On 12 July the expert visited the Fish Farm in Bouar with Mr G. Mievis. The farm is about 460 km from Bangui. Mr. F. Demoulin, associate expert, is working on this farm. The present task of the project is to produce Sarotherodon (Tilapia) nilotica fingerlings for distribution to local fish pond owners.
Seven years ago some common carp fingerlings were introduced from Cameroon for trial purposes. At present there are 3 females (approximate weight 7, 6 and 6 kg) and two males (approximately 5 kg each). Until now it has not been possible to successfully propagate these carps either naturally on kakabans or artificially by hypophysation. The carps were kept all the time with the Tilapia stock, which probably restricted them from any spawning activity.
On 13 July three females and one male were captured and examined to see if they were suitable for artificial propagation out of season. The females were in a well nourished condition but without the bulging abdomen which indicates ovary development. Only one female showed some signs (reddish sexual opening and swollen anus) of ripeness for induced propagation. The male did not give even a drop of milt when squeezed on the belly (this is the usual test for ripeness in the males). The breeders were transported in a concrete tank. The sexes were separated by a chicken-wire screen. The tank had a continuous water supply. The females and male were injected with acetone dried carp pituitary glands twice. The male had 3.2 mg at the first injection and 3.2 mg at the second with 10 hours interval. Females were given a first injection of 3.2 mg and 32 mg for the second (this corresponds to about 0.5 mg for the first and 5 mg for the second injection per 1 kg body weight).
The water temperature in the concrete tank was a maximum 27°C at noon and 22°C at night. According to the calculated hour grade at 07.00 h the next morning the promising female gave about 250 thousand eggs at the first and about 150 thousand at the second stripping (the females were sutured). The eggs seemed healthy and were fertilized with ample milt produced by the only male. The technique to eliminate the stickiness of the eggs was then applied (salt-carbamide and tannin).
One of the other two females gave a few ripe eggs only. The third female was treated after 24 hours of the introductory injection but produced a few unfertilizable eggs after the calculated time.
Two hatching jars (funnel type incubator made of plastic) were installed on the side of the cement concrete tank with a continuous water supply. The egg development was observed from time to time with a magnifying glass. The development was promising up to the gastrula stage (16.00 h). Even the gastrula began to grow around the yolk material of the egg (18.00 h). The temperature was a maximum 27°C during daytime (carp eggs were incubated in 29°C water in Madagascar 1978).
In spite of the promising start of the development, during the night most of the eggs died. Only a few were alive the next morning and these were deformed (the embryos had a poorly developed head and the tail was shorter than normal).
The destruction of the eggs was certainly due to the fact that they were forcibly ovulated before they were ready. They were ripe enough to be fertilized, but not ripe enough for normal embryo development. This phenomenon occurs when propagation is induced out of the propagation season. Most, if not all of the eggs, are unable to develop. A part may develop up to the hatching stage. In such cases most of the hatched larvae are deformed and unable to survive. In the present case the unripeness was more serious.
The expert also investigated the plankton of the ponds of the station, most of which have a dense phytoplankton population. The phytoplankton are mostly small of the Chlorophyceae group. The bigger phytoplankton are eaten by the Tilapia population. The smaller ones which cannot be filtered by the Tilapia survive and proliferate. Some ponds were as green as “pea soup” due to the abundant phytoplankton. Among the phytoplankton plenty of rotifers can be found and there were many Cyclops and in some ponds small cladocerans (Moina sp.). The plankton content of the ponds can be termed rich in spite of the dense Tilapia population. The bottom fauna was investigated in two drained ponds and could also be described as rich. There were plenty of red chironomids, larger size Ephemeroptera larvae and Odonata larvae on the bottom. The ponds of Bouar are richer than the ponds in Landjia.
The expert considers that these ponds are an obvious case for the introduction of filtering fish such as silver carp. Because of the rich bottom fauna the Tilapia ponds could also be stocked with a few common carp (5–8 fish per are) which may make the pond water more turbid and prevent the light penetration and consequently the large-scale development of the small phytoplankton). The carps would stir up detritus particles from the bottom and the crustacean fauna could develop more abundantly to the benefit of the Tilapia.
Artificial induced ovulation techniques on the common carp were demonstrated to the fish culture experts. It is hoped for success the next time the breeders are ripe for induced ovulation. This time will most probably be end October/ early November.
