In nature, many fish never reach adult size because they are eaten by other animals or predators or die from disease or lack of oxygen. Fish culture in ponds try to control the situation in order to produce more fish. In ponds predators can be controlled so that the pond yields more fish than the natural waters. Growth of fish in ponds is mainly due to the fact that fish cannot escape, and feeding, breeding, growing and harvesting the fish is carried out in a well-planned way.
Fish culture is practised in ponds. These are small shallow bodies of water in natural conditions and completely drainable, usually constructed artificially.The natural ponds differ from the lakes in having a relatively large littoral zone and a small profundal zone. Their source of water may also vary.
Growing fish in ponds is a very ancient practice. Fish were cultured as long ago as 2698 B.C. in China. Fish culture seemed to occur whenever civilization was settled for a long period of time. Fish culture was done in ancient Egypt and in China, which has had a continuous civilization for over 4000 years. The first written account offish culture in ponds was by Fan Lai, a Chinese fish farmer in 475 B.C. Ancient Romans introduced carp from Asia to Greece and Italy. By the seventeenth century, carp culture was being practised all over Europe.
Why fish grow in ponds
The practice of fish culture in ponds is more advantageous. It is easier to catch fish from a pond than it is to catch them from a natural resource. Fish growth can be controlled. Fish can be fed extra food to improve their market value. Natural enemies can be kept out from killing the fish in the ponds. Fish can be protected from diseases. In ponds, the production offish can be increased with scientific management and more income can be generated. Fish farming can help a farmer make the best use of the land. Fish fanning can also provide extra income.
Types of fish farms
There are two major kinds offish farms mainly based on the nature of rearing.
1. The fish farms in which fishes are bred to raise the fry and fingerlings.
2. The fish farms in which the fry or fingerlings are raised to marketable size. The farmer has to decide what type of fish farm he is going to start.
Based on water supply to ponds, they are classified into 5 types.
Spring water ponds : Spring water ponds are supplied by ground water, either through natural springs at their bottom or through others lying adjacent to them. The spring water is good for fish culture because it is clean and has no unwanted fish or fish eggs in it. If the spring has covered a long distance before draining into the pond, it may have contaminants and should be filtered before its use.
Rain water ponds : These are also called as sky ponds. These are filled with rain water and the extent of their filling depends upon the amount of the rainfall.
Well water ponds: These are filled with well-water and considered very good for fish culture. They may be adequately supplied with water which has no contaminants.
Flood plain ox–bow ponds : Water for these ponds is supplied by the stream. These are highly productive due to the accumulation of organic materials and periodic flooding.
Water course ponds: These ponds are placed on the course of flowing water and divided further into two main types.
Based on water supply, soil and topography the ponds are of five types.
Many aspects of the construction of these ponds are the same. The main difference between these is the water source. These are :
Barrage ponds: These ponds are usually filled by rainfall or by spring water. A spring, for example, sends water flowing through a small valley or down a slope into a low place. Or, a spring bubbles from the ground into a natural depression. The pond is formed by collecting water at the base of the valley and in the low places. The farmer does this by building a wall or dam which holds the water inside, what now, is the pond area. The number of pond walls that must be constructed depends upon the land and the drainage system. A barrage pond usually need only one wall – the main wall between the water source and the pond area.
Diversion ponds a) Rosary system b) Parallel system
One kind of drainage system called a sluice cart be used to let water both in and out of the pond. There are also a number of simple drainage systems which can be used that do not require any complicated construction.
Barrage ponds (Fig. 5.1) should not be built where the flow of water is too great as it is difficult to keep the water from breaking down the wall if the pressure of the water is too great. Brooks and streams which flow well, but not too strongly, make good sources for barrage ponds.
Even when the flow of water is not great, however, barrage ponds require overflow channels. Because barrage ponds are usually built in low areas, they are likely to get filled up during heavy rains. Overflow channels constitute any kind of a system which can be set up to stop the pond from collecting too much water. The overflow takes extra water away from the pond. If this extra water is not drained out, the pond wall may break.
These ponds are made by diverting water from another source like a stream or river. Channels are dug to carry the water from the water source to the pond. Diversion ponds can be made in a number of ways. Sometimes a pond is dug in flat ground or can be made by slightly enlarging a natural depression in the land. These ponds require walls depending upon the topography of the land, the drainage system, etc.
In diversion ponds (Fig. 6.2), the water is always brought to the pond through diversion channels instead of running directly into the pond. Water can be diverted in a number of ways. A small stream which gets its water from a larger stream nearby can be dammed and used as a diversion channel to feed a pond. Diversion ponds can be built in two ways.
Rosary system :
These ponds are built one after another in a string. All the ponds drain into each other and must be managed as if they were one pond. If the first pond in the series with a water inlet is full of predators which must be poisoned, all the other ponds have to be harvested and drained before the first pond can be poisoned.
Each pond has its own inlet and outlet. Therefore, each pond can be managed as a separate pond. The parallel system is a better system. But rosary systems are cheaper and easier to build. If the water source is good, and can be kept free of predators, and if management of the pond is done well, this is a cheaper and better system.
Diversion ponds are always better than the barrage ponds. This is due to the fact that they are less likely to overflow and the water source is more dependable throughout the year. Barrage ponds, however, require less construction and are likely to be cheaper.
Ponds may also be classified according to their size and usage in a fish farm into five types
These are constructed in accordance to the requirements of the fish or its stages of life cycle. These are:
Head pond :This pond is usually constructed near a perennial source of water. The main purpose of the pond is to meet the water requirements of the entire farm, taking into consideration the losses through seepage, evaporation etc.
Hatching ponds : These are also called as spawning ponds. These are small and mostly in the form of small tanks or plastic pools, made near the spawn collection centres. Hapas are fixed in these ponds. The eggs are collected and kept in the hapas for hatching. Similar ponds are also constructed in the fish farm. These are slightly deeper with water circulation. Here also, the hapas are fixed inside the ponds. The brooders are released into the hapa after giving them hormonal injections. Spawning takes place inside the hapa and the eggs are also allowed to hatch here.
Nursery ponds: These are also called transplantation ponds. These are seasonal ponds and are constructed near the spawning and rearing ponds. The main object is to create a suitable condition of food availability and growth of fry because at this stage they are most susceptible to hazards like the wave action and predators. These should be small and shallow ponds 0.02-0.06 ha. in size and 1-1.5 m. in depth. In the nurseries, the spawn (5-6 mm) are reared to fry stage (25-30 mm) for about 15 days. These ponds are usually rectangular in size. Extra care should taken for rearing the young stages, otherwise heavy mortality may occur. Sometimes the spawn are cultured for 30 days also. The pond bottom should gently slope towards the outlet to facilitate easy netting operations. Small and seasonal nurseries are preferred as they help in effective control of the environmental conditions. In practice about 10 million spawn per hectare are stocked in nursery ponds.
Rearing ponds : These should be slightly larger but not proportionally deep. These should be located near the nursery pond and their number may vary depending upon culture. They should preferably be 0.08-0.10 ha in size and 1.5-2.0 m in depth. The fry (25-30 mm) are reared here upto the fingerling (100-150 mm) stage for about 3-4 months. Carp fry grown in nursery ponds are relatively small in size and not fit enough for their direct transfer into stocking ponds. In stocking ponds bigger fishes are likely to be present which may prey upon the fry. Hence, it is desirable to grow the fry in rearing ponds under proper management practices upto fingerling size so that their ability to resist predation will be improved.
Stocking ponds : These are the largest ponds and are more deep, with a depth of about 2-2.5 m. The size of the pond may vary from 0.2-2.0 ha., but these should preferably be 0.4-0.5 ha in size. These are rectangular in shape. The fingerlings and advance fingerlings are reared upto marketable size for about 6 months. One year old fishes may grow upto 1 kg. or more in weight.
Management of nursery ponds is one of the most important aspect for successful fish culture practices. The hatchlings or spawn are reared to fry stages in small ponds called nursery ponds. The hatchlings, spawn and fry are extremely delicate, these should, therefore, be reared with utmost care to get a very good survival rate.
Nursery management has to be started right from the summer, so that the raising of a good crop of fry is possible. Drying up of nursery ponds in summer helps mineralization, removal of organic detritus and destruction of predators and aquatic weeds, which are more in perennial nurseries. The ponds have to be desilted, but the fine layers of the desilted earth containing rich humus matrix could be used to fill up the sides or eroded bundhs inside the nursery ponds. This helps in the manurial value of the rich superficial layer of earth and adds to the productivity of the pond. The outlets, inlets and strengthening of bunds have also to be attended to during the summer. The vegetation on the bunds are excellent breeding grounds for insects, hence, these should be destroyed and the vegetation burnt during summer.
If drying of the ponds is not possible, it is better to go in for poisoning of the pond. Poisons like endrin, tafadrin, derris root powder and Mohua oil cake are used to eradicate fish enemies. For successful nursery pond management the following pre and post stocking management techniques are to be followed.
Pre–stocking pond management
It involves site selection, eradication of weeds, insects and predators, liming, manuring, etc.
Green manuring in the pond:
The growth of plants in a pond bed is a necessity so as to enrich the soil. This process is known as green manuring. The short term crops of the leguminous family members like peas, beans, etc. help in enrichment of the soil with nitrogen. After the growth of the plants, the pond bed is ploughed and levelled with the roots of the plants in the soils. The nodules of these plant roots enrich the soil with nitrogen and are beneficial for enhancing pond productivity, resulting in a high survival rate and fast growth of fry.
Eradication of aquatic weeds and predators:
Aquatic weeds create certain problems in the ponds such as providing breeding grounds for aquatic insects, enabling to harbour predatory insects, restricting the free movement of fry, causing obstruction during netting and resulting in depletion of plankton production. Hence, the weeds should be cleared during summer either mechanically or by applying chemicals.
Predators injure the spawn and are responsible for a high mortality rate. Hence, the predators should be eradicated from nursery pond. The predatory fishes are Channa sp., Wallago attu, Heteropneustes fossilis, Clarias batrachus, Anabas testudineus, etc. which cause maximum harm to spawn, and use them as food. Weed fishes such as Salmostoma sp., Amblypharyngodon mola, Barbus sp., Esomus danricus, etc. are small sized and uneconomic fishes, which prey on carp spawn. They breed in the pond and compete with carp spawn in space and food.
Complete draining of pond is the best and simplest method to eradicate undesirable fishes. The drag nets should be used repeatedly for fishing. However, as most of the predator fishes are bottom dwellers, netting may not solve the problem. Therefore, the fish toxicants are used for eradicating them totally. Endrin at 0.01 ppm, dieldrin at 0.01 ppm, aldrin 0.2 ppm and nuvan at 30 ppm are useful to eradicate the forage fishes and all other fish enemies. These poisons are effective for 1-2 months and it is not advisable to use them repeatedly. The poisons get accumulated in the pond bed and it is impossible to remove them afterwards. These should be treated about 60 days prior to stocking.
Derris root powder (4 ppm) is good to eradicate forage fish from nursery pond and it is effective for one week. Mahua oil cake (Madhuca latifolia) at 250 ppm is lethal to forage fish. It should be applied a fortnight before stocking. After its lethal effect on forage fish, it is useful as manure later on. Sugarcane jaggery at 1% concentration is also lethal to the fish and its active poison is saponin. Tea seed cake is lethal to fish seed at the rate of 600 kg/ha. Application of 3-5 ppm of powdered seed kernel of Croton tiglium, 2-6 ppm of powdered root of Milletia pachycarpa, 20 ppm of powdered seed of Barringtonia accutangula, 12 ppm of powdered unripe Randia dumetorum and 10 ppm of powdered bark of Walsula piscidia is also effective.
Liming is most essential to maintain the pH of water. The water should be slightly alakaline as it is useful for the eradication of microorganisms in the pond and also to help maintain the hygienic condition of water. Lime is useful to neutralise the acidic condition which will result while manuring. Lime is applied at the rate of 250 kg/ ha. Its dose has to be increased upto 1000 kg/ha in highly acidic soils.
While watering the pond, care should be taken to see that no forage fishes enter into the pond either at the egg, young or adult stage. For this, water should be let in through a fine sieve. The nursery pond has to be filled with water upto a depth of one metre.
Manuring has to be done after filling the pond with water. The main objective of manuring is production of adequate quantities of plankton, which is useful as natural food of carp seed. Several types of manures are available to increase the productivity of the pond. The most common , best and cheap of all the manures is raw cattle dung (RCD). Raw cattle dung at the rate of 10,000 kg/ha produces a good bloom of zooplankton in 10 days. The application of 5,000 kg/ha of poultry manure also produces good amount of plankton in pond. However, it is better to find a suitable manure which produces plankton within 3-4 days. A mixture of 5,000 kg/ha raw cattle dung, 250 kg/ha of single super phosphate and 250 kg/ha groundnut oil cake (GNO) has been found to yield plankton in about 3 days. This mixture is soaked in water, mixed thoroughly and spread on the surface of the water, so that the manure gets mixed thoroughly in water, thereby enhancing the pace of plankton productivity. It should be applied initially for about 10 days earlier to stocking and remaining seven days after stocking. If two or more crops of fry are to be produced from the same nursery pond, then the pond should be fertilized with 2,000 kg/ha of cattle dung a week before each subsequent stocking.
Inorganic manures are useful to fertilize the soil instead of water. 10:1 elemental ratio of N:P is required for phytoplankton growth. Inorganic fertilizers are usually applied in 10 equal monthly instalments at the rate of 100-150 kg/ha/yr.
Eradicating insects and other harmful biota:
Insects are usually found in large numbers in ponds over the greater part of the year, especially during and after rains. These insects injure the spawn and so have to be eradicated. Hence, the insects should be eradicated prior to stocking to ensure maximum survival of the spawn. Notonecta, Ranatra, Cybister, Lethoceros, Nepa, Hydrometra and Belostoma are highly destructive to the carp seed. The insects can be eradicated by using oil emulsions. After manuring the nurseries, they should be treated with oil emulsion.
The spraying of oil emulsion is 12-24 hours before stocking the spawn in nursery pond so as to eradicate the insects. The oil emulsion with 60 kg of oil and 20 kg soap are sufficient to treat one hectare of water. The soap is dissolved first in water and it is added to the oil and stirred thoroughly to get a brownish grey solution. It is then spread on the surface of the water. All the aquatic insects die because of suffocation due to the thin oil film on the surface of the water. The spiracles of insects are closed by the oily film so that they die.
An emulsion of 56 kg of mustard oil and 560 ml of Teepol is also useful to treat one hectare of water. An emulsion can also be prepared with diesel boiler oil and any detergent. Since soap has become very costly, one effective method is to use 50 cc of Hyoxyde-10 mixed in 5 litres of water with 50 litres of high speed diesel oil for a hectare of water.
The mixture of Herter W.P (0.6-1.0 ppm) and oil extracted from plant Calophyllum inophyllum is effective to insects as well as prawns like Paleamon lamenii, which is usually found in nurseries. A mixture of 0.01 ppm gamma isomer of benzene hexachloride and ethyl alcohol is also highly toxic to insects. Application of biodegradable organophosphates like Fumadol, Sumithion, Baytex, Dipterex, etc. (0.25 to 3 ppm) are useful to kill the insects.
Whenever an oil emulsion is applied, there should be no wind as it disturbs the oil film, and its effectiveness will not be felt on the eradication. Birds like king fishers, herons and cormorants are destructive to fry and fish. Thin lines stretched across the pond are the most effective means of controlling them.
After satisfying the physico-chemical nature of the water and plankton growth in the nursery pond, the spawn can be stocked in the ponds at the rate of 5-6 million spawn/ha. The stocking should be done either in the early morning or late evening after gradual acclimatization of the spawn to the pond water.
Post-stocking pond management
After preparing the nursery pond, it is better to maintain optimal physico-chemical properties and plankton. Brown colour of water reveals rich zooplankton growth. Green or blue colour reveals predominance of algae in the plankton. Dirty colour reveals suspension of silt in the water column. Maintenance of one metre water depth is enough in nursery ponds.
Among the chemical properties, 3-8 ppm dissolved oxygen is good for stocking spawn. Carbon dioxide above 15-20 ppm is lethal to fish life. A pH ranging between 7.5 to 8.5 is highly productive. The total alkalinity of 100-125 ppm is highly productive in water. 0.2 to 0.4 ppm of phosphates are good for plankton production and 0.06 to 0.1 ppm nitrates are considered enough for fish growth. 1 ml of plankton in 50 litres of water in nursery ponds is considered to be conducive for stocking spawn.
After stocking, during one or two days most of the plankton will be consumed by the spawn. Survival and growth of spawn are influenced by quality and quantity of food available in the pond. To ensure healthy growth of spawn, artificial feeding is necessary and is restored from the next day after stocking. The major carp spawn of 5-6 mm length weighs 0.0014 mg. The most commonly used artificial feeds are groundnut oil cake, rice bran, coconut, mustard cakes, etc. Finely powdered and sieved groundnut oil cake and rice bran mixed at 1:1 are used. The feeding schedule is as follows.
1-5 days after stocking – double the initial body weight of the spawn. 6-10 days after stocking – thrice the initial body weight of spawn.
11-15 days after stocking – three to four times the initial body weight of the spawn.
The level of artificial feeding has to be decided by the fish farmer based on the study of physico-chemical parameters and plankton.
In 15 days of nursery rearing, the spawn grows to 20-30 mm size fry. At this stage, these fry could be transferred to rearing ponds. Supplementary feeding should be stopped a day before harvesting. The harvesting should be carried out in the early morning. In the same nursery pond, 3-4 crops of fry can be raised in a season.
Rearing Pond Management
Its management is similar to stocking pond management except stocking material and stocking densities. This stocking material is fry stage, which is reared up to fingerling stage for about 3 months. The stocking density of fry is 0.2-0.3 millions/ha.
Stocking Pond Management
After rearing the fish seed upto fingerlings in rearing ponds, these fingerlings are reared to marketable size in stocking ponds. The management techniques in rearing and stocking ponds are almost similar.
To get maximum quantity of fish utmost care should be taken through the most economic management measures. It should be clear that much of the success of a fish pond depends upon careful planning. The principles in the rational management of stocking ponds are increasing the carrying capacity of ponds by fertilization and supplementary feeding, optimal utilization of ecological niches in the pond by stocking manipulation, maintenance of water quality, the culture of quick growing species and fish health monitoring.
It includes site selection, conditioning of the ponds, watering and fertilization of ponds.
Conditioning the pond:
If the pond is an old one from which the fish have been harvested, it should be completely ploughed. Ploughing helps in drying of pond bottom, increases the mineralisation, removes the obnoxious gases accumulated in the mud and destroys aquatic weeds and undesirable organisms. Ploughing of the pond bottom improves soil condition, but it should not be so deep so as to bury the fertile top layer and bring up the sterile layer to the surface. Desilting of the pond is essential to maintain productivity. The pond bottom should be cleared of any twigs, branches and stumps or dead fish. Then the bottom should be smoothened again. When the pond has dried enough, the soil will have large cracks in it. That means restoration of pond bottom is most essential now to improve the physical, chemical and biological condition of the soil.
Control of aquatic weeds:
The growth of aquatic weeds deprives the pond soil of nutritive elements, restricts the movement of fish, interferes with netting operations and harbours predatory and weed fishes and insects. Hence, the aquatic weeds should be controlled. The best way of weed control is pond drying and ploughing.
Eradication of undesirable organisms:
The real problem arises during the rearing of fish, when the other animals eat the fish. Frog, snakes and birds eat young fish and must be kept out of ponds. The worst predators are carnivorous fishes, which should be prevented from entering into ponds by screening the water inlets.
The common predatory and weed fishes (Fig. 5.3) in ponds are Channa sp. Clarius batrachus, Heteropneustes fossilis,Wallago attu, Notopterus notopterus, Mystus sp., Ambasis ranga,Amblypharyngodon mola, Salmostoma sp., Esomus danricus, Puntius sp., etc. The weed fishes are small sized and uneconomical fishes and are usually found in ponds. The undesirable fishes enter into ponds accidentally, through incoming water along with carp spawn. The predatory fishes are harmful to all the stages from the spawn to the adult stages of carps and prey on these carps as well as compete with them for food and space.
In any pond, all trash fishes and predators must be removed before stocking the pond. The simple methods of draining and drying of the ponds and then ploughing them are most effective in controlling them. If the draining is not possible, the pond as completely as possible, the undesirable fishes should be removed from ponds by repeated drag netting. However, many fishes escape the net by staying at the edges of the pond. The bottom dwellers like murrels, climbing perches, magur, singhi, etc., which burrow themselves in the mud are difficult to be caught by netting. Dewatering is the best method, wherein the water should be removed by pumping, although this is an uneconomical method. In this case, the best way to get rid of the undesirable fishes is to poison the water in a pond which cannot be drained.
Various types of fish poisons are available in the market. These are classified into 3 groups -chlorinated hydrocarbons, organophosphates and plant derivatives. Chlorinated hydrocarbons are most toxic to fish. These are accumulated in fish tissues and are stable compounds, which are not metabolised. Organophosphates are less toxic to fish, but they have adverse effects on aquatic flora and fauna. The accumulation is less in fish tissues and relatively less persistent in water. Hence, the plant derivatives are good fish poisons.
The best natural poisons are mahua oil cake, rotenone of derris root, quick lime (160 kg/ha), tea seed cake (150 kg/ha), camellia seed cake (50 to 200 kg/ha depending on water depth), tobacco waste (150-200 kg/ha) and powdered cotton seed (Table 6.1). Another safe chemical is saponin , which is a compound of tea seed cake and is applied at a dose of 0.5 ppm in the pond. Most of the natural poisons will degrade and disappear from the water in 7-12 days. Mahua (Mahuca latifolia) oil cake is an excellent poison, which breaks down after 10 days and is useful as a fertilizer. The chemicals like endrin, dialdrin and DDT should be avoided in ponds, as they can last in the ground for years and later kill all the pond fish.
Eradication of aquatic insects (Fig. 5.4) is discussed in nursery pond management.
Fig. 5.4 Aquatic insects
- a) Eretes b) Peschatius c) Dineutes d) Laccophilus e) Stemolophus f) Rhantaticus g) Limnometra h) Anisops i) Diplonychus j) Regimbartia k) Notonecta l) Hyphoporus m) Laccotrephes n) Cybister o) Lithocerus p) Hydrophilus q) Ranatra r) Hydaticus s) Sandracott
Lime is frequently applied in aquaculture practices to improve water quality. After the pond is ploughed, cleared and smoothed, it should be conditioned with lime. Liming increases the productivity of a pond and improves sanitation. It is both prophylactic and theuraptic. The main uses of lime are;
Fig. 5.5 Aquatic weeds
- a) Pistia b) c) Azolla d) Eichhornia e) Lemna f) Ceratophyllum g) Chara
a) Naturalize the acidity of soil and water.
b) Increase carbonate and bicarbonate content in water.
c) Counteract the poisonous effects of excess Mg, K and Na ions.
d) Kills the bacteria, fish parasites and their developmental stages.
e) Build up alkaline reserve and effectively stops fluctuations of pH by its buffering action.
f) Neutralises Fe compounds, which are undesirable to pond biota.
g) Improve pond soil quality by promoting mineralisation.
h) Precipitates excess of dissolved organic matter and this reduces chances of oxygen depletion.
Fig. 5.6 Aquatic weeds
a) Nymphaea b) Nelumba c) Jussiaea d) Marsilia e) Potamogeton f) Najas
i) Acts as a general pond disinfectant for maintenance of pond hygiene.
j) Presence of Ca in lime speeds up composition of organic matter and releases CO2 from bottom sediment.
k) Lime makes non-availability of K to algae.
New ponds can be limed before they are filled with water. The limestone should be evenly spread over the dry pond bottom. In ponds with water, it is better to spread evenly on surface of water. Whether the pond is new or old, a layer of lime should be placed on the bottom of the pond. The lime should be added to the pond two weeks before the water is pumped into the pond. The best time for lime application is during the period when fertilization has been stopped. Lime should not be applied while the pond is being fertilized.
The highly acidic soils (pH 4-4.5) need a dose of 1000 kg/ha lime, whereas slightly acidic soils (pH 5.5-6.5) need about 500 kg/ha lime. Nearly neutral soils (6.5 to 7.5 pH) require only 200-250 kg/ha lime. The pH of the pond soil should be brought to nearly neutral for maximum benefits.
After the lime has been applied to the pond bottom for at least two weeks, the water should be let in slowly. The water should fall from the water inlet into the pond, so that the water mixes with oxygen from the air as it falls into the pond. The water should not go in to the pond too quickly. If the water enters too fast, the pond bottom will get stirred up and thus make the water muddy. Screens should be used at inlets, so that the unwanted fishes and other organisms will not enter into the pond. The pond should be allowed to be free for a few days after it has been filled. The quality of water in the pond should be checked before the fish is released into it.
Fishes require certain elements to grow and reproduce. These elements are C, H2, O2, N2, K, P, S, Ca and Mg. Some other elements, called trace elements like Cu, Zn, Mn, Mo, B, etc., are needed only in small amounts. If these elements are missing or present in very low quantities, the fish will not grow well. Fish get these elements from the pond soil, the pond water and the food they eat. Some fish ponds lack elements that are necessary for fish growth and productivity. In these cases, it is necessary to add fertilizers to the water. The fertilizers are simple materials which contain the missing elements. The elements most often missing or in short supply in fish ponds are N2, P and K. Fertilizers consisting these missing elements are added to the fish pond to help the growth of the fish and of the plankton, which the fish use as food.
A pond rich in phytoplankton is often bright green in colour. The colour indicates a bloom of algae. In a normal bloom, the secchi disc disappears at about 30 cm depth; when the secchi disc disappears at 20-40 cm depth, the pond is very productive and fertile. No fertilizer is needed in a pond under these conditions.
Sometimes a pond can become too fertile. If the secchi disc disappears at only 15 cm, the bloom is too thick. The thick layer of green blocks the sunlight in the pond and no oxygen can be released by the phytoplankton. In this case, there is too much fertilizer in the pond, and hence some of the thick layer of algae formed at the surface of the water should be removed. These ponds do not need any fertilizer.
If the secchi disc can still be seen at 43 cm depth, the plankton in the pond is not sufficient. It is, therefore, necessary to add fertilizer to the pond water in order to prepare a fertile pond. Another factor which determines the need for fertilizers is the quality of the soil. If the soil is highly productive, the need for fertilizers is less; if the soil is not so productive, the need for fertilizers is greater.
The choice of fertilizers can be decided on the basis of physical composition of soil. In sandy or sandy loamy soils with low organic matter, fertilization is carried out with organic manures. In loamy soils with medium organic matter, a combination of both organic and inorganic fertilizer should be applied. In highly clay soil with rich organic matter, fertilization is carried out with only inorganic fertilizers. Amount of fertilizers to be applied to ponds may be worked out on the basis of the productive potentiality of the pond. The ponds can be categorised on the basis of N, P, organic carbon and alkalinity (Table 5.1).
In case of deficiency of potash, it can be included at the rate of 25-50 kg/ha/yr. The NP ratio should be 2:1. In addition, cow dung may be applied at a rate of 10,000-15,000 kg/ha/yr. The best way to use this
animal manure is to make a soup of it in a tank by mixing it with water. This soup should be spread in the pond. Fertilizer should be applied at a rate determined by the area of pond. Area is the length of the pond, multiplied by the width. For example, if a pond measures 20 m in length and 10 m in width, it has an area of 200 square metres (m2). This is equivalent to 2/100 of a hectare. To fertilize a 200 m2 fish pond with cow dung, at the rate of 1000 kg/ha, you must use only 20 kg.
Fertilization should be done 2 weeks prior to stocking the fish, so that, sufficient natural food is available in the pond. 1/5 of the total quantity of organic manure is required as an initial dose, and the rest is applied in 10 equal instalments. Organic and inorganic fertilizers may preferably be applied alternating with each other in fortnightly instalments. The amount of fertilizers required in general for fish ponds is 10,000 kg/ha/yr of cow dung, 250 kg/ha/yr of urea, 150 kg/ha/yr of single superphosphate and 40 kg/ha/yr of murate potash. In large ponds, fertilizers may be applied by using boats.
Stocking is used to describe the act of placing the fish into the pond. The stocking density is used to describe the total number of fishes, which can be stocked in a pond. The stocking ponds are generally stocked with fingerlings which are about 75-100 mm in size. For increasing fish production, the selection of fish with desirable qualities is the most important biological factor. Since fish with the shortest food chain give the highest production, phytophagous, herbivores, omnivores and detritus feeders are preferred for culture in stocking ponds. For rearing of fish, either monoculture or polyculture in any species, combination may be carried out, most preferably the polyculture. The desirable stocking rate is 5,000 fishes per hectare. In a monoculture pond, the stocking rate is the same as the stocking density because there is only one kind of fish. There is enough food and room in a pond for a particular number of fish. Good growth of fish depends upon the right number of fish cultured in the pond.
The stocking rate depends on the volume of the water and on the oxygen balance of the pond rather than the size of the pond. The ratio of fish to the volume of water should not be less than 1 fish to 2 m3 of water where there is no forced aeration.
As far as possible each pond should be stocked with silver carp and catla, the surface feeders. This should not be more than 30 to 35%, otherwise it would affect their growth adversely. Rohu is a column feeder and it should not be stocked more than 15-20%. Bottom feeders such as mrigal and common carp together can be stocked to the extent of 45%. Availability of aquatic weeds in the pond decides the stocking density of grass carp. It should preferably be about 5-10%.
Rearing of fingerlings to table-size fish may continue for one year or only 6 months. In the latter case, the stocking density may be reduced. In this system, harvesting is done monthly and the number and species of harvested fish are replenished with a new stock of fingerlings. This is possible only where the supply of fingerlings is available throughout the year. Under these conditions the production is much higher than with the annual or 6 monthly stocking and harvesting.
In a polyculture of Chinese carp, the stocking density is about 20,000 fingerlings per hectare. The stocking rates are 5,000 grass carp, 5,000 bighead carp and 10,000 silver carp. If common carp is also included, then in a stocking density of 7 Chinese carps, 2 fish would be grass carp, 3 would be common carp, and there would be only one each of bighead and silver carp. In Malaysia, the ratio of carp stocking has been suggested at 2:1:1:3 for grass carp, bighead, silver carp and common carp.
If fishes are stocked in a pond, there should be enough oxygen, no temperature difference between the stocking water and the pond water. When the fingerlings are transported from a far away place, in order not to stress the fish, the bags with fingerlings are placed in the pond unopened until the water temperature inside the bags is about the same as the temperature in the pond. When it is same, the fingerlings are allowed to swim out of the container into the pond water by themselves. The fingerlings should not be poured into the pond water, as they die because of the shock of hitting the water.
Water quality Management
Water quality managment is discussed in detailed
The feed management is discussed in detailed in chapter 6.
The health managment is discussed in detailed in chapter 7
The fishes are harvested after a one year with the help of gill nets. Five to Six fisherman depending up on the size of the pond enter into the pond from one side, move to wards the other end with gill net and catch the fishes.
Aquatic weeds and their controle
Aquatic vegetation is described as aquatic weeds. Any undesirable vegetation which causes direct or indirect damage to the fishes or hamper the fishery operations may be described as weeds. In the tropical regions of the world, aquatic weeds grow luxuriantly causing nuisence to fisheries, water transportation and water supply systems, and provide conducive habitat for factors of several diseases. In India, ponds and tanks usually have fertile soil and water and so they invariably overgrow with all types of aquatic vegetation. For successful farm management, a strict watch on the growth of unwanted vegetation is necessary. With the presence of excess vegetation it becomes very difficult to net fishes in weed infested ponds.
Reasons for control of weeds
Uncontrolled vegetation growing excessively hinder fisheries interest in many ways. The weeds in the water reduce the yield of fish just as the weeds in the field reduce the yield of cultivated crop. It is necessary to control the weeds in fish ponds. Some of the reasons for this are quite obvious.
1. Due to the presence of aquatic weeds in the pond, the fishes cannot swim properly, thus restricting their ability to browse and hunt for food.
2. Weeds absorb nutrients for their growth and multiplication, thus absorbing nutrients essential for planktonic food of fishes which causes depletion offish food. Due to their presence, water loses its fertility to sustain fish stock.
3. Weeds offer shelter to unwanted predatory and weed fish, which hunt upon or compete with the cultivated varieties.
4. By profuse growth, weeds choke the entire water column, restrict netting and make navigation impossible.
5. The presence of weeds in water reduces the water holding capacity of the area and water loss due to evaporation through leaves occurs. In case of few weeds, the evaporation is much more than that from the open surface.
6. Weeds cause wide dirunal fluctuation in dissolved oxygen, temperature and other physico-chemical parameters to make the water inhospitable for fishes.
7. The weeds accelerate the process of siltation of the water area, ultimately turning it into a swamp.
8. Weeds harbour harmful insects, frogs, snakes and other predators enabling them to breed and multiply.
9. Weeds choke the gills of the tender young fishes.
10. The weeds interfere with the circulation and aeration of water, restrict the diffusion of sunlight and upset the normal chemical balance of the system.
11. The toxic gases in the pond bottom ooze produced by rotting organic matter cannot be easily eliminated into the atmosphere if the water surface is choked with weeds. In these conditions very few fish could survive in the water.
12. Aquatic weeds are responsible for minimising water depth and ultimately cutting down the soil-water interaction which is so essential for recycling of nutrients for the fishes.
13. Thick algal blooms deplete the oxygen in the water during dark hours or when they die or rot and cause sudden mortality of the fish stock.
14. Some kinds of algae cause allergic irritations on human skin and make it difficult for people to get into the pond.
15. The fish yield is reduced in weedy infested water bodies. 16. Weeds affect water irrigational potential.
Advantages of weeds
Weeds do not always have harmful effects. The weed mass can be turned to some productive use which will recoup some of the losses involved in controlling them. The extra advantage of the utilization method lies in producing valuable end products. Different methods of control and utilization of weeds should be seen as useful tools in an integrated system of aquatic weed management. The aquatic weed are advantageous and help in the development and maintenance of a balanced aquatic community. The advantages are:
1. Aquatic weeds produce oxygen during photosynthesis and this oxygen is utilized by the fishes.
2. Weeds provide shelters for small fishes.
3. Weeds provide shade for fishes.
4. Weeds provide additional space for attachment as well as food for aquatic invertebrates which in turn serve as food for fishes.
5. Weeds help in the precipitation of colloidal clays and other suspended matters.
6. Weeds, after removal, can be used as bio-fertilizers and even used in fish farms.
7. Aquatic weeds are used as food for fishes like grass carp.
8. Weeds are also used for pollution abatement.
9. Weeds are used as a source of energy production.
Weeds as food for fish
There are a number of herbivorous fishes which directly consume aquatic weeds. The grass carp is a fast growing fish that feeds on aquatic weeds. The fish utilize submerged weeds like Hydrilla, Najas, Ceratophyllum, Ottelia, Nechamandra and Vallisnaria in that order of preference. The young fish prefer smaller floating plants like Wolffia. Lemna, Azolla and Spirodela. In composite fish culture the production is greatly enhanced by inclusion of grass carp because of its fast growth. It also occupies an ecological niche, which otherwise remains unfilled with the fear that the grass carp may breed and compete with the native fish population in natural waters, only the triploid grass carp which is supported to sterile is being allowed to be introduced.
The other herbivorous fish which utilize aquatic weeds are Pulchelluspulchellus, Oreochromis and Etroplus. Though an omnivore, Cyprinus carpio feeds well on filamentous algae like Pithophora and Cladophora. The manatee, Trichechus sp., a large air-breathing herbivore, is being utilized for the clearance of aquatic weeds in the canals of Guyana.
These advantages of water plants become negligible when they are present in excess and their control then, is essential. The methods to be adopted to control the aquatic vegetation can be formulated only after the plants are identified.
Factors contributing to profuse growth
A number of factors either individually or jointly influence favourable growth of weeds in cultivable waters. These are :
1. Climatic condition and geographical situation of the area.
2. Water depth – lesser the depth, more is the growth of vegetation especially the submerged rooted or emergent vegetation.
3. Clarity of water or turbidity – more suspended material adds more turbidity thus retarding penetration of light in the pond which has an effect on the growth of vegetation.
4. Silt deposition at the bottom, promotes excessive growth of aquatic weeds.
5. Quality of water – fertile condition of water has its impact on the propagation of vegetation.
6. Infestation from other sources – the minute generative vegetative components like spores and cysts may be carried through the water supply, wind, flood, birds, cattle, etc.
Types of aquatic weeds
The aquatic weeds (Fig. 5.5 and 5.6) are classified on the basis of habitat of plants – rooted weeds and floating weeds.
1. Bottom rooted weeds : Plants are rooted at the bottom of the water body and spread within the bottom layers of water. Vallisneria, Ottelia
2. Submerged rooted weeds : The plants are rooted in the bottom soil on the deeper margins of the pond and ramifying in the volume of water. e.g. Hydrilla, Chara, Potamogeton
3. Marginal rooted weeds : Plants are rooted on the marginal region of the surface layer of water and ramify on the surface of water and also on the adjoining land. e.g. Marsilia, Ipomoea, Jussiaea
4. Plants are marginally rooted and ramifying within the marginal region of the water volume. E.g. Typha, Scirpus, Cyperus, Panium
5. Emergent rooted weeds : Surface plants which are rooted in the bottom of the pond but their leaves float on the water surface or rise above the water level. They prefer shallow parts and shores of the pond. g. Nymphea (Lotus), Nymphoides, Nelumbium.
1. Surface floating weeds : The plants are floating on the surface of water and with roots in the water. e.g. Eichhornia (water hyacinth), Pistia, Lemma, Azolla, Spirodele. Few surface plants, are floating on water but without roots g. Wolffia.
2. Submerged floating weeds : The plants are floating but submerged in the water e.g. Ceratophyllum, Utricularia.
3. We can also divide the aquatic weeds broadly as floating, emergent, submerged, marginal weeds and algal blooms and filamentous algae.
Methods of weed control
Based on the intensity of infestation and type of weeds, the aquatic weeds can be controlled by means of manual, chemical and biological methods.
a. Manual and mechanical method
When infestation is scanty and scattered, the weeds can be controlled manually only in small water bodies. This is an ancient method and is still practiced in most of the places. The pre-monsoon period (April-May) is more suitable for manual removal. In many parts of the country, advantage is taken of the drought to control the weeds as ponds and other water bodies dry up or register a sharp fall in the water area, and the plants can thus be removed. Where labour is cheap, manual labour is often employed to remove aquatic weeds. The weeds are controlled manually by hand picking, uprooting the emergent and marginal weeds and cutting the others with scythes.
Most of the floating plants like Pistia, Lemna, Azolla, Wolfia and Eichhornia can be effectively controlled by clearing manually with nets, whereas, the marginal weeds like grass, sedges, rushes, Typha, etc. may be controlled by repeated cutting. This method does not inflict any pollution and there remains no residual toxic effect as in the case of chemical treatment or shading. The weeds thus collected should be dumped far away, be converted into compost manure or burnt so as to have no chance of reinfestation.
Manual weed control is very expensive, time consuming and unsatisfactory. Therefore, mechanical devices have been developed. Cleaning of a weed infested water sheet through the mechanical method, becomes necessary where the water area is not shallow enough to walk through or small enough to uproot the weeds manually or cut them effectively with simple hand implements. Labour problem and an urgency of the work to eradicate the whole area of weeds within a stipulated time period before water level is raised, are the other factors which make it necessary to resort to mechanical methods for eradication of weeds.
A number of devices ranging from very simple barbed wire bottom rakers to sophisticated mechanical equipments like power winches with steel wire, under-water cutter, dredgers, mechanised removers, etc. are in vogue to use for the purpose. Broomfork, long fork, sickels or scythes, long knives, barbed wire netting, chaining and motor powered weed cutters are some of the specialised equipment used for this purpose.
Crusher boats are used to clear water bodies infested with water hyacinth. The rooted submerged weeds are dislodged mechanically by dragging with log weeders fitted with spikes and barbed wires. Mechanical winches are used for cutting and dragging of submerged weeds.
Another simple method of control of water hyacinth is to construct floating barriers which prevent water hyacinth from reaching other water bodies. The floating barriers reduce time, labour and cost as the accumulated weed is removed by draglines.
Laser rays are also used to control water hyacinth, usually of 10.6 nm wavelength. The irradiated plants are plasmolysed immediately.
Burning follows in proportion with the amount of laser energy applied. Many of the plants die within ten weeks. Daughter plants are stunted and turned pale due to destruction of chlorophyll.
A large number of chemical weedicides are used for control of aquatic weeds. It is a very effective and cheap method. The weedicide is to be selected in such a way that it should be cheap and easily available, non-toxic to fish and man, should not pollute the water and should not involve the use of special and costly equipment. The lethal action of the weedicide is either by direct contact or by translocation of chemicals from the treated part of the plant to the other areas of its system resulting in both cases in the death of the plant.
Different type of chemicals are in use for eradication of weeds. Many of these are poisonous, toxic or harmful for human and other animals. Their mode of action on the weeds are also different. The same chemicals may not be useful for the eradication of different types of weeds.
Chemicals used for eradication of weeds are broadly classified under three categories.
1. Compounds of heavy metals. e.g. Copper sulphate, Sodium arsenate, etc.
2. Hormone weedicides g.2,4-D, 2,4,5-T, etc.
3. Fertilizers. g. Superphosphate, Urea, Ammonia, etc.
According to the mode of action, a weed killer chemical can also be grouped into two categories.
1. Contact weedicides – which kill plants on contact.
2. Translocated weedicides – which are absorbed by plants and are killed.
The contact weedicides may be selective or non-selective killer types. The selective killer type of chemicals are effective only on some specific weeds whereas the non-selective type chemicals kill all types of weeds. Besides weedicides, some chemicals are used as soil sterilants. It shows that all chemicals are not suitable for killing all types of weeds and all the chemicals may not have all the qualities required for commercial use. Some chemicals are extremely poisonous for animals and human beings. Some chemicals like fertilizers are required to be applied at a very high dose which is neither economical not easy to apply. Endothal, Endothal amine salt, 2,4-D are toxic to fish. Diquot is toxic to fish and not advocated to apply in muddy water.
Of all the weed controlling measures, biological control of weeds through stocking the water with weed-eating fish, such as grass carp, Ctenopharyngodon idella, is found to be an effective and satisfactory method. Grass carp is a voracious weed eater and possesses strong pharyngeal teeth, which enables it to grasp and nibble at soft weeds like Hydrilla. The nature of its gill rakers helps it to sieve large quantity of microvegetation from the water body. Because of its efficiency for weed consumption and convertibility into flesh it is preferred for stocking in weed infested waters.
Grass carp usually eat the soft parts of the aquatic plants leaving behind the harder parts like stem. It shows a certain preference for soft submerged weeds like Hydrilla, Ceratophyllum, Najas, Vallisneria. Its lower preference towards Ipomea is due to the hard nature of the weed. Hydrilla verticellata is the most preferred as it has soft leaves which could be easily nibbled and are easily digested.
Control of weeds, especially the soft submerged type of weeds, through biological control by stocking the water with grass carp has certain advantages. It is not only the most economical due to its low cost of operation and easy application but also does not contaminate the water with toxic substances unlike chemicals used for control. Moreover, it gives economical returns by increased fish production.
Common carp, Cyprinus carpio and Katti, Acrossocheilus hexagonalepsis and ducks are also used for biological control of aquatic weeds. Beatles and stemborers are also recommended for the purpose.
Biological control of weeds may be done by shading. Increasing turbidity, covering the surface by controllable floating weeds, shading the water area by canvas or coloured polythene sheets to cut down sunlight in order to check excessive growth and vegetation are some of the methods also in use.
Whichever method is used for the control of aquatic weeds, employment of manual labour is necessary. In the mechanical method labour is necessary for the clearance of the remains of the vegetative parts of the weeds. Even if the chemical method is resorted to, the dead weeds which sink to the bottom have to be removed. A rational utilization of all methods suitable according to the local condition and also economical is to be resorted to for eradication of weeds. However, checking of excessive weed growth at the proper time is also one of the effective and important factors to keep the weed under control. Control measures should be adopted before the flowering season of the weeds. The time for control of weeds given below has been found to be appropriate under Indian conditions.
– Eichhornia, lotus – Duck weeds
– Utricularia, Ottelia – Jussiacea, Trapa,
Nymphoides, Pistia, Nechamendra
– Najas, Myriophyllum – Scrispus, Nymphaea
Water Quality Management
Successful pond culture operations mainly depend on maintenance of a healthy aquatic environment and production of sufficient fish food organisms in ponds. Water is the primary requisite to support aquatic life. Physical, chemical and biological factors play an important role in governing the production of fish food organisms and fish production in the pond. Water not only plays an important role in the fish production, but also it helps in the survival and growth of the fish. Hence, fish farmers should take a lot of care to maintain hygienic conditions in the pond, so that they get more profits. If the water quality is maintained with utmost care, the farmers need not spend much money for curing the diseases. If the water quality is maintained, the fishes also have a good taste. Water quality is influenced by physical, chemical and biological factors.
The physical condition of water is greatly influenced with depth, temperature, turbidity, light and water colour.
Pond depth has a vital bearing on the water quality. Depth determines the temperature, the circulation pattern of water and the extent of photosynthetic activity. In shallow ponds, sunlight penetration upto the pond bottom and facilitates an increase in the productivity. A depth of 1-2 metres is considered optimal for biological productivity of a pond. If the depth is very less, water gets overheated and thus has an adverse effect on the survival of the fish.
Temperature affects fish migration, reproduction and distribution. It depends on climate, sunlight and depth of the pond. Temperature varies vertically in the water body and also shows diurnal fluctuations. Fish posses well defined limits of temperature tolerance with the optimal being 20-32°C. Indian major carps can thrive well in the temperature range of 18-38°C. Wide fluctuations of water temperatures affect the survival of fish. In very low or very high temperatures, the fishes are strained, spend more energy and growth of the fish is affected. These temperatures also affect the chromatophores of prawns, and the prawns develop a red colour. If the temperature is maintained optimally, the red colour disappears. At low temperatures the food consumption offish and prawns decreases and gasses are produced at high temperatures. Hence, water temperature maintenance is very essential to obtain high yields. Fish and prawns or their seed have to be acclimatized whenever they are transferred from one pond to the other.
Water turbidity is mainly due to suspended inorganic substances like clay, silt, phyto – and zooplankton and sand grains. Ponds with a clay bottom are likely to have high turbidity. Turbidity reduces sunlight penetration and photosynthesis and hence acts as a limiting factor. If the turbidity is due to more suspended particles, they absorb nutrients in their ionic form, making them unavailable for plankton production. High turbidity also reduces the dissolved oxygen in the pond water. Turbidity is measured with the secchi disc. If the secchi disc disappears at 30-50 cm. the water is productive in nature. If it is not visible at a depth less than 25 cm, a dissolved oxygen problem could anse during the night. If it is more than 50 cm, the plankton produced is less in the pond water. In less turbid waters, the aquatic weeds growth is more. In highly turbid waters, the sand grains accumulate in the gills of the fish and prawns, causing suffocation and excessive secretion of mucous. High turbidity can be reduced by adding lime and alum. If the water is more turbid, it should be stored in sedimentation tanks and then used for fish culture. If the turbidity is more due to phytoplankton, water m the pond should be changed. Fertilizers have no effect in high turbid waters, hence fertilization of the pond should be stopped.
Availability of light energy to a fish pond greatly influences its productivity and photosynthesis. In shallow ponds, light penetrates to the bottom and is responsible for luxuriant growth of aquatic weeds. In high turbid waters, the light will not penetrate to the bottom. Due to this, the vegetation at the bottom will decay and produce harmful gasses, which affect the fish and prawn life.
Water gets its colour due to phytoplankton, zooplankton, sand particles, organic particles and metallic ions. Water used for fish or prawn culture should be clear, either colourless or light green or blue in colour. Water colour is golden or yellow brown if diatoms are more. This type of water is best for prawn culture. Brownish green, yellowish green and light green coloured waters are also good for prawn culture. Water becomes greenish in colour when phytoplankton is more, develops a brown colour due to zooplankton and mud colour due to more sand grains. Water with black, blackish green, dark brown, red, yellow colours are not good for culture. These colours are due to the presence of more phytoplankton, bad pond bottom and acids in the water. The red colour of water is due to the presence of high levels of iron and death of phytoplankton (phytoerythrin released).
The chemical factors like pH, dissolved oxygen, alkalinity, hardness, phosphates and nitrates influence the productivity of the pond.
pH is the hydrogen ion concentration, which ranges from 0-14. Water is slightly alkaline in condition, with the optimal range of 6.5-8. Less than 5 and more than 10 pH is lethal to fish and prawns. The pH of pond water undergoes a diurnal change, it is alkaline during the day time and slightly acidic just before day break. The fluctuations of pH are similar to dissolved oxygen. pH fluctuations are more in phytoplankton and weed infested waters and water with less hardness. No sudden pH fluctuations in brackish water and sea water occurs due to their buffering capacity.The difference in pH from morning to evening should not be more than 0.5. When pH increases, ammonia and nitrites become toxic, when it is reverse H,S becomes more toxic. pH below 6.5 and above 8.5 is responsible for reduction of growth and resistance of parasitic infection increases in acidic waters. Whenever pH falls, lime should be added to the pond water. When pH is high, lime should not be used. Urea should not be used to reduce pH. This is because NH3 becomes toxic at high pH. It is always better to add new water to maintain an optimal pH. Alum or aluminum sulphate can be used to reduce the pH and turbidity. Alum removes phenolphthalin alkalinity. 1 ppm alum reduces 1 ppm phenolphtahlin alkalinity. Fish, prawns and their seed should be acclimatized to new water whenever they are transferred from one pond to another.
Dissolved oxygen is one of the most important chemical parameters, which has a great influence on the survival and growth of fishes and prawns. The pond water gets oxygen mainly through interaction of atmospheric air on the surface water of the pond and by photosynthesis. It is produced only during daytime, reaches a maximum at 3 PM, then gradually decreases upto early morning. During the night it decreases and it reaches a minimum during the early hours. It is due to nil production of dissolved oxygen at night and instead, consumption of oxygen by plankton, weeds, fishes and prawns. During overcast days, the production of dissolved oxygen during the day is less and during the subsequent nights it decreases drastically. When water temperature rises, oxygen is released into atmosphere. When salinity increases it is dissolved in water. The optimum dissolved oxygen is 5-8 ppm. If less than 5 ppm the growth rate decreases the fish and prawns are prone to get diseases and less than Ippm of dissolved oxygen results in death. More than 15 ppm results in gas bubble disease in fishes and prawns. Whenever the animals are under stress due to less dissolved oxygen the food consumption temporarily decreases. When oxygen decreases, prawns accumulate on the water surface and near the pond shores and are found stationary at one place or show weak movements. Fishes come to the surface and engulf the air. Prawns get milky white spots when dissolved oxygen is continuously less. It decreases gradually from the surface to the pond bottom and CO,, NH3 and other gases increases, hence prawns are under more stress. Farmers should take precautionary measures at nights, especially during the early hours to increase oxygen levels. If it is very less, the water surface should be disturbed by beating water with bamboo poles or by rumming boats or by using aerators.
Alkalinity is caused by carbonates and bicarbonates or hydroxides of Ca, Mg, Na, K, NH4 and Fe. Alkalinity is less in acidic soils and in ponds with more organic load. Alkalinity is more in clay soil ponds and is increased if water is pot exchanged. The optimal level of total alkalinity is 40-150 ppm. Alkalinity has direct effect on the production of plankton. ‘
Hardness is caused by Ca and Mg. Water with less than 40 ppm is soft and more than 40 ppm is hard water/ The pond water with a hardness of 15 ppm or more is satisfactory for growth of fishes and prawns and do not require additional lime. If water has less than 11 ppm hardness it requires liming for higher production. If it is less than 5 ppm, the growth rate is affected and causes eventual death of the fish.
Na, C12, Ca, Mg, K, bicarbonates and sulphates are responsible for salinity of the water. Salinity is an important parameter for survival, growth and high production in brackishwater culture systems. Salinity ranges between 0-40 ppt in brackishxvater and 35 ppt in sea water. The optimal salinity for prawn culture is 15-20 ppt. The prawns can survive at 2 ppt and 40 ppt. but their growth rate decreases. If the salinity is high, the water should be exchanged. Due to heavy rains more freshwater enters into the ponds and sudden decrease is found in salinity levels which affect the life in the pond. To avoid this, two outlets (one at high level and other at low level) should be provided to send out freshwater and sea water separately from the pond. The animals should be acclimatised before introducing them into new water.
CO, is produced during respiration and consumed during photosynthesis. CO, is less during daytime and more at nights. The optimal level of CO, is 5 ppm. At high CO, levels, pH decreases, CO, is accumulated in the blood of the animals and water becomes acidic. The animals become sluggish, loss of resistance occurs, they cannot utilize dissolved oxygen and they ultimately die. Whenever CO, increases lime should be added to the pond. 1 ppm of lime reduces 0.9 ppm of CO,.
Dissolved ammonia and its compounds
NH3 is found in excreta and is also released due to decomposition of organic matter. It is an important compound influencing the growth of phytoplankton in the aquatic ecosystem. The optimal limit of NH3 is 0.3-1.3 ppm and less than 0.1 ppm is unproductive. Whenever NH3 increases pH also increases, but dissolved oxygen decreases. CO2 reduces the toxic effect of NH3. NH3 also increases with feed due to high protein levels and death of phytoplankton. When NH3 is more in water, animals may not get excreta with NH3. NH, accumulates in the blood and oxygen transport in the blood reduces. – Gills become black, biochemical tissue is damaged and gasous exchange is affected. NH3 levels can be reduced with good management like no excess feed, optimal stocking and water exchange. Lime should not be added when NH, is high. Optimal level of nitrites is 3.5 ppm.
H2S is produced in anaerobic conditions by the action of-micro-organisms on sulphur compounds. H,S is toxic to fish and prawn. It should be less than 0.05 ppm in pond water. H2S is responsible for respiratory problems. When H,S increases, lime should be added.
The biological factors like plankton, weeds and disease causing agents also play a role in water quality maintenance.
Plankton are free living smaller plants and animals, which move along with the waves. Plankton are natural fish food organisms, which consists of 60% easily digestible proteins. Phytoplankton produce food and O, by photosynthesis. Plankton density variations depend upon the fertilizers used and fish species cultured. Carbon, oxygen, H,, P, N,, S, Fe, K, Na, Mn, Mo, Zn, B and Cl, are essential for plankton production. Out of these, N, P, K, are most important elements for plankton production.
To increase plankton production, organic and inorganic fertilizers should be used. Lime is also essential for plankton production. Fertilizers and lime should be used at regular intervals. This helps in production of plankton in sufficient quantities. Excess production of plankton, especially myxophyceae members settle on the water surface and form algal blooms. This hampers photosynthesis and oxygen depletion is observed, esp£Cially during nights. CO, levels increase in the pond and affect water quality.
Disease causing agents — water quality
The most important aspect of water quality management in the culture system is to maintain fish without disease causing agents and under hygienic conditions. The diseases in fishes and prawns are caused by bacteria, virus, fungi, protozoa, helminth, and crustacean parasites. These parasites enter into the pond along with water, fish or prawn seed and nets from other infected ponds. Due to the unhygienic conditions these parasites cause diseases in fish and prawns, and the fish and prawns become less resistant to diseases. Due to the parasitic infection the growth rate reduces and finally they die. To avoid these bad effects, use good and healthy material and fish and prawns should be examined once in 15 days. Abnormal behaviour offish and prawns is observed in infected ponds. These should be observed and immediate action should be taken, otherwise, whole crop could be wasted / destroyed.
Aquatic weeds – water quality
Excess growth of aquatic weeds in fish pond is not a good sign in aquaculture systems. Weeds utilize the nutrients and compete with desirable organisms. Weeds also compete for oxygen, especially during nights and space with fishes. They obstruct the netting operations too. Hence, the weeds should be removed from ponds by mechanical, chemical or biological methods. Application of lime, fertilizers and feed are some of the important measures to maintain the water quality. These should be applied whenever required. Excess application leads to the poor condition of water quality.
Role of aerators in the water quality management
Atmospheric oxygen dissolves in the water at water surface. In this layer, dissolved oxygen increases quickly, but not at the pond bottom./To get oxygen even in the bottom layer, the pond water should be disturbed. To gedhis aerators are very essential. Aerators produce the air bubbles, which disturb the water in the pond, so that more oxygen dissolves in the water.Aerators, therefore play a vital role in aquaculture to increase fish and prawn production.
Different types of aerators are in operation to increase aeration in the ponds. Diffused, air lift pumps, U-tube and splashers are some of the common aerators (Fig 5.7) in operation in aquaculture.
In diffused type, the blower or compressor is arranged on the dyke, and this is connected to a porous tube, which is arranged on the pond bottom. Compressor produces air, which comes out of the porous tube in the form of air bubbles and disturbs the water to produce more dissolved oxygen. The capacity of the aerator depends upon the compressor energy and pond depth.
In air lift pump aerator, air is sent into a tube, which opens on surface of the water. Air bubbles travel through the tube and enhances the dissolved oxygen. This aerated water falls on water surface and increases dissolved oxygen water further.
In U-tube aerator, the U-tube has 12-18 metres depth. At one end. air is pumped with the help of blower and the air bubbles travel to the other end i.e., air bubbles have more contact time with water. These aerators are more efficient, but need more expenditure for construction. Splasher type of aerators are also known as surface aerators. Propeller of the aerator is arranged near the water surface and water is sprinkled which helps in enhancing the oxygen in the pond. Paddle wheel surface aerators are also used in fish ponds. Sprinklers are used in fish ponds where porous pipes are arranged on the water surface and pump the air is pumped with engines into the pipes. This gives good aeration in the pond and produces successful results (such as those obtained in Kolleru area).
a) Diffused type b) Air lift type c) U-tube type d) Splasher type
Role of filters in the water quality management
Aquatic culture systems contain living organisms in water.These organisms require inputs, such as food and they excrete other materials. The inputs must be mixed with or dissolved in water to be available to the organisms, whose outputs will also become mixed with or dissolved in water. Excessive output and/or input can become toxic if the concentration is allowed to increase in the culture water. The process of removing excess materials is called filtration. It consists of passing the water through a thick layer of sand and gravel which act as strainers. Suspended and colloidal matter in the water and also a large number of bacteria are caught in the interstices of the sand during its passage. The mechanical, biological and airlift filters are generally adopted in aquaculture practices to manage and control the water quality for intensive rearing and culture.
A mechanical filter (Fig 5.8 a) is an under drained water tight basin in which the filtering materials are placed. The size of a mechanical slow sand filter unit may be about 30 to 60 m x 15 to 30 m or more and about 2.5 m to 3.5 m deep according to desired flow. Water after passing through the filter is collected in an outlet chamber, which is equipped with a flow regulating arrangement. The filtering material about 90 cm to 150 cm of which about 60 cm to 90 cm is fine sand, is laid on top of the under drainage system in five or six layers in progressively smaller sizes towards the top.
a) Mechanical filter b) Airlift filter.
The sand is supported on two or three layers of graded gravel, with the finest layer immediately below the sand and the coarsest material at the bottom of the filter, packed around the drains. The gravel layers must be graded sufficient to prevent the material from mixing and the sand being drawn down.
The following thickness may be taken for the filtering materials from the bottom towards the top.
1. 10 cm to 15 cm of broken stone 40 mm to 65 mm size
2. 8 cm to 15 cm of gravel 20 mm to 40 mm size
3. 5 cm to 10 cm of gravel 3 mm to 6 mm size
4. 15 cm of coarse sand and
5. 60 cm to 90 cm of fairly uniform fine sand.
When the resistance in the filter (due to sand and clogging) i.e., loss of head, is equal to the total depth of water on the filter, the operation will stop. The loss of head should not be greater than the depth of the filtering sand. When it becomes excessive and before a negative head is formed the filter should be cleaned. The level of the filtered water at the outlet chamber should not be below the level of the surface of the filter sand.
The rate of filtration is 120 litre per minute when the graded layers are 1′ sand of 0.05 to 0.1 mm, 6″ sand of 0.1 to 0.5 mm, 6″ gravel 2 to 5 mm and 1′ metal 5 to 10 mm at the total filtering surface area of 144 square feet.
It comprises the mineralisation of organic nitrogenous compounds, nitrification and dentrification by bacteria suspended in the water and attached to the gravel in the filter bed.
Heterotrophic and autotrophic bacteria are the major groups present in culture systems. Heterotrophic species utilize organic nitrogenous compounds excreted by the animals as energy sources and convert them into simple compounds, such as ammonia. The mineralisation of these organics is the first stage in biological filtration. It is accomplished in two steps; ammonification, which is the chemical breakdown of proteins and nucleic acids producing amino acids, and organic nitrogenous base and deamination in which a portion of organics and some of the products of ammonification are converted to inorganic compounds.
Once organics have been mineralised by heterotrophs, biological filtration shifts to the second stage which is nitrification, it is the biological oxidation of ammonia to nitrite and then to nitrate by autotrophic bacteria. Those organisms unlike heterotrophs require an inorganic substrate as energy source and utilise carbondioxide as their only source of carbon. Nitrosomonas and Nitrobacter sp. are the principal nitrifying bacteria in culture systems. Nitrosomonas oxidises ammonia to nitrite, Nitrobacter oxidises nitrite to nitrate.
The third and last stage in biological filtration is dentrification. This process is a biological reduction of nitrate to nitrite to either nitrous oxide or free nitrogen. Dentrification can apparently be carried out by both heterotrophic and autotrophic bacteria.
Air lift filter
It is the most trouble free means of filtering water through synthetic sponge layer by pumping the water with air lift (fig 5.8b). In culture applications, lift pipe extends below water level and the filter chamber rests above the top water surface. The suspended or colloidal impurities upto the size of 0.002 mm can be filtered out through this system. By pumping 5 cm3 air /sec/. 2 litres of water per minute can be filtered when the diameter of the lift pipe is 1 cm.
Fish culture is practised in ponds. These are small shallow bodies of water in natural conditions and completely drainable, usually constructed artificially.The natural ponds differ from the lakes in having a relatively large littoral zone and a small profundal zone. Their source of water may also vary.
Nursery ponds are also called transplantation ponds. These are seasonal ponds and are constructed near the spawning and rearing ponds. The main object is to create a suitable condition of food availability and growth of fry because at this stage they are most susceptible to hazards like the wave action and predators. These should be small and shallow ponds 0.02-0.06 ha. in size and 1-1.5 m. in depth. In the nurseries, the spawn (5-6 mm) are reared to fry stage (25-30 mm) for about 15 days. These ponds are usually rectangular in size. Extra care should taken for rearing the young stages, otherwise heavy mortality may occur. Sometimes the spawn are cultured for 30 days also. The pond bottom should gently slope towards the outlet to facilitate easy netting operations. Small and seasonal nurseries are preferred as they help in effective control of the environmental conditions. In practice about 10 million spawn per hectare are stocked in nursery ponds.
Rearing ponds should be slightly larger but not proportionally deep. These should be located near the nursery pond and their number may vary depending upon culture. They should preferably be 0.08-0.10 ha in size and 1.5-2.0 m in depth. The fry (25-30 mm) are reared here upto the fingerling (100-150 mm) stage for about 3-4 months. Carp fry grown in nursery ponds are relatively small in size and not fit enough for their direct transfer into stocking ponds. In stocking ponds bigger fishes are likely to be present which may prey upon the fry. Hence, it is desirable to grow the fry in rearing ponds under proper management practices upto fingerling size so that their ability to resist predation will be improved.
Stocking ponds are the largest ponds and are more deep, with a depth of about 2-2.5 m. The size of the pond may vary from 0.2-2.0 ha., but these should preferably be 0.4-0.5 ha in size. These are rectangular in shape. The fingerlings and advance fingerlings are reared upto marketable size for about 6 months. One year old fishes may grow upto 1 kg. or more in weight.
The pond management consists of pre-stocking, stocking and post stocking management phases.
Pre-stocking pond management involves site selection, eradication of weeds, insects and predators, liming, manuring, etc.
Post-stocking pond management involves water quality management, feed and health management and harvesting.
Based on the intensity of infestation and type of weeds, the aquatic weeds can be controlled by means of manual, chemical and biological methods.