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October 7, 2011
Semi natural or hormone induced reproduction in ponds/tanks as describe above has not proved to be a reliable method for mass production needed for large fish farms or distribution centres of catfish fingerlings. Therefore artificial propagation under more controlled condition, stripping of the eggs, collection of the sperm followed by fertilization of the eggs has been developed.
The artificial reproduction of the African catfish, as for all fin-fishes, is a chain of activities which is more or less similar to that of natural reproduction. It starts with the selection of broodstock from nature or from the broodstock ponds after which they are transferred to the holding tank in the hatchery. Ideal broodfish weighs between 300–800 grams, Larger fish are difficult to handle, resulting in a substantial losses of eggs prior to stripping. Mature broodfish are selected according to the following criteria;
• A well distended, swollen abdomen from which ripe eggs can be obtained by slightly pressing the abdomen toward the genital papilla. Ripe eggs are uniform in size and a experienced hatchery operator can see the nucleus as a small dark point in the centre of the egg.
• A swollen, sometimes reddish or rose coloured genital papilla.
Note: From a practical point of view can be stated that all females are “ripe” once some eggs can be pressed out and if the eggs are more or less uniform in size (just put some of the eggs on the nail of your thumb andadd a few drops of water and look).
The most common technique to induce final maturation and ovulation in the African catfish is to inject the female with hormones or pituitary gland material.
The required quantity of powdered acetone dried pituitary material or the number of whole pituitaries are pulverised in a porcelain mortar. Then, the required quantity of 1 ml per fish of physiological salt solution (9 gram of common salt in 1 litre of water) is added. A syringe is filled with the suspension and the injection can be given.
The most common method to administer the hormone solution, is a injection intramuscular into the dorsal muscle.
Note: fill the syringe, put the needle on it and empty the syringe again into the mortar, when this is possible you can start to inject the fish. This procedure has to be followed as often the needle gets blocked by pituitary material which is not completely crushed and it is unpleasant for the fish and annoying for the operator to resolve this problem once the needle is stuck into the fish.
The process of final maturation and ovulation of the eggs will start after the administration of the hormonal material. The speed of the process is dependent on the water temperature, the higher the temperature the quicker the eggs ovulate.
Stripping of the female spawners is done by gently pressing their abdomen with a thumb from the pectoral fin towards the genital papilla. Ovulated eggs will flow out
easily in a thick yet from the genital vent if the females responded well to the hormonal treatment. The ovulated eggs are more or less transparent, flattened and a gram of egg contains approximately 6003 eggs. Under normal condition a “ripe” female a quantity of eggs which equals to 15–20 % of her own body weight. If the fish is stripped to early the eggs are coming out difficulty and they have a “flushy” if they are stripped to late.
It is recommended that you determine the number of eggs per gramme of the broodstock used at the station.
Note: The temperatures at the several stations is rather fluctuating, with higher temperatures at day time. It looks like it that the eggs are stripped to early which results in very low hatching percentages (5–10%). It is always much safer to strip the egg to late then to early because if you are to early you will lose all your eggs, if you are to late you will loose some eggs.
The males of the African catfish cannot be stripped and consequently the sperm can only be obtained by sacrificing a male.The male is killed and thoroughly dried after which the testis is dissected and placed in a glass and cut quickly into small pieces with a scissor and finally the milt is pressed out with a pestle or a teaspoon. The sperm and pieces of testis material is added to the stripped eggs. The eggs are fertilized by adding about the same volume of clean water. The water and egg mass are mixed by gently shaking of the bowl.
Note: Stirring of the egg must be continued until the eggs are placed in the hatching tanks as the eggs become sticky and without stirring the will stick together in one clump. After about 60 seconds fertilization has taken place and the sperm has lost its activity and the fertilized egg are ready for in tanks or happa's.
With this method it is not necessary to preserve or dilute the sperm in a physiological salt solution. Once large number of females are needed to be stripped at one morning (5 or more) it is an advantage to take the sperm first and preserve it in a 0.9 saline solution.
Note: First a female is stripped and then the milt of the male is taken. One drop of water in the bottle with sperm will destroy the sperm completely while one drop of water in the bowl of eggs will only destroy some eggs.
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Semi natural or hormone induced reproduction in ponds/tanks as describe above has not proved to be a reliable method for mass production ...