An understanding of the following basic principles of freshwater pond fish culture is essential.
Like agriculture, fish culture is also based on a series of processes involving reception and transformation of solar energy. In the pond ecosystem solar energy is utilized for primary production by chlorophyll-bearing plants such as planktonic algae and macrophytes. This conversion of solar energy into chemical energy (food) is guided by the photosynthetic and chemosynthetic activities going on in the aquatic plant community and the rate at which this is carried out is called primary productivity of that ecosystem.
A part of the primary production is cycled through different trophic levels resulting in fish production. Here comes the community of consumers that comprise microscopic as well as large animals, which are unable to synthesize their own food and feed upon primary producers. Different forms of pond life are linked together through predator-prey relationship (Fig.1). This chain of food production, which follows a general pattern, primary producers - herbivores-carnivores - appears too simple and straight. But, in fact, it is a complex food web with various cross linkages.
Fish populations may be classified into several trophic levels, depending upon their position in this food chain. Phytophagous fish such as grass carp and silver carp belong to the second trophic level as they feed upon the first trophic level organisms. Likewise, zooplankton, feeding upon phytoplankton, also belong to the same category. Carnivorous fish communities thriving upon zooplankton or herbivorous fishes occupy the third trophic level while other predatory fishes preying upon carnivorous fishes belong to the fourth trophic level (Fig. 2). A relatively simple food chain operates in fish ponds, but a complex one occurs in lakes and other larger aquatic ecosystems. The picture becomes even more complicated in large water bodies such as rivers and seas where complex food chains are referred to as food webs which in fact represent several interconnected food chains. There are some fishes which occupy mixed positions, between different trophic levels. They consume both plants and animals and as such, cannot be naturally categorised into any one particular trophic level.
A properly managed pond presents an example of a simple food chain under simple conditions. Here the number of food chains is reduced by encouraging the growth of phytoplankton. The macrophytes such as rooted green plants, floating plants, etc., are not allowed to grow. Phytoplankton is consumed by the zooplankton in the water column, whereas its detritus is utilized by benthic invertebrates. Phytoplankton, zooplankton, detritus and benthic organisms serve as food for the stocked fishes such as the desired carp species. Thus, as much of the available solar energy as possible is utilized for fish production by proper pond management.
Primary productivity is dependent on light, carbon dioxide, temperature and essential nutrients, each of which can be a limiting factor. Of these factors affecting primary production in ponds, the one that can be manipulated easily is the quantity of nutrient elements through the application of nitrogenous, phosphatic and potassic fertilizers, as in agriculture. In ponds, only the top 2 to 5 cm of soil is concerned with nutriention exchange, and the soil below plays a negligible role in the production cycle. Undrainable ponds receive dissolved nutrients and sedimentary particles carried by rain water from the catchment area. Besides, production and decomposition of minute plant and animal organisms in ponds also modify the properties of the pond bottom to a great extent. The nature and quantity of fertilizers determines the species composition to be used in a culture system. At low phosphate concentration, diatoms are common, but with increasing concentrations green algae become more frequent, eventually giving way to blue-green algae. In addition, excessive phosphate gives rise to phytoplankton blooms which check the light penetration and thus lower the pond productivity through ‘autoshading’ (Prowse, 1968).
Figure 1. Pond Ecosystem
Figure 2. Food Pyramid
Light energy is one of the major inputs in primary production and hence the success of fish culture depends largely on the efficient utilization of incident light. When incident light strikes the water surface, it is partially reflected and partially transmitted into the water where part of it is utilized in the process of photosynthesis and the rest is scattered or absorbed by suspended particles. In turbid waters, more light is scattered or absorbed, thus allowing the light penetration only to shallow depths. The rapid disappearance of light in such waters affects adversely the growth of diatoms. The bottom layer of water, being devoid of photosynthetic plants and also being in close contact with the decaying organic matter, suffers from oxygen depletion causing critical stress conditions for the fish. Thus, it is important that primary producers must provide oxygen to support the total biological respiration during darkness and also during the less favourable (warmer, overcast or rainy) days apart from providing food for the second and third trophic-level fish.
| Temperature (°C) | Solubility of oxygen (mg/l) | Temperature (°C) | Solubility of oxygen (mg/l) |
|---|---|---|---|
| 15 | 9.76 | 26 | 7.99 |
| 16 | 9.56 | 27 | 7.86 |
| 17 | 9.37 | 28 | 7.75 |
| 18 | 9.18 | 29 | 7.64 |
| 19 | 9.01 | 30 | 7.53 |
| 20 | 8.84 | 31 | 7.42 |
| 21 | 8.68 | 32 | 7.32 |
| 22 | 8.53 | 33 | 7.22 |
| 23 | 8.38 | 34 | 7.13 |
| 24 | 8.25 | 35 | 7.04 |
| 25 | 8.11 | ||
The concentration of dissolved oxygen in the water, which depends on the temperature, is an essential component of the aquatic environment to govern the carrying capacity of a pond. Variations in concentration of dissolved oxygen may occur due to the following three important factors:
the saturation level of oxygen in water decreases as the temperature rises;
supersaturation is an unstable state, and
plants not only photosynthesize to produce oxygen, they also respire and consume oxygen.
The saturation value for dissolved oxygen available for fish life at 20°C water temperature is more than that at 30°C at a particular atmospheric pressure (Table 1). Dissolved oxygen (Do) concentration is always high at lower temperatures and gradually decreases with increase in temperature. In natural waters, including undrainable fish ponds, DO values are constantly changing because of biological, physical, and chemical processes (Fig. 3). The air above the pond water surface may be considered to have a more or less constant percentage of oxygen. However, the partial pressure of oxygen in the air may vary slightly at a given location because of differences in atmospheric pressure. Transfer of oxygen from air to water will occur when water is undersaturated with DO, and oxygen will diffuse from water to air when water is supersaturated with oxygen. However, the diffusion of oxygen into the pond water is very slow, except under conditions of strong turbulence, hence the most important source of oxygen is that generated during photosynthesis. As discussed earlier, light is the most essential source in photosynthesis where penetration into the water column is regulated to a large extent by suspended or colloidal particles (turbidity) and also by dense plankton levels. Sometimes, phytoplankton blooms or algal scums limit light penetration causing reduction in photosynthetic rates, even in waters with adequate nutrient concentrations. Oxygen production by phytoplankton is greatest near the surface and decreases with the increase in depth because of self-shading. When heavy infestation of aquatic weeds and dense bloom of plankton occur, the situation becomes much more complex. On the other hand, these are additional sources of oxygen at daytime; but on the other hand, they also respire and consume oxygen throughout day and night. At times the pondwater is supersaturated with oxygen during the day, which is a highly unstable state, while during the night, a greater proportion of oxygen is used up for their respiration, thereby reducing the availability of oxygen to fish. Thus, it creates a wide fluctuation in the level of dissolved oxygen, adversely affecting fish life. Figure 5 shows a situation created by algal bloom or weed infestation where wide variations between actual and expected oxygen production do occur (Figs. 4 and 5). In fact, under such situations oxygen production increases to its maximum during the daytime leaving surplus for the fish even after consuming for their own respiration, but at night this surplus level drops down to critical level. Under conditions of heavy algal blooms and weed infestation, the phytoplankton and aquatic weeds actually consume more available oxygen during day and night than they produce during the whole day (Fig. 6). During cloudy days, when the incident light is inadequate for phytosynthesis, the situation in terms of availability of DO becomes worse.
Aerobic decomposition of organic matter by bacteria is also an important drain on the oxygen supply in ponds. Aerobic decomposition requires a continuous supply of oxygen and proceeds more rapidly when DO concentrations are near saturation. However, decomposition also occurs under anaerobic conditions, but the rate of degradation of organic matter is not as rapid and complete as under aerobic conditions. Under aerobic condition, the end product of decomposition is primarily carbon dioxide. At times high rate of bacterial decomposition of dead organisms and other organic bottom deposits lead to a condition favouring the increase of the level of carbon dioxide and other abnoxious gases, with a simultaneous depletion of DO, resulting in fish kills and planktonic collapses (Radheyshyam et al., 1986). Therefore, it is important that the pond water should provide adequate oxygen to support the total biological respiration during the hours of darkness.
Figure 3. Oxygen Cycle in Pond
Figure 4. Effect of Algal Bloom on Oxygen Production
Figure 5. Dial Oxygen Production/Consumption Pattern under Algal Bloom/Weed Infestation
Figure 6. Relation between Stocking Density and Production
The choice of fish species is very important in maximizing production, both in terms of quantity and quality.
Since considerable amount of energy is lost in successive trophic levels of the food chain, efficient fish culture always aims at making the chain as short as possible. Because of this, herbivorous fishes are always preferred to carnivorous fishes, the latter being mostly excluded because of their longer food chains. Mixed species farming or polyculture yields a higher production than single species farming. It is obvious that any single species cannot utilize all the available food in a pond because of its specific feeding habit and hence a combination of compatible species with complementary feeding habits are usually stocked to make better use of the natural food available in the pond. Selection of the species should be based on the productivity of a pond, availability of artificial food resources, availability of seed and the marketing prospects. The principal considerations in species combination are that they have complementary feeding habits, they occupy different ecological niches, they attain marketable size at more or less the same time, they tolerate each other, and they be non-predatory in nature. A combination of plankton and macrophyte feeders is most usual. Ungrazed phytoplankton is fed upon by zooplankton and to utilize them, the zooplankton feeders are included in the combination. The combination of the phytoplankton-feeding silver carp (Hypophthalmichthys molitrix), the zooplankton-feeding bighead carp (Aristichthys nobilis) and the weed-eating grass carp (Ctenopharyngodon idella) is well known in China and Southeast Asia. In India, under composite fish culture, six species of fish viz. catla (Catla catla), rohu (Labeo rohita) and mrigal (Cirrhinus mrigala) along with three Chinese carps such as grass carp, silver carp and common carp (Cyprinus carpio) are stocked together so as to utilize most of the fish food organisms present in the pond (Lakshmanan et al., 1971; Sinha et al., 1973; Chaudhuri et al., 1976). Other similar combinations may work just as well, but the most important aspect is to try to establish a balance between the species based on the food spectrum of the pond (Sinha, 1971).
Stocking density: Normally fish production increases with the increase in the number of fish stocked per unit area upto a level and then starts decreasing (Fig. 6). Higher stocking density results in increased total production, as there is better utilization of the available food, but in such cases the individual weight and size is reduced. On the other hand, lower stocking density yields larger individual fish. Proper stocking rate for a pond is that optimum level which results in a given time, usually a year, in a production which is highest in quantity and quality of fish, and most profitable. In ponds where no artificial feed is used, the total crop becomes dependent on the primary production and in such cases simply by increasing the stocking density, the increase in the total production is not possible. Even with supplementary feeding the scope of increasing stocking density and fish yield is limited; it increases to an optimum level and then starts decreasing.
Under crowded conditions fish compete for food and space and are stressed due to aggressive interaction. Fish under stress exhibit decreased feed consumption and slow growth and are predisposed to many parasitic and microbial infections. Increase in stocking density simultaneously increases the total oxygen demand with obvious dangers. In undrainable ponds, accumulation of excretory products of the fish population also suppresses their growth rate. With efficient removal of such metabolites by aerating the pond water, the stocking rate can be increased further, thereby enhancing production.
It has been observed that under identical conditions of management levels and stocking density fish grow bigger in larger ponds. In Malaysia, grass carp, Puntius sp. and monosex Tilapia mossambica grew bigger in ponds of a larger area indicating the living space phenomenon (Chen and Prowse, 1966). In other words, the rate of production in a 0.2 ha pond will be more than double that of a 0.1 ha pond, despite the fact that the stocking rate per unit area is the same and all other management components including the genotype of the stocking materials and ecological conditions remain the same. The ponds having larger surface area are subjected more often to wind action resulting in greater rate of diffusion of atmospheric oxygen into the water. Larger ponds have other advantages also, viz. better cooling action by wind. In smaller ponds, water tends to stagnate and in hot weather tends to heat up quickly.
Though it is preferable to have ponds of a large size, there is a physical limitation. Large ponds are difficult to fill and even more difficult to harvest. There must be an optimum size and shape of the pond to balance size with practicability of management, i.e. large enough to allow proper growth of fish, but at the same time small enough to be manageable. Recommended optimum size is 0.4 ha – 1.0 ha (Sinha and Ramachandran, 1985).
With the increase in carrying capacity of the pond either by aeration or circulation of water, fish growth can be increased further by supplementing the natural food with some artificial feed. This is the single most important management component for increasing production. In intensive and semi-intensive culture of fishes, supplementary feeding is indispensable. The quantity of feed and the form in which it is offered affect the rate of consumption. Temperature, dissolved oxygen level, crowding and health condition, etc., affect the rate of food consumption.
Organic matter and mineral constituents of the pond soil supply the required nutrients for chemical and biochemical production processes. The pond bottom also provides a suitable environment for the decomposers like bacteria and fungi to mineralise organic components of the pond sediment and release soluble nutrients. Sometimes such nutrients are not available in sufficient quantity in the pond and hence they are added from outside in the form of fertilizers. Since plankton production is often limited by inadequate quantity of phosphorus which is essential for the assimilation of nitrogen into cellular matter, phosphatic fertilizers are widely used in fish culture.
Unlike phosphorus, availability of nitrogen does not depend on the inherent status of soil, since it is brought to the soil by different processes. Nitrogen fixation by azotobacteria, blue-green algae, atmospheric electric discharges and photochemical fixation are some of the potential sources of pond nitrogen. In a tropical climate, the fixation of atmospheric nitrogen by blue-green algae is of considerable importance. However, when nitrogen is added from outside, its form, viz., ammoniacal or nitrate is also very important. It is advisable to use the ammoniacal form of nitrogen in acid and neutral soils and the nitrate form in alkaline soil (Saha, 1969). Though there is a considerable loss of nitrogen from the ammoniacal form in alkaline soil, the use of ammonium sulphate in low doses is usually recommended, keeping in view the role of sulphate in reducing the soil alkalinity.
Potassium is the other essential nutrient for plant growth. In ponds it is easily available both in soil and water and does not form insoluble salts and is rarely deficient except in acid peaty soil. Yet, a little potassium when added to the pond, stimulates the production of plankton.
Organic matter of the pond sediment is also an essential factor regulating the bacterial activity. In this context, the ratio of organic carbon to total nitrogen (C/N ratio) is important. Periodic application of organic manures ensures to a certain extent replenishment of nutrients and also provides an energy base for bacterial activities. Apart from this, the organic matter and the bacterial flora are also directly consumed by zooplankton and some fish species.
Various intensification approaches such as increased stocking rates, increased feeding, fertilization programmes, etc., sometimes result in nutrient accumulation, frequent appearance of algal blooms, dissolved oxygen deficiency and other water quality problems in undrainable ponds. As a result of such water quality and environmental problems, the infectious diseases and their control assume importance. A fish farming system is unique in that the farmed animal is poikilothermic and lives in water where respiratory oxygen level compared to air is limited and becomes critical at times. Further, metabolic waste products, left-out feed materials and organic load of the pond bottom can affect certain exposed vital organs and tissues of fish. All such factors affect fish health and contribute to the risk of disease outbreaks.
The above basic facts need careful consideration while planning for freshwater pond fish culture. The habitat of an undrainable pond is very varied and dynamic, but can be monitored and managed for increasing fish production.
