Coche (1967) discusses the socio-economic importance of fish culture in rice fields and points out the deficit of animal protein in densely populated rice growing areas. In these areas usually all the available field space is used for rice leaving little space for cattle breeding and sheep and poultry do not produce enogh meat (Schuster, 1955). Thus fish grown in the paddy fields, will be ideal use of land and would also be an easy source of cheap and fresh animal proteins. Thus fish culture can greatly contribute to the socio-economic welfare of rural populations of especially developing countries. An added advantage also is that unlike sea fish or other animal proteins, the fish from the local paddy fields would cause no transport problem and would be most fresh and healthy.
The production of a fish crop between the rice crops gives the farmer an off-season occupation (Hora and Pillay, 1962). Hickling (1962) points out that this “increases the income without increasing expenses”. Apart from the additional income available from rizi-pisciculture, the combined culture leads to a reduction in labour in weeding and an increase in the yield of paddy by 5 to 15%. Schuster et al (1955) report an increase of 6.2% in rice production in Indonesia, 10% in Malasia and 10 – 15% in China. In Zimbabwe - Marr (1959) reported an increase of 6%. Using an improved strain (CR 1108) Sinhababu et al (1983) observed that rice growth with Indian carps in rizi-pisciculture trials showed an increase of 3.8 – 6.2%. According to Coche (1967) the increase in rice production is ascribed to various factors, namely:
Increase in organic fertilization by fish excreta and remains of artificial feed.
Better tillering of the rice seedlings due to the activity of the fish.
Reduction in the number of harmful insects, such as paddy stem borers, whose larvae are eaten by fish.
Reduction in rat population due to increase in the water level.
Increased mineralization of the organic matter and increased aeration of the soil resulting from the puddling of mud by benthic feeders.
Control of algae and weeds (by phytophagons fish) which compete with rice for light and nutrients.
In some cases a small reduction in rice production has been recorded, (Chen, 1953; Ardiwinata, 1957) mainly due to the lesser space for rice - trenches in which fish grow, taking space, but the higher price of fish always offsets the loss due to reduced yield of rice. Another major advantage of growing fish in rice fields is that they serve as control of vectors such as snails and mosquitoes - malacophagous and larvivorous fishes - these have a great sanitary, potential - this may be specially so in regions like Africa (Coche, 1967) where population pressure is not high and therefore more land could be put under rizi-pisciculture (see “control of algae, weed, molluscs and mosquitoes” - note following).
In U.S.A., the primary reason for adopting this culture appears to be the aid in conserving soil fertility by crop rotation and also in reclaiming new areas. In winding up the discussion on advantages, Coche, (1967) cites Hora and Pillay's (1962) statement that “fish culture is only a subsidiary activity and the cultural operations should be modified to suit the requirements of paddy production”. It is worthwhile also recalling Tonolli's (1955) statement on the beneficial effect of fish on Paddy production that “a sort of Mutual Symbiosis takes place, in which each of the two components is not damaged but rather favoured, upto a certain point, by the presence of the other”.
Coche (1967) lists the following disadvantages in fish culture in rice fields.
A greater supply of irrigation water and a greater water depth required for fish culture. Especially in view of the shortage of water due to increase in human demands this will be a very serious difficulty in future.
Extra investment and labour involved in raising and strengthening fields bunds.
The need for rice variety tolerant to deep water and to low temperature.
Fish may damage the young seedlings by uprooting them (carps) or eating them (Tilapia rendalli).
Certain parts of the field is lost for fish culture by the construction of trenches and refuges for fish.
Additional costs are involved in fertilizing and feeding the fish.
In certain types of soil continuous inundation may not be possible.
However, there are several remedies for these. Often genetically improved and suitable rice varieties such as those which stand deep water, salinity, low temperature etc. are available. Fish stocking can be delayed after transplanting (10 days for carps and 3 weeks for T. rendalli).
Rice and fish in rotation - advantages and disadvantages:
Some of the advantages and disadvantages mentioned already (rizi-pisciculture) persist, but the major drawback there would be adequate supply of water. These are decidedly specific advantages, especially as proved in Arkansas, U.S.A.
The major advantage as mentioned is the independence of the two operationsrice and fish. In modern technology chemicals for fertilizer and biocides used to sustain high yielding rice are so harmful to the fish - and for growing fish usually a greater depth of water is demanded. In rotating rice and fish the separate operations can be maintained even though some chemicals/biocides will be retained in the soil for the succeeding crop and these doses though sublethal can cause some harmful effects by accumulation, but by and large crop rotations is more advantageous. In Arkansas where deep water (1 – 2 metre) can be retained, stocking with bottom dwelling fish, in their search for food “help in levelling the rough and torn ground of newly cleared and flooded fields (wood-lands)” - here fertilization increased rice production by 25% or more due to higher fertility and better tillage of soil (Hogan, 1959; Jones, 1960). The clearing of timberland itself becomes cheaper while growing fish in reclaimed woodlands. Reclamation of saline swamps also becomes cheaper while fish culture is practised in these swamps as in Bengal (Pillay, 1958).
The greatest difficulty in rice-cum-fish culture is one not listed above, that of the improved modern technologies of agriculture in vogue in cultivating rice - large amounts of chemical fertilizers, herbicides and porticides are used in growing rice - most of these are harmful to fish. In judicious application of these, to an extent some reduction in damage is possible, but most insecticides even in the lightest dose kill the fishes. The organochlorines are in this way more harmful than organophorphorus insecticides. For example, among several insecticides screened the organochlorine, ‘Endosulfan’ kills carp and T. mossambica 100 – 1000 times faster than the organophorphorous, ‘Lebaycid’ and ‘Phosvel’ (Kutty et al, 1977). An aspect which would be considered here is that many insecticides are easily bio-degradable and given a few days time their toxicity is reduced greatly. Thus is there a possibility for choosing some less harmful and quickly bio-degradable biocides to protect both the paddy and the fish? Perhaps we can also control the time of application i.e. by applying the biocides soon after transplantation before the fish are introduced, by then the potency of the biocides would have become least. But inspite of such an approach the possibility of accumulation of pesticides which may not kill, cannot be ruled out. As discussed earlier it appears the whole ecosystem of each crop, is upset drastically by the use of chemicals and biocides - the whole biological complex is changed. And several pests have developed immunity to various chemicals applied to control them. Thus the ultimate cannot be in increase of chemical applications to protect the crop, but to evolve more harmonious methods in line with sound biological principles. This has already been proved in some cases in crop protection, by use of biological control methods. Let us hope that such methods, which are within the reach of man, will be evolved so that fish and paddy can be grown together in the vast expanse of water which is available on the global surface as paddy fields.