Grow-out and polyculture - Carps

Grow-out and polyculture

Of all the important carps used in aquaculture, it is probably only the common carp that is produced in monoculture, whereas both Chinese and Indian major carps are almost always grown in polyculture. Even common carp is now increasingly used in polyculture with some of the species of Chinese or Indian carps, especially with (i) grass carp and silver carp, (ii) catla and rohu or (iii) tilapia, the grey mullet and silver carp.

Stocking rates

In the grow-out of carp, either in monoculture or in polyculture, the basic objective is the production of an optimum quantity of the desired size of fish, at minimum cost. There are a number of interdependent factors that affect productivity and cost. Stocking rate or the density of fish in the pond, the quality and quantity of food produced by fertilization or artificial feeds supplied to the pond, water temperatures, availability of oxygen and build-up of metabolites in the pond are all factors that influence growth rate and production. The size of fish at stocking, the duration of culture and the size at which the fish are harvested will also influence the total yield. The growth potential of the genetic strain used is another important factor to be considered. To this should be added the influence of natural productivity of fish food in the ponds, even when fertilization and feeding are adopted.

Many fish farmers adopt the system of multi-size stocking, which involves stocking fry, fingerlings and young adults belonging to different size-groups in the same pond, in order to utilize the food resources more efficiently. This practice involves periodic harvesting of the marketable fish and in some cases even additional stocking. There is also the practice of multistage stocking which consists of stocking fish in progressively larger ponds as they grow in size, reducing the stocking rates as required.

From the above it would be clear that the formula for determining the number of fish to be stocked, obtained by dividing the expected total production by the expected individual growth and adding it to the expected loss due to mortality, can only give a general indication. As many of the influencing factors are site-specific, it is necessary to decide on the stocking rate or population density in a pond according to the culture practices adopted, environmental conditions and market requirements.

Several stocking rates and grow-out practices are in use in different areas. In Europe, because of climatic conditions, it generally takes three years to grow the fish to the preferred market size of 1000–2000g weight, except in southern Europe where marketable fish can be produced in two summers. This involves keeping finger-lings and yearlings in wintering ponds. 

In tropical and semi-tropical regions, fish of 600–1000g or more can be grown in one year or less. In monoculture, under normal management, a stocking rate of 4000–5000 fingerlings, of 2.5– 5cm length, per ha or 2000–3000 fingerlings, of 5–10cm length, per ha is recommended. With intensive feeding and aeration of ponds, higher rates of stocking can be adopted. If the fish are to be raised to market size in a shorter time, a lower stocking density of 3000–5000 per ha may be necessary.

Polyculture

Polyculture is now the most common practice of carp culture and several species combinations and stocking rates have been developed. These combinations are not always targeted to obtain the maximum biomass of fish from a unit area, but are often based on one or two species as the main crop for which there is the highest market demand, and the others as subsidiary compatible species that will utilize parts of the food resource that may be wasted otherwise.

Polyculture is adopted even in the rearing of fingerlings in Chinese carp culture. However, because of the overlap in feeding habits, when bighead is a major species, silver carp is excluded; similarly grass carp and black carp are seldom raised together. Some typical stocking rates are summarized in Table 16.1. The duration of fingerling production from 30–40 days old fry ranges from 120 to 270 days, extending from about early July to January, and in some cases up to March or April. Adult fish for the market are raised from these fingerlings in one to two years. Bighead, silver and mud carp reach market size in one year and grass and black carp in two years. The rate of growth and the size reached vary considerably between different parts of China, depending on climatic conditions.

Besides carp, a number of other species are stocked in polyculture, the more common ones being tilapia (Tilapia mossambica), Wuchang fish (Megalobrama amblycephala), crucian carp (Carassius auratus), red eye (Squaliobarbuscurriculus) and white croaker of white amurbream (Parabramis pekinensis). Snakehead (Ophicephalus (= Channa) argus) and man-darin fish (Siniperca chautsi) are sometimes added to feed on weed-fish and other unused aquatic organisms. Many farms adopt the system of multiple stocking and harvesting in rotation. Ponds are stocked with fingerlings in high densities and as they grow in size the bigger fish are harvested and additional fingerlings added. In this way, grass carp can be harvested up to six times a year and mud, black and common carp up to twice a year. As is to be expected, annual yield per ha varies between farms and regions within China, from 310kg/ha in extensive farming to about 3000kg/ha in semi-intensive systems and about 8700kg/ha in intensive systems. Much higher production has been shown to be possible under experimental conditions.

Hepher and Pruginin (1981) reported on the results of polyculture with comparatively fewer species in a sub-tropical climate in Israel. Higher weights of the stocking material, intensive fertilization and supplementary feeding and temperature conditions contributed to the higher yield of up to 10.5 tons/ha.

In polyculture of Chinese carp in fresh-water ponds in Taiwan, tilapia and some of the coastal species such as milkfish and mullet are included. A small number of the carnivorous sea perch (Lateolabrax japonicus) are also added to control small weed-fish and young tilapia that may be produced by wild spawning in the ponds.

The traditional Indian major carp culture in India is an extensive production system with only limited fertilization. The different species are stocked in varying ratios, the most common being; Catla 30 per cent, rohu 60 per cent, mrigal 10 per cent. When calbasu is included, the percentage of rohu is reduced to 50 to provide for 10 per cent calbasu. By altering these ratios according to the primary production in the ponds, and more intensive stocking and supplemental feeding with locally available feedstuffs like oilcake and rice bran, higher rates of production have been obtained. In recent studies in polyculture of composite culture in India, the common carp and some of the Chinese carps (mainly silver and grass carp) have been added.

In later studies, this combination has been further enlarged by adding a small number of carp hybrids (calbasu male x catla female), grey mullet and a carnivore, chital (Notopteruschitala), to control weed-fish. With a high stock-ing density and greater use of fertilizers, higher rates of production of marketable fish have been achieved in culture periods of one year or less. It is believed that the maximum yield that can be obtained in polyculture under Indian conditions is around 7 tons/ha in eight months of rearing and 10 tons/ha in one year (Tripathi, 1983).

Evaluating production performance in different cropping patterns in polyculture of a six carp species combination (catla, rohu, mrigal, silver carp, grass carp and common carp) in Orissa, India, Jena et al. (2002) showed that a two-crop system performed better than single stocking – multiple harvesting and single crop-ping systems in a yearly cycle, as indicated by better yields and feed conversion values. The net production and FCR in the two-crop system, stocking the same combination and density (10000/ha) as in the other two systems, were 6828kg/ha/year and 1.67, while in the single cropping, the corresponding values were 5844 and 3.16 and for the single stocking – multiple harvesting system the figures were 6320 and 2.53 respectively.

With a view to establishing a genetically broad-based population and initiating a genetic improvement programme for developing a more productive stock of rohu (Labeo rohita), Reddy et al. (2002) evaluated six stocks of rohu in India (one locally farmed and five riverine, from five major rivers of Northern India) for growth and survival under monoculture and polyculture (rohu full-sib groups stocked along with catla, and mrigal).

 The stock effect on growth and survival of rohu was found to be inconsistent, but the interaction between production system and survival was significant, though much lower than the full-sib effect. This provides a good base for further fruitful work on selective breeding of rohu, especially as rapid inbreeding among the farmed stocks of rohu has already resulted in poor survival and growth (Eknath and Doyle, 1985; 1990).

Polyculture of common carp together with tilapia and other species like nilem (Osteochilushasselti), tawes (Puntius javanicus), kissinggouramy (Helostoma temmincki) and gouramy (Osphronemus goramy) has been practised in Indonesia. In polyculture of common carp, generally 80 per cent would be common carp and the rest tilapia. Two other common combinations are with tilapia as the major species and kissing gouramy or tawes as the major species.

Rainbow trout (Salmo garidneri) is some-times grown in combination with common carp in countries like Poland and Czechoslovakia. One-year-old trout are stocked at the rate of 1200–1500/ha, forming about 15 per cent of the stock in the pond. The trout feed on the abundant weed-fish and carp fry produced by wild spawning. Under favourable water conditions (mainly oxygen levels and temperature), an additional production of about 40kg/ha of trout is obtained, in addition to carp (Lavrovsky, 1968). Other polyculture systems which include carp are the stocking of the European catfish (Silurus glanis) in carp ponds to control weed-fish, and of carp and the grey mullet in eel ponds to utilize the large amounts of feedstuffs that eels leave uneaten.

Pond fertilization and feeding

Pond culture of carp is in most cases based on fertilization and supplemental feeding. Organic and inorganic fertilizers and their use in fish ponds have been described. In most countries, particularly in Asia, there is a distinct preference for the use of organic manures including green manure and compost in carp ponds. It has been demonstrated that manure increases zooplankton and chironomid production in carp ponds, probably as a result of the high production of bacteria and protozoa developing on the organic matter of the manure. The decomposing organic matter or detritus has a high protein content, possibly due to the growth of bacteria, protozoa and microalgae. The rate of application of manure has necessarily to be based on the environmental conditions and stocking densities. Besides the dosage, the mode of application is also important. Application of the entire quantity of manure required in one lot, or sporadically, sometimes results in the development of algal blooms, especially under tropical conditions. It also affects the oxygen concentration in the ponds. A dose of 100–120kg dry matter per day can be used safely in most situations. The quantity of manure to be applied has to be increased with increasing standing crop of fish in the ponds. Based on East European practices, Woynarovich (1975) recommended a lower dosage of 20–35kg per day per ha. The rate of organic manure application in India is 10000–20000kg/ha (wet weight) per year. Often inorganic and organic fertilizers are used alternately: inorganics at fortnightly intervals and organics at monthly intervals.

One of the major advantages in the use of chemical fertilizers is that, since the nutrient contents are generally standard, the dosages required can be easily determined and followed with some confidence. In intensive methods of culture with higher densities of fish in the ponds (up to 3000 fish per ha), fertilization with a standard dose of 60kg/ha single superphosphate and 60kg/ha ammonium sulphate or liquid ammonia every two weeks is considered necessary in Israeli carp ponds when the temperature is over 18–20°C. In medium to highly productive polyculture carp ponds in India, where supplementary feeding is adopted, NPK fertilizers are applied in the ratio of 18:8:4, at rate of 500kg/ha per year. In less productive ponds, low in nitrogen and phosphorus, the fertilization dose is increased to 120kg N and 90kg P2O5 per hectare.

The feeding habits of the different species of carp have been described earlier. The nutritional needs of common carp have been studied in some detail. The major advantages of the common carp are that it can digest carbohydrates and accepts several feed-stuffs such as cereals, legumes, oil cakes, slaugh-terhouse refuse, ground trash fish, etc. The most common feed used in Israel is sorghum, although broken or poor-quality wheat and bitter lupine have also been used. Out of a wide variety of feedstuffs, rice bran is most popular in both nursery and rearing ponds for carp in Asian countries. Rye and barley are the main feedstuffs used in European carp farms.

Several countries now use processed feed mixtures of pellets in carp farms, but usually as supplementary feed along with fertilization. Hepher and Pruginin (1981) give the following two formulations of carp feeds used in Israel:

a diet containing 18 per cent protein:

80–90 per cent finely ground wheat

5–10 per cent fish meal

5–10 per cent soybean meal.

a diet containing 25 per cent protein:

60–70 per cent wheat (partly replaced some-times by other cereals)

15 per cent fish meal containing 65 per cent protein

15–25 per cent soybean meal 3–4 per cent soapstock oil.

The diets formulated on the basis of nutritional needs of common carp appear suitable for Indian and Chinese carps as well, because the dietary protein requirements are very similar. The crude protein levels required for optimum growth at 30°C have been estimated as 450g/kg for common carp, rohu, mrigal and grass carp (Sen et al., 1978) and 410–430g/kg for grass carp (Singh, 1983).