Coche (1967) mentions that when fish and rice are grown together (rizipisciculture) three main sanitary problems which may arise, can be solved. They are:
the control of weeds and algae, which provide a breeding ground for mosquitoes and also compete with rice for the available nutrients and indirectly also with fish;
the control of snails which transit bilharzia;
the control of mosquitoes (eggs and larvae) which transmit several diseases such as malaria, yellow fever, filaria, encephalitis and dengu. Let us now look at each one of these biological control systems separately.
The presence of weeds in the rice fields is known to have reduced rice production as high as 50% (Chateau, 1957 a & b; Coche, 1967). The weeds can be controlled directly by use of herbicides (weedicides), which are not always successful It has been shown that herbivorous (macrophytophagous) and algivorous (microphytophagous) fish can be successfully used in the control of weeds and algae in the rice fields (Lemasson, 1955; De bont, 1955; Mortimer, 1957; Coche, 1960). The fish found useful in this purpose are Puntius javanicus in Asia, Tilapia rendalli and T. zilli in Africa, which are hervivorous and T. mossambica in Asia, which are algivorous. Coche (1960) found in Zaire that adult Tilapia rendalli (=melanopleura) stocked at 3 per are (300 per hectare) three weeks after the transplantation of one month old rice - seedlings kept the rice fields completely free of algae and weeds. Mortimer (1957) found that in Zambia stocking mixed age population of T. rendalli at the rate of 22 kg/ha one week after rice transplantation, successfully controlled the weeds and algae in the rice fields.
Bilharzia, a serious human disease, is for years known to be transmitted by water snails in the tropics. Schistosoma haematobium (Bilharz), a trematode occuring in Africa and Middle East, is parasitic in the lower intestine - these trematodes have a life cycle including an intermediate host, a specific snail, belonging to genus Biomphalaria (Syn. Planorbis), Bulinus and Physopsis. In South and Central America only S. mansoni is known to occur and in the Far East S. japonicum, whose intermediate host is a snail belonging to the genus Melania, is met with.
Besides the use of chemicals to control these snails, use of fishes is known to be one of the best methods of their biological control. Because of the toxicity of chemicals used for killing the snails, on fish as well, the chemical use in rizi-pisciculture cannot be resorted to. Experiments in the biological control of snails in rice fields by fish have been done in Uganda (Cridland, 1954; Greenwood, 1954), in Mozambique (Silveira da'Costa, 1957) and in Congo - Katanga (Zaire) (De Bont, 1956; De Bont and De-Bont-Hers, 1952, 1956; Coche, 1960).
When rice is not grown in the fields ducks can effectively remove the snails (Van Der Lingen, 1959). Periodical draining and drying of the rice fields, followed by liming if possible, and trimming down of bank vegetation, can eliminate snails. Eradications of algae and weeds (by use of phytophagous fish, has already explained) can also help in the control of snails which feed on the aquatic vegetation. Hickling (1962) considers these methods effective, but only in controlling snails which might harbour Schistosoma (sporocysts) (Cridland, 1957). The destruction of snails, if infested by Schistosoma (miracidia) is a necessity, and for this under rizi-pisciculture conditions, snail-eating fish are excellent.
In the far East, the black carp (Mylopharyngodon piceus), which is an excellent
food fish and grows to a large size (4 kg), is considered most useful in
controlling snails and their possible introductions to Africa for the purpose
has also been suggested (Hickling, 1962). Coche (1967) lists the following species
as suitable for snail control:
Astatorheochromis alluandi (Maemohen, 1960); Chrysichthys mabusi (De Bont and De-Bont-Hers, 1952). Clarias gariepinus (Mosley, 1951); Haplochromis bimaculatus in Ghana (Hickling, 1962) H. mallandi (De Bont, 1956; Coche, 1960). Protopterus aethiopicus (Lemasson, 1954; Hickling, 1962); and Synodontis spp. In lake Victorias Greenwood (1954) listed Astatorheochromis alluandi, Macropleurodua bicolor and 6 species of Haplochromis for snail control. Coche (1960), however reports that in experimental trials in Zaire (Congo - Katanga), stocking 3 adult Tilapia rendalli (=melanopleura) and 2 adult Haplochromis mellandi per are, for three years, removed almost all the snails, but complete eradication was not possible.
Paddy fields and any stagnant water, with weeds and algae, are often used as breeding ground of mosquitoes, in that larvae of mosquitoes grow here and directly cause as vectors increased incidence of epidemics such as malaria, filaria and yellow fever. Malaria is transmitted by the mosquitoes, Anopheles funestus and A. gambiae in Africa and A. aconitus in India and yellow fever by mosquitoes of the genus Stegomya. Several other mosquitoes also serve as vectors for other human diseases (filaria, (culex) encephalitis, dengu.), and it is significant that many mosquitoes have developed resistence of insecticides.
It is from this context that the biological control of mosquitoes by fishes are important.
There are two levels by which biological control of mosquito larvae in the paddy fields and other water bodies can be made. One is by the control of the algae and weeds growing in the water bodies, as the destruction of these algae and weeds cause the eggs and larvae of culicides which attach on them to disappear, an aspect which is stressed by Coche (1967). The second level by which mosquito control can be achieved is by the introduction of larvivorous fishes (those which eat the insect larvae) in the water bodies (Hora, 1952), an aspect apparently neglected by Coche (1967) in his review. In this case several fishes can be used, including carps, but most dramatic control of mosquitoes is achieved by introduction of larvivorous fishes such as Haplochelys, Oryzias and Lebistes by the Vector Control Research Laboratory of the ICMR at Poncheteriy in India, that they have in association with the Tamil Nadu Agricultural University (India) launched a programme for mass culture of lar - vivorous fishes. Eventhough the carnivorous fishes used in this case are small and not useful for edible purpose, some edible fishes have also been used in this context, as already explained.
Coche (1967) states that mosquito control in the paddy fields has been achieved by using combination of algae feeders and weed feeders among fishes. Hofstede and Botke (1950) proved that in the vicinity of Djakarta(Indonesia) malaria could be controlled by using Tilapia mossambica, eating green filamentous algae. The same species has been found effective in Malaysia as well (Hickling, 1962). In Zaire T. rendalli (=melanopleura) which feeds on floating and emergent vegetation has been found to be effective in malaria control (de Bont, 1949). When vegetation has been lacking, as in ponds with steep banks in Cameroon and Madagascar, no anopheline larvae were found (Lemasson, 1957). Apparently in paddy fields, rice plants may serve as shelter for larvae (Coche, 1967), but under rizi-pisciculture no serious breeding of Anopheles aconitus could be found (Hora, 1952), probably because the fishes were eating the mosquito larvae directly and also the weeds which would otherwise harbour the larvae.
In conclusion, it can be said that sanitary improvement of irrigated rice fields can be achieved if mixed populations of snail, algae weed and larvae feeders among fishes can be introduced in the rice fields. Periodic clearing of bunds for removing weeds will also be helpful in keeping mosquito larvae under check. Coche (1960) has successfully demonstrated in Zaire that a mixed population of T. rendalli (=melanopleura), T. macrochir and H. mellandi effectively controlled weeds, snails and mosquitoes.
Perhaps judicious stocking of well known minor carnivorous fishes, though non-edible, which by their numbers can also serve as forage species for larger edible carnivorous fishes, in the rice fields, can control mosquitoes while stocked together - for snail and weed control, snail eating and phytophagous fishes would also have to be introduced, depending on their availability and need.