APPENDIX C: SYNTHESIS REPORT

Synthesis Report prepared for the FAO/CIFA/NACA Expert Consultation on the Intensification of Food Production through Aquaculture in LIFDC Countries

C. Saha, P.V.G.K. Reidy, R.K. Jana, S.N. Mohanty, B.K. Mishra, R.K. Dey and S. Adhikari

Introduction

Due to rapid urbanization and industrialization of developing countries, the available agricultural land and water areas have been greatly reduced. The growth of population is inversely proportional to reduction of these resources. To cope up with the situation, there is a need to develop technologies that can provide scope for enhancing production levels from an available unit area. The production technologies pertain to the monoculture and polyculture of cultivable species under extensive, semi-intensive, intensive/super-intensive levels as well as raising fish using running water facilities, integrated farming, cage and pen culture practices, etc. Technologies for many of these systems are already available but may need suitable modifications to suit local conditions. In some cases, standard technology packages have yet to be developed using the components of genetics and biotechnological tools.

The genetic potentials of many economically important species have yet to be fully tapped in these countries. In India there has been an initiation of conservation and preservation of wild fish populations. With regard to aquaculture genetics, methodologies have been already developed for genome manipulations and selective breeding of carps. Development of planned and systematic breeding programmes could help in production of good quality fish seed with traits showing high survival, better growth and increased resistance to disease and other adverse environment factors.

The classical selective breeding and the modern genetic and chromosomal engineering or a combination of these technologies may form a potential support to the farming systems to reduce the culture period and cost of production, thereby making it a sustainable and profitable endeavour.

Based on information from country status papers, fish culture practices followed by participating countries are more or less the same and mainly based on extensive, semi-intensive and, to some extent, intensive systems. Integrated farming developed in China is not common or practiced on a large-scale in some countries of the region.

The recommendations for intensification of culture of food fishes should be based on the nature of water bodies available in a country. Water bodies existing in participating countries are in the form of ponds, tanks and reservoirs. Except for Bhutan, where the mountain nature of land restricts fish culture, enough water bodies of different categories exist, in countries like India, Bangladesh, Pakistan and Sri Lanka, which are suitable for aquaculture purposes. The information provided in the status papers also suggests that: (a) the various aquaculture systems followed are more or less the same; and (b) cultured species mostly include the Indian and Chinese major carps besides the local varieties.

While recommending the technologies and protocols for intensification of aquaculture, it is important to take into account the criteria (e.g. type of water bodies available and their nature) and the category of farmers (e.g. resource rich, marginal, poor, etc.).

Nature of Water Bodies Available for Culture

Reservoirs

Fish farming is practiced in reservoirs which are formed from to dam construction across rivers and those found in urban/rural areas. These are generally categorized (based on water area) as large (above 5 000 ha), medium (1 000-5 000 ha) and small (up to 1 000 ha) reservoirs.

Ponds for Table Fish Raising

· Large ponds (above 10 hectares)

· Small ponds (below 10 hectares)

Ponds for Seed Raising

· Perennial

· Seasonal

· Irrigation ponds

Available Culture Practices

· Extensive

· Semi-intensive

· Intensive

· Super-intensive

· Integrated farming

· Cage-culture

· Pen-culture

· Paddy cum fish culture

· Flow-through or running water facilities

Recommendations

Technologies

Recommended technologies suitable for different water bodies are described below.

i) Reservoirs (riverine)

Depending on the fertility (natural productivity) and type of food available, the species to be stocked need to be selected. The ratio of species also depends on the type of food available in the ecosystem. Apart from direct stocking and rearing, reservoirs, due to their large area and high water depth, may serve as one of the most ideal environment to develop cage culture systems, more or less along the lines of cage culture of Atlantic salmon and rainbow trout in Norway.

ii) Small Reservoirs

Though not very common, there exist (in some places in India and possibly in other participating countries) relatively large tanks with good catchment area which are usually termed as reservoirs for storing water either for irrigation purpose or for drinking. If it is for irrigation purpose, fish culture can be taken up in these water bodies at an extensive level.

iii) Ponds Above 10 ha

These large-sized water bodies may be suitable for intensive and super-intensive fish rearing. In such bodies, average depth is between 2-3 meters, hence, high stocking density is possible. For intensive culture using high stocking densities, heavy input (e.g. fertilizers, feeds, aeration, etc.) becomes one of the important aspects of the technology package, not only for activating the metabolic rate in the fish but also for relatively rapid mineralization of the unutilized feed items and to maintain the oxygen level in the medium. These types of ponds are mostly suitable for resource rich farmers.

iv) Ponds below 10 ha

These water bodies may be suitable for undertaking semi-intensive farming as these are relatively easy to manage. These are also suitable for middle-income farmers.

v) Ponds/Tanks

Some perennial or even seasonal type of water bodies fall under this category, with water spread of at least one ha. These water bodies can be used for fish culture by applying extensive culture technologies. This system will be quite suitable for marginal or even resource poor farmers.

Ponds or tanks which are around 0.5 ha or less and seasonal in nature can be used for rearing fry/fingerlings for culture purpose. This can be taken up in rural areas as employment generation and poverty alleviation programmes to raise two to three crops of fry/fingerlings.

vi) Cage/pen culture

Reservoirs are well suited for cage culture. Other larger water bodies like bheels in Assam and some lakes may be suitable for pen culture in an enclosed area.

Suitable Candidate Species

Depending on the preference of the consumer of a country or in a particular region of the country, the candidate species for culture have to be selected and the type of water bodies also have to be taken into account. The following are important considerations:

· Reservoirs and other larger water bodies are suitable for carp culture or large catfish culture.

· Medium water bodies may be well suited for culture of carps, prawns (fish-cum-prawn), small size catfish like Clarias batrachus, Heteropneustes fossilis, etc.

· Seasonal or other small size ponds may be much suitable for spawn rearing or culture of minnows such as Amblypharyngodon sp., Puntius sp. Gobids, etc., species which have shorter life cycles and at the same time relished by the rural poor. Almost all these species form a part of the daily meal among the rural people in these countries. However, these species are not cultured in a regular and systematic manner as no proper technologies are yet developed. They are naturally available in ponds and other water bodies and are harvested to meet the local demands. Development of systematic culture technology for these species is needed.

Other species of fish suitable for culture in smaller ponds are the murrels or snakeheads belonging to the family Chennidae (formerly Ophiocephalidae), consisting of only one genus, Channa. Four species are common: (a) C. marulius,(b) C. striatus,(c) C. punctatus, and (d) C. gachua. These are highly preferred food fish especially among the rural people in India. Murrels are mostly bottom living fish. They burrow into the upper layer of the mud in the pond and can remain there for considerable time. They are very strong and hardy fish and can remain alive out of water for considerable time, making them available for sale in live condition like catfish, and thus attract customers.

Presently, the problem posing the culture of these species is mainly the availability of seed in sufficient quantities and proper feed (mostly high animal protein feed).

Constraints to Development of Aquaculture

Availability of Seed

The first and foremost constraint in aquaculture farming is the availability of adequate quantity of pure seed of the desired species.

Transport

Once the seed is available, then the next constraint is its transportation. Most fish farms are usually located away from the hatchery site. Obtaining carp seed, is not as difficult as obtaining seeds of other cultivable species like the catfishes, minnows, shrimp/prawn as the production of seed of these species is low.

Food and Feeding

Appropriate feed for various stages, from eggs to fry, is one of the most essential aspects in up keeping of sustainability of production. The cost of feed ingredients is increasing, adding to the cost of production. It is therefore essential to develop affordable and efficient feeds. Feed being one of the main inputs in intensive farming, which plays a major role when considering the viability and sustainability of the aquaculture industry in an eco-friendly manner.

Available Feed Inputs and Future Guidelines

Present Status in Various Countries

Consequent to several nutritional studies in India (and elsewhere), feed formulations for different culture species of fish and shellfish have now been made possible. However, in most south Asian countries, farmers still use their own farm-made feeds due to non-availability of prepared balanced dry pellet feeds. Most often, fishmeal or any other animal protein is incorporated in order to improve the biological value of different plant feeds. In Sri Lanka, for example, feeds consisting of local ingredients like rice bran, fishmeal and “coconut poonac” are used to support subsistence level of carp farming. The feed scenario for freshwater giant prawn in the country underwent a transformation to the use of pellet feed comprising 20 percent sprat heads, soybean powder and egg mixture, 29.5 percent rice bran, 10 percent wheat flour, 0.5 percent vitamins and minerals. Chicken feed is also used for small-scale prawn farming. A diet containing 20 percent protein was used for cage culture of tilapia on experimental basis.

In Myanmar, feed made up of cake-bran mixture is used by farmers. Egg yolk or egg custard is used for catfish fry rearing in addition to the use of Moina and Artemia nauplii. Pellet feeds manufactured by a government feed mill are also reported to be used in catfish farming.

Farmers of Bhutan use wheat bran, rice bran, kitchen wastes, etc. as feeds. Attempts are made in government fish farm to use other ingredients like mustard oil and soybean cakes in addition to wheat bran for carp broodstock management including grass carp which are also feed with plant feeds. Use of starter feed and formulated feed (prepared from oil cakes and soybean) is limited to government farm in Bhutan. Although 83 fish feed ingredients of both plant and animal origins are identified in Bangladesh, supplementary feeding using cake-bran is still the trend. This kind of feed is sometimes fortified with growth promoters, salts, minerals and vitamins for gonadal development in brood fish as reported in some hatcheries in Bangladesh. The feeds incorporated with fishmeal or dried blood meal containing 25-30 percent protein are used in seed rearing practices. Improved feed scenario with formulated feed containing 30-40 percent protein, 30-35 percent carbohydrate, 5-7 percent fat and 1-2 percent vitamins and mineral premix has been reported for shrimp/prawn farming to a limited extent.

It is evident from the foregoing account that feeding using improved supplementary feed or balanced feed to intensify production levels of fish and prawn is yet to take up a momentum and efforts are required to standardize aquaculture feeds for every growth stages of candidate species.

Future Guidelines and Recommendations for Feeds

Based on the experience and studies conducted at CIFA and elsewhere, the following guidelines may be considered for effective feed and feed management. The artificial feeds of carp and prawn should contain 25-30 percent protein, 8-10 percent lipid, 25-26 percent carbohydrate and 3.5-4 kcal/g gross energy with P/E ratio of 88-113 (mg/kcal energy) and also 1-2 percent vitamins and minerals especially when feed pelletization is contemplated. Protein levels in catfish diets are kept as high as 30-40 percent, which could be significantly reduced by providing them adequate quantity of lipid as much as 30 percent. The ten essential amino acids and the two essential fatty acids (n-3 and n-6) need to be balanced in feed formulation. In prawn diets, 0.25-0.5 percent sterol and cholesterol are required.

Feeds should be prepared using locally available ingredients of both plant and animal origin. For the commonly used blood meal, available ingredients like rice bran wheat bran, groundnut oil cake, soybean meal, fish meal, meat meal, silk worm pupae and several other indigenous materials may be used for this kind of feed depending upon their availability in a locality and costs. Particle and pellet size feeds are important considerations and should be adjusted according to the size of fish and prawn. Larval diet should reflect the biochemical composition of zooplankton/Artemia for their amino acids, molecular weight and essential fatty acids with proper particle size. It is further suggested to use probiotics and phospholipid compounds in carp larval diets.

Improved supplementary/balanced artificial feeds should be made available in conjunction with adequate natural food supply. Plankton production can be increased using suitable fertilization packages at regular intervals. This is most important for extensive farming where plankton is the only source of nutrients and energy. For semi-intensive systems, supplementary feeds need not necessarily be nutritionally balanced but should be adequate to supplement the deficiencies. Feeding of single ingredient as supplementary feed is not beneficial. Understanding that interactions between natural fish food supply and artificial feeds economize feed cost and greatly influence the dietary utilization is essential. Feeding of formulated diet along with natural food supply in semi-intensive systems helps in increasing the production level beyond 3-4t/ha/yr.

A large proportion of alternative protein sources should be used for preparation of freshwater fish and prawns feeds. It is important to consider the formulation of less-expensive feeds to support sustainability of aquaculture. The information generated also needs to be disseminated to the private sector for commercial production, and subsequently made available to farmers and entrepreneurs at affordable prices.

Use of locally available ingredients (e.g. plant materials) for feeds should be encouraged in order to lessen dependence on expensive feeds such as fishmeal. Feed ingredients and formulated feeds have a reported of up to a maximum of 3-4 months under tropical conditions depending on the variety used; vitamin pre-mixes are preserved for a maximum period of 6 months under cold conditions.

Feeding Practices

Carp

Finely powdered feeds are used for both seed and small fry; granules for bigger fry. Dry pellets, when used, are grounded or crushed. In advanced laboratories or hatcheries, automatic feeders are used at desired intervals. A feeding frequency of 30 min to 1 hour interval, commonly practiced in hatcheries, is reported to yield good results in terms of high growth and better survival. In grow-out ponds, feed doughs or dry pellets using feeding basket suspended in ponds are commonly used. Feeding twice a day (morning and evening before sunset, in equal amounts) was found to be beneficial. The table below shows the feeding requirements used for different stages of carp.

Table 1. Feeding requirements of different stages of carp.

Magur

The magur eggs are reared for 12-14 days indoor with mixed zooplankton, Artemia nauplii, molluscan meat, tubifex and egg custard as larval feed. Egg custard is supplemented with vitamins and minerals. No feed is provided until 4th day of hatching and as soon as yolk absorption starts small quantity of feed is given. Larval feeds are provided from 5th day onwards.

Initial feeding starts with feed particle/organism of 20-30µ and the size is increased gradually to 50-60µ for one week old fry. Bigger-sized fry are removed and shifted to other containers. Unutilized feed and excreta of fry need to be removed through 50 percent water exchange with constant aeration. Fry which are 10-12 days old are stocked in large containers using appropriate density, and fed with reduced quantity of Artemia nauplii supplemented with zooplankton, Tubifex, molluscan meat or fish flesh for 10-12 days. Advanced fry and fingerlings are fed artificial feed comprised of finely minced trash fish/molluscan meat and rice bran (1:1 ratio) or any other formulated feed at 5-10 percent of body weight, daily during morning and evening hours.

Prawn

It is observed that prawn larvae of different stages require 5-50 brine shrimp nauplii (2-10 nauplii/ml) and should be provided with feed at the rate of 50-150 µg/day/larva. The feed quantity depends on feed utilization by the larvae and is decided by the hatchery operator through visual estimation. In the event of over-feeding, it is necessary to exchange 70-90 percent of the culture water.

Table 2. Level of acceptance by different larval stages to different types of diet.

For feeding the prawns in grow out ponds, feeds are provided at 5-10 percent of body weight, and pellet size 2-3 mm in diameter. In polyculture of carps and prawn, feeds are provided at 2-5 percent of body weight. Artificial feeding normally starts after seven days of stocking. About 60-65 percent of the ration should be provided during night hours. In grow-out, feeds are provided along the water edges of pond or simultaneously in check-trays suspended in water 2-3 m away from the pond dikes. The feeds may also be provided using feeding trays/baskets suspended in water at least at 6-7 positions in the ponds, depends on the size.

Health and Environment Monitoring

Monitoring of health and environment in aquaculture is important, particularly in intensive/super-intensive systems, in order to harvest healthy crops with high survival and optimum growth. Due to heavy inputs of feed and fertilizers, diseases occur more frequently in intensive culture systems. The following measures are suggested for health and environment monitoring.

Health

Diseases have, during the last decade, become significant constraints for successful aquaculture and hampered production and trade of the aquaculture sector. Extensive farm level survey conducted by NACA in 16 countries of the Asia-Pacific region suggested that diseases and related problems have probably caused annual losses of more than US$ 3 billion/year. Besides this negative economic impact, pathogen transfer and subsequent disease outbreak in natural aquatic ecosystems have caused significant impact on aquatic biodiversity.

Disease Emergence in Various Types of Culture Systems

In the traditional or extensive system of fish/prawn culture, less disease outbreaks occur because of low stocking density and absence of any inputs. In semi-intensive, intensive and super intensive systems with very high stocking densities and high input of supplementary feed and fertilizers, outbreaks of infectious diseases and occasional appearance of non-infectious diseases caused by environmental deterioration and nutritional deficiencies are reported.

Occurrences of Diseases in the Participating Countries

Bangladesh

A number of diseases are of major concern to fish growers. These include infections by protozoan and metazoan parasites, bacteria and fungi. However, most serious losses have been caused by Epizootic Ulcerative Syndrome (EUS) which appeared first in 1988. EUS caused severe losses in total fish production, drastic reduction in market prices and also consumer rejection. Some wild species of fish may be under pressure of extinction due to EUS. More recently, the frequency of EUS is reported to have decreased. Shrimp disease such as White Spot Disease (WSD) is also a serious problem in brackishwater. Ectoparasites such as Argulus and Lernaea cause serious disease problems particularly in broodfish.

Bhutan

No information is available about the types of fish diseases occurring in ponds or tanks and their management strategies practiced although culture of indigenous warm-water fish species have been undertaken by the National Warm Water Fish Culture Centre (NWWFCC) under Gelephu Integrated Area Development Project (GIADP).

Pakistan

No major disease problems were reported from farms in Pakistan until 1996 when the first outbreak of EUS was confirmed and heavy mortality occurred in the Punjab province. Isolated cases of fish diseases caused by protozoan and metazoan parasites, fungi and bacteria have been recorded.

India

Two of the most serious diseases creating concerns and constraints in the aquaculture sector of the country are EUS and WSD. EUS affected a wide variety of fishes both in the wild and cultivated conditions throughout the country from 1988 until present. WSD affected brackishwater prawns from 1993 onwards. Various stages of carp and catfish culture are affected by various diseases such as: (a) bacterial diseases (e.g. bacterial septicaemia, columnaris disease, fin and tail rot, edwardsiellosis; (b) fungal diseases (e.g. saprolegniasis, achlyasis, branchiomycosis; (c) parasitic diseases (e.g. ichthyophthiriasis, trichodinosis, myxoboliasis, dactylogyrosis, gyrodactylosis, argulosis and lernaeasis).

Although there are no reports of any serious disease outbreaks in the mature freshwater prawns (M. rosenbergii and M. malcolmsonii) several diseases caused by viral, bacterial, fungal, parasitic, ectocommensals, nutritional and environmental deterioration have been reported. In hatchery and in grow-out systems. These include larval mid-cycle disease (MCD), bacterial necrosis, exuvia entrapment disease, epibiont fouling disease and black spot disease.

Sri Lanka

EUS was first observed to occur in 1987 in natural water bodies affecting a wide variety of fishes of which C. striatius and C. punctatus were the most severely affected ones. The disease spread to almost all natural water bodies including every small stream causing heavy mortalities in natural fish population. In culture practices, parasites (e.g. Piscioodinium sp., Cryptobia sp., Trichodina, Scyphidia, Ichthyophthrius, Myxobolus, Dactylogyrus, Digenian metacercariae, Lernaea, Ergasilus, Synergassilus) have been reported to cause frequent losses. Bacterial disease in Catla catla and mrigal fingerlings in ponds and gill rot in grass carp brooders have been reported causing severe losses; while fungal disease caused by Saprolegnia sp. has been reported in Labeo rohita.

Remedial Measures (Preventive/curative)

Good husbandry management

The most effective and recommended procedure for controlling disease is good management of ponds. Good water quality, appropriate feeding, reducing stress, etc., are important components of good management.

Disinfection of ponds^[4]

Drainable ponds should be thoroughly dried and exposed to sunlight for about 10-15 days. In non-drainable ponds, bleaching powder (Calcium hypochlorite 50 ppm) should be applied in order to get rid of all unwanted fishes, molluscs, tadpoles, crabs, etc. It also serves as disinfectant for soil and water.

Disinfection of materials and equipment

Net, gears, plastic wares, ‘hapas’, and other materials should be thoroughly cleaned and sun-dried or immersed in concentrated solution of disinfectant like potassium permanganate before and after use.

Proper Feeding

Good quality supplementary feed in optimum quantity is essentially to maintain and raise a healthy crop of fish.

Separation of Year Class Fish Population

Older fish may serve as carriers of disease causing organisms. To avoid such risks the best course is to separately maintain older fish.

Immediate Removal of Dead/Diseased Fish From Pond

Dead or moribund fish should be immediately removed from ponds.

Chemoprophylaxis^[5]

Use of chemotherapeutants for prophylactic treatment is an effective measure against bacterial diseases. Dips, baths and oral routes of administration and pond treatments are commonly practiced. Oral administration of therapeutants is also applied in prophylactic treatments for systemic infections.

Occasional application of potassium permanganate (2-3 ppm) is recommended. Dip treatment in 500-1 000 ppm of potassium permanganate (KMnO₄) solution for 60-120 sec before releasing fish back in ponds is a very effective prophylactic measure. Dip treatment in 2-3 percent common salt solution for seeds before stocking is a very inexpensive prophylactic measure against a wide range of parasitic and microbial diseases.

Immunoprophylaxis

Immunization programme is an emerging and important measure for preventing communicable fish diseases. Vaccines against some of the bacterial pathogens of fish (e.g. Aeromonas salmonicida, Yersinia ruckeri, Vibrio anguillarum) and a parasite Ichthyophthirius multifilis and several viral pathogens are now commercially available in some developed countries.

Curative Measures

Epizootic Ulcerative Syndrome (EUS)

A number of chemicals and chemotherapeutics have been tried to control EUS with inconsistent results. However, lime (CaO) at 600kg/ha/m of water in 3 equal parts at 5-7 days interval was found extremely effective against the disease in India. A chemical preparation named “CIFAX” and an herbal treatment using turmeric in combination with lime were also found highly effective against the disease.

White Spot Disease in Shrimps

A number of chemicals, antibiotics and immunostimulants are reported to have been used to control the disease but results are highly inconsistent.

Bacterial Diseases^[6]

Bacterial Hemorrhagic Septicaemia

· Evidence of using streptomycin at 20-25 mg/kg body weight or in combination with penicillin (20 000 1.U/kg body wt.) through injection.

· There are incidences where terramycin in feed at 75-80mg/kg body wt with feed for 10-12 days.

Columnaris Disease

· Dip treatment in 500 ppm potassium permanganate (KMnO₄) for 1-2 min

· Oxytetracycline at 50 mg/kg body wt. with feed for 10-12 days.

Fin and Tail Rot

· Treat fish with oxytetracycline mixed with feed at 50 mg/kg body wt for 10-12 days.

Edwardsiellosis

· Treat fish with oxytetracycline mixed with feed at 50 mg/kg body wt for 10-12 days.

Vibriosis in Prawns/Fish

· Sulphamerazine at 8-12 mg/45.3kg of fish /day for 3 days with feed is used.

· Oxytetracycline at 70-80 mg/kg body wt/day for 3 days with feed is used.

Fungal Diseases

Fungal diseases such as saprolegniasis, achlyasis and branchiomycosis in carp and catfish and also fungal infections in prawns are cured by:

· Bath treatment of 250 ppm formalin^[7] for few minutes

· Pond treatment of 15-25 ppm formalin

Protozoan Diseases

Protozoan parasites like Ichthyophthirius multifilis, Trichodina, Cyclochaeta can be controlled in ponds by a combined treatment of malachite green (0.1 ppm) and formalin (25 ppm). However, use of malachite green is banned and this procedure should not be used any more.

Since no chemotherapy is available for the myxosporidian parasites like Myxobolus, Thellohanelus and microsporidian parasites, fish ponds should be disinfected with 50-60 ppm bleaching powder (Calcium hypochlorite) before stocking

Helminth Diseases

Infections by the monogenean trematodes Dactylogyrus and Gyrodactylus in carps and catfish can be controlled by dipping the infected fish in 2-3% common salt and also application of 0.25ppm malathion (an insecticide) in ponds.

Crustacean Diseases

The two most serious and frequently occurring crustacean parasites namely Argulus and Lernaea in major carps are controlled by 0.25 ppm malathion application in ponds in 3 equal weekly portions. Use of pesticides for controlling fish diseases is discouraged and should be avoided. Possible human health and environmental implications of such uses must be duly considered.

Nutritional Disease

Scoliosis and lordosis, diseases caused by vitamin C deficiencies, are often encountered during culture operations of carps and catfish. Incorporation of optimum levels of vitamin C in supplementary diets is the remedial measure for such diseases. Liver lipid disease in Catla catla due to rancid feed intake is cured by providing fresh feed with high protein content (about 22%).

Losses Due to Diseases

· There are no reliable estimates of economic loss due to diseases and related problems in the participating countries of this workshop, with the exception of Sri Lanka who reported (Balasuriya & Jayasekera 1990) the following loss estimates:

· >Rs. 1 M losses in 1988 and 1989 due to EUS;

· Rs 4 M losses due to Myxobacterial gill rot disease in Catla catla

Guidelines/Recommendations for Tackling Disease Problems in Aquaculture Practices of the Participating Countries

· Diagnostic facilities for timely and precise identification of various disease problems may be established and strengthened by adopting recently developed quick diagnostic procedures (PCR techniques, dot-blot and immunoflourescent techniques).

· Prophylactic and preventive measures against commonly occurring diseases in fish/prawn should be standardized and strictly adhered to during culture practices.

· As far as possible curative treatment of diseases through chemicals, insecticides, antibiotics, etc. should be avoided and injudicious application of these chemical should be strictly prohibited.

· Studies and researches on application of indigenously available plant and herbal materials as substitutes for chemicals treatment for controlling and managing disease should be encouraged.

· Scientists and technical personnel involved in disease investigations should be continuously trained.

· Quarantine and health certification programmes should be introduced through governmental legislation in all participating countries in order to prevent the entry and spread of diseases/pathogens from one region to the other that may result from introducing exotic varieties from other countries.

· Regular exchanges of scientific information on disease among fish disease specialists and technical personnel of participating countries should be ensured through workshops, seminars, symposia and networking, etc.

· Epidemiological studies and reliable statistical data about losses due to diseases should be meticulously undertaken by participating countries.

Environment

Present Status in Participating Countries

Bangladesh

In Bangladesh, typical freshwater food fish production systems are extensive and semi-intensive polyculture systems with supplementary feeding and fertilization and management. Rice bran, wheat bran, mustard oil cake, sesame oil cake, etc. are some of the important feed ingredients widely used in the country. Both inorganic (chemical), organic and compost are used to economize the cost and to maintain balance with supplementary feed. The inorganic fertilizers used are N-P-K, urea, TSP and MP (Murate of Potash), SSP; organic fertilizers include cow dung, poultry droppings and compost. Expensive commercial artificial feeds are not yet in use. The uncontrolled use of pesticides and chemicals may cause irreparable damage to the environment. The newly constructed ponds and other water bodies are treated with extended doses of organic fertilizers to prevent resist seepage problems. As the culture system is still dominated by extensive or semi-intensive methods, husbandry practices are employed to maintain fish health.

Regular water exchange, bio-filter, water treatment, feed quality assurance, aeration and other quality attributes are maintained for health management. The over-growth of plankton is controlled by restricting the use of fertilizers and growth enhancers. The application of lime at monthly interval (at varying dose from 0.25 kg to 1.0 kg/ha) prevents the outbreak of common fish diseases.

To ensure production of fish food organisms, both organic and inorganic fertilizers are used. Fertilizers are applied 5-7 days after lime treatment. Application, in one ha pond, of 1 200-2 500 kg cow dung, 700-12 000 kg poultry litters, 2 000-3 000 kg compost, 25-30 kg urea, 10-15 kg MP can enhance the production of plankton (e.g. Cladocera, copepod, etc.) growths within 10-15 days of fertilization. The use of 1 kg/ha mustard oil cake accelerates rotifer growth. Mustard oil cake, rice bran, wheat bran, fish meal, dried blood meal, etc. are used as supplementary feed for carp seed rearing. The mixture of the ingredients applied as supplementary feed can ensure 90 percent survival of seed until fry level. The mustard oil cake, with small amount of urea and mixed with rice bran or wheat bran, is soaked in water for 1-12 hr before application in pond. During the initial 5 days, the use of only mustard oil cake results to better seed production of carp.

The production inputs viz., spawn, feed, fertilizer, etc. need operational capital. Lack of sound financial management among operators limits intensified seed production practice. The use of different chemicals, insecticides, pesticides during pond preparation phase, clearing out of predatory fish and other animals, combating insect infestation and disease control in production phase have negative impacts on environment.

Bhutan

The mountainous nature of the land has restricted fish cultivation in large scale. Fish culture system mainly depends on organized farming along with supplementary fertilization and feeding. Water bodies have very low alkalinity and hardness, therefore it will respond positively to lime. The pond is disinfected using chemicals such as KMnO₄. The culture system is mainly extensive/semi-intensive.

Sri Lanka

In integrated fish farming and culture of freshwater prawn, use of cow dung, lime and feed are generally practiced. Feeding is done with chicken feed by broadcasting three times a day. Cow dung application at higher doses is reported to cause gill rot disease.

Pakistan

Freshwater fish culture in the country is mainly semi-intensive. Before stocking the fry, 0.25 ppm Baytex or other organophosphate pesticides are generally used. Feeding is usually done 2-3% of the body weight. Ammonium sulphate is used as fertilizer. Nothing has been mentioned regarding other fertilizers. The basis of ammonium sulphate has not been mentioned. Ponds are limed and manured properly.

Ideal Environmental Parameters and Guidelines for Their Management for Sustainable Fish Culture

Environmental parameters play vital roles in aquaculture and its sustainability. If the parameters become adverse, the fish/prawn in the culture systems become stressed, they subsequently succumb to diseases. The water quality parameters of significance to fish/prawn health in tropical climate are described below.

Water Quality Management

Dissolved Oxygen

The optimum dissolved oxygen (DO) content of pond waters should be in the range of 5 mg/l to saturation level for good growth of fish. Below are some guidelines for dissolved oxygen for fish health management:

· 5.0 mg/l - optimum for normal growth and reproduction in tropical waters;

· 1.0-5.0 mg/l - may have sub-lethal effects on growth, feed conversion and tolerance to disease;

· 0.3-0.8 mg/l - lethal to many species if sustained for a long period.

Oxygen depletion in water is rectified by the following aeration methods:

· Manual. In this method, water surface is splashed with bamboo sticks. This helps in dissolving atmospheric oxygen in water.

· Mechanical. A diesel water pump is operated through method. Water is pumped out and simultaneously sprayed in again into the water body. This helps in dissolution of atmospheric oxygen.

· Aerators. Aerators are mechanical floating devices.. Their rotating blades churn the water helping in dissolution of atmospheric oxygen in water. Depending upon the concentration of oxygen in waters, the number and placement of such aerators are determined.

Other steps taken to control the oxygen level are:

· Care should be taken to feed fish in the afternoon or evening in heavily stocked pond systems as oxygen requirement in fish after feeding increases and dissolved oxygen is minimum in pond during early morning.

· Organic manure application in a water area should be done carefully as organic material consumes oxygen during decomposition. Therefore, the quality of manure to be applied without the risk of oxygen depletion can be calculated taking into consideration the availability of dissolved oxygen during the 24 hr period.

· During collapse of phytoplankton bloom, decomposition occurs and in the process oxygen requirements of micro-organisms increase. Thus, special care has to be taken during this time.

· Special care has to be taken as fish more oxygen are required with increasing temperature.

Ammonia

The total ammonia concentration in water comprises two forms, namely:

NH₃ = unionized ammonia (Free ammonia) and NH₄⁺ = Ionized ammonia

They maintain equilibrium as per the equation: NH₃ + H₂O ® NH₄⁺ + OH^-

The un-ionized ammonia fraction is more toxic to fish and the amount of the total ammonia in this form depends on the pH and temperature of the water. As a general rule, the higher the pH and temperature, the higher is the percentage of the total ammonia present in the toxic un-ionized form. Below are guidelines for un-ionized ammonia level in fish health management:

· 0.02-0.05 mg/l - safe concentration for many tropical fish species;

· 0.05-0.4 mg/l - sub-lethal effects depending on the species; and

· 0.4-2.5 mg/l - lethal to many fish species.

There are a number of measures to maintain safe ammonia concentration in pond water. Normally at high dissolved oxygen and high carbon dioxide concentration, the toxicity of ammonia to fish is reduced. Some recommended measures to reduce the effects of ammonia are:

· Aeration will increase the dissolved oxygen concentration and decrease the increasing pH thereby reducing toxicity.

· Healthy phytoplankton populations remove ammonia from water. Care should be taken while using fresh manure with high ammonia content. The manure should be dried to allow ammonia gas to escape before application to the pond.

· Biological filters may be used to treat water for converting ammonia to nitrite and then to harmless nitrate through nitrification process.

· A high quality feed that contains no more nitrogen (crude protein) and phosphorus than actually needed by fish should be used in ponds and also over-feeding should be avoided.

· Excessive liming should be avoided as it raises pH and high pH favours ammonia toxicity to aquatic animals.

· Water exchange can reduce ammonia concentrations in fish and prawn ponds. From both economic and environmental perspectives, water exchange should only be used when necessary.

· Formalin can be used to remove ammonia from fishponds.

Nitrite

Nitrite is an intermediate product in the biological oxidation of ammonia to nitrate, a process called nitrification. In most natural water bodies and in well maintained ponds, nitrite concentration is low. In water bodies with high organic pollution and low oxygen concentration, nitrite concentration may increase. Guidelines for nitrite value in fish health management are as follows:

· 0.02-1.0 mg/l - sub-lethal level for many fish species;

· 1.0-10 mg/l - lethal level for many warm water fish species.

Measures to maintain safe nitrite level in water are:

· Correct stocking, feeding and fertilization practices should be maintained. The ponds should be kept well oxygenated.

· Bio filtration is done through special filters by which biological conversion of nitrite to harmless nitrate occur.

Temperature

Temperature sets the pace of metabolism by controlling molecular dynamics (diffusibility, solubility, fluidity) and biochemical reaction rates. Under favourable conditions, the optimum temperature range for many coldwater and warmwater fishes are 14-18°C and 24-30°C, respectively. Water temperatures can be adjusted to optimum levels in controlled system such as hatcheries. It is difficult to adjust water temperature in large water bodies. Operation of aerator during calm and warm afternoon helps to break thermal stratification by mixing warm surface water with cool subsurface water. Planting of trees on pond banks to give shade will reduce stratification but at the same time, reduces the beneficial effects of wind mixing and restricts solar energy for photosynthesis.

Turbidity

Turbidity in culturable water is the resultant effect of several factors like suspended soil particles, planktonic organisms, humus substances produced through decomposition of organic matter, etc. Turbidity is measured by secchi disk visibility. Optimum secchi disk visibility in fish ponds is considered to be 40-60 cm. Turbidity resulting from plankton is generally desirable. Guidelines for suspended soil particles value in fish health management are:

Pond waters turbid with suspended soil particles can be controlled by application of 500-1 000kg/ha organic manure, 250-500 kg/ha gypsum or 25-50 kg/ha alum.

Hydrogen Sulphide

Freshwater fish ponds should be free from hydrogen sulphide (H₂S). Hydrogen sulphide is produced by chemical reduction of organic matter that accumulates and forms a thick layer of organic deposit at the bottom. Unionised hydrogen sulphide is toxic to fish, but the ions resulting from its dissociation are not very toxic. Guidelines for hydrogen sulphide value in fish health management are:

· 0.01-0.5 mg/l - lethal to fish and any detectable concentration of hydrogen sulphide in water creates stress to fish;

· 0.1-0.2 mg/l - prawn lose their equilibrium and create sub-lethal stress;

· 3 mg/l - prawn die instantly.

Measures to rectify increase in hydrogen sulphide levels include:

· frequent water exchange to prevent building up of hydrogen sulphide in the water body;

· when pH of water is increased by liming, the toxicity of hydrogen sulphide decreases; and

· potassium permanganate (6.2 mg/l) can be used to remove hydrogen sulphide (1 mg/l) from water.

pH

pH is a measure of the hydrogen ion concentration in water and indicates how much acidic or basic the water is. Water pH affects metabolism and physiological process of fish. pH also exerts considerable influence on toxicity of ammonia and hydrogen sulphide as well as solubility of nutrients and thereby water fertility. Guidelines for pH value in fish health management are given in Table 3 below:

Table 3. Effects of different pH levels.

Measures for rectifying alkaline and acidic water bodies are provided below.

Alkaline Waters

· Rapid fluctuations in pH caused by excessive phytoplankton populations may be rectified by ensuring good water management. Water body should have an alkalinity of more than 20 mg/l as CaCO₃.

· Application of acid forming fertilizers.

Acidic Waters

· Calcium carbonate (CaCO₃), calcium hydroxide (Ca(OH)₂), calcium oxide (CaO) or dolomite is used to rectify the acidic water bodies depending upon the pH.

· Salt water like sea water may be flushed through water bodies of coastal farms to neutralize acidity.

Alkalinity

Alkalinity refers to the concentration of bases in water and the capacity of water to accept acidity, i.e. the debuffering capacity. In most waters, bicarbonates and carbonates are the predominant bases. Guidelines for alkalinity in fish health management are:

· 300 mg/l - create stress to fish;

· 20-300 mg/l - ideal for fish;

· <20 mg/l - create stress to fish.

High alkalinity can be rectified by treatment with lime.

Total Hardness

Contents of alkali earth metals, mainly calcium and magnesium constitute the total hardness of a water body. Guidelines for hardness value in fish health management are given below:

· 20 mg/l - satisfactory for pond productivity and helps protect fish against harmful effects of pH fluctuations and metal ions;

· <20 mg/l - creates stress to fish.

Measures for rectification of low hardness:

· Ponds with low hardness can be treated with lime for rectification.

Carbon Dioxide

Carbon dioxide is present in the atmosphere in very small quantity. For this reason, in spite of its high solubility in water, its concentration in most water bodies is low. It occurs in water in three closely related forms, namely: (a) free carbon dioxide, (b) bicarbonate ion (HCO₃^-), and (c) carbonate ion (CO₃^2-). The amount of each form presents, depends on the pH of water. For example, in neutral or acidic waters high concentrations of free carbon dioxide, i.e. the toxic form is frequently found. Guidelines for carbon dioxide value in fish health management are:

· 12-50 mg/l - sub-lethal effects include respiratory stress and development of kidney stones (nephrocalcinosis) in some species;

· 50-60 mg/l - lethal to many fish species with prolonged exposure.

Measures for controlling high carbon dioxide concentration include:

· repeated aeration of water;

· increasing the pH of water by hydrated lime can control high carbon-dioxide concentration. Experiments have shown that 1.0 mg/l of hydrated lime can remove 1.68 mg/l of free CO_2; and

· phytoplankton population and the organic loading in a water body should be regulated by correct stocking, feeding and fertilization.

Bottom Soil Management

Role of bottom soil in determining productivity of a pond is well documented. Production of various primary food organisms depends largely on the availability of different nutrients. Dynamics of availability of most of these nutrients, in turn, is determined by the condition prevailing in the bottom soil. Considering this significance, bottom soil is designated as the chemical laboratory of a pond. However, suitable soil quality problem is common in aquaculture ponds, and therefore, many methods are used for the purpose of improving pond soils.

Soil Texture

Many important physico-chemical properties affecting the fertility of fish ponds are influenced to a great extent by the relative proportion of the different size fraction of the soil. An ideal pond should not be too sandy to allow leaching of the nutrients or should not be too clayey to keep all the nutrients adsorbed in it. When the pond is constructed on sandy soils, then heavy doses of organic manure application is essential to control seepage loss of water. In general, the dose of raw or composted farmyard manure varies from 10 000-15 000 kg/ha/yr.

Soil Acidity

Soil may be acidic, alkaline or neutral. The ideal range for fish pond soil is pH 6-8. Water passing over acid soil tends to be acidic with low alkalinity and hardness. High concentration of metal ions particularly aluminium and iron also may be present. Acidic ponds do not respond well to fertilization.

Liming is the only way to improve water quality with acid soils. Recommended rate of application of lime at different soil pH has been given under the section on fertilizer schedule for fish ponds.

Acid Sulphate Soils

Acid sulphate soils from mine spoils and coastal mangroves tide swamps contain high levels of pyrite (FeS_2, 1-6 percent). As long as sediments containing pyrite are submerged and anaerobic, they remain reduced and there is little change. However, as they are drained and exposed to the air, oxidation results and sulphuric acid is formed.

Sulphuric acid reduces the pH of the water when pond is filled. In ponds, problems with acid sulphate soils usually originate in pond dikes. Pond bottoms are usually flooded and anaerobic, so sulphuric acid does not form. However, dikes dry and sulphuric acid formed during dry period enters pond in run-off water after rains. Acidity on dikes can be controlled by liming (0.5-1.0 kg/ m²) and establishing good cover with an acid resistant grass species.

A procedure for rapid reclamation of ponds with acid sulphate involves drying and filling of the soil to oxidize pyrite, filling the pond with water and holding till water pH drops to < 4 and then draining the pond, repeating the procedure until the pH stabilizes at a pH >5, followed by liming the pond with 500 kg of calcium carbonate/ha.

Bottom Soil Oxidation

Dissolved oxygen cannot move rapidly into water-saturated soil, and pond soil becomes anaerobic below a depth of few mm. Aeration and water circulation are beneficial in improving bottom soil oxygenation, but the surface layer of soil may still become anaerobic in intensive fish culture ponds. When the redox potential is low at the soil surface (anaerobic conditions), hydrogen sulphide and other toxic microbial metabolites diffuse into the pond water. Sodium nitrate can serve as a source of oxygen for microbes in poorly-oxygenated environment; the redox potential will not drop to a level enough for the formation of hydrogen sulphide and other toxic metabolites.

Some Other Environmental Parameters

Nutrient Removal

It is possible to precipitate phosphorous from pond water by applying sources of iron, aluminium or calcium ions. These ions precipitate phosphate as insoluble iron, aluminium or calcium phosphates. Alum (aluminium sulphate) and ferric chloride are commercially available sources of aluminium and iron, respectively. Alum is cheaper and more widely available than ferric chloride. Gypsum (calcium sulphate) is a good source of calcium, because it is more soluble than liming materials. Treatment rates of 20-30 mg/l alum and 100-200 mg/l gypsum have lower phosphorus concentration in pond waters. Alum is acidic and more suitable for use in waters with 500 mg/l of total alkalinity and above. Gypsum is better for use in low alkalinity waters.

Phytoplankton Removal

Algaecides are used to reduce the abundance of phytoplankton in intensive fish culture ponds. Copper sulphate pentahydrate (CuSO₄.5H₂O)^[8] is recommended for reducing phytoplankton abundance and the abundance of blue-green algae in particular. The usual recommendation is to apply a dose of copper sulphate equal to 1/100 of the total alkalinity. The best approach to phytoplankton control is to regulate nutrient inputs by moderate stocking and feeding rates, but it may be feasible to use alum or gypsum to precipitate excessive concentrations of phosphorus.

Sanitation of Ponds (Chlorination)

Hypochlorus acid and hypochlorite (free chlorine residuals) are the chlorine products which are used for disinfecting pond water. However, chlorination of water containing fish or prawn is both dangerous and unbeneficial. It is possible to sterilize bottoms of empty ponds and water in newly filled but unstocked ponds by applying chlorine products. When this is done, enough chlorine should be applied to overcome the chlorine demand and provide 1 mg/l or more of free chlorine residual. The residuals will detoxify naturally in a few days so that ponds can be stocked safety.

Water Exchange

There are reasons to exchange water in specific instances, such as to reduce salinity, to flush out excessive nutrients and plankton or to reduce ammonia concentrations. However, daily water exchange usually does not improve water quality in ponds, and pumping costs are a liability. Ponds are highly efficient in assimilating carbon, nitrogen and phosphorus inputs, which are not converted to fish or prawn flesh, but if water exchange is great, these substances are discharged from ponds before they can be assimilated. Thus, the pollution potential of aquaculture ponds increases as a function of increasing water exchange. From both economic and environmental perspectives, water exchange should only be used when necessary.

Fertilization of Nursery, Rearing and Stocking Ponds

Natural or inherent fertility of nurseries often remain unsatisfacotry due to deficiency of one or more of the nutrient elements in soil and water including other environmental conditions. Correction of deficiencies by application of manures or fertilizer containing these deficient nutrients in suitable form and in optimal amount is necessary to accelerate biological production. Accordingly, to facilitate effective control or manipulation of environmental condition, small ponds either seasonal or having shallow depth, are preferred for nurseries.

Use of Organic Manures

Both organic manures and chemical fertilizers are widely used for improving productivity of nurseries. Cow-dung is the most widely used organic manure compared to others and applied at the rate of 5 000 to 15 000 kg/ha in one instalment well in advance, preferably a fortnight prior to stocking with spawn. The amount is reduced to 5 000 kg/ha when mohua seed cake is used as a fish toxicant in nurseries with shallow water depth. Sometimes, to hasten the process of decomposition of added manures, nurseries are limed at 250-350 kg CaCO3/ha after the application of manure. In other cases, spaced manuring with cow-dung at 10 000 kg/ha 15 days prior to stocking followed by subsequent application of 5 000 kg/ha seven days after stocking have been practiced for sustainable production of zooplanktons in nurseries. When more than one crop is raised, nurseries may be manured with cow-dung at 5 500 kg/ha immediately after the removal of the first crop. Besides cow-dung, a combination of mustard oil cake, cow-dung and poultry manure using the ratio of 6:3:1 at 1 100 ppm has been used for the culture of desirable species of zooplankton for carp.

Inorganic Fertilizer

Inorganic fertilizers containing a fixed percentage of individual nutrient element or a combination of more than one element are also able to enhance biological production in nurseries. A ratio of nitrogen to phosphorous of 4:1 (N:P) is considered most effective for increased production in nurseries. Weekly application of N:P:K mixture in the ratio of 8:4:2 is suitable for increased production of fish food organisms. Use of N:P:K in the ratio of 18:8:4 at 500 kg/ha after liming at 200 kg/ha is quite effective in enhancing the production of slightly acidic and unproductive soils used for nurseries.

Nitrogenous fertilizers containing different forms of nitrogen (e.g. amide, ammonium-cum-nitrate and ammonium) are suitable for management of nurseries. These three forms of fertilizers (e.g. urea, calcium ammonium nitrate and ammonium sulphate) are effective for slightly acidic to neutral, moderately acidic and alkaline soils, respectively and the rate of 80 kg N/ha is most suitable for rearing of rohu in nurseries.

Organic and Inorganic

Combined use of both organic and inorganic fertilizers are also suitable for increased production of either fish food organisms or fry. The combination of mustard oil cake and N:P:K (ratio of 6:8:4) on equivalent nutrient basis at 12 kg N/ha is suitable. However, on equivalent basis (N:P:K) organic manure (cow-dung) is most suitable for management of carp nurseries.

Rearing and Stocking Ponds

Since acidic pond soils reduce the microbial activity and availability of nutrients to pond water, either native or when added externally, application of lime is the first step of management for all stages of fish culture. Liming raises the soils pH to a desirable level (near neutral) and establishes a strong buffer system in the aquatic environment.

Liming stimulates the microbial decomposition of organic matter, supplies Ca to the pond, increases HCO₃^- content in the pond and maintains sanitation in the pond environment. Generally, ground limestone is extensively used and spread over the dry bed to ensure the complete benefit or broadcast over the water surface in a single dose at least 15-20 days before stocking. On the basis of soil pH, specific dosages of lime are usually applied to ponds as described below. Besides initial application, some compensatory applications of lime in the range of 100-200 kg/ha may also be made in the stocking pond from time to time to neutralize acidity developed through application of acid farming inorganic fertilizers and organic manure and also when fishes are diseased or distressed.

Table 4. Specific dosage of lime applied to different pond conditions.

In India, organic manure is more commonly used than inorganic fertilizers. A variety of agricultural wastes, cow-dung, poultry droppings, pig manure, bio-gas slurry, etc. can be used as organic manure. In rearing ponds, application of raw cow-dung or bio-gas slurry is observed to give better results. Depending on the organic carbon content of pond soil in rearing pond, application of raw cow-dung or bio-gas slurry in the range of 3.0-7.0 or 5.5-12.0 t/ha, respectively; and addition of 5.0-15.0 or 10.0-30.0 or 2.5-5.0 t/ha/yr of cow-dung, bio-gas slurry, or poultry droppings, respectively, in stocking ponds give good results. In rearing ponds, 50 percent of the total requirement is usually given 15-20 days prior to stocking of fry and the remaining portions in two equal monthly splits during the entire rearing period. In stocking ponds, on the other hand, 20 percent of the total requirement is applied initially and the rest is given in equal monthly splits. But if the ponds are treated with mohua seed cake to eradicate unwanted fishes, the initial application of the organic manure can be dispensed in both culture systems.

Efficiency of N-fertilizer in enhancing the productivity of pond depends largely on their forms. The commonly used N-fertilizers are urea, ammonium sulphate and calcium ammonium nitrate. Among these forms, urea is suitable for slightly acidic to neutral soil, ammonium sulphate for alkaline soil and calcium ammonium nitrate for acidic soil. Depending on the available nitrogen content of the pond soil, application of 50-70 kg N/ha (i.e. 108-152 kg urea/ha; 200-280 kg calcium ammonium nitrate/ha; 250-350 kg ammonium sulphate/ha) in rearing ponds and 75-150 kg N/ha/yr (i.e. 163-326 kg urea/ha/yr; 300-600 kg calcium ammonium nitrate/ha/yr; 375-7 500 kg ammonium sulphate/ha/yr) in stocking ponds give good results. The fertilizer should be applied in equal monthly splits alternately with organic manure with a fortnight interval.

Single super phosphate (SSP) is most commonly used as phosphate fertilizer in fish ponds. Depending on the available P₂O₅ content of pond soil, application of 25-50 kg P₂O₅/ha (i.e. 156-312 kg SSP/ha) and 40-75 kg P₂O₅/ha/yr (i.e. 250-468 SSP/ha) in rearing and stocking ponds, respectively, give good results. To get better utilization efficiency, P-fertilization should be applied at weekly intervals and the first instalment is to be given 7 days after initial organic manuring.

Muriate of potash (KCl) and sulphate of potash (K₂SO₄) are commonly used as potassium fertilizers in fish ponds. Application of 10-20 kg K₂O/ha (i.e. 16-32 kg KCl/ha or 20-40 kg K₂SO₄/ha) and 25-40 kg K₂O/ha/yr (i.e. 41-66 kg KCl /ha or 52-83 kg K₂SO₄/ha/yr) in rearing and stocking ponds, respectively, give good results. The fertilizer should be applied in equal monthly splits. In order to avoid depletion of oxygen, application of manure and fertilizers are to be suspended, if thick green or blue- green blooms develop in the pond..

Conclusion

As all the participating countries are developing in nature, many similarities can be expected in socio-economic pattern. The country status papers indicate that the species cultured in these countries and the aquaculture systems practised fall under some or all systems discussed earlier. Farmers are also, more or less under similar categories as previously mentioned, i.e. resource rich, middle and marginal or resource poor categories.

Fish culture technologies so far developed or available and under practice are mainly based on management considerations of species cultured, stocking density, health and environment monitoring. These have demonstrated higher production levels to the level of 10-15 t/ha/yr with carps. However, further improvement in the production, as rightly felt by the aquaculture scientists, depends on the development of technologies that can provide sustainable higher yields and that which are cost effective and environmental friendly.

One of the potential means to develop such technologies is to exploit the genetic potentials of the candidate species which are hitherto not properly tapped. Several genetic techniques are available and some of them (e.g. selective breeding) have already demonstrated their potential for producing genetically improved strains, with better survival and growth, strong resistance to disease and adverse environment, etc. All these traits help in enhancing production of high quality products. Use of genetically improved strains may also bring down the requirement of feed as their efficacy in converting the feed to fish flesh is usually much better than the normal individuals. This in turn brings down the cost of production and also may reduce the culture period due to their faster growth. Lower inputs of feed and fertilizers also make the technology environmental friendly. Genetic technologies may be particularly useful in intensive or super- intensive systems. Other technologies, purely management in nature, may be quite suitable for smaller ponds/tanks and seasonal water bodies. Integrated farming practices may also be suitable for extensive and semi-intensive systems.

The immediate need, in most of the participating countries for aquaculture practices, appears to be the availability and supply of fish seed particularly to farmers in the rural and remote areas where transportation of the seed is a problem as well as financial assistance for procuring inputs (e.g. feeds and fertilizers). Feed is one of the most important aspects in enhancing production in aquaculture. Urgent attention is required to develop suitable feed formulations and feeding methods. As already pointed out, while considering nutritional aspects, besides effective formulation of feed items, it is also important to identify cheap, effective and easily available ingredients. Fish health management is one of the most important managerial aspects for successful aquaculture since optimum survival of stocked populations and harvesting of good quality fish products depend on disease-free and healthy conditions of fish/prawn during culture periods. Regular monitoring of health and pond environment is highly essential for sustainability.

Making use of all the readily available water bodies is also one of the most important aspects in aquaculture for the over-all increase in the fish production of a country. In bigger countries like India, regional preference for a particular variety or type of fish also exist. In some states like West Bengal, Orissa or some of the eastern regional states, more people prefer Indian major carps and some other small varieties. In states like Punjab, consumers prefer fish with fewer bones, such as catfish or pomphret. In southern India, beside carps, murrels (Channidae) are preferred. In Karnataka State, Cyprinus carpio and Catla catla are preferred among other carp species. Similar preferences might also be present in other participating countries. Therefore, depending on the consumer preference, the species to be cultured can be selected and their farming should be encouraged and developed.

In order to further develop aquaculture in South Asia, technology exchange programmes along with human resource development are very important considerations, where countries like India and Bangladesh can play an important role. Networking of people and enhanced exchange of knowledge and experience should be encouraged. A focal station, either in India or in Sri Lanka, may be selected for updating database available from different countries and make them available or accessible to all countries.

Considering the present practice of aquaculture and economic status of the aquaculturists in the participating countries, emphasis may be given for low and medium input technologies and appropriate measures for transfer of technologies to the grass root levels. In this regard, necessary emphasis should be given for rural development having aquaculture as the main thrust area and judicious use of resources like water. Employment generation and poverty alleviation also can be incorporated in programmes for aquaculture development in the region. Since some of the countries in the region have already developed irrigation systems for supply of water for agriculture and allied activities, it would be a very useful and effective if water is used in an integrated way for multiple uses, employing flow-through system of aquaculture, horticulture along with agriculture.