6. PRINCIPAL FISH AND PRAWN SPECIES UNDER CULTURE IN RICE FIELDS - CHARACTERISTICS OF SUITABLE RICE AND FISH VARIETIES

Before we review the species of fish used in culture in the rice fields, we shall look up the characteristics of both the rice and fish used in rizipisciculture. In spite of the vast work of rice agronomists, a review of rizipisciculture indicates that very little information in their characteristics - the recent studies of the International Rice Research Institute (IRRI) and associated institutions in S.E. Asia (e.g. Grover, 1979; Singh et al, 1980; Khoo and Tan, 1980) and some Agricultural Universities in India, where riceagronomists and fish culturists are collaborating in evolving improved practices for rizi-pisciculture, (e.g. Mudanna et al, 1970; Natarajan et al, 1980) are in this direction. From the available information we shall first discuss the characterists of suitable rice varieties.

6.1. Characteristics of rice varieties used in rizi-pisciculture

As we have already pointed out there is a clashing of interests of the rice grower and fish grower in rizi-pisciculture in the following respects:

1. the rice grower wants to reduce the depth of water retained in the field (consume less water) but the fish grower wants to grow fish in deeper water.

2. the rice grower wants to have short duration crop, whereas the fish grower wants his fish to be cultured for longer time for attaining marketable fish size.

3. the rice grower wants to apply modern technology for growing rice, by applying more chemicals (biocides, fertilizers), which is against the interest of the fish culturists.

As we have already discusses under “advantages and disadvantages of fish culture in the rice fields”, there are ways of overcoming these problems and a compromise is demanded in view of the need for using all available waters for fish culture and the demand for more cheap protein.
Of the problems raised above, the first can be solved by choosing varieties of rice which can tolerate deeper water ie. long stemmed or floating varieties - some of these varieties are already available, as referred to earlier, and others can be bred in areas where there is need. From the available information the average maximum depth for wet rice is 20 cm, the requirement changing with the growing period of rice (Singh et al, 1980 - see Fig. 2). We have already pointed out that provision of trenches and sumps (refuges) with relatively deeper water can allow fish to have their requirement satisfied, while the paddy crop will be submerged to a small depth only. The second problem raised is also solved by recourse to the trenches and sumps in the rice fields where fish would seek shelter while the first paddy crop is harvested and the second crop is started. Also the fish could be transferred from the rice field temporarily during the harvest to a pond tank and then restocked at convenience. The third problem is to be solved by use of rice varieties which are more resistant to insect attacks and diseases and also by the use of biological control methods. We should also use low persistent chemicals.

From the above, we can formulate the characteristics (requirements) of rice varieties suitable for rizi-pisciculture:

1. tolerate deeper water - deep water or floating varieties of paddy are already in use in certain parts as in India, especially in cases where water is filled in the fields by tidal flux. Also new varieties can be evolved.

2. Rice varieties should be resistant to insect attack and associated diseases - alternately as explained biological control methods can be applied, but such techniques are not always available. One can also use less persistant pesticides.

While a longer duration rice crop would be preferred for rizi-pisciculture, this would not be practicable, because the profit of the rice grower is enhanced by having a short duration crop. The farmer not only gets money quicker on his investment, but also is able to save on the operations (labour, fertilizer etc). While the older rice varieties were of 3 – 4 month duration crops (160 days even), the new varieties evolved are of 105 to 125 days and many more new varieties are being evolved, actively in most rice growing countires of the world. Rice breeding as most other crop breeding programmes has already a highly accomplished and rewarding technology. Fish culturist, or fish breeders to be precise, may yet emulate them. Several rice varieties have been evolved at IRRI and their specific resistance ratings studied, (Table III), so that rice varieties suiting to the conditions may be chosen.

6.2. Characteristics of fish species suitable for rizi-pisciculture

Coche (1967) and Vincke (1979) list the following as characteristics of fishes suitable for rizi-pisciculture:

1. The fish must tolerate (grow) in shallow water

2. They must tolerate high temperature and low oxygen, conditions often present in the rice fields on hot days

3. The fish must have the capacity to grow fast and reach the marketsize.

4. They must tolerate high turbidity

5. They must not escape (by their behaviour) from the enclosed field.

A desirable trait for fish to be recruited for rizi-pisciculture is the better resistance to pesticides. It is thus important to know the tolerance limits and sublethal effects of commonly used pesticides to commonly cultured fishes. Some information of this type exists especially with reference to common carp and tilapia (Table IV). In Table IV is included the LC₅₀ of several pesticides to common carp and their level of persistance. The tolerance limits of tilapias have been studied by Kutty et al (1977) and by Gurure (1986). Gurure's observations are given in Table V. While screening pesticides, as indicated already, it is important to know if the insecticide residues accumulate in tissue (persistant) and also stay on in the environment after use. As obvious non-persistant pesticides such as osgamophosphorus products (e.g. Folithion, Lannat) and Carbonates (Sevin and Furadan) are preferred for use (Table IV).

Coche (1967) lists 14 species among the main fish (all finfish) cultivated in the rice fields and considers the common carp and Tilapia mossambica as the most important among those cultured. He does not list any crustacean or anyfish cultured under brackish water conditions. Vincke (1979) gives a list of 35 species (finfish only); this list includes those grown in brackishwater conditions as well, but no crustacean are included, eventhough in the text he does mention that the prawns, Palaemon carcinus, Penaeus semiculcatus, P. carcinus, Metapenaeus monoceros and M. brevicornis, are cultured in the rice fields (pokkali) in India. Vincke mentions that all the species listed in his table are not important and names the following as the species more commonly cultured in the world:

Cyprinus carpio

Tilapia mossambica

Trichogaster pectoralis, Clarias batrachus

Channa striatus

Table VI gives a consolidated list of species taken from various sources.

Fig. 2. Change in water depth in rice fields at different phases of growing rice (From Singh et al, 1980)

Table III

Resistance ratings of IRRI varieties in the Philippines
(From Singh et al, 1980)

Note:
R - resistant,
VR-very resistant,
MR-moderately resistant,
MS-moderately susceptible, and
S-susceptible.

The IRRI varieties IR26, 30, 32, 36, 38 and 40 have been cultured along with tilapias and carp (Arce and Dela Cruz, 1978).

Table IV

Toxicity of selected insecticides used in rice cultivation as regards the common carp Cyprinus carpio (From Micha and Vincke, 1985)

(1) Active-ingredient content expressed in %

Table V

24, 48 and 96 hour median lethal concentration
(LC₅₀) in ppm. of two pesticides for the fingerlings
of Oreochromis niloticus and Tilapia zillii

(From Gurure, 1986)

Table VI

List of fish and prawn species cultivated in the rice fields of the world
(sources: George et al (1968), Raman & Menon (1963), Coche (1967), Jhingran (1973) and Vincke (1979))