The tender spawn starts external feeding after about 3 days of hatching and thus, thereafter need a congenial environment with enough feed for growth and survival. The spawn of 5 – 6 mm size (Fig. 6) is nursed in well-prepared nurseries and grown to fry measuring 25 – 30 mm size in about 2 weeks period. The package of practices, evolved at CIFRI, ensures 60 – 70% survival rate of fry against about 5% under the traditional system in the country.
Selection of ponds
Nursery ponds are usually small, 0.02 – 0.05 ha in area, with water depth between 1.0 and 1.5 m. Seasonal ponds are preferable to perennial ones since seasonal exposure of the pond bed to direct sunlight helps in improving pond conditions. It is preferable also because such ponds are relatively free from weeds and unwanted fishes. Aiming at providing the delicate young ones enough moving space and sufficient feed in an environment free of pond-dwelling enemies, nurseries are specially prepared just before monsoon season. Care is taken to prevent entry of ducks in such ponds in view of their predacious habits.
Clearance of weed
A weed-free nursery facilitates free movement of the growing fry and is also conducive to the production of their natural food. Such ponds, being small and shallow are cleared of vegetation conveniently and economically by manual labour. This may be done during the summer months, April and May.
Eradication of unwanted fishes
Predatory fishes directly prey upon the young ones besides competing with them for space and oxygen. The weed fishes compete with the cultured species for their demand for food, space and oxygen. In view of the harm caused by these undesirable fishes, their complete eradication from the pond before stocking is of utmost importance in scientific nursery management.
The commonly encountered species of predatory fishes are: Channa spp., Clarias batrachus, Heteropneustes fossilis, Pangasius pangasius, Mystus spp., Ompok spp., Wallago attu, Glossogobius giuris etc.
The common weed fishes are: Puntius spp., Oxygaster spp., Ambassis spp., Amblypharyngodon mola, CoUsa spp., Rasbora spp., Aplochei/us spp., Laubuca spp., Esomus danricus etc
Repeated netting of the pond is one of the methods for eradication of fishes. But many fishes escape the net, particularly in deeper waters. Another method of eradication is dewatering of the pond. But this method can only be applied in small water bodies with an additional water source to refill the pond. Hence, to ensure complete removal of the existing fish population use of fish toxicants is necessary. This may be economically done during pre-monsoon season when the water level is minimum. The date for use of fish toxicant may be adjusted about 5 weeks earlier to the anticipated time of spawn availability.
However, seasonal ponds which dry up during summer months are generally devoid of such unwanted fishes and thus, need no such treatment. The suitability of fish toxicant is judged on its properties like effective minimum dose, its cost, conformability of the killed fish, the least adverse effect on the pond biota, short duration of the toxicity, the non-cumulative residual effect in the pond, commercial availability and simplicity of application etc. Out of a long list of known fish toxicants, suitable ones are mahua oi1cake, tea seed cake or any other plant derivative; Ammonia and Bleaching powder. Depending on availability, cost and convenience, one may choose any of the following fish toxicants.
Mahua/cake: The most extensively used fish toxicant in the country IS oil cake of Mahua containing 4-6% saponin, It kills fishes at 200250 ppm in 6-10 hours. The fishes, thus killed, are fit for human consumption. While the toxicity in water lasts for 15-20 days, the oilcake serves as organic manure in the pond subsequently.
The quantity required is calculated on the basis of water volume in the pond. At the above-mentioned dosage, mahua oil cake required is 2000-2500 kg per hectare for every meter of average depth. The required quantity of mahua oil cake is powdered, soaked in water and broadcast uniformly over the water surface. Repeated tirring of water thereafter ensures proper mixing of the oil cake. Drag netting helps removal of affected fishes besides mixing of the toxicant.
Tea seed cake: Seed cake of tea (Camellia sinensis), may prove to be a substitute for Mahua oil cake wherever commercially available. A dose of 75-100 ppm is sufficient to obtain a complete kill in the pond with toxicity lasting for 10-12 days. It also ultimately acts as a fertilizer in the pond. The treated fishes are fit for human consumption. The method of application is the same as in mahua oil cake.
Other plant derivatives: Other fish toxicants of plant origin like stem bark (20 ppm), seed (15 ppm) and root bark (10-15 ppm) powder of Barringtonia acutangula, seed powders of Croton tiglium (3-5 ppm) and Milletia piscidia (4-5 ppm), root powder of Milletia pachycarpa (3-6 ppm), unripe fruit powder of Randia dumetorum (15 ppm) and Cassaria arevaolaus (25-30 ppm), seed husk of Tamarindus indica (510 ppm), bark powder of Walsura piscidia (5 ppm) and whole plant of Euphorbia thirucalli have been found to be effective as fish toxicant under laboratory conditions.
Ammonia: Anhydrous ammonia @ 20-25 ppm has been found as an effective fish toxicant. The killed fishes are safely consumable. The cost of ammonia as a fish toxicant is off-set to some extent by its fertilizer value which has been estimated to be about 36% of the cost of ammonia applied. Ammonia acts as herbicide as well. Toxicity of ammonia in water lasts for about 5-6 weeks.
Anhydrous ammonia from a cylinder is introduced into the water through a. hose and a 1.2 m long G. 1. pipe applicator with delivery holes. The applicator is suspended from a boat or held in position by long ropes by two persons standing on opposite bank. The cylinder is partly immersed in the water near the shore to prevent excessive cooling and condensation of ice. Ammonia is lighter than water, the applicator is kept as far below the water level as necessary to effect the bottom dwellers. However, the applicator should be kept well above the bottom soil to prevent loss of gas from absorption by the soil. Its efficacy also depends on the pH of water, the effect being quicker with increasing pH.
Bleaching powder: Bleaching powder, Calcium hypochlorite, as fish toxicant has been found to be effective in 3-4 hrs. at 25-30 ppm. Its toxicity lasts for about 7-8 days in the pond. It also possesses disinfecting effect besides oxidising the decomposing matter on the pond bottom. In view of the limited supply of mahua oilcake, bleaching powder is an effective substitute with easy availability and lower cost. The powder is dissolved in water and made into the form of a slurry. The solution is sprayed on the water surface immediately and the water stirred for thorough mixing. Distressed or killed fishes are removed by subsequently repeated netting and are fit for human consumption.
Application of lime
With a view to helping mineralisation of organic matter and for prophylacticreasons, quick lime is applied in the pond. After about two week of eradication of unwanted fishes, application of lime @ 250-300 kg/ha is generally recommended
Manuring of pond
Manuring is done with the objective of encouraging growth of plankton, particularly animalcules which form the natural food of the spawn. Nurseries are, thus, manured with only cattledung (organic manure) @ 10,000 kg/ha about 15 days before the anticipated date of stocking by broadcasting all over the pond. In ponds where mahua oilcake is used as piscidice earlier, the dose of cattledung may be reduced to half
Aquatic insect control
Manured nursery ponds are populated with large number of aquatic insects, particularly during monsoon months. Most of the aquatic insects, either in their larval and/or adult stages, prey upon spawn and early fry and, in addition, also compete with them for food. Hence, it is necessary to eradicate all such insects from nursery ponds before stocking with spawn. The most commonly occurring aquatic insect group are: Coleoptera (Beetles): Predaceous diving beetle (Cybister), water scavenger beetle (Sternolophus) and whirling beetle (Gyrinus), etc. Hemiptera (Bugs): Back swimmer (Anisops), Giant water bug (Belostoma), water scorpion (Ranatra), water stick insect (Laccotrephes), etc.
Since the complete eradication of insects by means of seining is not possible, selective treatments are necessary. The commonly used methods to control insects in fishery waters are as follows. However, choice of a method depends onthe convenience of application, local/easy availability of the items and cost involved etc.
Use of soap oil emulsion: The generally recommended CIFRI method is to spray a soap-oil emulsion on the surface of the water for killing insects which come up to the surface for respiration. An emulsion of any cheap washing and vegetable oil at the rate of 18 kg soap: 56 kg oil per hectare is prepared by heating the mixture for a short while. The emulsion is applied 12-24 hours before stocking by uniformly broadcasting over the water surface of the nursery. Care is taken to keep the uniform film undisturbed for a couple of hours. Hence, calm dry days are chosen for the application
Soap can be substituted by Teepol B-300 ( a synthetic detergent) in the abovesaid emulsion. The recommended dose of Teepol is 560 ml to be emulsified with 56 kg of vegetable oil.
Use of kerosene oil: Spraying of kerosene oil over the surface of water @ 80-100 litres/ha has also been found to be useful by West Bengal State Fisheries Department in killing the aquatic insects in nursery ponds.
Use of L.S.D.: Another treatment, found useful by Maharashtra State Fisheries Department in killing aquatic insefcts, is spraying an emulsion prepared by mixing light speed Diesel oil (1 litre), emulsifier Hyoxid 1011 (0.75 ml) and water (40 ml) at the rate of 1040.75 ml per 200 square meter of water surface.
Use of turpentine oil: Turpentine oil @ 75 litres/ha when sprayed on the water surface has been reported to kill the insects completely.
Stocking of spawn
If piscicide is earlier used for the preparation of the pond, it is essential to ascertain detoxification before stocking. This may be conveniently done under field conditions by fixing a hapa in the pond and releasing some spawn in it. Comfortable behaviour of spawn for about 24 hours confirms complete detoxification. Nursery ponds are stocked with about 3-4 days old spawn usually in the morning hours after about 5 weeks of piscicide application, if done. The moderate rate of stocking may be 25-35 lakhjha.
Since it becomes difficult, even after manuring to maintain the desired level of natural food for the growing fry in the pond, it becomes necessary to provide supplementary feed from outside. A mixture of finely powdered groundnut/mustard oilcake and rice bran/polish in equal proportion by weight is supplied to the fry. Other feed items that can alternatively be used are a mixture of powdered aquatic insects, prawn and cow peas or fish meal and groundnut oilcake etc. It is recommended that cobalt chloride or manganese sulphate @ 0.01 mg/day/spawn may be added to the feed. Addition of yeast increases survival of fry. Feed may be broadcast all over the pond once daily in morning hours commencing from the day of stocking . Feeding may be stopped a day earlier to harvesting.
Harvesting of fry
The fry in about 2 weeks generally grow to 25-30 mm size. They are harvested with fine meshed (1.5 mm) drag net in the cool morning hours avoiding the cloudy days.
During a normal breeding season lasting about 3 months, 3-4 crops can be raised from the same nursery pond. The fry are then raised to fingerlings in rearing ponds.
- Central Inland Fisheries Research Institute, Barrackpore