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The total fry (fish seed above 8 mm.and up to 40 mm.in length) requirements for the presently cultivated waters have been estimated to 165.18 crores, as against the meagre production of 21.11 crores in 1964-65. If all the existing readily available water areas are to be adequately stocked, the total requirements to meet the above demand would be 219.66 crores and the requirement will increase further with the progressive reclamation of more water areas for fish culture. The available source of fry production in the country are:
Riverine collection contribute only 5.90% of total fry production in the country. The estimated fry production from riverine sources are 124.61 lakhs (estimated in 1964-65) contributed by State like Delhi, Gujrat, Madhya Pradesh. Madras, Mysore, Orissa, Punjab and Uttar Pradesh primarily. The list of important carp fry collection centres in these States are given below:
In 164-65, 1986 31 lakhs of fry have been produced from rearing spawn, which consists 94.10% of the total fry production of the country. For the rearing of spawn sufficient nursery space is required to augment the fry production. Requirement of nursery space Increased seed production will get a serious setback without required nursery facilities. The present nursery space available in different states of the country is only 1922.7 acres. The Fish Seed Committee has estimated the optimum requirement of nursery areas on the basis of average stocking rates of spawn per acre, per crop, with an average of two crops per season. This worked out a total of about 5911.87 acres of nursery space required to bring all the readily available water areas under fish culture in the country. Nursery management Nursery management is based on the principle of bringing about suitable and proper ecological, physicochemical and biological conditions in pond, where by protection from harmful external agencies and promotion of optimum growth of fish fry, will be ensured in natural conditions or through human manipulation. The various operations involved in nursery managements are directed to meet the above principle. Nursery ponds Any small body of water, either perennial of seasonal, but with shallow depth of 0.5 to 1.0 metre can be used as a nursery pond. A pre-requisite of nursery pond is maintenance of a steady water level for two to three months during the fry rearing period and sufficient fertility, of the soil and water for the natural production of planktonic organisms. Cement, revetted and stone or brick lined ponds can also be used as nurseries but the yield may not be satisfactory, as the pond water does not get the advantage of natural mineral fertility of soil (Hickling, 1962). Ecological conditions The inter-relationship of primary productivity, plant and animal life, which compete with each other and complete the food chain in a nursery environment bears great significance in successful nursery management. Ecological factors such as abundance of macro-vegetations and of higher animal life in the form of fish, insect and amphibian population hardly play any important and useful role in the rearing of the fry. An alternation of the natural ecology through human agency is necessary to make a nursery is, therefore, to be limited to a short cycle ending with zooplankton organisms. Physico-chemical conditions Wide fluctuations in the water level, temperature and chemistry of water will be unfavourable for the survival of fry. High turbidity either due to suspended slit or due to profuse growth of phytoplankton is also harmful in nursery tanks. Nurseries, being shallow, are relatively free from thermal stratification and the water temperature in ponds of warm water region may range between 250 and 350 C during fry rearing season. This relatively high temperature is conducive for accelerated growth of fry (Backiel and Stegman, 1968). The range of other important physico-chemical conditions in the normally productive nurseries are usually as follows:
Fluctuations in the levels of one or more factors either in the pond itself or due to transfer of fry from one water to another may have far reaching adverse effect on them, resulting even in large scale mortality. Normally nursery with alkaline water and fair level of phosphate and nitrate can yield satisfactory growth and survival of fry. In fry rearing, the various operations are classified as pre-stocking, stocking and post-stocking techniques, which are to be adopted in time sequence. (Anon, 1956). Pre-stocking management An abundant growth of vegetation is undesirable in the nurseries in view of the wastage of soil nutrition, arresting of planktonic production, hindrance to the free movement of fish fry and to netting operation. Among four important methods of weed control e.g. chemical, biological, mechanical and manual, perhaps economical clearance of weeds in nursery waters which are small and shallow, can be best conducted by manual efforts. Eradication of predatory animals Enemies of fish fry in nursery pond include frogs, snakes and birds besides almost all species of large sized fishes. In view of the known high degree of destruction that may be caused by unwanted fishes to the hatchling and advanced fry, their complete eradication is the most important pre-stocking nursery operation. The most efficient method of eradication of predatory and unwanted fish population is by draining ponds. Poisoning of pond water increasingly resorted to complete eradication of fish population. The widely used fish poison is derries root powder with rotenone as the active principle. This contact poison which damages the respiratory system of fish can be applied without much difficulty and a 4.0 ppm. (0.20 ppm. rotenone) concentration is effective to kill almost all species of fresh water fish. The quantity required for a nursery can be easily calculated on the formula. LxBxDx4.0=Weight of derris root powder in gm. ( L,B & D represent the water edge length, breadth and average depth respectively expressed in metres). After application of the poison, fishes come up in distress to the water surface in about one hour, when they can be collected easily and either transferred to other non-toxic waters for revival or used for consumption without any adverse effect. Due to non availability of this preferred piscicide in this country, Tafdrin-20 with 20% endrin as its active poison, is being used for clearing ponds. This highly toxic chemical liquid insecticide is applied by spraying over water surface at a concentration of 0.01 ppm.and its quantity requirement is calculated as LxBxD (in metres) x 0.05 1 Vol. Of Tafdrin in c.c. Though an effective poison, the lingering residual toxic effect of endrin up to a period of nearly 2 months is of great disadvantage as compared to 7 to 10 days period in case of derris root. It is generally desirable to use toxic elements of plant origin rather than powerful chemical toxins as piscicide. The suitability of a fish poison should, however, be judged on its properties such as effective minimum dose, revivability of affected fish, consumability of poisoned fish, least adverse effect on pond biota, short duration of toxicity persistence, non-cumulative residual effect on pond, commercial availability and easiness of application. Some species of cyclopoid copepods are also known to attack fry at their earlier stages and contribute towards mortality rate. Pond fertilization In nursery rearing, one of the major factors determining survival and growth of carp fry is the availability of proper type of food in requisite quantities (Alkunhi, 1955). In nursery ponds, where stocking is done at high densities, the normally available food reserve gets used up in a very short time. It is, therefore, necessary to continue manuring in such a way that maximum planktonic growth is maintained for a prolonged period. Under the existing knowledge on plankton productivity it is extremely difficult to synchronise high plankton production with fry stocking. However, it is generally observed that a relatively high density of zooplanktons could occur about 15 to 20 days after heavy organic manuring. Taking advantage of this feature, nursery ponds are manured with fresh cow dung about 15 to 20 days prior to the probable stocking date. Satisfactory production of plankton is caused by manuring at a rate of 12,000 to 15,000 kgs.per hectare of water surface area of pond by throwing the manure along the marginal waters. In addition to this initial fertilization, the pond is further fertilized at a lower rate of 5000 kgs. per hectare after stocking, in case of planktonic density is unsatisfactory. This additional manuring helps in the maintenance of the standing crop or to get a further pulse of their growth. In addition to raw cattle dung, vegetable oil cakes (mustard oil cake or ground nut oil cake) are also used for its effectiveness and cheapness. Two different manuring schedules are given below:
b) Manuring with 10,000 R.C.D./ha.of water surface area-suitable for stocking 24 hours after manuring. c) 200-400 kg. Of mustard oil cake/ha.of water surface area-suitable for stocking of spawn 15-20 days after manuring. d) 300-500 kg.of ground nut oil cake/ha.of water surface area-suitable for stocking of spawn 15-20 days after manuring.
Treatment with lime is generally done especially in ponds with acidic soil water at a rate of 200-250 kgs./ha. Liming treatment may either be combined with or preceded fertilization, which enhances fertilization effect. Inorganic fertilizer, stimulate profuse growth of phytoplanktonic bloom, which is not desirable in nursery ponds due to its non utility as nutritive food of carp fry and lead to extreme fluctuations in O2 content of the pond water resulting large scale mortality of fry. The desirable forms are the zoo-planker comprising, Infusoria, Rotifera, Cladocera and Copepods on which fish fry feed on voraciously. Assessment of food in nurseries The qualitative and quantitative nature of the standing crop of plankton following fertilization of nurseries is to be ascertained at short intervals by filtering 45 litres of pond water through a plankton collection net made of No.21 bolting silk. A rich production and dominance of phyto-planktonic organisms in the nursery at the time of stocking suggests its immediate unsuitability for stocking and a sediment volume of about 1.0 ml. Of zoo-plankers, consisting of rotifers, copepod nauplii and cladocera is to regarded as a good food reserve for the hatchlings. Eradication of insects Aquatic insects and their larvae have been observed to cause large-scale destruction of hatchlings stocked in nurseries. A simple method adopted for the eradication of insects is by the application of an emulsion prepared with any vegetative oil and cheap soap. The principle of this technique is to spread a thin film of oil on the water surface and thereby cut off direct contact of surface water with atmosphere resulting in respiratory difficulties for aquatic insects. The emulsion is prepared by mixing oil at the rate of 58 kg.per hectare with one third of its weight of cheap soap in solution. The treatment has no adverse effect on carp spawn and, in order to avoid repopulation of the treated pond by migrating insects from adjacent ponds, the soap-oil emulsion is applied immediately before or within 24 hours prior to stocking. Low speed diesel oil is found to have replaced the vegetative oil effectively on economic point of view. Stocking of spawn Usually spawn are transported from breeding or collection centres to nursery ponds. The physical and chemical characteristics of water in which they are transported is more often different than those of nursery ponds. Since sudden changes in water conditions are harmful or even fatal for the tender fry, partial acclimatization to the new habitat is beneficial. For this the fry container may be kept immersed in the pond water for temperature equalisation and small quantites of pond water added for the gradual change in physico-chemical conditions of water. The spawn are then released by gradual tilting of the container on the water surface. Determination of stocking density is an important aspect of nursery management. A nursery being a closed ecosystem, the stocking rates are influenced mainly by the amount of available food and space that can support the optimum number of fry till they attain about 25 mm.size within a time limit of nearly 15 days. In a properly prepared nursery with moderate growth of zooplanktonic organisms the stocking rate practiced is 2.5 million per hectare. But stocking density can be increased to 6.0 million per hectare with artificial feeding. In 10 liters of water with fresh zooplanktonic feed and removal of excreta from the aquarium showed a very high survival rate of spawn for a period of 15 days.
In plastic pool of 1.8 metre diameter and 1.2 metre height with a water height of 0.9 metre yielded 77,000 of 16-18 mm.fry in 10 days from one lakh of spawn, when fed with 5 ml.of freshly caught seived plankton per day. Stocking should be done either during morning or late evening, when water temperature is relatively low. Post stocking management Supplementary feeding Supplementary feeding in nurseries with high stocking density is a must in order to get a higher survival rate. The criteria for good fish feed are (I) ready acceptability to the early fry, (II) easy digestibility and (iii) high conversion value. The food requirement of fry are judged by the conversion rate, which is equivalent to the quantity of feed divided by weight increase in flesh, also known as growth coefficient. The fish food should also be easy transportable, abundantly available, with good keeping quality and low cost. On considering the above points, in India the artificial fish feed consisting of grain bran and powdered vegetable oil cake (in equal weights) is found to be ideal. The powdering should be done so finely so that it can be easily taken by spawn and early fry. The daily ration is estimated on the basis of fry population and their advancing growth with the approximate initial weight of each fry 0.0014 gram. The total quantity to be fed according to different workers are given below:
Broadcasting of feed on the surface of water is the normal procedure. All the food broadcasted is not utilised by the early spawn and fry to full extent. A large quantity of feed drops down at the bottom of the pond, decomposed and forms organic manure. Common carp is the best utilizer for artificial feed. Among Indian carps; mrigal utilize the artificial feed best. Catla do not utilize artificial feed to greater extent. Mixture of dry finely powdered notonecta, small prawns and cheap pulses in the ratio of 5:3:2 is also used as artificial feed at the same rate as that of grain bran and vegetable oil cake. Survival and growth Survival rates in nurseries have been registered as high as 88% and an average of about 50% with size of fry ranging between 20 and 30 mm.
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