9. POND MANAGEMENT

Carp culture in ponds is basically a three-tier culture system where the first step begins with the rearing of spawn up to fry (2–3 cm) stage for 2–3 weeks in nursery ponds followed by rearing of 2–3 weeks old fry for about 3 months up to fingerling stage (8–12 cm) in rearing ponds before they are finally released in stocking ponds for growing up to table size fish. To ensure high rate of survival and growth during all the three stages of rearing, a package of management practices should be strictly followed, and slackness at any stage of the management procedure may affect farm productivity and profitability adversely. Techniques of management involve (i) manipulation of pond ecology to ensure optimum production of natural fish food while maintaining the water quality parameters within tolerance limits of the stocked fish species; and (ii) the husbandry of fish through stock manipulation, supplementary feeding and health care. Broadly, the various steps involved in the management of ponds at all the three stages of culture may be classified as (i) pre-stocking, (ii) stocking and (iii) post-stocking management operations.

9.1 Pre-stocking management

Pre-stocking management aims at proper preparation of ponds to remove the causes of poor survival, unsatisfactory growth, etc., and also to ensure ready availability of natural food in sufficient quantity and quality for the spawn/ fry/fingerlings to be stocked. Pre-stocking part of the management involves the following sequential measures.

9.1.1 Eradication and control of aquatic weeds and algae

Aquatic weeds are unwanted plants that grow within the water body and along the margins. Unlike in temperate climate, the pond fish culture in tropics face serious problems due to weed infestation and frequent appearance of algal blooms. They remove a large quantity of nutrients from the water, which otherwise would go into the production of planktonic growth. Even the poor fish crop that is produced in weed chocked water is difficult to harvest. The fishes are subjected to stress due to dissolved oxygen depletion and wide fluctuation between the dissolved oxygen values of the day and night. Decomposition of the dead aquatic weeds further creates the oxygen problem. Dense growth of the submerged weeds restrict fish movement and interfere with fishing operations. Filamentous algae often get entangled in the gills of the fish and suffocate them to death. Floating weeds such as water hyacinth, pistia, etc., very often cover the entire water surface cutting off light drastically, thus resulting in critical reduction in primary productivity of the pond. Common aquatic weeds creating problems in fish culture ponds (Fig. 34) are broadly classified according to their nature of occurrence, into four major groups. They are floating, emergent, submerged and marginal. In addition, algal blooms and mats also create serious problems in terms of dissolved oxygen and production of certain toxic materials in some cases. Aquatic weeds of common occurrence in undrainable ponds are grouped in the following Table (Table 20).

Figure 34. Common Aquatic Weeds in Underainable Ponds

Control measures for all the above mentioned classes of weeds and blooms fall into four major categories, viz. preventive, manual and mechanical, chemical and biological. Any of these methods or at times a combination of methods may be taken up depending on the nature of infestation, pond condition, cost involvement and availability of required inputs.

9.1.1.1 Preventive control

Taking into consideration the high cost of controlling aquatic weeds, certain preventive measures are to be followed to reduce the chances of their infestation.

The preventive measures have to be taken well in advance. The measures include trimming of pond margins, dewatering and desilting of old ponds, uprooting or burning of dried marginal weeds during the summer and providing barriers to prevent the entry of floating weeds.

9.1.1.2 Manual and mechanical control

Manual removal of aquatic weeds is an age-old practice and holds good even today in rural areas. The free floating groups of weeds are either hand picked or dragged by wire or strong coir rope nets. In bigger ponds they should be removed part by part from the marginal areas and finally the centrally located weed mass is dragged towards the banks and lifted out. Certain small and light floating weeds such as spirodela, lemna, azolla, wolffia, etc., are easily skimmed out by twisted straw ropes or fine meshed nets. The manual removal of submerged weeds from a heavily infested water body is relatively much more difficult. They are either pulled by hand or hand-drawn bottom rakes or uprooted with bamboo poles having a cross piece tied strongly at the terminal end. Repeated cutting of the aerial shoots and leaves of rooted emergent plants are also useful. Implements used for manual control are mostly hand scythes for cutting, and hand forks, strong nets and bamboo poles with terminal cross piece for twisting and uprooting (Fig. 35).

Mechanical devices used for clearance of rooted submerged weeds are steel cables, cutting chains and diesel operated winches (Mitra, 1956).

9.1.1.3 Chemical control

The manual removal of weeds from heavily infested large water bodies is difficult and time consuming. Under such conditions certain commercially available chemicals (herbicides) can provide an efficient means of eradication of undesirable aquatic plants. Total kill and disintegration of weeds can be achieved by this method ensuring full return of the nutrients back to pond soil and water for production of natural fish food. As a matter of fact there is not a single chemical known so far which can eradicate all types of weed infestation. Therefore, one must know the weeds and its species, appropriate herbicide and its rate and time of treatment. In larger ponds where dense infestation covers a substantial portion of the water, the herbicide should be applied part by part if the pond is already stocked with fish. As discussed earlier most herbicides are selective in nature and when applied to a mixed population of weeds, growth of some tolerant weeds may be encouraged at the cost of susceptible ones; likewise, when surface or floating weeds are destroyed, the submerged weeds develop. Under such conditions subsequent application of appropriate herbicide should be taken up.

Floating weeds: Water hyacinth is one of the most important weeds of this group. Depending on its degree of infestation, they are categorized in three groups, viz. small, medium and big, based on their wet weight per unit area. The recommended doses of the herbicide 2–4-D are 2,7 and 12 kg/ha for small (13 kg/m²), medium (23 kg/m²) and big (35 kg/m²) (Ramchandran, 1969; Patnaik and Das, 1983). Addition of a detergent (0.2 % concentration) to the aqueous solution gives better results. The dilution for better coverage has been estimated at 400 l/ha. The foliar spray (spraying over the leaves) is undertaken with the help of a foot pump/hand pump sprayer with a three-action nozzle. Field application of herbicide, especially towards the interior of thick water hyacinth infestation, is a difficult task. In such cases a pair of stout bamboo poles should be laid on the top of the infestations so that the operators can walk over them. Normally, the complete kill of plants takes around 25 days. This chemical is available in two suitable forms as sodium and amine salt.

Figure 35. Hand tools Used for Manual Control of Aquatic Weeds

Water lettuce which often causes a serious problem in fish ponds can be controlled with 0.1–0.2 kg of paraquat/ha. This infestation could also be controlled by foliar spray of aquous ammonia (1%) at the rate of 50–75 kg/ha along with 0.2 % of any commercially available detergent as a wetting agent.

The aquous ammonia is broadcast as foliar spray over the infestation with a foot pump sprayer and a small funnel–shaped sprinkler 3–4 cm in diameter, provided with 10 pin-sized holes pierced on the diaphragm covering the mouth of the funnel. The stem of the sprinkler is connected to the sprayer through a 30 m long polyethene tube, so that the sprayer is kept on the shore and only the sprinkler is taken inside the infested area in a boat.

The area to be treated inthe field is divided into small plots (20–30 m²size) and solution is sprayed at the rate of 5 000 1/ha.

Salvinia forms a thick surface mat in ponds and can be conveniently controlled by the application of foliar spray of paraquat at the rate of 1 kg/ha. Usually it takes 30–40 days for the weeds to be killed and settled in the pond.

Smaller floating weeds, e.g. Spirodela, Lemna and Azolla can also be cleared with 0.1 kg/ha of paraquat.

Emergent weeds: Water lily, lotus, and floating heart can be cleared by spraying the herbicide 2–4-D at the rate of 8–10 kg/ha with detergent (0.25%). The chemical is diluted at the rate of 300 l/ha and sprayed through a footpump sprayer.

Submerged weeds: Ottelia, Vallisneria, Hydrilla, Najas, Potamogeton and Ceratophyllum can be controlled by paraquat at the rate of 3–4 ppm within two weeks. It can also be controlled by application of anhydrous ammonia at the rate of 15–20 ppm.

Marginal weeds: Ipomea, Jussiaea, etc., could be controlled by spraying the herbicide 2–4-D at the rate of 8 kg/ha.

Algal blooms and mats: Due to overdose of fertilizers or enrichment of the water through treated sewage or agricultural fertilizer, the minute algal cells multiply fast turning the pond water bright green or sometimes brickred. Some of the more harmful blooming algae are microcystis, anabaena and euglena. A number of chemicals have been employed to control these algal blooms. Copper sulphate is perhaps the oldest and a very widely used algicide. The recommended doses are 0.2 to 1.0 ppm, but it is not very effective in ponds having high pH (pH above 8.6), Microcystis bloom is cleared with 0.3 to 0.5 ppm of Diuron. Simazine also clears the bloom in 16–20 days and the rate of application is 0.3–0.5 ppm. Both the chemicals do not have harmful effect on fish. It has been observed that the sudden kill of blooms is likely to cause oxygen depletion which might cause mortality of fish. In order to avoid this a prophylactic dose of diuron (0.1 ppm) should be applied in the very early stage of bloom development. Usually the chemical is sprayed over the affected portions of the ponds. The common mat forming algae which occur in fish ponds are Spirogyra, Pithophora, Oedogonium and Cladophora. Although repeated netting can reduce the infestation to a considerable extent in nursery and rearing ponds, application of Diuron at the rate of 0.3–0.5 ppm is recommended. Various chemicals and the dose of application is summerised in the ready reckoner given below (Table 21).

9.1.1.4 Biological control of aquatic weeds

Another important controlling method is by introduction of weed-eating fishes. Common carp, gourami, tilapia, pearl spot, the grass carp and a species of puntius are the fishes of known weed-eating habits (Table 22).

Grass carp is the most effective biological control agent against most of the submerged and floating weeds except the water ferns. Grass carp normally consumes choiced aquatic weeds, at least 50% of their body weight in a day. About 300–400 fish, each of about 0.5 kg weight, are enough to clear 1 ha of Hydrilla infested water body in about a month. Normally Hydrilla infestation density ranges from 5–25 kg/m² (Alikunhi and Sukumaran, 1964).

9.1.2 Eradication of unwanted fish

Predatory fish prey upon the spawn, fry and fingerlings of carps and the weed fish compete with carp for food, space and oxygen. Therefore predatory and weed fish should be completely eradicated from nursery, rearing and stocking ponds before these ponds are stocked. The commonly encountered predatory and weed fish in undrainable ponds are listed below (Table 23).

Absolute removal of these unwanted fish by thorough and repeated netting is not possible and hence dewatering and poisoning the pond are the only alternative methods. If situation permits, dewatering should be the preference as it ensures complete eradication of unwanted fishes and disinfects the pond bottom. Dewatering also offers the opportunity to desilt the pond bottom. However, where it is not possible, which is true in most situations, the pond should be treated with fish poison. From an economic point of view the poisoning should be done during pre-monsoon season when the water level is usually low, requiring the minimum quantity of poison material. The date of poisoning, however, should be fixed about three weeks before the anticipated date of stocking. Seasonal ponds which dry up during summer months need not be treated with fish toxicants.

9.1.2.1 Fish toxicants

Although a number of chemicals and plant derivatives are available in the market which are poisonous for fish, only a limited number of such toxicants are safe and suitable for fish culture purposes. Based upon the following criteria a suitable fish poison is selected.

• Poisoned fish should be safe for human consumption

• Least adverse effect on the pond biota

• Toxicity period should be of short duration

• Should not have residual effect

• Easy commercial availability

• Simplicity of application

• Cost considerations.

Mohua oil cake, bleachng powder and ammonia are considered suitable.

9.1.2.2 Application of toxicants in ponds

Mohua oilcake: Of all the fish poisons of plant origin, the most extensively used fish toxicant in undrainable ponds is oil cake of Mohua (Basia latifolia). It kills all the fish species within a few hours when applied at the rate of 250 ppm (CIFRI, 1968). It contains about 4–6% of active ingredient, the saponia, which on dissolving in water haemolyses the red blood cells and thus kills the fish (Bhatia, 1970). The required quantity of mohua oilcake should be soaked in water and uniformly broadcast over the entire pond surface. Following this operation, repeated netting should be done to ensure proper mixing of the poison and removing the affected fishes which are suitable for human consumption. The toxicity of doses up to 250 ppm lasts for about 96 hours (Jhingran and Pullin, 1985) and subsequently it serves as organic manure in the pond. It should be applied at least two weeks before stocking the ponds.

Bleaching powder: Bleaching powder or Calcium hypochlorite (CaOCl²) is another practical and safe fish toxicant. It kills all the predatory and weed fish of the pond when applied at the rate of 25–30 ppm (Tripathy et al., 1980). However, during storage, significant chlorine content is lost and hence it is always safer to use the commercially available bleaching powder at the rate of 35–50 ppm or 350–500 kg/ha/m of water. Fish kill occurs within 1–3 hours and the toxicity lasts for 3–5 days. Plankton and benthic fauna start developing from the 7th or 8th day after treatment. Chlorine content of the bleaching powder thoroughly disinfects the pond which is essential in undrainable ponds where disinfection by sun drying is not at all possible. Disinfection of the pond is one of the essential measures for maintaining proper health condition of the fish. Besides, it also satisfies the lime requirement of the pond soil.

The method of application is also relatively simple. The powder is mixed with water and uniformly spread over the entire water surface. Distressed and dead fish are removed by netting. Chlorine killed fish are safe for human consumption.

Ammonia: Anhydrous ammonia when applied at the rate of 20–25 ppm kills the predatory and weed fishes. Besides, it also controls the aquatic weeds and later acts as nitrogenous fertilizer. Toxicity of ammonia lasts for 4–6 weeks.

Details of doses for commonly used fish toxicants are summerised in the following table (Table 24).

* Requires additional dose of lime at the rate of 150 kg/ha

The nursery ponds require subsequent poisoning for selective killing of the larger planktonic copepods. These copepods are predatory in nature and instead of serving as food for the delicate spawn and early fry, they attack and prey upon them resulting in poor survival. For this reason 4–5 days prior to stocking of spawn, the pond should be treated with malathion at the rate of 0.25 ppm (active ingredient) for selective killing of the planktonic copepods. This treatment significantly increases the survival in nursery ponds (Kumar et al., 1986). Such treatment is not required in rearing and stocking ponds.

9.1.2.3 Calculation of dose

The required quantity of poison can be calculated using the following formulae.

For rectangular ponds:

= Required amount of poison in kg.

For circular ponds:

= Required amount of poison in kg.

9.1.3 Eradication of predatory insects

Many aquatic insects in their larval and/or adult stages, prey upon fish hatchlings and fry and also compete with them for food. The common insect predators are beetles, bugs and dragonfly nymphs (Fig. 36). Among beetles, diving beetle (Cybister), water scavenger beetle (Sternolophus) and whirling beetle (Gyrinus) are more dangerous forms. Back swimmers (Anisops) appear in swarms in manured ponds during rainy season and cause heavy damage. Other predatory members of this group are water scorpion (Laccotrephes), giant water bug (Belostoma) and water stick insect (Ranatra). Dragonfly nymphs are highly predatory on carp spawn.

Proper prepration of nursery ponds for stocking with spawn thus also aims at total eradication of such predatory insects. The basic method is to apply a thin oily film over the pond surface which chokes the respiratory tubes of aquatic insects. The spawn and fish food organisms remain unaffected. Some of the common treatment methods are presented in the following table (Table 25).

ADVERSE ENVIRONMENT FAVOURS PATHOGEN PROLIFERATION AND CUASE STRESS TO FISH	BETTER ENVIRONMENT FAVOURS FISH PREVENT QUICK PROLIFERATION

HOST PATHOGEN ENVIRONMENT-INTERACTIONS RESULTING DISEASE-OUTBREAK		HOST PATHOGEN ENVIRONMENT INTERACION RESULTING IN ’NO DISEASE‘ CONDITION
	H-SUSCEPTIBLE HOST
	AE-ADVERSE HOST
	P - VIRULENT PATHOGEN

Figure 36. Common Insect Predators in Nursery Pond

Except for soap-oil emulsion other mixtures or emulsion are easily prepared by simple mixing. For making soap-oil emulsion, the soap is mixed with oil and gently heated for some time with vigorous stirring. These emulsions are applied by spraying over the pond surface about 12–24 hours prior to stocking of spawn. It is the film of the emulsion which is important and hence care is taken not to disturb the film for a few hours. Windy days should be avoided as it will break the film.

Malathion application in nursery ponds also controls the predatory insects population and hence subsequent treatment for control of insect is not required. However, if swarms of these predatory insects are seen in the nursery pond, treatment should be applied immediately.

9.1.4 Fertilization of ponds

Fertilization schedule involving both organic and inorganic fertilizers starts 10–15 days prior to stocking and is prepared on the basis of nutrient status and chemical environment of the pond soil and water.

9.1.4.1 Basis of fertilization

In undrainable ponds where the frequent change of water is a remote possibility, the physico-chemical properties of pond water governing the biological production cycle are more or less a reflection of the bottom soil. The organic and mineral constituents of the soil play their part in releasing the required nutrients into water for pond productivity through chemical/biological processes. Pond bottom soil also provides suitable substrates and necessary environment for the microbial decomposers - the living fertilizer factory of the pond. Thus it is the soil condition and its nutrient status that forms the basis of pond fertilization by using either organic manure or inorganic fertilizer or a combination of both. Important characteristics of pond soil which influence fertilizer use is briefly described here.

Texture of the soil: The texture of pond soil, i.e. mechanical composition of the soil comprising sand, silt and clay and organic matter content, basically influences the economy of both inherent and added nutrients. Sandy and very clayey soil are not desirable as in the former the nutrients are lost due to heavy leaching; while in the latter, due to high adsorption capacity, the nutrients from the water are trapped. Clay minerals and organic matter of the bottom mud are both colloidal in nature and thus exhibit colloidal properties like adsorption and cation exchange phenomenon. Sandy soils, on the other hand are low in colloidal substances and also deficient in organic humus. These are important considerations for deciding the application of fertilizers and manures.

Soil pH: As in water, pH of soil is also one of the critical factors affecting pond productivity. Under anaerobic condition the decomposition of organic matter is slow and the products of decompositions are mainly reduced compounds and short chain fatty acids thus making the soil strongly acidic. Soil pH also influences transformation of phosphorus into available forms and controls the adsorption and release of essential nutrients at the soil-water interface. Both for soil and water a slightly alkaline pH is considered favourable for fish ponds.

Availability of essential mineral nutrients such as phosphate, nitrogen, potassium, carbon and calcium is a consideration which determines the quality and quantity of fertilizers to be applied. Nitrogen is required in large quantities as it is the basic and primary constituent of protein and chlorophyll. Although, phosphorus is required in a small quantity compared to nitrogen, it is considered as the single critical element for maintaining aquatic productivity. Banerjee (1967) classified the undrainable ponds into low, medium and highly productive groups, on the basis of their nutrient status considering mainly nitrogen, phosphate and organic carbon (Table 26).

9.1.4.2 Fertilization schedule

Proper analysis of soil and water is essential before deciding on the fertilization schedule. Detailed recommendations have been made in the chapter on pond environmental monitoring (Section 9.3.3).

Liming: Diurnal changes in pH values ranging from pH 5 during the night and pH 11 during the day due to community respiration and photo-synthesis is a common experience but such wide variations impose stressful conditions for the fish. An adequate level of calcium in the pond provides a buffering system as shown in Figure 37.

Liming helps to raise the total alkalinity level and consequently the reserve CO₂ will increase the availability of carbon for photosynthesis by raising the bicarbonate concentration in water. This raised level of reserve CO₂ will also prevent biological decalcification.

Figure 37. Mechanism of Buffering Action of Line

Depending on the pH of the soil, the dose of the liming should be Adjusted as per the following table (Table 27). Alkalinity can also be used as an indicator of the need for lime in fish ponds.

The total dose of lime calculated as per the table, need not be applied at one time. It may be divided into 3–4 doses and the first dose may be applied about a week prior to the manuring of the pond. It helps in faster mineralisation of organic matter in the pond sediment and acts as a prophylactic agent as well. The same dose is applicable for nursery, rearing and stocking ponds. However, as and when needed during the culture period, additional doses of lime can also be applied.

Manuring: Organic manuring besides being important as means of adding the nutrients, is also equally important for improving the soil texture. A combination of organic manures and inorganic fertilizers is considered more effective than using either of these alone. However, in nursery ponds, use of mineral fertilizers is not recommended as the application may cause blooms of algae which may persist and may harm the young fry. Cow dung at an initial dose of 10 000 kg/ha may be applied in the nursery ponds about two weeks prior to anticipated stocking. If the pond is poisoned by mahua oil cake, then the dose should be restricted to 5 000 kg/ha. If two or more crops of fry are to be produced during the season from the same nursery ponds, then the pond should be fertilized with 2 000 kg/ha of cattle dung about a week before each subsequent stocking. In case of poultry manure the dose should be only 33% of the cattle dung. Rearing ponds are initially manured with the raw cattle dung about two weeks prior to stocking. The rate of application is between 5 000 – 7 000 kg/ha in 5 instalments. If the pond is treated with mohua oil cake then the dose of organic manuring is reduced to half. Dose of inorganic fertilizers may be regulated as per pond soil productivity determined by detailed analyses. In the absence of soil testing facilities a general recommendation should be followed. In such cases inorganic fertilizers are applied at the rate of urea 140 kg/ha and triple superphosphate 60 kg/ha in 4–5 instalments.

In stocking ponds a combination of organic and inorganic fertilizers is considered more effective. Initial manuring with organic manure at the rate of 20% of the total requirement is done 15 days prior to stocking and the remaining 80% of the organic manure is applied in 11 equál monthly instalments during the rearing period. However, if mohua oil cake is applied earlier, the initial manuring is not essential.

The total quantity of inorganic fertilizers to be applied is decided according to soil type (Table 28) and applied in equal monthly instalments. The monthly instalments of organic and inorganic fertilizers are applied alternately allowing a gap of a fortnight between the two applications. Nitrogenous fertilizers are selected on the basis of soil pH.

In the absence of proper soil testing facilities fertilization schedule in stocking ponds may be followed as per the following table (Table 29).

9.2 Stocking

Complete detoxification of the piscicide applied earlier should be ensured before stocking the nursery, rearing and stocking ponds. One or two days prior to stocking, a hapa should be fixed in the pond and some stocking materials should be put inside the hapa. Absence of distress and mortality after 24 hours confirm complete detoxification and the pond should be regarded as ready for stocking.

9.2.1 Stocking of nursery ponds

Carp spawn requires natural feed immediately after stocking and hence it is essential to have a minimum plankton value of 30–40 ml/m³ in case of stocking at a moderate rate (1.5–2.5 million/ha). When a higher stocking rate is to be adopted, plankton population is also required to be increased accordingly. In case the stocking density is over 5 million/ha, the plankton volume should be around 100 ml/m³.

Self-produced or procured 3–4 days old spawn should be stocked in the morning at the rate of 4–6 million/ha. The stocking density must be according to the condition of the pond and the amount of fish food organisms available. The rate of stocking in a well prepared nursery pond with adequate fish food organisms can be as high as 10 million/ha. However, the survival level decreases with the increase in stocking density (Sen, 1976), (Table 30).

Combined rearing of two or more species of spawn should not be done in nursery ponds. The pond should be stocked after three days of hatching when their sizes range from 0.6–0.75 cm and counts on an average about 500 numbers/ml. The required number of spawn are measured with the help of metallic or plastic sieve cups of known volume. Spawn are reared in nursery ponds up to fry stage for about 2–3 weeks when they usually attain 2–3.5 cm in length and 0.15–0.75 g in weight. At higher stocking density the growth is relatively slow. It is possible to raise 3–4 crops of fry from the same pond during the same breeding season and in addition, the pond can also be utilized for rearing of common carp seed during January to March.

9.2.2 Stocking of rearing ponds

Rearing of fry to fingerling stage is done in rearing ponds where fry are stocked at the rate of 0.25–0.30 million/ha with a survival level of 60–80% under proper pond conditions. Either monoculture or polyculture methods can be adopted for this rearing.

In the case of polyculture the species combination and their ratio should be decided on the basis of their habit, feeding, availability of feed, etc. Some of the possible combinations are - catla, rohu, mrigal, common carp (3:4:1:3); silver carp, grass carp (1:1); silver carp, grass carp, common carp (4:3:3); catla, rohu, mrigal, grass carp (4:3:1.5); silver carp, grass carp, common carp, rohu (3:1.5:2.5:3), etc. Combination of too many species should be avoided as it invites excessive handling at the time of harvesting for species segregation. Fry are reared in ponds for about 3 months when they usually attain 100–150 mm in length and 15–40 g in weight. For healthy fry rearing it is recommended that the size of the fry at the time of stocking in the rearing pond should be as uniform as possible. This can be done by size grading at the time of fry harvesting from nursery ponds. Prior to stocking the rearing ponds the pond waters must have a plankton level of about 30–50 ml/m³.

9.2.3 Stocking of grow-out/stocking ponds

After proper preparation, the pond should be stocked with 100–150 mm long fingerlings of desired carp species. In case the fingerlings are not available, the pond can also be stocked with advanced fry or early fingerlings in absolutely predator-free ponds. The stocking rate depends primarily upon the volume of water and on the oxygen balance of the pond. Quality of available natural fish food in the pond and the capacity of the farmer to provide supplementary feed, are also matters for consideration. Usually a pond having average water depth of 1.5–2.5 m should be stocked at the rate of 5 000 fingerlings/ha. The volume of water available for fish in an undrainable pond should not be less than 2 m³/fish if there is no provision of artificial aeration. In composite fish culture, rearing of six species of carps, viz. catla (Catla catla), rohu (Labeo rohita), mrigal(Cirrhinus mrigala), silver carp (Hypophthalmichthys molitrix), grass carp (Ctenopharyngodon idella) and common carp (Cyprinus carpio) is considered to be the ideal combination. However, depending on the availability of quality fingerlings of these carp species, three or four species combinations can also be taken up. Ratio of different species in the combination is also equally important. However, there are certain general guidelines for selecting species combinations (Table 31).

* Lower units in shallow ponds

Availability of weed in the pond or in the vicinity decides the stocking density of grass carp. In older ponds where the soft sediment layer of the pond bottom is usually very thick and anaerobic in nature, the ratio of bottom feeder and especially the common carp should be kept at a higher level. Likewise, the relative density of column feeder-rohu should be kept on the high side in deeper ponds than in shallower ponds, whereas ponds showing consistently higher zooplankton population should have a higher ratio of surface feeders. Based on the performance of individual species in the combination and availability of seed, combinations can be modified in subsequent years. Silver carp, however, should be stocked 1 or 2 months later. Interspecies competition for food between catla and silver carp to some extent is the key point for such differential stocking. The stocking pond also should have a desired level of plankton population of about 30–50 ml/m³.

9.2.4 Method of stocking

Stocking of spawn, fry and fingerlings should be done very carefully to avoid any post-stocking mortality due to shock or infections. To minimize post-stocking mortality the fry/fingerlings should be slowly and gradually acclimatized to the temperature and quality of the water in the stocking pond. To do so, open the mouth of the seed transport bag/container and gradually add the pond water in phases and after 15–20 minutes slowly dip and tilt the bag/container in the pond so that the spawn/fry/fingerlings are free to swim out. Stocking should preferably be done in the cool evening hours. Apply prophylactic treatment to seed prior to their release so as to avoid any post-stocking infections (Section 9.3.4).

9.3 Post-stocking management

Post-stocking management involves harnessing the pond productivity in the form of natural fish food, maintenance of pond environment congenial to the cultivated fish and fish husandry, mainly feeding and health care.

9.3.1 Feeding

Soon after stocking, the fish start grazing natural food available in the pond irrespective of their stage of life cycle. Spawn feeds voraciously on plankton. Therefore, immediate steps must be taken for providing supplementary feed. In the case of nursery ponds where spawn are reared for about a fortnight up to fry stage, supplementary feed is broadcoast on the pond surface in the form of fine powder daily in the morning hours at prescribed rates (Table 32).

The following schedule of feeding should be followed for nursery ponds (Table 33).

At the time of stocking, the spawn of 0.65–0.75 cm average length weigh about 0.0014 g each, and a mixed collection of 0.1 million weigh about 140 g.

Grass carp is fed its preferred aquatic vegetation or green animal fodder as per the following table (Table 34). See Fig. 38.

Figure 38. Feeding Enclosure for Grass Carp

The form in which the supplementary feed is given is also important. In the nursery ponds the feed should be provided in finely powdered form and may be broadcast over the pond surface. In the case of rearing, stocking and brood stock ponds, the supplementary feed mixture should be mixed with enough water to make a dough and applied into feeding trays fixed in the ponds. Better results can be obtained if the feed mixture is pelletized and fed to fish (Fig. 33B). The pellets may be of the sinking or floating type, but both types should be water stable. The sinking type of pellets are put in feeding trays fixed in the pond.

The standing crop of fish is estimated every month on the basis of sample netting for growth and health check and feeding schedule is adjusted accordingly. Periodical netting should be done strictly on a monthly basis and with the help of hand nets and spring balance (Fig. 39), the average weight of each species should be recorded (Table 35). The average weight of individual species, monthly increment in weight, total standing crop and amount of feed to be given should be estimated on the basis of data thus available.

The feeding tray should be cleaned daily before the application of fresh feed. Fish normally stop feeding if they are sick or the temperature is far below normal. In such situations a proper health check is required and the feeding rate is adjusted. Grass carp should be fed until they stop eating. Usually they consume aquatic vegetation, about 50% of their body weight on a daily basis.

Av. wt. - Average weight

9.3.2 Periodic fertilization

The next step in post-stocking management is the periodic fertilization which ensures replenishment of nutrients and consolidation of the energy base for microbial decomposition activities. The desired total quantity of fertilizers are best applied in small equal doses at periodical instalments throughout the rearing period so as to ensure maximum utilization of these fertilizers. The mode, sequence and timing of application of fertilizers are important or achieving best results. Lime should be applied first followed by the organic manure and finally the inorganic fertilizers an the same order is followed subsequently. These fertilizers should be applied only when the physical conditions of the water are most suitable such as plenty of sunlight, adequate oxygen, optimum temperature, adequate water level and low wind velocity. Turbid water with a high content of suspended solids are not preferred. Fertilizer should be sprayed or distributed properly over the water surface during the day time when the top layer of water is warmer and lighter. Inorganic fertilizer application must be stopped temporarily when the nitrate and phosphate content of water show a level of 0.5 ppm or above at any stage during the periodic pond environment monitoring. Similarly, organic manuring may also be stopped if the soil organic carbon level goes beyond 2%. However, normal application may be resumed after the specific nutrient level goes down. Care should be taken to see that the phosphatic fertilizers dissolve properly in the water since powdered orgranular fertilizer may often solidify after coming in contact with water. It is more effective if doses are divided further so that application is more frequent. The results are encouraging when organic manures are applied in daily doses in pons. The desired amount of cattle dung is mixed with water and uniformly spread over the entire pond surface. In nursery ponds the first manuring is done two weeks prior to stocking and if more than one crop is nursed, fresh manuring should be done a week prior to every subsequent stocking.

Figure 39. Hand Net and Spring Balance

A periodical fertilization schedule is summarized in Table 36. The rate of fertilization by organic and inorganic manures has already been discussed (para 9.1.3.2).

9.3.3 Pond environmental monitoring

9.3.3.1 General considerations

Proper pond management involves a regular and steady supply of nutrient for sustained production of fish food organisms. The supply of nutrients could be from within the pond itself or from outside. It is also required to regulate the physico-chemical parameters of the pond ecosystem within the safe tolerance limits of the cultured fish species. This necessitates periodical monitoring of pond environment and taking corrective measures in time. Olah and Sinha (1984) have developed a practical monitoring system of perennial undrainable ponds which offer the monitoring of basic architecture and production processes of such pond ecosystems in tropical monsoon lands. The system needs simple instrumentation, little working time and labour and reveals sufficient information about the actual nutrient level of pond sediment and water. Most of these parameters can be easily measured at the pond site while some require laboratory facilities. The monitoring system gives reliable guidelines for fish farmers to optimize fish production.

9.3.3.2 Parameters to be monitored

It is essential for extension workers to name and code-number the ponds in their area. Such coding may be based either on postal district/unit/village farmer's name, etc. The fish farmer should record the following information on his fish farm:

Nature of pond: Perennial or seasonal; nursery pond, rearing pond or stocking pond.

Water area: Measurement of the water area is essential in order to know the size of the pond for proper fish stocking and quantifying the production processes. This can be done easily with the help of a bamboo pole of known length.

Age: Age is one of the most important parameters, since it has direct relevance with the productivity of the pond which usually varies from one year to several hundred years.

Management: Management status should record the existing management techniques and its level (intensive or extensive). The species of fish present, details of culture activities, stocking structure and density, fertilization, feeding, harvesting, marketing, etc, need to be recorded. To obtain qualified data on the organic carbon and biogenic nutrient load it is necessary to know the number of livestock and human population associated with the particular pond.

The fish farmer should also monitor the following parameters on a routine basis.

Water colour: The visual colour of the pond water is a simple but important reflection of the basic production processes.

Water transparency: Water transparency measured with a Secchi disc is intended to quantify the result of those processes which determine and modify the visual colour. However, a low transparency may result either from high turbidity alone or from dense algal population and thus cannot reflect the correct trophic or production level of the water. However, the Secchi transparency readings together with the visual colour provide valuable information on the productivity of the water.

Water depth: The primary water source is usually the rainfall during the monsoon. After the rainy season the water level gradually decreases which results in a very shallow water column by the end of the dry season. The water depth can be measured with a 4–5 m long bamboo pole fitted at its base with a wooden disc of 25 cm dia.

Soft sediment depth: A soft sediment layer is usually present in the pond bottom. The depth of this layer can be measured with a 6–8 m long bamboo pole having a wooden disc of 10 cm dia at its base.

Solid sediment depth: In older ponds, in addition to the soft sediment layer, a solid sediment layer with a low water content is also present. The thickness of the layer can be measured with a 6–8 m long bamboo pole with a sharp end. The total thickness of the soft plus solid sediment layers has a direct relation to the age of the fish pond, at times the sediment layer measures more than 2 m. Such thick sediment, having a rich nutrient content, is anaerobic in nature with slow bacterial decomposition and mineral cycling rates. This should be properly utilized for fish culture.

Chemical environment in the water column: The water is chemically characterized by pH, alkalinity, NH₄-N, NO₃N and PO₄-P measurements following standard methods. Normally the pH and alkalinity do not change from pond to pond on the same types of maternal soil. The measurements of NH₄-N, NCO₃-N and PO₄-P indicate the basic inorganic nutrient status of the pond.' Simple chemical parameters such as dissolved oxygen and pH may be measured using field kits. Slightly alkaline water (pH 7.0–8.5) and oxygen levels of 6–9 ppm indicate optimum condition.

Dawn oxygen: Fish ponds usually exhibit wide fluctuations in the dissolved oxygen content from day to night. This diurnal oxygen fluctuation is normally measured to calculate the community metabolism of the whole pond while quantifying the production and respiration processes in the ecosystem. A single measurement just before sunrise would be an important indicator of the risk of fish kill due to oxygen depletion. Desirable ranges of various pond environment parameters are presented in Table 37.

A simple schedule for monitoring the important parameters is presented in Table 38.

9.3.4 Fish health monitoring

In most of the situations, cultured fish are healthy even in the continuous presence of pathogens. However, when environmental stresses occur and the balance shifts in favour of the disease, the characteristic pathogens flourish. Under such circumstances if the fish fail to adjust adequately or if corrective measures are not taken timely, outbreak of diseases may occur. By resolving environmental problems and applying effective therapeutics, the original balance between the host and the pathogen may be restored. Thus a disease outbreak may often be a symptom of environmental imbalance and it gives a distress signal so that the adverse environmental conditions may immediately be corrected to prevent fish losses. The approach to health care in composite fish culture in undrainable ponds is essentially one of management of ecosystem and fish husbandry.

9.3.4.1 Host-pathogen-environment linkage

Susceptible fish, the virulent pathogen and the aquatic environment in which they encounter each other are the three contributing factors in fish disease outbreaks (Snieszko, 1974). The fish itself possess a varied and complex defense system, the immune system, the potency of which determines the susceptibility or resistance to the particular pathogen under a particular circumstance. Several environmental components effectively influence the normal immune mechansim of the fish when their value exceeds the normal tolerance limits. A virulent pathogen, when present in the surrounding, is usually capable of causing an infectious disease to fish under stress. The causative agents of the disease and their fish hosts carry on their struggle in the aquatic environment and the environmental parameters which influence this encounter may shift the balance from one side to the other and often determine whether the host will overcome the infection or the pathogen will flourish (Fig. 40).

Some of the infectious and parasitic agents can survive only in live fish, and in such cases the disease transmission is from fish to fish. Such disease-producing agents are true pathogens. Others are extremely adaptable organisms which can survive outside the fish and cause infections whenever fish are weakened or otherwise predisposed to disease due to environmental stress. Most of the fish disease agents belong to this category.

9.3.4.2 Health monitoring programme

Health protection of cultured fish is considered to be one of the most important aspects of modern aquaculture systems including the composite fish culture which requires a programme basically to check the health status of the fish quite frequently and employment of fish health management measures. This enables timely detection of any disease outbreak and taking up proper treatment measures at the initial stage. Otherwise, in advanced stages of the disease, control and treatment measures do not provide economical and effective.

A fish health monitoring programme should consist of the following components:

1. Daily observation of fish in each pond.

2. Sampling and examination of fish at regular intervals for health check and diagnosis of the disease if any.

3. Monitoring of pond quality and sanitation.

4. Sampling and examination of fish at the onset of distress, disease outbreak or mortality.

Figure 40. Effects of Environmental Changes on Fish-Pathogen Relationship

The sampling for health check of fry and fingerlings should be done at weekly and fortnightly intervals respectively, while in composite fish culture ponds it should be at least once a month. A thorough health check of fry/fingerlings is required 1 or 2 weeks before netting out for stocking in grow-out ponds or before transfer to another pond. Such an examination will provide sufficient info rmation for planning.

Diseased fish may exhibit either or both physical and behavioural signs, the most common of those are listed below:

Behavioural signs:

• slowing down or a complete stoppage of feeding;

• loss of equilibrium, swimming erratically or in spirals;

• surfacing for gulping air and scraping against the floor and sides of the pond.

Clinical symptoms:

• excess mucous secretion;

• change in normal colouration;

• erosion of scales, part of fins, skin, etc.;

• decolouration or paling of gills;

• abdominal swelling;

• bulging of eyes;

• presence of cysts, spots or patches over the body and gills, etc.;

• appearance of lesions, haemorrhagic spots and greyish or brownish areas over the body.

Laboratory examinations:

Thorough visual examination for external signs of the disease should be followed by detialed but quick laboratory examination by pathomorphologica, pathoanatomical and microscopical studies of squash and smear preparation from different organs/tissues. Diagnostic procedures in brief are presented below (Table 39).

In situations where a disease problem is suspected, only those specimens exhibiting symptoms of distress or disease should be selected. Live moribund speciments are preferred, but if necessary, freshly dead specimens may also be collected for laboratory examination.

Smear preparation of selected tissues and organs may be made on the spot by smearing the material on a slide. Slides can then be dired, stained and examined immediately. Bacteriological media can be inoculated with materials from various organs, especially kidney, heart, etc., employing aseptic techniques. On-site disease diagnosis permits the immediate application of chemotherapy or remedial measures to control or eradicate the disease. However, accurate diagnosis of disease is of utmost importance if proper treatment is to be applied and this is possible only through experience and training. At times, may disease conditions occur which cannot be properly diagnosed without specialized laboratory facilities and in such conditions samples should be sent to such laboratories under proper preservation, packing and shipment (Dey, et al., 1982). As far as possible the specimen for examination to reference laboratories should be always sent live but when circumstances prohibit live delivery, specimens may be forwarded packed in ice. Specimens for parasitology examinations may be preserved in 5–10% formalin solution. In case of larger specimens incision may be made to facilitate effective penetration of the fixative. The volume of fixative should be at least five times the volume of materials to be preserved.

9.3.4.3 Health management measures

Understanding and managing the undrainable pond environment is the key to successful fish health management and profitable fish culture, and to ensure this the knowledge of the role of various environmental components in the occurrence of disease outbreak is essential. The main thrust of such measures is directed toward:

• minimizing the stress on cultured fish;

• prevention of the introduction of serious disease agents;

• confinement of disease outbreaks to affected areas;

• minimizing losses from disease outbreaks.

The following important measures are the key components of successful fish health managements (Figure 41).

Surveillance and maintenance of water quality: Abrupt and wider fluctuations in some of the environmental parameters such as dissolved oxygen content, pH, turbidity, temperature, additions of some chemicals, detergents, pesticides and naturlaly produced toxic substances such as hydrogen sulfide, ammonia, dinoflagellate toxins, etc., often cause stress in fish and predispose them to infectious diseases. Anything that alters the environment of the fish is a potential stressor and efforts should be made to identify and avoid them. Undrainable ponds offer great protection against spreading of disease outbreaks by confining the outbreaks only to the affected ponds. However, the recent trends of intensification in aquaculture involve high stocking rates, increased feeding and fertilization programmes resulting in nutrient accumulation leading to appearance of algal blooms that lead to dissolved oxygen and other water quality problems. In older ponds, cases of excessive accumulation of organic matter have been observed, resulting in the appearance of bacterial bloom and related oxygen depletion (Radheyshyam et al.,). For health and optimum growth, the dissolved oxygen level should not drop below 5 mg/1. Carbon dioxide concentration up to 20–30 mg/l may be tolerated by fish provided oxygen is near saturation. At lower levels of dissolved oxygen, toxicity of carbon dioxide increases. When pH values remain above 9.5 or below 6.0 for extended periods, fish will be under stress and may not grow well. Liming agents may be used for low pH corrections. Ammonia concentration above 1.0 mg/1 indicates organic pollution. Hydrogen sulfide toxicity increases with decreasing pH and it is harmful even at 1.0 mg/l concentration level. Making the pond environment more congenial and hygienic, eliminates the risk of stress and provides safety to fish. Proper and timely management of soil and water qualities by manipulating feeding, fertilization, liming, addition of clean water, bottom raking, aeration of water by recirculation or splahsing, etc., reduces most of the environmental problems and provides congenial conditions for the health growth of fish. An interval of about 15 days between the pond poisoning and the stocking eliminates most of the pathogens from the environment.

Figure 41. A Model for Integrated Fish Health Management System

It is always advisable to stock the pond only with healthy and genetically vigorous fry and fingerlings so that they may have better growth rate and resistance towards diseases. Prior to stocking, samples of the stocking material should be examined to check their health status. This avoids any risk of introducing infected stock in the pond. However, the stocking materials should also be prophylactically treated before releasing into the pond (detailed under Chemoprophylaxis).

Overstocking may lead to biological crowding resulting in waste build up, decreased availability of natural food, depletion of dissolved oxygen, deterioration of water quality, etc., and hence it is advisable to follow the recommended stocking density for nursery, rearing and stocking ponds.

Minimizing handling stress: The rougher the handling, the greater is the stress and the risk of disease (Kumar et al., 1986). Care should be taken not to break the protective mucous coating of the skin. During summer months netting should always be done early in the morning and it is better to have minimum possible handling during hauling. High temperature during hot water causes increased metabolic activity and induces more stress upon them.

Measures in pond management:

Poisoning of pond - Wild fish population is one of the most potential sources of disease-producing organisms. Use of chlorinated lime (bleaching powder) is the most suitable material for this purpose, since it kills all the wild fish species, molluscs, tadpoles, frogs, crabs, etc., and also disinfects the pond water and soil. It is applied at the rate of 40–50 ppm (Tripathy et al., 1978). Mahua oilcake is also a widely used piscicide, but it fails to disinfect the pond. In nursery and rearing ponds it is desirable to have second poisoning with malathion at the rate of 0.25 ppm 4 or 5 days prior to stocking. It eliminates the larger copepods which do appear in large numbers after organic fertilization. These copepods prey upon young fish larvae and also serve as vectors or carriers of many infectious pathogenic organisms. Some of the common crustacean fish parasites also get killed. Malathion application has significantly increased the survival level in nursery ponds (Kumar et al., 1986).

Disinfection of appliances - All required appliances such as fry carriers, hapas, utensils, buckets, nets and gears, etc., require thorough cleaning and disinfection before being put to use. Some of the pathogenic organisms are found adhering to them and may cause disease if they are allowed to come in contact with the host fish species. Disinfection can be done by washing or immersing in a concentrated solution of disinfectant. Some of the most effective and easily available disinfectants for such use are chlorine, sodium hydroxide, sodium chloride potassium permanganate, etc. Chlorine is probably the most widely used disinfectant in fishery management and is easily available as a solution of sodium hypochlorite and powder of calcium hypochlorite (bleaching powder). Solution of 1–2% chlorine is active against bacteria, viruses and fungi but is extremely toxic to fish and hence their residues must be thoroughly rinsed from the disinfected items before being brought into contact with fish. Sun drying of nets, hapas, etc., is also a practical method of disinfection.

Proper feeding - In addition to the natural fish food which is made available by fertilization, an adequate amount of good quality supplementary feed is essential for maintaining healthy growth of fish. Any deficiency in quantity and quality of feed may cause various diseases by increasing susceptibility to many infections.

Prevention of entry of unwanted fish: Most undrainable ponds lack proper embankments. Most of these ponds have channels in the embankments connecting them with outside waters during the rainy season. Most of the ponds lack even proper embankments. These channels are the vulnerable sites through which some of the wild unwanted fish species or other animals get entry to the pond. Fixing fine meshed screen into these channels may eliminate the risk of entry of unwanted fish species into the pond. Pond embankments may also be raised to prevent risk of inundation and entry of undesirable animals and fish species. Some fish eating birds, molluscs, etc., serve as intermediate hosts for many parasites that infect fish. Tadpoles and frogs may also act as carriers of certain parasites and bacteria which ultimately may infect carp species and hence such animals should not be allowed in the pond.

Separation of young and brood fish: Brood fish may serve as carriers of disease causing organisms without exhibiting any clinical symptoms. They sometimes become survivors of previous epizootics due to built up immunity but retain some of the pathogens. To avoid such risk, the best course is to separate the young ones from the adults.

Removal of dead fish from the pond: Dead and apparently sick fish should be removed. A daily log of losses must be kept. Such records will provide valuable insight into the problems and may lead to their solution.

Holding the fish in a hand net and dipping it into a concentrated solution of the drug for one minute or less is used as prophylactic treatment in case of mild diseases. A short bath is useful when facilities for a rapid flow of water are available. Water flow is stopped and relatively high concentration of the drug is added. Exposure time should not be longer than one hour. A long bath is a very effective method for prophylactic treatment of pond fish for external parasites. The oral route is used in prophylactic treatment to prevent certain infections. It is generally conceded that feeding medicated feed to fish is a prophylactic rather than a curative measure.

Prophylactic use of streptomycin and penicillin at the rate of 25 mg of streptomycin sulphate and 20 000 I.U. of penicillin has been found to be very effective in preventing outbreak of columnaris disease in rohu in a field-oriented experiment (Kumar et al., 1986). Feeding antibiotics with feed has successfully prevented the occurrece of CE (Carp Erythrodermatics) in European carp culture. Prophylactic treatment of pond with locally available organophosphorous insecticide (malathion) at the rate of 0.25 ppm of active ingredient successfully prevents occurrence of trematode and copepod infections.

Occasional application of potassium permanganate at the rate of 2 or 3 ppm is recommended for increasing dissolved oxygen concentration and hauling prophylaxis. Dip treatment in 500–1 000 ppm solution of potassium permanganate for a few seconds before releasing adult fishin ponds is also a very effective and practical prophylactic measure. Short bath for a few minutes in 2 or 3% common salt solution is also a safe and inexpensive prophylactic measure against a wide range of parasitic an microbial pathogens.

9.3.4.5 Immunoprophylaxis

Immunization is becoming one of the most important ways of preventing communicable diseases in animals, including fish. Several commercial vaccines are now available and being used in many developed countries. Vaccines for some of the bacterial diseases of carps which do occur in undrainable pond culture systems are also available. These vaccines are against Aeromonas hydrophila and Flexibacter columnaris. Viral vaccine against Spring Viremia of Carp (SVC) is also being used on a commercial scale very successfully.