.
Crabs are one of the worst predators and competitors in ponds causing heavy damage to the stock. Their dike boring activities are the major causes of water leakage in ponds.
Some snails compete for natural food in the pond. The occurence of snails in shrimp ponds appears to coincide with soft shelling in shrimps.
Frogs are considered the most common amphibian predator. The adult frogs are harmful when present in sufficient numbers.
Water snakes predate directly on shrimp larvae and adults and are therefore extremely harmful when present in sufficient numbers.
There are a number of wading birds which cause serious problems in shrimp farms such as the kingfisher and grey heron. The grey heron can swallow a whole prey of 15–20 cm in size. These birds are especially harmful when the shrimps concentrate in the surface or other shallow areas providing opportunity for these aerial predators.
The lower mammals such as otter often destroy their preys more than predates. The animal usually causes serious damages in a shrimp farm by killing shrimps as they enter the pond.
6.5.2 Methods in controlling undesirable species in shrimp pond
There are two usual methods used in controlling undesirable species in shrimp ponds - physical and chemical methods.
Physical method - The most effective method in this category is drying the ponds. Other methods include installation of appropriate screens in the oulet/inlet gates to prevent entrance of undesirable species, proper maintenance of dikes and water gates to prevent leakage and to eradicate boring organisms like crabs ans snakes and setting up traps and bird-scaring devices like strings of colored cloth streching across the surface of the pond to scare predatory birds. During the culture period, selective harvesting or the use of cast net can be resorted to minimize the impact of undesirable species.
Chemical method - Eradication of undesirable species is very effective, less cumbersome, efficient and fast when using chemicals. This is because chemicals act as contact or systemic poison. There are several types of chemicals used and collectively are known as pesticides. The use of organic pesticides such as Aquatin, Brestan, Endrin, etc., is not recommended in shrimp farming because these have residual effects which destroy the fertility of the ponds as well as being non-selective or broad-spectrum compounds in terms of biocidal activity. In selecting pesticides, plant-extracted compounds are recommended because these are biodegradable and in most cases contribute to the fertility of the fertility of the pond soil. The commonly used pesticides are:
Rotenone - This chemical which is extracted from grounded Derris sp. roots has been used as piscicide for centuries in South America. It is a selective poison affecting readily the finfishes but not the shrimps at certain concentrations. Commercial rotenone is in powder form containing 4–5% rotenone.
Effective dosage for commercial rotenone is 2 g/m3 for common predators and 8 g/m3 for eel. Crude rotenone can be extracted from Derris plant by cutting the root into small pieces, crushed and soaked overnight in water. The crushed roots are squeezed to obtain as much extract as possible. The extract is then added to the pond at the rate of 4 g of dry root/m3 of water. Fresh derris root is more effective than the dry one.
Saponin - It is extracted from tea seed cake which is a residue from oil processing of Camellia sp. seed. It contains 10–15% saponin. Saponin is widely used to eradicate finfishes without toxic effect on crustaceans especially shrimps. The effectiveness of saponin decreases with decreasing salinity. The recommended levels of application are 12 and 20 g/m3 for salinities above and below 15 ppt, respectively. Saponin is used in shrimp farms in Thailand, Malaysia, Singapore and Taiwan even during the rearing period because of its piscicide activity.
Calcium carbide - It is applied into the crab hole and enough water is poured in the hole to activate the calcium carbide which kills the crab.
Tobacco dust - Nicotine is the potent principle in tobacco dust and is non-selective type of poison. Since this is toxic even to shrimp, it is generally applied during pond preparation activities. The pond should be flushed well before stocking. A dosage of 200 kg/ha is used to kill undesirable species especially snails which will take about six months before the population re-establishes.
Ammonium sulphate - This chemical compound which is also a fertilizer (21-0-0) is effective in eradicating undesirable species when used in combination with lime. The toxicant is ammonia which is released from the reaction of ammonium sulphate with lime. This chemical is applied together with lime during pond preparation at the undrainable portions of the pond at a dosage of 1 part of ammonium sulphate to 5 parts of lime. Lime must preferably be applied first to raise the pH since the rapid release of ammonia from ammonium sulphate is dependent on high pH (above 8.0).
Liming is the application of calcium and magnesium compounds to the soil for the purpose of reducing soil acidity. It is usually applied during or after the pond drying stage.
6.6.1 Action of liming
The favorable actions of liming are: (a) kills most microorganisms especially parasites due to its caustic reaction, (b) raised pH of acidic water to neutral or slightly alkaline value, (c) increases the alkaline reserve in water and mud which prevents extreme changes in pH, (d) neutralizes the harmful action of certain substances like sulfides and acids, (e) promotes biological productivity since it enhances the breakdown of organic substances by bacteria creating a more favorable oxygen and carbon reserves, (f) precipitates suspended or soluble organic materials, decreases biological oxygen demand (BOD), increases light penetration, enhances nitrification due to the requirement of calcium by nitrifying organisms, and (g) indirectly improves fine-textured bottom soil in the presence of organic matter.
Excessive liming, however, can be damaging because it decreases phosphorus availability through precipitation of insoluble calcium or magnesium phosphate.
6.6.2 Liming substances
The chemicals used for liming of soils are the oxides, hydroxides and silicates of calcium or magnesium since these are the ones capable of reducing acidity. Typical liming substances are the following:
(a) Calcium oxide, CaO
This is the only compound to which the term lime may be correctly applied. Calcium oxide is variously known as unslated lime, burnt lime and quicklime. It is manufactured by roasting calcitic limestone in a furnace. Calcium oxide is caustic and hygroscopic and is sold commercially in powder and granular forms.
(b) Calcium hydroxide, Ca(OH)2
Calcium hydroxide is known as flaked lime, hydrated lime or builder's lime. It is prepared by hydrating calcium oxide. It sold commercially in powder or granular forms.
(c) Calcium, CaCo3 and mixed calcium-magnesium carbonate, [CaMg (CO3)2]
The carbonates occur widely in nature. Among the common forms that can be utilized as liming substances are calcitic limestone which is a pure calcium carbonate and dolomitic limestone which is a calcium-magnesium carbonate with varying proportions of calcium and magnesium. Commercial calcium carbonate is known as agricultural lime. The carbonates are the least reactive of the three liming substances.
6.6.3 Determination of lime requirement
There are several methods used for the determination of lime requirement in ponds. One of these is the Boyd's method which is relatively easy to perform. The procedure is as follows:
Weight 20 g of the seived soil sample in 100 ml glass beaker and add 20 ml of distilled water. Stir intermittently for one hour.
Measure the pH of the above solution with a pH meter previously standardized and record the reading.
Add 20 ml of p-nitrophenol buffer to the above solution and stir intermittently for 20 minutes. Prior to reading its pH, set the pH meter to pH 8.0 with 1:1 mixture of the p-nitrophenol buffer and distilled water.
Read the pH of the soil sample (buffer/distilled water mixture) while stirring vigorously.
Use the values of the soil sample pH in distilled water and soil sample in buffered solution to obtain the liming rate from Table 1.
If the pH of the soil in the buffered solution is below 7, repeat the analysis with 10 g of soil sample and double the liming rate from Table 1.
Table 1. Lime requirement in kg/ha of calcium carbonate (neutralizing value of 100) to increase total hardness and total alkalinity of pond water above 20 mg/L
|Mud pH in water||Mud pH in brffered solution|
|(kg/ha of calcium carbonate required)|
|5.7||121||242||363||484||605||726||847||968||1 089||1 210|
|5.6||168||336||504||672||840||1 008||1 176||1 344||1 512||1 680|
|5.5||269||538||806||1 075||1 344||1 613||1 881||2 150||2 419||2 688|
|5.4||386||773||1 159||1 546||1 932||2 318||2 705||3 091||3 478||3 864|
|5.3||454||907||1 361||1 814||2 268||2 722||3 175||3 629||4 082||4 536|
|5.2||521||1 042||1 562||2 083||2 064||3 125||3 646||4 166||4 687||5 208|
|5.1||588||1 176||1 764||2 353||2 940||3 528||4 116||4 704||5 292||5 880|
|5.0||672||1 344||2 016||2 688||3 360||4 032||4 704||5 376||6 048||6 720|
|4.9||874||1 747||2 621||3 494||4 368||5 242||6 115||6 989||7 974||8 736|
|4.8||896||1 792||2 688||3 584||4 480||5 376||6 272||7 186||8 064||8 960|
|4.7||941||1 882||2 822||3 763||4 704||5 645||6 586||7 526||8 467||9 408|
Source: Boyd 1979
To prepare a p-nitrophenol buffer of pH 8.0. Dilute 20 g para-nitrophenol, 15 g boric acid, 75 g potassium hydroxide in one liter of distilled water. Make sure that the chemicals used are all reagent grade. Check the pH using a standardized pH meter and adjust to the right pH by adding 7.5% potassium hydroxide (7.5 g/100 ml distilled water) or 1.5% boric acid (1.5 g/100 ml distilled water). Use medicine droppers to dispense the solutions. Refrigerate the buffer while not in use. It will last indefinitely. Care must be exercised in the use of p-nitrophenol as it is toxic and reportedly carcinogenic. Potassium hydroxide is very caustic and should be handled with care. As a rule, never handle the chemicals with bare hands and flush with plenty of water should contact occur.
Generally, the liming material used in ponds is calcium oxide. It is therefore necessary to convert the liming rate to calcium oxide value by multiplying the above rate with 0.56.
6.6.4 Methods of liming
Liming can be done in three different ways:
by broadcast over dried pond which includes the dike walls.
by mixing with water and spraying over the pond, and
by liming the water flowing into the pond.
In general, any one of these methods may be employed. Certain cases, however, demand the application of a particular method. In using the first two methods, lime should be spread as uniformly as possible over the complete surface of the pond or pond water. Liming the pond water is usually carried out with the use of flat boat. The third method is uncommonly practicec although it saves the labor in spreading.
One usual way of increasing carrying capacity of a shrimp pond is to improve its natural fertility through the addition of organic or inorganic fertilizers. Pond fertilization is an important and necessary step in extensive and semi-intensive methods of farming operations.
(a) Organic fertilizers
The most common fertilizers are animal manures, rice bran, compost and sewage. Application of organic fertilizers especially in newly developed ponds is advisable because it serves as soil conditioner. The advantages and disadvantages of organic fertilizers are listed in Table 2. The rate of application for shrimp ponds ranges from 500 to 2,000 kg/ha.
(b) Inorganic fertilizers
Inorganic fertilizers are synthetic fertilizers that generally contain concentrated amount of at least one of the major plant nutrients like nitrogen, phosphorus and potassium. These major nutrients are expressed on a percentage by weight basis. Nitrogen is expressed at %N and phosphorus as % Phosphorus oxide (P2O5). Commercially available inorganic fertilizers are usually sold with such trade names as 16–20-0 (16%N-20%P2O5 -0%K2 O), 14-14-14 (14%N-14% P2O5 -14%K2O), 45-0-0 (urea). The rate of application ranges from 25 to 100 kg/ha during pond preparation.