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| Fisheries :: Culture Fisheries |  |
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Milkfish
The milkfish (Chanos chanos) is one of the most ideal finfishes for farming in coastal areas. They are fast growing, tolerates a wide range of temperature, oxygen and salinity. They feed mostly on filamentous algae from the bottom of the pond and are free from major diseases and parasites. Milkfish are cultured in large scales in countries like Indonesia, Philippines and Taiwan in ponds called “Tambak”. In India too the popularity of its farming is growing especially in Tamil Nadu and Kerala. The milkfish has a moderately compressed, spindle-shaped, elongated body covered with small scales and without scutes along the belly. In its early stages of life history, in natural environment, the fish enters estuaries, backwaters, lagoons and rivers, which serve as nursery grounds and spend early part of its life there for about a year or more.
Since milkfish does not mature and breed in confined waters and culture ponds, development of a hatchery technology has been difficult. However, induced breeding has been successfully carried out in this species, but the final survival rate has been less and hatchery operations hence are not economical. In this respect the most suitable method is collection of seeds from natural sources. In India, the seeds of milkfish measuring 2-7 cm in length occur along the coast of Orissa, Andhra Pradesh, Tamil Nadu, Kerala and Karnataka. The main fry season extends from March to June. Milkfish seeds prefer clear calm inshore waters of gently sloping beaches including tidal creeks, estuaries, brackishwater bodies and mud flat areas where the temperature is about 23-25 oC and salinity varies between 10 and 32 ppt. The seeds are collected from low-lying areas using scoop nets, dip nets and hand nets. In estuaries and lagoons, drag nets or seine nets may be used, while in mud flat areas, a scare line made of coir rope of about 3-3.5 m length with palmyrah leaves attached is dragged below the water surface and seeds collected in fine meshed cloth. Soon after collected the seeds are conditioned by keeping in a limited volume of clear water for a definite period without food. Seeds are transported in containers with diluted seawater of 10-15 ppt salinity and at a rate of about 100 fry/L. Nurseries are ponds for rearing the fry until they attain 5-7 cm in length. The area of nursery ponds ranges from 500 to 5,000 m2. At the nursery site the fry are acclimatized to the salinity of the pond water. Preparation of pond for stocking with fry has to be started about one or two months in advance. The ponds are drained and dried for about 10 to 15 days and later tilled and raked. Lime is added @ 1000 kg/ha and fresh water is let in. Pond water is fertilized with organic and inorganic fertilizers. Once the algal bloom develops more saline water is added to a height of 10 cm. Within 3-7 days, a complex of blue green algae, diatom, bacteria, nematode worms develop at the bottom of the pond called “Lab-Lab”. This algal consortium is most vital for developing frys of milkfish. Stocking to the nursery pond is usually done only after “Lab-Lab” has developed in the pond. The fry are stocked at densities of 20-50/ m2. The threat of predatory fishes, crabs and snakes can be screened from entering the pond using nets. Erecting poles along the embankments and crisscrossing with strings can discourage predatory birds. One serious cause of mortality of seeds in fry ponds is the sudden reduction of temperature and salinity due to heavy rains. Filling the ponds with brackishwater before rains may prevent such an eventuality. The fry feed actively on Lab-Lab and phytoplankton and grow rapidly. By the end of one month they measure 5-8 cm long and weigh 1.5-5 gm, when they are ready for transfer to the production pond or pen structures for rearing. They are captured by partially draining the fry ponds at low tides, when the fry usually congregate near the water gates, for which seine nets are used. Farming in earthen ponds The ponds where milkfish fingerlings are reared are called “production ponds” or “rearing ponds”. The production pond ranges from 0.5 ha to 3 ha in area and are rectangular in shape, with water depth ranging from 0.3 to 0.7 m. The best bottom for these ponds is a soft, jelly like, colloidal and biologically active mud containing about 4% humus and large amounts of clay. The silt, sand and clay should be in the proportion of 64%, 32% and 4%. In Taiwan, a system of canals is provided on the outer boundary of the ponds to give protection to fish against extreme conditions of temperature. About two months prior to stocking, the pond management begins for augmenting the growth of blue-green algae in pond bottom. These are similar to those practiced in nursery pond viz., draining of the water, drying, tilling, leveling and raking. Manuring of the ponds is always carried out. Usually, green manure is used, such as leaves and twigs of mangrove plants, rice straw, copra, rice bran, oilcakes, pig manure, chicken manure etc. The rate of fertilization usually varies from 500 to 2,000 kg/ha. Beside organic manure, inorganic fertilizers containing nitrates, phosphates and potassium (NPK) such as superphosphates, triple superphosphates, urea etc may also be applied at required rates. Water is then let into the pond and the depth is increased to about 30 cm gradually. Within two weeks the algal-periphyton complex (Lab-lab) develops at the pond bottom. Stocking in earthen pond follows only after the growth of Lab-lab. Usually fingerlings of 7 to 15 cm length are stocked at a rate of 2,000 to 10,000 per ha. When the growth of Lab-lab in the pond decreases, fresh dosages of manure are added. Farming in pens Inexpensive pen structures for farming the milkfish are constructed in shallow natural creeks, swamps, lagoons, lakes and bays, ranging in depth from 1 to 3 m. The bottom in pen culture sites should be of firm clay or mud so that poles and posts can be driven sufficiently deep to make them support the pen structure. Traditionally pens are made up of wooden planks, split bamboo etc. But in recent times, nets materials made of synthetic materials such as nylon, polypropylene, polythene etc are used commonly. A part of the vertical net barrier is buried inside the mud or ground with the aid of a footrope and small weights, secured to a chain link between concrete sinkers. At the upper level, floats are provided. Fingerlings stocked in usually feed upon the natural food in the lagoon or lake and no artificial food is provided.
Eradication of predators and pests Certain predatory fishes, crabs, water snakes are the common threat to milkfish culture ponds and pen structures. These can be prevented from entering the farming area through use of fine meshed screens. Apart from these larvae of certain insects also pose threat to fingerlings of milkfish. Application of 0.5 ppm Baluscide, lime, urea etc. in the initial preparation of the ponds helps to reduce their growth. Harvesting Milkfish has a higher growth rate in its first year in brackishwater, during when it grows to a marketable size of 30-45 cm long and 300-800 gm in weight. The periodicity of harvesting depends upon the number of batches stocked. During harvesting the pond is drained using pumps, while in the case of pens, the lowest tidal period is the best time for harvest. If trenches were provided in culture ponds, it would be easier to gather all the fish inside the trenches by draining the water and then capturing them. Usually, seine nets are operated for capturing farmed fish. The survival rates ranges from 80 to 95% amounting to a production ranging from 500 to 1000 kg/ha in ponds and 250 to 500 kg/ha in pens. (Source:www.vuatkerala.org) |
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