In growing fish in rice fields, we have to look at critically the physicochemical and biological environment of the rice fields, the purpose being two-fold - one is to choose proper species of fish and prawns for culture (species selection - besides the already well known species in use) and the other is to adapt the rice field and/or technology of operations to suit optimum growth and yield of both rice and fish under culture. An aspect of interest which should be mentioned here is the element of time. In combined culture of rice and fish the duration of rice crops is 100 – 150 days - and in cases where the same fish lot is permitted to grow for the next crop or for the fallow period there would be some abrupt change in the environment.
The various factors influencing fish culture in the rice fields have been discussed by Coche (1967), Huet (1972) and Vincke (1979). The factors concerned can be listed as under:
Altitude and latitude
Rainfall and water supply
Water temperature and dissolved oxygen
Water turbidity
Salinity and tidal flux (in coastal waters)
Fertility and richness of plankton and other fish food organisms
Polluting effects of chemicals.
We have referred to some of these factors in our discussion on advantaged and disadvantages of fish culture in rice fields.
Fish culture in rice fields is said to be practised between latitudes 20°N and 20°S and between 1,000 and 1,500 metres altitude (Schuster, 1955; Coche, 1967). Such regions are found in Preanger highlands of Java, the Batak areas in Sumatra, the inland areas in Madagascar and the Thailand region (Coche, 1957), but it is well recognized that several paddy growing areas exist at lower attitudes - the brackishwater paddy growing areas of Bengal (“Basabada”) and Kerala (“Pokkali”) in India are coastal based. Coche (1967) states that “at lower altitudes in tropical climates the possibilities of successful rearing of fish are more restricted: Water temperature increases, dissolved oxygen content decreases, water acidifies and carnivorous fish become predominant”. Coche does not favour coastal low water areas and swamps also as there are no proper drainage and oxygen content and mineralization rate are low. However, in the coastal saline water area in India profitable culture of prawns and brackishwater fishes is practised along with deepwater (floating) paddy. Perhaps the low saline flats have the advantage that high production technology for rice by using more fertilizer and pesticides is less applied there.
Schuster (1955) and Coche (1967) hold that a minimum annual rainfall of 1200 mm is required for fish culture, as “this amount of water makes the flooding of the fields possible for at least 90 days”. It is now well known that in areas with very little rainfall, 2 crops of rice are grown by use of irrigation water mainly in many parts of Asia and here fish culture in rice fields is practised. Therefore the minimum amount of 1200 mm rainfall does not appear to be critical as long as irrigation water is available
Rice fields are usually bounded areas in which either rain water or irrigation water is contained - in the latter suitable water supply systems, such as dams, reservoirs, tanks, weirs, canals etc, have to be constructed for ensuring water supply. The amount of water required would “depend on the climate (temperature and rainfall), the soil characteristics (e.g. texture, structure and retention power), the quality of the bunds (soil, compaction) and the intensity surveillance the farmer is apt to devote to his field” (Coche, 1967). An average value of usage of water in Indonesia is considered to be 1 – 2 litres of water per second per hectare. But quite often water is not let to flow through the paddy fields but is filled and retained for upto 10 days and only the water lost by seepage and evaporation is filled up again, thus a continuous flow of water indicated might be misleading (Hora and Pillay, 1962). The filling of water in saline swamps (Java, Bengal and Kerala) is dependent on flood water tidaal flow.
Depth of water retained is, however, critical on the rice variety and the season - varying often from 5 – 25 cm, but the deeper trenches and swamps (refuges) dug in rizi-pisciculture fields would ensure deeper (see “temperature”) water for fish. When fish is grown in rotation deeper waters can be held (20 – 60 cm) or even 150 cm as in Arkansas, U.S.A.
Local temperature, as obvious, varies greatly with season, latitude and altitude, In the tropical belts where rice is grown water temperature can often rise to lethal levels of fishes - 33 – 40°C (Schuster, 1955; Tonolli, 1955). The upper lethal temperature of Tilapia mossambica (Ananthakrishnan and Kutty, 1974); the snakehead (Channa punctatus) (Ananthakrishnan & Kutty, 1976); common carp and Indian carp fingerlings (Kassim, 1978) are below or within this range. Coche (1967) mentions that carp are usually fed between 16 and 32°C, and recommends stoppage of feeding above 27°C. Similarly low lethal temperatures are equally critical 20°C for T. rendalli (=melanopleura) and 15°C for T. mossambica for active feeding - (see also lecture on “Physico-chemical characteristics of water” - under Site Selection for Aquaculture). Deeper trenches and refuges would offer more tolerable water, temperature wise, so fish could seek protection therein.
An increase of 100 metres in altitude is expected to decrease temperature by 0.56°C (Coche, 1967), but this can be applied as obvious within limits only. As already discussed by us elsewhere temperature and dissolved oxygen content have an inverse relation i.e. the higher the temperature, the lesser the oxygen. Thus in tropical waters especially at the lower plains the DO content will be low, often critical, except for air breathing fish. When putrifying organic matter is present the water can be completely devoid of oxygen. Hickling (1962) recommends application of a light dose of fertilizer phosphate) in such waters so that an algae bloom could sustain oxygen supply, but this can also lead to depletion of oxygen further as the algae themselves would consume oxygen in darkness.
Besides the airbreathing fish, which are generally predators, more low oxygen tolerant and eurythermal species, such as carps and tilapias, can withstand to a great extent the temperature and oxygen changes in paddy fields.
Turbidity of the water in the rice fields is often very high. The only redeeming feature in this context is that the water depth is not high. The light cutting through the paddy stems can penetrate and still maintain high productivity (Vincke, 1979). The species chosen for culture should tolerate high turbidity. The carps and airbreathing fish do tolerate high turbidity. The airbreathing fish can efficiently reduce the gill ventilation and thus avoid gill clogging by their recourse to aerial breathing - the shallow waters ideally help them to swim up and down for gulping and release of air. As recently shown in India the swamps can be developed well for culture of airbreathing fish.
These affect only the coastal waters where salinity tolerant, longstemmed paddy are grown as in Bengal (‘Basabada’) and Kerala (Pokkali) in India and Java in Indonesia. Usually water is locked up during off season for paddy, and prawns and brackishwater fishes are grown and there is a large flux in salinity and depth of water, the latter to a lesser extent, which the fish and prawns have to tolerate. Most species indigenously available are marine elements (penaaeid prawns) which tolerate low salinity - some highly low salinity tolerant like Penaeus monodon and Metapenacus monoceros and less tolerant ones like P. semisculcatus and fishes like mullet, pearl spot (Etroplus) and cock-up (Lates calcarifer) are euryhaline. Tilapia mossambica is also successfully cultured in tidal ponds along with rice, when the salinity is often closer to that of fresh water.
Vincke (1979) describes the ecological conditions of rice field and mentions that rice fields ecology is characterized by shallow waters (5 – 25 cm) (see earlier discussion), high fertility rich fauna and flora, especially rich phytoplankton. The rice field water though shallow is often rich in nutrients and minerals, and Vincke states that these fields are richer than the many fish culture ponds. The nutrients and minerals often come through the fertilizers applied by the rice farmer. The water is usually acidic, but this is often corrected by manurial applications. Both phytoplankton and zooplankton develop in the field: cladocerans and copepods among zooplankton and desmids and Chlorococcales (Cyanophyceae), Microcystis and Anabaena (Cyanophyceae) and Bacillaricphyceae among phytoplankton (Vincke, 1979).
The paddy fields are also rich in dipteran larvae (especially chironomids) (Lemasson, 1955), which efficiently make use of the minerals and nutrients, especially nitrogenous materials unused, and the fish like carp which feed on the chironomid, make use of an otherwise untapped energy source in the natural cycle. Lemasson (1975) also points out that algae which make use of the nutrients and sunlight available are also very useful in pisciculture, for fishes like Tilapia mossambica and Trichogaster pectoralis (Sepat siam) feed on these algae. The excrements of the fish are also efficiently recycled, part going to the rice plant itself (Cf. symbiosis of fish and rice - Tonolli, 1955 - see earlier discussion) and part going to phytoplankton (algae) which become fish food and is thus fully recycled.
The disadvantages of the rice field for fish, as referred to, is the low depth of water and the stagnant water, the former is to an extent compensated, as explained earlier, by provision of trenches and swamps for refuge and also by the increased light penetration and consequent high production of phytoplankton and algae in the shallow depth. Compensation for stagnant water is made possible only to an extent by the addition and retention of fertilizers applied and in the case of fishes by adoption of polyculture technique (by right choice of species) so that recycling of nutrients is ensured (Vincke, 1979).
Both fertilizers and biocides (insecticides, fungicides) are used extensively in high yielding rice production, leading to green revolution in several developing countries, especially in S.E. Asia - these are also areas where fish culture in rice fields originated and are well established. There appears little compromise between modern technology in rice production and rizi-pisciculture. The ecology of the rice fields is highly influenced by these modern developments (see also earlier discussion). High amounts of chemicals injurious to fish are present in the water and soil and either these kill the fish or become retained in fish meat, both of which are harmful to the fish as well as the human consumer. As has already been stated before, a considerable expanse of water spread on the earth comprises rice fields and some ways fertilizing these waters for combined rice and fish production will have to be found, especially in view of the increased demand of proteins at all quarters in both developing and developed countries.
