Amongst the group of cyprinids known as the Indian carps, three species are of greatest economic importance. They are: catla (Catla catla Hamilton), rohu (Labeo rohita Hamilton), and mrigal (Cirrhinus mrigala Hamilton). Of lesser importance are: calbasu (Labeo calbasu), reba (Cirrhinus reba), white carp (Cirrhinus cirrhosa), bata (Labeo bata), and fringe-lipped carp (Labeo fimbriatus).
The original habitats of the three Indian major carps are the rivers and backwaters of Northern India, Pakistan and Burma. In addition, rohu inhabit the rivers of Central India and the south of Nepal (Terrai). The major carps have also been introduced into many other areas and countries. Though the Indian major carps have not gained such widespread cosmopolitan acceptance as the common and Chinese carps, which are more adaptable and tolerant to a wider temperature range, they are nevertheless the most important cultured fish species in India, Pakistan and Burma.
Under farm conditions, the Indian major carps grow rapidly, reaching marketable size within one year in tropical and subtropical countries. The catla is the fastest growing species. In natural waters, it frequently attains 300– 400 g in the first year, over 2 kg at the end of the second year, and 5–6 kg after three years. In fish farms, however, they can reach more than 1 kg during the first year. Rohu do not grow as rapidly as catla or as the Chinese major carps, attaining around 500–1 000 g in the first year under good farm conditions. The growth of mrigal is poorer than that of catla or rohu. As is the case for all cultured fish species, growth of farmed Indian major carps is a function of stocking density, availability of natural and supplementary food, competition with other species in the same pond, and environmental conditions.
The food and feeding habits differ between the three Indian major carp species. Catla are “column” feeders, mainly collecting the zooplankton at all depths. They occasionally also feed on the bottom, utilizing organic detritus, remnants of aquatic plants, and mud rich in organic matter. Thus, when stocked together, catla and bighead carp can be competitors for the natural food supply. Rohu is a mainly bottom and column feeder, preferring plant material including decaying vegetation. The food preferences of rohu thus overlap with other cultured cyprinids less than those of the other Indian major carps. Rohu have been used in Nepal to keep net cages free of fouling organisms such as sponges, bryozoans and algae. Mrigal have a narrower range of acceptable foods, being bottom feeders living mainly on decaying vegetation. They are therefore similar in feeding habit to the Chinese mud carp.
The Indian carps do not breed naturally in ponds or other confined waters. They are all river spawners, having semi-floating, non-adhesive eggs. The eggs have a large perivitelline space, and during swelling they increase their size by about ten times. Swollen, water-hardened eggs are rather delicate, and do not tolerate physical shocks well. This sensitivity to mechanical damage is a very important consideration when Indian carp eggs are to be incubated artificially.
The Indian major carps normally attain sexual maturity in their second year. However, as with other cyprinids, age at sexual maturity is temperature dependent. It can be delayed by 1–2 years in the northern extremes of the fishes' range.
The fecundity and breeding habits are similar in all the Indian major carps, and they can be considered collectively. Though fecundity naturally depends on the nutrition and living conditions of each individual, generally females spawning for the second and third time produce relatively the most ova, i.e., 100 000–200 000 eggs/1 kg of body weight. In nature, spawning coincides with the onset of the rainy season.
Not all details concerning the development of ova in the ovary are known. However, investigations have shown that in the northern hemisphere vitellogenesis starts in February and March. It ends after one or two months, and the ova are ready for final maturation and ovulation between April and June.
As the observations of the Brazilian scientist Rodolpho von Ihering demonstrated in 1936–37, most river spawning species of fish attain full ripeness of the ovary and become ready for induced propagation in confined, standing waters (ponds, lakes, etc.). The Indian major carps are no exceptions to this rule, and do not need any special treatment before hypophysation. The readiness for propagation is a natural requirement: the development of the ovocytes must be finished in good time before the breeding season, so that spawning can occur as soon as the environmental conditions become favourable.
Under natural conditions the Indian major carps spawn in groups once a year. Thus at any one time all the eggs in the ovary are in approximately the same stage of development. In cultured conditions, they can be induced to breed twice within the same spawning season, the second occasion following about two months after the first. However, it is not yet known whether the second batch of eggs are the result of a new vitellogenesis or merely the remainder of the first batch which did not undergo final maturation and ovulation the first time.
After natural spawning in selected stretches of the river, the semi-floating eggs drift downstream. To survive and develop successfully, the eggs and larvae must reach inundated areas where the post-larvae and fry can obtain enough food and avoid their enemies.
For centuries before artificial propagation techniques were developed (in 1960), the eggs and larvae of Indian major carps were collected from the rivers using large funnel-shaped, fine-meshed nets, in much the same way as for Chinese carps. The collecting nets were continuously watched day and night, and captured eggs and larvae transferred to the safety of specially made small earth pools on the banks. The larvae collected were a mixture of all the river-spawning carps and sometimes other carnivorous fishes. After a few days the number of larvae was estimated using small cups, and the fish were transported to fish farms for on-growing to market size.
Bundh breeding
Bundh is a local term which means a dike or an embankment. However, with regard to fish breeding, bundhs are special types of perennial or seasonal ponds or impoundments where riverine conditions are simulated during the monsoon season owing to the accumulation of rain water from the catchment areas. They can be categorized into two types, i.e., perennial types which are commonly referred to as wet bundhs and the seasonal ones, called dry bundhs.
Wet bundhs are ponds located in the slope of a vast catchment area. They have proper embankments equipped with inlets at the up-land side and outlets towards the low-land zone. During heavy rain, a greater portion of the bundh becomes inundated. The excess water flows out through the outlet. A wet bundh can be of any shape and dimension. Normally, a catchment of 20 to 100 times the area of the bundh is considered ideal for breeding.
A dry bundh is a shallow seasonal pond, or just a depression in the land, bounded by embankments on three sides which impounds freshwater from the catchment area during the monsoon season. It remains dry for the greater part of the year. In West Bengal, a catchment area of more than 5 times the area of the dry bundh proper is regarded as the best suited for breeding, while in Madhya Pradesh the recommended ratio for the same is 1:25. This ratio, however, depends to a large extent on the average rainfall in the catchment area. Dry bundhs in Madhya Pradesh are relatively bigger in size (0.2–2.5 ha) than those in West Bengal (0.1–0.5 ha).
In recent years, the dry bundhs in Madhya Pradesh have been equipped with a more modern system. These modern bundhs are generally masonary structures having a sluice gate in the deepest portion to facilitate complete drainage, as well as one or two spillways to allow excess water to flow out. Apart from the bundhpond proper, a dry bundh unit in Madhya Pradesh in most cases now includes a separate storage pond for stocking breeders, a watch tower equipped with a store room for keeping equipment, etc., and a battery of hatching containers to handle the fertilized eggs. Such modified dry bundh units are now also being introduced in West Bengal.
The seed produced in wet bundhs is not as pure as that obtained from dry bundhs because in addition to carps the perennial ponds of wet bundhs also harbour many varieties of unwanted fishes which breed along with the major carps. The wet bundhs being larger and deeper are also more difficult to manage than dry bundhs.
In common with other less important carp species, rohu and mrigal will spawn naturally in bundhs (also known as “acude” in Brazil) because river-like conditions are simulated during the spawning season. Fully mature broodfish are stocked into the bundh either immediately after the first rain water has accumulated (“dry bundh”) or when more water is present but levels are still low (“wet bundh”). When the monsoon begins, bringing torrential rain, the water level in the bundh rapidly increases, providing “fluviatile conditions” which stimulate natural spawning. The fertilized eggs are either allowed to hatch in the bundh or are collected with fine-mesh nets. The bundh spawning system might be considered primitive, but it has the advantage that it does not require hypophysation or specialized infrastructure. The technique is rather similar to the Dubisch method for stimulating spawning of common carp in ponds, and shares the common disadvantages that egg hatching and larval rearing cannot be well controlled and protected.
More up-to-date refinements of the bundh spawning system combine traditional methods with the use of hypophysation. Nowadays, attention is paid to the proper selection of breeders, sex ratio and a definite stocking rate in dry bundh breeding. In certain places some of the breeders from the selected lot (about 20 to 25%) are kept in separate hapas according to sex. The females are injected with an initial dose of pituitary extract at 3 mg/kg body weight. After about 4 to 5 hours, the second dose of pituitary extract is administered to the females at 8 mg/kg. At the same time, males receive their single dose of 3 mg/kg. Then the injected breeders are released back into the bundh amidst an uninjected group of selected breeders. The inlet and outlet of the bundh are opened to allow a steady flow of water through the bundh. The spawning activity of the injected breeders acts as a stimulus to the uninjected ones to follow suit. It results in mass spawning.
Recently, Chinese grass carp and silver carp have also been successfully induced to breed in a dry bundh in West Bengal.
Artificial propagation and fry production
The Indian carps are in every respect suitable also for artificial propagation. The broodstock can easily be raised in ponds if provided with the required space, healthy environment, natural and sometimes artificial feeds. Indian carps are not especially demanding in this respect, but it does appear that more consistent success is achievable with broodstock raised in big ponds than with fish held in small, shallow ponds.
Indian major carp brooders are normally raised in polyculture with other carps. Sufficient natural food can be provided by manuring the broodfish ponds.
The selection of male and female fish with good potential for use as broodfish is easy during or shortly before the normal spawning season. Females “ripe” for hypophysation have soft, bulging, but elastic abdomens; the sexual opening and anus are swollen and pink or red coloured. Males are more slender and usually ooze milt. The dorsal sides of the pectoral fins are rough, as are the scales and head.
Ideally, selected brooders should be injected and placed separately in ward tanks immediately after measuring or estimating their body weight. To ensure the best possible results all handling and treatment of brooders (capture, investigation of stage of maturity, transport, weighing, administration of injections, checking, etc.) should be made quickly to minimize stress.
Obtaining fertilized eggs
To induce spawning, a priming dose is administered between 16.00 h and 18.00 h. It contains 2 mg/kg of pituitary suspended in physiological salt solution and glycerine. Six hours later, females receive the decisive dose of 8 mg/kg body weight. Normally, freshly-collected pituitary glands are used. Induced spawning can be expected 4–6 h after the second injection. The optimal temperature is 24°C (acceptable range 24°–31°C). The lack of disturbance at night and at dawn helps stimulate the natural spawning.
The period during which Indian major carps can be induced to spawn is fairly long, for example in northern India from April to June, and in the southern part from June to August. In both cases these periods mark the onset of the monsoon.
Indian major carps can also be induced to spawn in “breeding hapas”: box-shaped containers made of close-meshed mosquito net cloth, kept stretched by ropes at each of their four top and bottom corners tied to bamboo poles. Breeding hapas can be 3.5 × 1.5 × 1 m, 2.5 × 1.2 × 1 m or 2 × 1 × 1 m. After the first injection, selected brooders are placed in the hapas in groups comprising one or two females with two or three males. Two equal doses of pituitary are administered, separated by an interval of six hours. The second dose is given at midnight, and brooders then spawn in the hapas during the night. The next morning, hapas are checked to ascertain whether spawning has been successful. After removal of brooders, the fertilized eggs are removed and their number estimated by volume before transfer to a “hatching hapa” for incubation. Hatching hapas consist of two layers of netting, the inner layer being of mesh size large enough to allow the passage of larvae through it and the outer layer of smaller mesh to retain them. Thus hatched larvae become separated from dead egg shells.
The Hungarian technology used for Chinese carps can also be successful. The priming injection consists of 0.5–0.8 mg of acetone dried common carp pituitary gland per kilogramme of body weight. The ground gland is made into a paste with a few drops of glycerine, then dissolved in physiological salt solution (7–8 g common salt in a litre of boiled drinking water); 0.5–1 ml is injected per kilogramme body weight. The decisive injection follows between 8 and 18 hours later.
The Indian major carps are group spawners, and males and females do not play together through all stages of sexual preparation as is usual with fishes which spawn in pairs. Therefore between final maturation (which is induced by the priming dose) and ovulation (which is induced by the gonadotropin deluge of the decisive dose) there is a sort of “waiting time” when the final maturation process has finished but ovulation not yet started. During this time all the eggs in the ovary can reach the same stage of maturation, ready for full ovulation a certain time after the decisive injection.
Fishes of most species treated in the above way give behavioural indications that ovulation has finished inside them. Alone or in company with males or other females they make typical spawning movements: head raised out of the water, spreading and twisting of the dorsal fin, swimming in circles (the so-called “swimming carousel”), quivering, etc. For most species the number of degree hours required between injection and stripping has been determined, but this is not yet known precisely for Indian major carps. It is also necessary to determine how long it takes after the proper spawning time for eggs to become over-ripe. This period varies between species, but is often less than one hour.
Ovulated eggs must be stripped immediately when the correct number of degree hours have elapsed or the females (with or without a signalling male) indicate behaviourally that ovulation has occurred. Any delay can reduce the success of fertilization.
To avoid losing eggs when unanaesthetized females are caught and removed from the water for stripping, special hand nets should be used, and at the same time the operator should hold the fishes' sexual opening closed. Hand-stripped eggs should be fertilized with sperm using the dry method as for common carp. Fertilized eggs are then transferred to incubation devices.
Incubation of the eggs
Many different incubation techniques have been practised with Indian major carps, with varying success. Where no water current is available, incubation hapas are used, but in these many eggs frequently suffocate and die. Nowadays some stations obtain eggs by induced natural spawning in circular or elliptical Chinese-type spawning tanks and the fertilized eggs are incubated in adjoining small round tanks receiving a continuous supply of water.
Where flowing water is available, various types and sizes of glass jars are used. However, it is not practicable to make big jars from glass, and preventing the eggs from washing out of the jars is also difficult. Consequently small glass jars are not recommended for incubation of Indian major carp eggs.
Instead, two alternative types of funnel-shaped incubators are recommended:
“Soft” incubators made of plastic sheeting with a sieve-cloth collar, designed for operation immersed in water. The usual volume of these is 40–50 l, having a capacity of 100 000–150 000 eggs.
“Hard” incubators made of fibreglass or other synthetic material. These are normally of 60–200 l capacity, and are equipped with filters to prevent the escape of eggs or larvae. On older models the filters are fixed, whilst on newer ones they are changeable.
The essential requirements of the developing eggs are:
Suitable water temperature. For Indian carps 27°C is optimal (range 24°–30°C).
Continuous supply of oxygen to every egg in the incubator. Inflowing water should contain at least 5–6 mg/l dissolved oxygen.
Removal of toxic metabolites (CO2, ammonia, etc.) which is effected automatically by the continuous water flow.
The water inflow should be regulated so that it does not cause mechanical damage to the eggs, which are especially sensitive in the early stages of development.
Generally, turbid water does not harm Indian carp eggs. Indeed, in natural conditions river-spawning fish often spawn in very turbid water.
The development time of Indian major carp eggs is temperature dependent and rather short, being similar to that of Chinese major carps.
Rearing of viable larvae until feeding starts
Separate, special larval-rearing devices, such as hapas, are now rarely used, but sometimes newly-hatched larvae are transferred to bigger (200 l) funnel-type systems. Normally, however, larval rearing is done in the same large incubators in which the eggs were hatched, with debris being removed by siphon when necessary. The environmental requirements of larvae are similar to those of developing eggs, except that they can tolerate stronger water flows.
The behaviour of Indian major carp larvae is very similar to that of Chinese carps. After hatching they first swim vertically, but after about a day they “lie down” and are moved passively with the water current in the incubators. One or two days later they begin to swim again, and soon fill their swim-bladders with air from the water surface. They are then ready to begin taking external food, and the post-larval stage of life begins.
The first food offered to post-larvae in rearing funnels is usually either boiled egg very finely ground or a microencapsulated egg diet. It is then advisable to stock the post-larvae as soon as possible into well prepared nursery ponds.
Nursing post-larvae
When 3–4 days old, larvae are stocked into a well prepared nursing pond.
Fertilization of the nursery pond is commonly done with 10 000–15 000 kg of organic manure such as cowdung, and 300 kg of lime per hectare. Generally lime is applied before manuring. Inorganic fertilizer is normally not used. The nursery should have plenty of fish food organisms of smaller size before the larvae are stocked. This is important since at that stage the yolk of the hatchling has been absorbed and it starts exogenous feeding. Rearing in the nursery lasts for about two weeks. The rate of stocking in a well-prepared nursery pond with adequate fish food organisms can be as high as 10 million/ha. The stocking density must be according to the condition of the pond and the amount of fish food organisms available. Two to four crops can be easily harvested in one breeding season, which normally lasts during the monsoon for 2–3 months. Before the next batch of larvae is stocked in the nursery pond, fresh applications of cowdung (5 000 kg/ha) and lime, are made to stimulate the growth of fish food organisms.
While zooplankton is considered to be the preferred food of Indian carp larvae and fry, some of the largest zooplankters such as Cyclops spp. and Mesocyclops spp. are reported to attack and kill fish larvae. In a well-prepared nursery pond, where carp larvae are generally stocked at a very high density their presence in large number causes heavy mortality of larvae. These copepods not only prey upon carp larvae and other zooplankton but also compete for food and space with the young fish. In order to have a better survival of the carp spawn, control of these copepods is essential. Soft organophosphorus insecticide, such as Malathion, given at 0.25 ppm five days prior to the stocking of carp spawn in nursery ponds kills copepods. Before stocking the carp larvae are given a bath of tetracyclin at 150 mg/5 l of pond water for l h followed by a weak solution of potassium permanganate for 30 seconds. The treated larvae then show a very high survival rate.
In fact, when copepods are eradicated rotifers start developing in abundance. These are choice food of carp larvae and fry.
Fry rearing
Rearing fry of about 20 mm length up to fingerling size (about 100 mm) is done in slightly bigger ponds of 0.1 ha. Preparation of these ponds involves clearance of weed, eradication of aquatic insects and undesirable fish, and fertilization with organic manure. Their management is basically the same as for nursery ponds. Since the rearing period for each crop of fry is 3–4 months, it is hardly possible to obtain two crops in one breeding season from the same fry rearing pond. With a stocking density of 0.3 million/ha, the fry grow up to about 70–100 mm fingerlings in 3–4 months, with 70 to 80% survival.
The Indian major carps are typical examples of fish which can realize their potential for productivity and growth only in polyculture.
Polyculture means stocking and raising different species of the same age group of fish, each species having different or partly different food and feeding habits. The co-habiting fish species generally do not compete with each other for natural food. If some competition occurs, it is balanced by the numbers of fish stocked, but it is not taken into consideration that they devour and to some extent compete for the same artificial feeds.
In polyculture the fish stocked are usually a mixture of plankton feeders and macrophyte (water-weed) feeders. The nutrients added to the water are taken up by both phytoplankton and the higher water plants. However, as with land plants, they may not grow at the same pace, so that one group may use up most of the nutrients leaving little for the other. In ponds it is attempted to make a balance by using both phytoplankton feeders and water-weed eaters. If stocking is done with fish that eat only water-weeds, these may be heavily grazed and the nutrients released may then be taken up by the phytoplankton, which becomes denser and denser, shading out the submerged water-weeds and preventing them from growing again. If stocking is done only with the plankton feeders, the phytoplankton may become so heavily grazed that the untouched water weeds grow very fast and use up all the nutrients. The plankton feeders will then starve. Sometimes not all the phytoplankton is grazed and zooplankton develops also. These need to be grazed down. Therefore, fish with different feeding habits are cultured together to achieve a balance where phytoplankton, zooplankton and water weeds can all be grazed on simultaneously.
Nowadays, Indian carps are often stocked in polyculture with Chinese carps. Examples of polyculture combinations are as follows:
Catla and silver carp
Catla and silver carp are both planktonophagous surface feeders, though catla eat predominantly zooplankton and silver carp phytoplankton. Because of their similar feeding zone and feeding habit, it is assumed that there exists some competition between these two species for the same ecological niche. Various combinations of catla and silver carp have been tried. Stocked in equal numbers in India, silver carp attained almost double the growth reached by catla at comparable survival rates. Even when silver carp is stocked at twice the density of catla, the former invariably grows faster.
Mrigal and common carp
When stocked in equal numbers, growth performance of mrigal and common carp appears to be similar, at least when supplementary feed is not supplied adequately. However, when supplementary feeds are given in large quantities, common carp stocked densely perform much better than mrigal, indicating their superior capability at utilizing artificial feeds.
Grass carp and other fishes
The association of grass carp with other carps has an important indirect benefit. Their excreta, consisting of semi-digested vegetable matter, serve as a food for bottom dwellers such as mrigal and common carp. However, grass carp is also a well known utilizer of supplementary feeds like rice bran and oil cakes. In order to avoid competition among grass carp, rohu, mrigal and common carp for supplementary feeds, it is always advisable to provide aquatic weeds in adequate quantity for the grass carp.
Stocking ratio
As far as possible, ponds are stocked with 10–20 percent of silver carp, this percentage depending on the availability of seed. Catla is also stocked and it is suggested that the combined stocking density of silver carp and catla should not exceed 30 to 35%. Both silver carp and catla feed mainly on the surface and thus they are grouped as surface feeders. Growth is normally adversely affected if their proportion in the stock is more than 30–35%. Rohu feeds in the underwater zone and is a column feeder. It grows well in deeper ponds. Therefore, ponds having more than 3–4 m depth of water need to be stocked with 15 to 20% of rohu. In shallow ponds the stocking density of rohu should not be more than about 10% of the total stock. Bottom feeders such as mrigal and common carp are stocked at a higher ratio which may together account for about 40–45%. Availability of aquatic weeds in the pond or in the vicinity decides the stocking density of grass carp. It is always desirable to keep 5 to 10% grass carp, and to feed them with aquatic weeds, vegetables or even with land grasses.
It has been seen in different parts of India that despite the higher numbers of Chinese carps used for stocking in polyculture, these species have recorded better growth rates than Indian major carps. However, their widespread use is dependent on the availability of their seed.
Supplementary feeding
The Indian major carps readily accept various kinds of feeds offered by the farmer in a systematic and organized way.
It has been experienced that through systematic supplementary feeding the production of the pond can be doubled or tripled. Such feeding is also beneficial in ponds where the soil and/or water are too poor to raise anything but a small crop of fish on the natural food.
The most important supplementary feeds used for the Indian carps are:
Agro-industrial by-products such as brans (wheat and rice) and oil cakes (groundnut and mustard);
Industrial wastes from tomato and juice factories, beer-breweries, etc.;
Grains and coarse flours form part of fish diets only when they have deteriorated or burnt, making them unsuitable for human or warm-blooded animal consumption.
For special purposes, such as farming broodstock, feeds of animal origin are used such as fish-meal, blood-meal and silkworm pupae.
Harvest
In fattening ponds, fish normally attain table-size within one-year. Catla, grass carp, silver carp and common carp under proper management can attain the marketable size of 1 kg in about 4–5 months. Periodic harvesting of such fishes and replenishment of the harvested stocks with fingerlings not only helps growth of other fishes by reducing the biomass, but also helps in raising more than one crop of a species in the same pond in a single year. Under intensive fish culture, with average survival of more than 80%, over 10 000 kg/ha/year of fish production can be obtained.
Final harvesting is done by seine net, either during the summer months when the water level falls or after the monsoon, or when the market demand is highest. However, harvesting in larger and deeper ponds poses serious problems. While surface feeders are easily caught with a seine net or drag net, bottomdwellers usually escape. It has been seen that the conventional drag net can collect about 90% of surface and column feeders, whereas only 20–40% of bottomdwelling fishes are caught with the same number of hauls in small ponds. To obtain the maximum catch of bottom-dwellers from large and deep ponds, a trap net is recommended.
