TYPES OF CULTURES

Air – Breathing fish culture

Murrels and cat fish are known for their esteem and good market demand owing to their low fat content and few intramuscular spines. The air breathing fishes are hardy and capable of breathing atmospheric air with their accessory respiratory organs. Due to the presence of these accessory respiratory organs these fishes can survive for few hours out side the water. These accessory respiratory organs are respiratory trees in Clarias, labyrinthine organ in Channa, air bladder in Heteropneustes, branchial chamber in the above fishes, etc. and are capable to engult air. These can be cultured in areas of low dissolved oxygen such as shallow foul waters, derelict ponds and swamps. Due to their ability to live out of water, their culture involves low risk and simple management.

In India, Andhra pradesh, Assam, Uttar pradesh, Madhya Pradesh, Tamilnadu, Karnataka, Maharastra, Bihar and Meghalaya support the most significant natural fishery of air breathing fishes. These fishes are carnivorous in nature and they adopt excellently to supplementary feeding. As there is not much wastage of energy through respiration by the growing air breathers of shallow waters, good yields could be expected.

The culturable species of air breathing fishes are Fig. 9.1

Channa straitus – Big or striped murrel or snake head fish

Channa punctatus – Spotted murrel

Channa marulius – Giant murrel

Clarias batrachus – Magur

Heteropneustes fossilis – Singhi

Anabas testudineus – Koi or climbing perch.

Out of these Channa striatus has highest demand in the markets and is also commands a higher price. Next best are Clarias and Heteropneustes. The culture of the above species are profitable.

a) Channa marulius b) Channa straitus c) Channa punctatus

Culturable areas

The culture of air breathing fishes needs shallow waters with a depth of 50 – 75 cm. Ponds for air-breathing fish culture need not be fertilized by chemicals. Air breathing fishes may also be cultured in cages in running water systems like streams, canals and unmanageable waters like reservoirs. The air breathing fish culture is equally adaptable in waters unsuitable for conventional culturable species of carps as well as in carp culture ponds. Shallow ponds are useful for fishes, in which the fish has to spend less energy in travelling to surface for intake of atmospheric oxygen.

Seed collection

The seed of murrel, magur and singhi are collected from the natural resources, inspite of success achieved in induced breeding. Even today, seed collected from nature continues to be the most dependable source of material for stocking. Murrels attain maturity in two years are known to breed throughout the year. The fry of 2-4 cm can be collected all round the year and from rainfed ditches and shallow water bodies with abundant weeds. However peak spawning is known to occur just before the monsoons.

a) Clarias batrachus b) Heteropneustes fossilis

The young ones emerging from the eggs move in shoals and their collection in large numbers is always easy. The fingerlings may not tend to move in shoals. Fry of giant murrel can be identified by their dark grey body and a lateral orange yellow band from eye to the caudal fin. Fry of stripped murrel have bright red body with reddish golden band and a dark black band from eye to the caudal fin. The spotted murrel fry can be recognised by their dark brown body with a golden yellow lateral band and a mid dorsal yellow line on the back.

In murrel culture, it is better to stock fingerlings rather than the fry. Cannibalism is found in murrel fry. The survival rate of fry which produced by induced breeding will be poor and to maintain the spawn and grow them to the fry stage is difficult. The spawn do not eat anything for two days after their emergence from eggs. Hence, the fry should be trained to accept supplementary food in separate ponds. The supplementary feed consists of boiled eggs, silkworm pupae, minced trash fish and worms along with yeast and vitamin B.

It is given for about 15 days at the rate of 20 % of their total body weight. The fry reach the fingerling stage of 4-6 cm length within a month.

The cat fishes breed twice in a year with the peak breeding season during rainy season. Magur fingerlings can be identified with their longer dorsal fin and slate colour. Singhi fingerlings are having a short dorsal fin and pink colour. Koi fingerlings can be identified by the dark spot on the caudal peduncle and greenish hue on the dorsal surface of the body. The magurs make a hole of 25 cm depth in the bund below the water surface. The fertilised eggs adhere to grass and are guarded by the males. 2,000 – 15,000 fry can be collected from each hole with the help of small fine meshed hand nets and reared in nurseries until they reach fingerling stage with about 5 cm in length.

Magur can be cultured in ponds for the production of fry. 1 X 1 m compartments of wire screen are made on the margins of the bund. At the centre of each compartment, a hole of 30 cm diameter is dug and is provided with few aquatic plants. After releasing both the sexes, about 5,000 fry can be collected from each compartment within 10 days. The males and females can also be reared in small earthern ponds. They can be stocked 20,000 / pond and fed either with filtered zooplankton or chopped fish meal and ground nut oil cake. The fry can be reared for 15 days in nurseries.

The peak season for the collection of seed of singhi is pre-winter period when paddy is harvested and the low lying fields get exposed.

Seed transport

The fry or fingerlings of air breathing fishes can be transported without oxygen packing. Polythene drums or iron drums are used for transport of fry or fingerlings. The carrier must have enough of space for their habitual surfacing to breath atmospheric air. The carrier should have a small amount of aquatic weeds like Vallisneria, Hydrilla, Myriophyllum and Ceratophyllum. The weeds may help to avoid jumping of the fish during transportation. If the distance is more, it is better to transport them in oxygen packed polythene containers.

Pond management

Nurseries are about 10 – 15 m2, having a water column of 50 cm. These are stocked with 0.2 – 1.5 million fry / ha. Prior to stocking, manuring is done with raw cattle dung at the rate of 500 Kg/ha alone. The soap – oil emulsion to eradicate insects is applied to the nursery water. Fry and fingerlings of magur and singhi collected from natural resources require nursery management, but murrels have to be trained in nursery ponds before stocking. After nursery management the fingerlings are to be transfered in stocking ponds.

Stocking

Uniform sized fingerlings are chosen for stocking. The fingerlings are disinfected with 2 % KMNO4 solution for 5 minutes or dipped in 200 ppm formalin solution for 50 seconds before stocking. Wounded fingerlings are treated with 0.3 % acriflavine for 5 minutes.

These fishes may escape through climbing or burrowing. Hence, the pond bunds should be firm with heavy log or wood, or fenced with bamboo cane or wire screens to a height of about 50 cm.

More fingerlings can be stocked in their culture system. 40,000- 60,000 systems.

fingerlings/ha of cat fishes can be stocked in monoculture In polyculture systems 20,000 – 30,000 fingerlings/ha of cat

fishes may be stocked. In monoculture systems, 15,000 fingerlings/ha of giant murrels, 20,000/ha in case of striped murrel and 20,000 – 30,000 / ha in case of spotted murrelare stocked. In polyculture systems, striped and spotted murrel may be stocked at a rate of 20,000 fingerlings / ha in the ratio of 1:1.

Polyculture of murrels – carps and catfishes – carps is also possible with proper care and management. The seed of air breathing fish should be stocked only when the carps have grown to a minimum of 300 gr, so that air breathing fishes may not prey on the carps. With this, not only an additional income can be obtained through the yield of air breathing fish, but also the growth of carps can be enhanced. The later is possible, as the trash fishes which may compete with carps for food and space, are eradicated by the growing air breathers.

Feeding

To maintain an abundant food supply for growing air breathers, the stocking pond must be rich in animal food source like frog tadpoles and trash fish. If this food source is not sufficient tilapia may also be grown in murrel and cat fish ponds. Dried marine trash fish also used in fish culture and is more economical. Feeding can be given to catfishes with fish offal or slaughter house waste or dried silkworm pupae mixed with rice bran and oil cake in the ratio of 1 : 1 : 1 : 1. A mixture of oil cake, rice bran and bio-gas slurry in the ratio of 1 : 1 : 1 has provided successful low cost feed for singhi. Rice bran and poultry feed in 3 : 1 and biogass slurry and rice bran in 1 : 2 also be given at the rate of 5 – 8 % of body weight.

During the eight months semi-intensive culture in stagnant ponds, the air – breathing catfish stock may be fed at the following rate daily during dark hours of the day to obtain better feed utilisation (Table 9.1).

The feed may either be broadcast in the pond in small amounts from the bund or may be served in feed baskets lowered near the bank in addition to broadcasting of feed to ensure availability of feed to all the fishes in the pond. Light traps can be installed in murrel ponds, by which the insects may be attracted by light and utilised by murrels as a protein-rich food.

Trained murrel fingerlings will also accept cheaply dried marine trash fish soaked in water, which may be provided as per the following feeding schedule (Table9.2). Slaughter house waste and silkworm pupae as a of source animal protein can also be used.

Growth and production

Murrels and cat fishes attain marketable size in a period of months respectively. If the management practices are proper, giant and striped murrels can attain a growth of 1 – 2 Kg/yr. and 0.75 Kg/yr. respectively, whereas spotted murrels grow to 160 gr. in 8 months. Cat fishes are known to grow slowly when compare to murrels. Magur and singhi grow to 0.2 Kg and 0.1 Kg respectively. The conversion rate with recommended feed is approximately 2 : 1.

Murrels with forage fish as supplementary food yield about 4 tonnes/ha/yr. Magur with dried trash fish and rice bran supplementary feed, give the production of 10 tonnes/ha/yr. Singhi give an yield of 4.4 /tonnes/ha/yr. Polyculture of murrel and koi, fed with rice bran, mustared oil cake and trash fish, give a production of 11.8 tonnes/ha/yr, while magsur and singhi fed with rice bran and trash fish give an yield of 5 tonnes/ha/yr. Mixed culture of 3 species of murrels produce 4 tonnes/ ha/yr when fed with soaked and dried marine trash fish and fresh silkworms pupae as food . In the intensive culture magur can give 7 tonnes/ha/5 months.

Culture with carps

With a stocking density of 5000/ha of Indian and Chinese carps and 1000 magur fingerlings produce 2518 Kg/ha/yr of carps and 3711 Kg/ha/yr of magur. This indicates that the polyculture is more profitable, and it is useful to include magur in the carp culture system. With a stocking density of 20,000/ha of magur along with left over carps (after partial harvesting of carps) production of 3.96 tonnes/ha/yr is obtained with 50 : 30 : 17 : 3 ratio of rice bran, fish meal, groundnut oil cake and minerals as supplementary feed. The magur is found suitable for composite fish culture of carps in place of common carp. Magur, koi and singhi are also suitable to culture along with a highly priced carp makhana, Euryale ferox.

Harvesting

Summer season is ideal for harvesting air – breathing fishes from ponds. The pond is drained and the fishes are harvested with the help of scoop nets or hand nets. Due to their high demand and market price, the culture of these air – breathers provide profitable income to fish farmers with simple management techniques.

Cage culture

The air – breathers can be cultured in cages also. The cages are prepared with mats made up of split bamboo in running waters. The optimal cage area measures 2m X 1m X 1m in size. The top of the cage is half covered with mat and the uncovered part is covered with a net to facilitate feeding and to prevent escape of fishes. Synthetic fibre mesh is also used to prepare cages.

Magur are stocked at a rate of 200/cage, fed with 10% of body weight on dried trash fish, oil cake and rice bran and produce 10 – 12 Kg/cu.m./yr. Singhi produces 12-20 Kg/cu.m./yr with a stocking density of 100 – 150/cage and 10 % of body weight feed of silkworm pupae, rice bran and mustard oil cake. Koi produce 4.2 Kg/cu.m./yr with a stocking rate of 50 – 100 /cage with the same food as singhi. Spotted murrel produce 4 Kg/cu.m./yr with trash fish and rice bran. Hence, the air breathing fish culture is highly proftable, as well as a rich source of animal protein. This fish is considered as a delecacy, and commands a very high price and continuous demand in the markets.

TROUT CULTURE

Trout is either grown as a food fish or sport fish, are released into natural waters for sport fishermen. Trout is popular because it is an attractive, active fighting fish and provides very high quality meat. Trouts have been released and cultured in water all over the world. Trouts have been grown on a commercial scale in USA since a very long time. Its culture in Europe dates back 400 years. It is a cold-water fish. It mainly inhabits rivers, streams, brooks, lakes and ponds. In India it is found in Kashmir, Himachal Pradesh, Uttar Pradesh, Nilgiris, Kodai hills and Munnar high range.

Many species of trout are grown, but the three most common of them are the rainbow trout, Slamo gairdneri or Oncorynchus mykiss, the Eurorean brown trout, S.trutta (Fig. 9.2)and the brook trout, Salvelinus fontinalis. Trouts have a streamlined body, narrow gill openings and reduced gills. Trouts are adapted to highly oxygenated waters and freezing point temperatures. Trouts have great power of locomotion with clinging and burrowing habits. Mouth is modified with rasping lips for food collection from pebbles, rocks, etc..

Spawning

The spawning season of S.gairdneri is from September to February, S.trutta is from October to December and S.fontinalis is from October to January. Trouts prefer gravelly substratum to safeguard their eggs and the eggs stick to gravel and debris. Trout build nests and spawn in streambeds. Culturists allow artificial fertilisation, because streambed fertilisation results less hatching rate than artificial fertilisation. Manipulation of the photoperiod and water temperature can be used to induce gonadial maturation, so that young fishes are generated throughout the year. Trouts are caught at or near maturity as they are swimming upstream and raised to maturity ro ponds. The brood fish are placed in small ponds with flowing water and are often covered with netting to prevent them from jumping. The milt of a single male can be used to fertilise two females, so that more females are stocked with few males.

Trouts exhibit sexual dimorphism. Males become more brightly coloured and the lower jaw develop a hooked beak during the breeding season. Females develop extended bellies and the genital papilla becomes larger and reddish. When they are fully matured, milt or eggs comes out with little pressure on the abdominal vent. When the trout is ripe, the female fish are stripped and eggs collected in a black coloured enamel or plastic container to which the milt of the male is added and mixed thoroughly with a quill feather for fertilisation. Water is added after mixing and the water causes the eggs to swell. Water should not be added before the mixing, since motility of the sperm is greatly reduced in the presence of water. To ensure a better survival rate, the eggs may be collected in a small quantity of saline solution (10 lit. fresh water +90 gr. common salt + 2 gr. potassium chloride + 3 gr. calcium chloride). The fertilised eggs develop a green tinge and are known as ‘green’, which are then transferred to hatcheries. Before transferring remove the foreign particles and dead eggs.

Transportation of trout eggs

The fertilised and hardened eggs (hardened for 24 hours) of trout are transported in cardboard cartons of 20 X 30 X 20 cm size. The inner side of the card board box is lined with styrofoam lining. Two moist sponges or cotton pads are arranged, one at lower side and other at upper side. Porous polyethylene bags containing about 4,000 eggs are placed in between the moist sponges and cotton pads. A polyethylene bag with IKg broken ice is kept for maintaining low temperature, above the upper pad. These cardboard cartons are transported to various places.

Hatchery techniques

The trout eggs are incubated by keeping them in concrete troughs with flat and horizontally arranged trays, incubators or jar. Hatcheries should be provided with circulating filtered and silt – free freshwater. In olden days baskets were used for incubation. Vertical flow incubators are the most common. It has many stainless steel, of fiberglass, aluminium, or wood, or PVC, or plastic trays, arranged one above the other. The bottom of the trays are provided with perforated zinc sheets, glass grills or mesh cloth for ensuring the passage of water through the different trays. The size may vary from 180 X 30 X 10 cm to 500 X 100 X 50 cm. Each tray has an upper egg basket and a lower perforated compartment on which basket rests. The eggs are placed in the basket for incubation. The water is introduced to the tray in such a way that it flows up through the basket containing the eggs, then down to the tray below and up through that basket and so on through the incubator. This upward flow of water through the eggs allows increased aeration and facilitates removal of metabolites.

Hatching jars are also used for the incubation of trout eggs. It consists a galvanised screen of 0.5 mm mesh with gravel bed at the bottom, just above the inlet. This gravel bed is useful as filter to remove the unwanted particles. The eggs are placed above the filter for hatching. Water passed through the inlet, upwells through the filter and eggs and drains through the outlet. After hatching, the hatchlings are maintained for some time in the jars.

The eggs are highly sensitive during the hatching period. Newly fertilised eggs can be killed if directly exposed to sunlight. During incubation, water must be moving and have a high oxygen content. Incubation normally takes place in water with 8° -12°c temperature. The fry can be held in the trays until they become active and are able to begin to feed. They can be released for stocking in natural waters.

Culture of trouts

The fry are reared in small rearing troughs before they have completely absorbed their yolk sac, and introduce to live on artificial feeds. Then they are transferred to nursery ponds for rearing to advanced fry stage. The nursery ponds may be concrete or stone-walled with 2.5 X 1 X 0.75 m to 9 X 1 X 0.75 m size. The water flow may be maintained 100 lit/min. inside the nursery pond.

The advanced fry are reared to adults in rearing pond and raceways. Rearing pond is a natural body of water, and a raceway is merely a running water fish pond. The size of raceways should range from 20 -100 m2 with a depth of 1.5 m. A series of raceways are constructed either side of the stream or river. Each raceway gets water from stream and water goes out of the raceway through the outlet which is found on the opposite side. Zinc plate screens are used at inlets and outlets. The water flow is maintained 50 lit/sec, into the ponds from river. Circular and oval ponds are used in USA. The stocking rate may be limited to produce 5-10 Kg/m2. High production of 200 Kg/mis also possible in raceways, if management is good.

Cage culture of trout is also common. In an experiment, fingerlings were stocked at 1.4 Kg/m2 in cagesand fed 3 % of their body weight daily. These trouts grew to 27-88 gr. in two months. The feed given to trouts includes cattle spleen, heart and lung and marine or freshwater trash fish. Many commercial trout feeds are available in the market. Trout are fed 3 – 4 times daily. There are number of ways of giving feed to trouts. The feed is either sprayged on the surface of the water, or the feed can be kept in a bag or in a container in the corner of the pond. It is used for the demand feeders, in which whenever a trout bumps into the trigger the feed is released into water, or automatic feeders can also be used. Jars and drums are also used for rearing trout fry.

Sewage – Fed Fish Culture

Sewage is a cloudy, dirty and odorous fluid from our toilets and kitchens of our houses. It has minerals and organic nutrients in a dissolved state or dispersed in a solid condition. Disposal of sewage has become a global problem because of urbanization. It is an effect of demophora, i.e. an unabated growth of human population. In recent years, sewage has become a major pollutant of inland waters, especially rivers. It is a source of many epidemics. It is responsible for a serious threat to soil and water ecosystems. The approach towards waste water disposal should be utilization of this residue with the concept of their reuse or recycle through an ecologically balanced system involving mainly aquaculture. The utility of sewage effluent to enhance fertility of freshwater ponds has long been known in many countries of the world.

The amount of sewage produced is India in 3.6 mm3/d (million cubic meters per day) or 800 mg/d (million gallons per day). About 30% (1.9 mm3/d) is produced at urban centers. Only 1.3 mm3/d (20.4% of India’s one-day total) is treated at these centers. Nearly 80% of the country’s one day total still remains to be treated and utilized. The amount of manure obtained from one-day production of sewage in India is about 0.126 m.tonnes. This is equal to 46 m.tonnes/year. The manure from one-day sewage is enough to cultivate 0.1 m.hectare of annual crop of fish. Sewage is also useful to cultivate fishes. In India only 130 plus sewage-fed fish farms are found covering an area of 12,000 hectares. The Vidyadhari sewage-fed fisheries near Calcutta is an example, where fishermen have taken full advantage of the sewage disposal systems of Calcutta. Here the fish yield is about 1,258 Kg/ha. The high manurial capacity is combined with the potentiality to serve as an additional source of water for fish culture and enhance the fish production.

Composition of sewage

The composition of sewage varies from place to place and according to season. Water is a major component of sewage (99%) and the solid suspension in sewage amounts to 1% only. On an average the sewage of Indian towns contains 52 ppm nitrogen, 16 ppm phosphorus, 45 ppm potassium and 350 ppm biodegradable organic matter. The organic carbon component is 25-40 ppm, the ratio of carbon and nitrogen being 1:3. Salts of several heavy metals such as Zn, Ni, Cr, Pb, etc. are also found above the permissible levels in sewage. The organic refuses in the sewage have proteins, carbohydrates and fats in varied proportions depending on the nutritional status and food habits of the population. Among carbohydrates, readily degradable starch, sugar and cellulose are detected.

Some ecological features of different waters are mentioned in Table 9.3. Sewage water has high BOD (Biological Oxygen Demand) and Oxygen Consumption (OC) values. Dissolved oxygen becomes depleted in sewage water due to high oxygen demand and low photosynthetic rate. Photosynthesis is low because of poor illumination as the suspended solids in sewage water obstruct sunlight. On an average, strong, medium and weak sewage consist of 1200 ppm, 720 ppm and 350 ppm of total solids respectively, out of which 850 ppm, 500 ppm and 250 ppm occurs in a dissolved state and 350 ppm, 220 ppm and 100 ppm is found in suspended form. Dissolved salts being very high in sewage water, manifest high specific conductivity. Production of acids in high amounts render the water acidic, making the medium unfit for supporting life (Fig. 9.3). Acidity of water below pH4 is known to kill the flora and fauna.

Sewage enriches water with organic matter that begins to decompose aerobically thereby depleting dissolved oxygen and leading to anoxic condition. Anoxia causes non-mortality of animals, adding organic matter further to the already rich organic content. In the absence of dissolved oxygen the organic matter undergoes anaerobic decomposition as a result of which obnoxious gasses like H2S, CH3 and CO are produced. These gasses besides being toxic, react with water to form acids.

Immediate effect of sewage on the biota is eutrophication. Sewage water stimulates rapid growth of phytoplankton leading to an algal bloom followed by rapid increase in zooplankton. For utilizing sewage in aquaculture, the properties such as the concentration of dissolved and suspended solids, organic carbon, nitrogen and BOD are essential.

Microbiological charactaristics

Harmless and even useful non-pathogenic bacteria are present in much greater numbers in domestic sewage as compared to pathogenic bacteria comprising mostly the intestinal microorganisms found in the community producing the waste. Usual load of coliform bacteria in raw sewage ranges between 108 and 109 MPN/100ml.

Site selection and construction of sewage-fed fish farm

Fish farm in the vicinity of an urbanized area has the scope to receive domestic sewage for the recycling of nutrients. Any area adjacent to a municipal sewage treatment plant is ideal for the location of a sewage-fed fish farm. The fish farm site should be at a lower level than the treatment plant so that the sewage can easily enter into the pond through a pipeline by gravity. The fish farm should have facilities of draining out water from the ponds.

The plan of the fish farm depends upon the source of the sewage, system of culture and topography of the land. Nearly 75% of the total area is converted into ponds leaving the rest for dykes and other purposes. Rectangular fish ponds of 0.3 to 1 ha are constructed with a slope of 1:3 for the embankment and maximum depth of 1.5m. Each pond should have proper drainage facilities.

The effluent is collected in a sump at the farm, from where the effluent is taken into the ponds through the distributing system. Additional arrangement is made to connect the pipelines with freshwater supply for emergency dilution.

Sewage treatment

Sewage treatment is necessary to kill the harmful microbes, prevent anoxia, raise the pH to an alkaline level, increase photosynthesis, reduce organic content, etc. The treatment has to be inexpensive and one which induces in sewage water the conditions prevailing in a natural freshwater pond. Sewage is treated in following three ways – mechanical treatment, chemical treatment and biological treatment.

Mechanical treatment:

Solids and organic matter are removed to a large extent by mechanical treatment, which involves flowing, dilution and sedimentation. Usually screening and straining of sewage it is done to remove the waste solids. The liquid and semisolid wastes are then

subjected to treatment for the removal of colloidal and semisolid suspension by dilution, H2S, CO2, CO, NH3, CH3 concentrations are brought below the normal levels. Thus, through primary treatment the supernatent effluent is separated from the sludge.

Chemical treatment:

In chemical treatment, several dissolved substances, harmful germs and aggressive odours are eliminated. Inexpensive precipitants,

coagulants, chelating substances, disinfectants, deodorising agents, etc. are used in this treatment. The sewage water is also treated with chlorine, bleaching powder and copper sulphate. It is also known as secondary treatment.

Biological treatment:

In biological treatment of sewage care is taken to promote bacterial growth. Bacterial action promotes oxidation of organic matter. The end products nitrogen oxides, bring about rapid growth of algae, particularly the blue green Microcystis. This arrests anoxia of water by raising the dissolved oxygen, lowering the CO2 content and by increasing the pH from acidic to alkaline levels. The algal bloom reduces the concentration of dissolved salts in the sewage water.

Pond Management

Fertilization

Fertilization of sewage-fed pond is done in two phases, pre-stocking and post-stocking fertilization. In dewatered and sun dried ponds, primary treated sewage effluent is taken up to a depth of 60 – 90 cm during premonsoon months (April – May). The effluent is then diluted with rain water or freshwater till the pond BOD reduces to 50 ppm. Periodic fertilization with sewage effluent is carried out after two months of stocking to maintain nutrient status and productivity of the pond at a desired level. The quantity of sewage effluent to be allowed into a pond solely depends on its quality determined on the basis of BOD values.

Liming

Application of lime in sewage-fed ponds is most essential. It is a useful promotor of fertility in ponds and act as a disinfectant against harmful microorganisms. Prestocking liming is recommended at a rate of 200 – 400 Kg/ha as initial dosage. Subsequent liming of 150 – 200 Kg/ha on standing crop is necessary throughout the year during sewage intake and during winter months, when parasitic infection is more.

Stocking

The cultivable species of freshwater fish such as Indian major carps and exotic carps can be grown in sewage-fed waters. Considering the high carrying capacity and high productivity of sewage-fed ponds with respect to plankton and benthic fish food concentration, fish are usually stocked at a reasonably higher density. The stocking rate recommended 10,000 – 15,000 /ha of carp fingerlings of about 10 gr. each and it is preferred to stock more of omnivorous scavengers and bottom feeders to maintain fish pond hygiene for higher yield. The ratio of carps for better output is rohu 2.5 : catla 1: mrigal 2.5 : common carp 2 : silver carp 2. Omnivores and bottom feeders directly consume the organic detritus of sewage-fed ponds, and thereby directly helping in keeping the pond aerobic. The stocking rate of fish is kept on a higher side considering the profuse growth of algae which will otherwise grow, decay, putrify and finally deplete the oxygen concentration of fish pond.

Ecological considerations and algal control

Maintenance of aerobic conditions of the sewage-fed pond is highly essential and as such early morning dissolved oxygen level should not deplete below 2 ppm for carps. The BOD should be below 30 ppm for better survival of fishes. CO2 concentration should not be allowed to increase beyond 20 ppm to keep the toxicity level within tolerance limit for fish and to control algal blooms. Liming helps in regulating CO2. Heavy metal pollution, if any, can be controlled by introducing water hyacinth at the pond margins and barricading them with bamboo poles to prevent spreading of the weed throughout the water surface of the pond.

Algal control is a must to maintain proper dissolved oxygen. It should be more than 2ppm and optimal 5 – 6 ppm in a sewage-fed pond. The presence of silver carp regulate the algae in the culture system. When biological control of algal bloom is not possible, application of simazin at rate of 0.5 – 1 ppm is recommended.

Control of aquatic insect

Aquatic insects are found in sewage-fed ponds, especially more during winter months. The insects of the pond mainly comprises hemiptera, coleoptera, odonata, zygoptera and trichoptera. Dipteran insects dominate, especially the larval stages of Chironomids associated with annelid worms of tubificidae.

Other insect larvae of the sewage-fed ecosystem belong to tubanidae, anthomyiodae, tetanocoridae, etc. The predacious hemiptera, coleoptera and a few odonata, zygoptera are needed to be controlled. An emulsion of soap and vegetable oil at a rate of 4 Kg/ha and in the ratio of 1:3 is applied to control these insects.

Harvesting and yield

After 5 – 6 months culture, when the biomass grows to an optimal level, the stocking density is thinned out through periodical and partial harvesting. The water depth of the pond is reduced by dewatering for final harvest when the fishes are removed by repeated drag netting.

In a mixed culture of five carp species in sewage-fed ponds, the yield rate varies from 5.4 – 8.6 t/ha/yr with an average production of 7 tonnes/ha/yr. The fishes are around 500 gr. to 1000 gr. during culture operations.

The recurring expenditure on sewage-fed fish culture is meagre compared to that of fresh-water fish culture. This culture is lucrative and a fish farmer can obtain an income, on an average of more than Rs. 40,000 /ha/yr. If murrels are cultured in oxidation ponds and the excess sewage is utilised for the cultivation of crops, the revenue could be further augmented.

Full utilization of sewage has manifold benefits. Outbreak of epidemics can be prevented. Biogas from sewage can be used as fuel to ease the pressure on LPG, electricity and fuel wood. Slurry from biogas plants can be used as a manure. Water reclaimed from sewage can be recycled for irrigation and pisciculture. Besides, scientific handling of sewage generates employment opportunities to educate youths. More than all these water bodies, rivers, particularly can be saved from sewage pollution by proper management.

Utilisation of Biogas Slurry for fish culture

In our country, especially in rural areas, mere has been a tremendous growth of biogas plants as a source of non-conventional energy. Biogas is also called as gober gas. The biogas plant is a device for conversion of fermentable organic matter, especially cattle dung into combustible gas and fully matured organic manure or slurry by anaerobic fermentation. The nutrients of the generated slurry can be harvested for production of feed and food and replace conventional inorganic fertilizers. Due to lack of knowledge and communication to farmers, most of the generated slurry is not used properly. The biogas plant can also digest night soil, poultry and piggery droppings, weeds and other fermentable materials along with cattle dung. Biogas slurry consists of 1.52 mg/lit nitrogen, 0.82 mg/lit of phosphorus and 0.83 mg/lit of potash. Biogas slurry is rich in humus and contains nutrients mostly in the available form. The oxygen demand for its decomposition is much less than for raw cattle dung or any other organic manure. Due to the high nutrients value of biogas slurry, it can be used as a fertilizer in fish culture ponds. Slurry application improves the soil structure. It enhances zooplankton production in water.

Gober gas plant is a composite unit of a digester and gas holder. Gas holder floats on the top of digestor, wherein gas is collected. In the plant, the whole system is based on continuous operation. The organic manure to be fermented is fed in semi-fluid form at the one end and the fermented spent slurry is extracted at the other end periodically with disturbing the whole system. Slurry is odourless, free from flies and other sources of infection.

In a preliminary experiment, the slurry from plant is drained into a fish pond of 0.15 ha area, which is stocked withrohu, catla, mrigal, common carp, silver carp and grass carp at a density of 7,500 fishes/ha, resulted in production of 5080 kg/ha/11 months (762 kg/ha/0.15 ha/ 11 months).

This experiment indicates that the high production potentiality of the pond using only biogas slurry as fertilizer. In Madurai Kamaraj University, the experiments conducted with Oreochromis mossambica by using only biogas slurry as fertilizer and found the enhanced production. They indicated that males grow larger than females. They got the production of 2.4 tonnes/ha/125 days with a stocking density of 30,000 juveniles/ha and initial size of O.Sgm. They also got 4.4 tonnes/ ha/125 days with a stocking density of 60,000 juveniles/ha and initial size of 0.5 gm.

In a polyculture experiment with Indian major carps at ratio of 4 rohu: 3 catla : 3 mrigal at a density of 5000/ha by using only biogas slurry (0.15% concentration every three days) as feed and fertilizer resulted 5500 kg/ha/yr. The fishes grow well with only slurry, without any supplementary food and other fertilizers, this reduces the cost of feed and fertilizer. But there is little chance of microbial attack, it can be controlled with good management. In an experiment at ANGRAU with biogas slurry in different dosages – 5000, 10,000 and 15,000 kg/ ha/yr applied in different fish ponds 1/3 of the slum’ was applied initially and the remaining slurry was applied in equal fortnightly instalments. Catla, rohu, mrigal, common carp, silver carp and grass carp were stocked at a ratio of 2:2:1:1:2:2 at the rate of 5000 kg/ha. The production was obtained was 1956. 2096, and 2052 kg/ha/yr in 5000, 10,000 and 15,000 kg/ha/yr biogas slurry treated ponds without any supplementary feed, or organic and inorganic fertilizers. The fish production obtained was 5470, 7230 and 6050 kg/ha/yr in the above three slurry treated ponds with supplementary feed, but without organic and inorganic fertilizers. Supplementary feed was given in the form of rice bran and groundnut oil cake in the ratio of 2:1 at the rate of 5% body weight of fishes.

The experiments indicate that high production offish in biogas slurry treated ponds and at the same time the expenditure is lesser than normal culture systems because organic and inorganic fertilizers and supplementary feeds are not used. By using the waste of biogas plant in the form of slurry, profitable fish production can be obtained. Fish produced through recycling of organic manure is more healthy and has less fat accumulation. The recycling system, however, requires effective management. One of the problems is the difficulty in balancing the expertise needed in fish animal husbandry. Over concentration on one system may be detrimental to the other. The monitoring of dissolved oxygen level in pond water is absolutely essential when the integrated systems are adopted. Excessive manuring causes water pollution. It rapidly decreases oxygen level in the water, produces toxic gases like ammonia often leading to fish kills. Application of manure should be regulated according to the dissolved oxygen level which is very essential for the rapid growth of fishes. No serious health hazards due to slurry was noticed, though animal excreta is a potential source of infection. Moreover, fermentation of the manure in a biogas plant kills and destroys the eggs of parasites.

Cage and Pen Culture

Cage culture

Fish culture in ponds is the primary method of freshwater and brackish water fish culture. However, there are other methods of fish culture used in places where pond culture is not possible. Other methods of fish culture are those carried out in dams and reservoirs, cages, pens and rice fields. Due to exponential growth in population and the great pressure on land for habitation and agriculture, the large water resources such as tanks, lakes, reservoirs and canals, which have been not exploited so far can be used for augmenting fish production. Due to the large water bodies, the management has complex problems. The best thing seems to be captive, regulated culture of suitable fishes in impoundments installed in them.

A practical approach to increase the aquaculture production could be takeup as fish husbandry in cages, pens and other enclosures in large water bodies like tanks, swamps, lakes, reservoirs and canals along with open ranching, without prejudice to their other use. By virtue of the short gestation period, these unconventional systems yield quick results with minimum conflict of interaction on land demand with agriculture and other animal husbandry practices. Enclosure aquaculture can play a significant complementary role in augmenting yields from our capture fishery resources, especially those having large predatory fish population.

Cages and pens could be utilized as nurseries for raising fish seed and for the grow-out of table fish. They dispense with the need for land based nursery forms cutting down on the cost of seed production. Investment on long distance transport of fingerlings for stocking reservoirs and handling mortality can be avoided by insitu rearing of fry in cages and pen installed in them. One of the impoundment cultures is in cages. Many countries are practicing cage culture of fishes and prawns successfully. Cage culture has also been started in India only recently.

Advantages of cage culture

The advantages of cage culture are

Large water bodies could be utilized better for fish culture.
The flowing water could be better utilized for fish culture.
Cage culture reduces demands on prime agricultural land for fish farm construction.
Free exchange of water.is possible in cages.
High density stocking and intensive feeding of the stock can be achieved, which gives high yield per unit area.
Decomposition and degradation of concentrated waste products do not arise in cage culture.
Oxygen depletion can not be found in cages. Monitoring growth of the stock, diseases is easy.
Considerable reduction or extreme compactness in the production area is thus achieved in cages.
Several units of cages could be installed in a water body for gainful employment and income.
Harvesting is simple and easy.
Considerable indirect employment will be generated.
With ca’ge culture, the animal protein production can be increased.
The left over feed, faecal matter and metabolites enrich the water body in which cages are installed.

Location of cages

The ideal location for cages is weed-free shallow waters. Flowing water is best for cage culture. The site should have adequate circulation of water. The wind and wave action should be moderate. The water should be free from pollution and weeds. The area should be easily accessible. Cage culture can also be practiced in areas like swamps where there is water not being used for any other purpose. Seed should be available in the vicinity. A ready market for fish should be available near the site. Flowing waters with a slow current of 1 – 9 m/minute’lare considered ideal for cages. The cages should be a little away from the shores to prevent the poaching and crab menace.

Types of cages

Cages can be circular, cubic and basket like and the shape has little effect on yield rate. Cages may be floating at the surface, just submerged or made to sit on the bottom. Floating cages may be the most appropriate for Indian conditions and the experiments conducted in our country for seed rearing, grow out, nutrition and biomonitoring have been in such enclosures. The size of the cage depends on the type of culture operation and the support facilities available. Large cages are difficult to handle. Although the cost of small cages is higher, handling is easy with low risk of losses. The nursery cages are generally of the floating type, while the ground cages may be floating or immersed depending on the species cultured.

Construction of cages

The type of material used for cages (Fig 9.4) will depend on the type of culture whether they are used for fry or table fish rearing. Bamboo interspred with wooden planks for cages is commonly used in Indonesia, Vietnam, Thailand and Kampuchia. Thick polythene fibers are used for cages in Japan. Metallic grills are used in—LISA. Aluminum frame and nylon webbing is used for fabrication of cages in USSR and West Germany. In our country, fairly fine mesh nylon netting are used. The cage material are used mainly depending on their cost and availability.

Small cages with mats of locally available plant materials such as palm leaves. Cyperus stem, Phragmites stem and split bamboo are used in India. These cages are of 1 – 2 m2 area. Split bamboos are joined with the help of coir rope or nylon twine. The cages are installed in the water body with bamboo supports at the four comers and the bottom. Materials other than bamboo mats are decayed by the third month and collapsed within a year. Split bamboo cages remain for over a year. Circular cages with thick bamboo stipes tied with nylon twine the durability of over 3 years.

Cages made up of monofilament woven material of 1 – 3 mm mesh size and 0.3 – 1 mm thickness are light and easy to handle, but remain for 6 to 12 months. The exposed part become brittle and gives way. Knotless nylon webbing of 3 – 6 mm mesh size and knotted nylon webbing of 7 -15 mm mesh have been found to be most durable. Cages made of water – proof surface painted light conduit pipe frames with a 10 m2 area are light in weight and have long durability. A battery of cages is enclosed with a bamboo catwalk and the whole structure floated by sealed empty barrels of 200 1. capacity.

The circular cages with conduit pipe structures which can be easily assembled have been designed with nylon webbing in different dimensions. These cages are floated freely on the water surface with the help of 3 – 4 sealed HDPP jerry cans. These arc extremely useful for cage culture. Due to their circular is shape the wave action in minimum. These can be moved from place to place with least water resistance. Due »their circular shape, the rearing space is maximum in side. The aeration and water circulation is better in these cages. Fishes can move in the cages with least obstruction.

Auto-floating, highly durable HDPP pipe frame nylon net cages with 36 m2 area are also used. These are light in weight and not need floats to float on the water surface.

The size of the cages depend on die scale of culture, species cultured, infrastructure, financial and management resources. The size varies from 2- 10m3 in India, 100 – 150m3 in Indonesia, 60- 180m3 in Kampuchia. 40 – 625 m3 in Vietnam and 30 m3 in Holland. Large cages are operated in Germany with 42 m diameter and 16,500 m3 at the water depth of 12 m. These are provided with automatic or water jet pump-feeding, special handling and harvesting accessories.

Calturable fishes in cages and their stocking

The fishes used for the cage culture should be adaptable to captive culture, fast gro\vng, hardy and disease resistant. The Indian and Chinese carps, tilapia and magur can also be cultured where trash fish is cheaply and abundantly available. In Thailand and Kampuchia the cat fishes, Pangasius species are being cultured in cages successfully. Koi and Singhi are also cultured in India in cages.

In India, the nursery cages are stocked with carp fry at the range of 150-700 fry/in2 in caaes with different materials. In Japan 15.000-62.000 fry/nr2 of grass carp fry are stocked in nursery cages. The common carp stocking density is 150/nr2 in Kampuchia, 133 -417/nv1 in Indonesia and 80 – 360/nr2 in Vietnam. In Thailand Pangasius sutchi, P. larmmdi and P. micronemus fry are stocked at densities of 150-300/ nr2 in cages of size 1-10 m2 area with a depth of 1.5m. .

The number of fish that can be stocked in a cage is variable and depends on the canying capacity of the water area, water quality and rate of circulation, the fish species, the quality and quantity of feed supplied. A safe level may be about 3000 to 6000 fish/ ha. In able – fish rearing cages in India, the fingerlings of carps are stocked at density of 30 – 38no /m2 . The tilapia, Oreochromis mossambicus can be stocked a rate of 100 – 200 m-2. Murrels can be stocked at density of 40-100m2.

Management and yield

The cage culture can be taken up in two phases – nursery phase and table – fish rearing phase. In nursery phase of cage culture, the spawn or fry are reared to fingerling stage in 2-3 months. Different feeds can be used for culture in nursery cages. Groundnut oil cake, rice bran, egg yolk, soyabean cake, soyamilk and soya flour are used as food for fry in nursery cages. The silkworm pupae are also tried as supplementary food.

The initial size of fish to be stocked in the cages will depend primarily on the length of the growing season and the desired size at harvest. The carp fingerlings for stocking in 16-20 mm mesh cages should be over 10 gr. to expect a final size of over 500 gr. within 6 months. It should be ensured that the fingerlings used for stocking are healthy and disease free. All the fish should be actively moving. It is ideal to stock cages in the cool part of the day.

In India, the growing season is almost year round, except for December – January in northern parts, where the temperature is low during these winter months. Very little natural food such as plankton, insects and various other organisms enter the cages and is available to fish. However, supplementary feeding is essential in the cage culture to get high production. The types of feed used will depend on the species cultured and their prevailing market prices. Murrels, for example, require to be fed with fish, shrimps or other animal matter. Most of the fish cultured are omnivorous and they accept both plant and animal byproducts such as oilcakes, brans, fish meal and silk worm pupae.

Cage fish are generally fed at least once daily throughout the growing period to get better growth. The quantity of feed to be given is important, since under-feeding will reduce growth and production, while over-feeding will waste costly feed and can affect the water quality. A method used to estimate the daily feed to be give in cages is based on the total weight of the fish. The feed is usually expressed on percentage of body weight. In carps, the feeding rate is 4 – 5 % of the body weight per day until they attain approximately 100 gr. And thereafter at 2 – 3 %.

In table-fish rearing phase, involving the high-tech system of saturated stocking and feeding on enriched formulated feeds, the production recorded in common carp is 25 – 35 Kg m° month’1 in foreign countries. The channel catfish, Lactarius punctatus in USA yielded a production of 20 – 35 Kg/nr3. In Africa, tilapia yielded 17 Kg/nr3and trout produced 15 Kg/nr3. The food quotient in these cultures varied from 1.3 – 2.1. In India, a production of 1.5 – 2.5 Kg nr: month’1 common carp was achieved with mixed feed of silk worm pupae, ground cake and rice bran. Catla yielded 1.4 – 2.7 Kg nr2 month’1 with groundnut cake and rice bran with the food quotient 5.6. Tilapia produced 1 – 1.6 Kg nr2 month’1 with a mixture of rice bran, groundnut cake and commercial cattle feed and food quotient ranged from 1.8- 2.3 . About 1 Kg nr2 month”’ of murrel and 0.3 – 1.5 Kg nr2 month’1 of catfishes, singhi and Koi are obtained.

Cage culture of prawns

The freshwater and marine prawns are also cultured in cages. The cages are stocked with wild or hatchery reared post larvae. Commercial scale rearing of post larvae in floating and fixed nursery cages (3.7 X 2.7 X 1.3 m) has been done with considerable success. They are fabricated from fine mesh (0.5 mm) nylon netting, supported by bamboo poles which are driven into the bottom of the water body. The optimal stocking density reported is 30,000 post larvae/cage (2 .310 m’3). Feed is provided in trays fixed inside the cages. Initially, the post larvae are fed on a paste of finely ground trash fish, later are fed with fresh mussel meat.

Pen culture

Recent results in the use of cages, pens or enclosures and recirculating water systems suggest some ways of compact intensification of production in aquaculture given the accessory inputs. This practice may provide great possibilities in the future in certain selected and suitable areas.

Aquaculture in open waters through the use of pens or enclosures is also a means of minimising the limiting effect of metabolities and pollutants on cultivated stock. Greater production in very limited space has been found possible under those situations. Production figures from these types of aquaculture environments approximates to 4 -10 t/ha/yr in Laguna lake in Philippines.

Selection of sites for pen culture

i) Low tidal amplitude

ii) Fish pen – site must be sheltered as much as possible against high winds

iii) Depth not less than 1 meter during lowest water level

iv) The best site is on the leeward side of the prevailing winds with moderate flow of current especially in a place where current in overturning

v) Water with stable PH slight variation is best. Avoid turbid and polluted water.

vi) Muddy clay and clay – loam soils are best types of bottom soil. Too much still and decomposing organic matter must be avoided.

Construction of pens

Pens can be constructed with the help of bamboo screens and nets

a. Construction of pens with bamboo screens

Split bamboo should not necessarily be shaped and rounded. They are soaked in water for two weeks and then dried for one week. During the soaking and drying period, bamboo poles are prepared and staked at the chosen site according to thedesired size and shape of the fish pen. After stacking poles, bamboo splits are closely woven extending to a length of more or less five meters and made into a roll. After weaving, these are set by stretching them from one pole to the other interrurned or just set inside or outside close to the poles from bottom to top. They are tied every pole by rubber and one provided with sliced rubber around, liming one on top and one at the bottom. These splitted rubber prevent them from wear due to wave action. Nursery nets which should be 1/16 th to 1/10 th of the area of the fish pen can be set before constructing the fish pen or after it is set.

b Construction of pen with nets

Construction of a fish pen made out of synthetic netting is easier than one made of bamboo screens. Netting materials can be kuralon, nylon, cremona, tamsi. etc. An ordinary fisherman can connect the nets into the fish.pen after taking into account the desired height or depth of the pen site. After the net is constructed , the poles are staked in mud after making a provision for the front rope and tie rope at the interval of 1.0 – 2.0 m per stake and also the provision for float rope. In preparing the poles, all nodes are cleaned except one node with brunch protending one inch which is staked in the mud from 15 – 30 cm or more depending upon the depth of soft mud. With this node the foot rope is tied, and these together with the bottom net are staked in the mud. Boulders can be used as sinkers in the absence of lead sinkers. Bamboo tips of 1-1 Vi m are also used to stake the bottom net with a foot rope firm into the mud to avoid escape of the fish stock. Construction of the nursery net may be done before or after the construction of the fish pen. They should have a free board of about 1 meter above the normal water level to prevent entry or exit of fishes by jumping and as a precaution against water level fluctuations. Metal and metal coated with HDPP screens are often used for pens which is highly durable.

Culture

Pen culture is extensively practiced in Japan, Peru and Philippines. Fish formers in Laguna debay and Sansabo Kekes stock milk fish fingerlings in pens and grow them to marketable size (200 g or above). Prawn are also similarly cultured. Very little work has been done on pen culture of fishes in India.

Traditional trapping and extensive culture of tiger prawn, milk fish, pearl spot, mullet, bekti and thread fins are done in some sort of pens and enclosures in canals joining the backwaters in Kerala and in the shallow areas of Chilka lake (Janos) in Orissa. The pens are made by weaving split bamboo or with netting. The enclosing of fishes is done usually after the monsoon season upto late autumn and the culture period lasts for about 6 to 8 months. The size of Janos in the Chilka lake varies from 5 to 500 ha. Since the stocking and harvesting are not done systematically, precise production S3* figures areajatavailable. The yield, however, is estimated to be about 60 Kg/ha/season.Seed rearing experiments were conducted in a split bamboo enclosure of 247.5 m2 reinforced with a nylon netting in Punarswamy Bhavanisagar (Tamilnadu). It was stocked with mrigal (size 7 mm) and Labeo fimbratus (size 5 mm) spawn at the rate of 4.6 million/ha and usual farm practices were followed. In 30 days mrigal attained a size of 38 mm and fimbriatus, 28 mm. At the time of conclusion of the study after 3 months, the former had attained a size of 88 mm and the later 75 mm. The overall survival obtained was 27.8 %.

Major carp seed rearing in pens is being done every year from 1982 onwards in the Tungabhadra reservoir in Karnataka. A shallow bay of the reservoir near Hampusagara is cordoned off with bamboo mats reinforced with Casuarina poles and lined with mononlament cloth during the summer months, prior to the reservoir getting filled. The pen is divided into several compartments with bamboo mats, lined with mononlament cloth. When the nursery pen, get water with the filling of reservoir, they are stocked with spawn of carps. The stocking density varies from 5 to 20 million spawn ha. The feed given is a mixture of ground nut cake and rice bran (1 : 1). After 2 to 4 months the fingerlings

are enumerated and released in the reservoir. A survival varying from 11 to 30 % is obtained from the varies nursery pens.

A pen culture experiment for raising cattle and rohu in Mamkamaun a flood plain lake in Gandak basic yields a computed production of 4/ ha/6 months. The experiment was conducted in a bamboo screen pen (1000m) and the stock was fed with a mixture of nee bran and mustard cake, apart from a feed formulated from the aquatic weeds collected from the lake. Since intrusion of fishes from outside including predators is possible in pens. It is important to stock larger fingerlings (over 50 g size) to ensure better survival. It is be desirable to have scale pen culture. The species mix and stocking rates will mainly depend on the natural food supply, supplemental feeding strategy, water depth and the duration of rearing.

Supplementary feeding

The fish pens that are densely stocked with 10-20 fish per square meter, generally need regular feeding at the rate of 4 -10 % of the total body weight of the stock at least once 3 week, or it could be divided into daily feeding. The amount of food to be given depends on the condition of the culture fish which could be checked through sampling at least once a month.

Management

Management offish pens is more laborious and demanding than a fish farm, because there are more risks in managing fish pens. Fingerlings are liable to escape once a single bamboo split breaks or a small portion of the net is torn. Every now and then the fish pens have to be checked for any holes or breaks.

The fish pen site has to be laid idle at least one month a year so that excess food and other organic matter are completely decomposed before stocking with new fingerlings. If the site is not sheltered it would be advisable to remove the net or split bamboo screen during the stormy season and repeat during fine weather condition.

Summary

The culturable species of air breathing fishes are Fig. 9.1

Channa straitus – Big or striped murrel or snake head fish

Channa punctatus – Spotted murrel

Channa marulius – Giant murrel

Clarias batrachus – Magur

Heteropneustes fossilis – Singhi

Anabas testudineus – Koi or climbing perch.

Source: Aquaculture

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