Artificial propagation of Clarias lazera is already practised both at the laboratory level and also on a larger (commercial) scale.
A suitable keeping technique exists in Landjia for Clarias broodstock which ensures egg production throughout the year using the same female stock. Investigations have shown that one female of 1 kg body weight can produce 100 000 eggs monthly. Propagating 10 times a year, production can be 1 million eggs annually. Three induced ovulations per year in common carp of the same size will only produce 0.4 million eggs.
Techniques of stripping, fertilizing, incubation and hatching have been developed, and it has been possible to produce greater numbers of early fry.
The expert first had to determine if the stickiness of the eggs can be eliminated by using a 5‰ salt or urea-salt solution during the fertilization and swelling period, and finally a brief and carefully dosed tannin treatment at the end of the swelling.
Eliminating the stickiness of the eggs would permit their easier incubation in jars and in greater quantities. The newly hatched larvae could be allowed to swim from the jar into a suitable larvae collector.
During the first trials, the eggs did not stick in 5‰ common salt solution or in carp egg fertilization solution (4‰ salt and 3‰ urea (carbamide)). As the Clarias eggs did not stick immediately after coming into contact with water (like the sticky eggs of most other fishes), clean water could be used for 1 minute during fertilization and thereafter sticking of the eggs could be prevented with the above-mentioned solutions.
The tannin treatment should be carried out very carefully with only 4–5 g tannin in 10 l of water.
The first trial showed unusually high numbers of deformed (malformed) embryos (the exact percentage could not be determined). It is not likely that this was caused by the chemicals used (salt or urea or tannin) but was most probably due to the strong water current in the jar when the cleavage of the animal pole occurred (fish eggs are very sensitive to mechanical effects at this stage of development.) This harmful effect could be prevented by keeping the water inflow as low as possible.
Keeping the non-feeding larvae up to the feeding stage presents no problem provided the necessary oxygen is available. Greater quantities of non-feeding larvae could be kept in big funnel-type incubators (50–200 l incubators) such as those used for carp larvae in commercial hatcheries in Hungary.
Feeding of the Clarias larvae for a longer period in the hatchery involves difficulties which could hardly be solved in commercial scale hatcheries.
The production in basins of rotifers or small cladocerans such as Moina sp. is possible but difficult. The problem is to produce the great quantities frequently needed during a day. Production of Crustacea on a continuous basis in basins is also very difficult. In Hungary, Tubifex cut into small pieces is used for large-scale feeding of European catfish larvae. Tubifex is taken by professional collectors in heavily-polluted brooks or canals. It has been demonstrated that the Clarias larvae need live food for growth and development. The encapsulated egg diet was found inadequate for long periods (see Chow, 19801).
The following solutions can be adopted in Landjia when larger-scale fingerling production is initiated.
The larvae are kept in the hatchery only up to the feeding stage. They could be fed once or twice with boiled or capsulated egg diet and then released into a well prepared, freshly inundated fry-rearing (nursery) pond. The rearing pond should be prepared specifically, be easy to manage and drain. The best shape is elongated with perpendicular walls. If possible the intrusion of spawning frogs should be prevented. The ponds should be kept free of water plants and dry when not in use. The larvae should be kept here no longer than 20 days. Losses will be rather high but can be compensated.
Experiments should be carried out to determine the smallest size of Clarias fingerlings which promises an acceptable survival rate in the farmers' ponds.
The larvae could be kept in mosquito-net boxes (hapas) in the Tilapia ponds. The bottom of the box could be made of unrottable PVC or similar. After keeping for 10–15 days they can be released into a well fertilized pond. The net box would prevent the early losses of larvae due to various predators.
The larvae could be raised in troughs with a semi-circular bottom. This type of trough is successfully used for raising European catfish larvae and Acipenser ruthenus larvae in Hungary. It has the advantage that the debris and faeces collect on the axe of the trough from which they can easily be removed.
Larvae 10–15 days old could be fed with zooplankton plus artificial feed. Zooplankton can be collected in large quantities with a special net from a well manured pond. The zooplankton collecting net has a 30–40 cm × 15–20 cm rectangular opening, is made of 150–180 micron mesh netting, is about 50–60 cm long and has a big easy-to-handle plastic container at one end. The plankton should be filtered through a 500–600 micron mesh net before being fed to the larvae.
It is very important that the larvae rearing technique should be as simple as possible and adaptable to local conditions.
Also to be considered is whether the loss of larvae during the nursery period could not be compensated with easily available quantities of fertilized eggs or non-feeding larvae.
Further trials with Clarias eggs
Clarias eggs are very resistant to treatment with the chemicals used in the fish propagation practice.
The eggs which were treated for 5 minutes after fertilization (in water) with a light tannin solution (3.5 g tannin in 10 l of water) stayed alive, did not stick to each other or to a substrate; they had swollen normally and the larvae hatched.
In another case, the tannin treatment did not cause any hindrance in development when the eggs were treated after 20 minutes of the fertilization.
It can be concluded that the tannin eliminates the stickiness of the Clarias eggs but does not impede the swelling and does not influence egg development.
For the remainder of the eggs the following technique was applied:
The stripped egg mass was fertilized with a small amount of clean water;
After 1–2 minutes ample salt-urea (carbamide) solution was poured over the eggs;
The eggs were washed 2–3 times with the above-mentioned solution during the one hour following:
After approximately one hour the eggs were treated with tannin solution (3.5 g tannin in 10 l of water);
After washing the eggs 2–3 times with clean water, they were put into incubator jars. The water flow in the jar was kept very low to avoid mechanical damages during the cleavage time (the inflow water did not rotate the eggs except for a few at the bottom of the egg mass);
Normal water current (inflow) was effected when the eggs arrived at the gastrula stage (stripping was at 07.30 h; the gastrula stage was reached at 14.00 h; water temperature 25°C).
This technology seems to be suitable to incubate Clarias eggs in greater masses on a commercial scale. It needs further practical investigations to determine the mortality and hatching rate of the eggs, and the viability of the larvae.
The productivity of the tropical fish ponds can be maintained only by regular manuring or fertilization.
Duck-raising on the pond (fish-cum-duck culture) can provide this great advantage. Unfortunately, the ducks kept in Landjia are not accustomed early enough (3–7 days of age) to life on the water; and therefore they do not leave the keeping platform and do not exploit the other advantages of the pond (collecting as food, water weeds, insect larvae, tadpoles, frogs, insects).
The expert explained the technique of making the ducklings used to the water in their early days.
Because of the lack of sources of cheap manure in Landjia, it would be advisable to extend duck-keeping to all the most important bigger ponds.
The expert investigated the plankton present in the ponds with an approximative quantitative field technique. Those ponds which receive an organic manure supply, have a satisfactory amount of micro-phytoplankton and rotifer plankton (bigger plankton was eaten by the Tilapia stock).
Those ponds which do not have a regular supply of manure were very poor both in phyto- and zooplankton. The production capacity of these ponds is very low. Moreover, water weeds were growing over these ponds making them inadequate for intensive fish culture. Mechanical or chemical control of the water weeds is very costly and needs continuous attention.
