Many different hormonal preparations have been successfully used to induce spawning in African catfish. Some of these include HCG, DOCA, Carp Pituitary Suspension, Progestagen, Pimozide, and LH- Rha with different proprietary names such as Ovulin, Ovaprim and so on. However, the catfish farming community in Africa is now using a homoplastic pituitary gland suspension to induce spawning. The method has been found to be highly reliable and, in comparison to synthetic hormone analogs, the technique is cheap and practical.
The homoplastic pituitary hormone is obtained from the base of the brain of either a sexually matured male or female catfish. The whole pituitaries are removed from sexually mature adult catfish during the spawning season and are either used immediately or are stored in absolute ethanol or acetone, with no loss of efficacy. The pituitaries can be taken from males or females and they are homogenized in sterile water or pure rainwater and injected into the females. The dose is calculated on a 1.5:1 (donor: recipient weight basis).
Female catfishes with suitably developed eggs can usually be stripped 12 hours after receiving a single dose at a temperature of 28oC, or 20 hours at a temperature of 22oC. At this stage, the eggs have hydrated and have gone through the process of ovulation. The ovaries at this stage can occupy up to 70% of the abdominal cavity. Broodstock females usually vary between 1 and 2 kg in weight. Owing to high levels of aggression, the effects of which can be quite severe, broodstock females after injection are usually separated from each other in the holding tanks by way of sturdy screens. A simple and completely reliable method of testing the readiness of the eggs for fertilization is by holding the females in a head-up vertical position. If the eggs begin to run freely from the genital pore, they are ready to be fertilized.
To increase genetic variability, a minimum of two males are used to fertilize batches of eggs. To obtain adequate quantities of sperm, males are sacrificed and their testes removed. Fertilization is best effected by first diluting the sperm in physiological saline after that, the solution is mixed with the eggs. Fertilization can also be effected by squeezing sperm directly onto the eggs which have been stripped into a bowl, adding some water (which activates the sperm) and then thoroughly mixed.
The fertilized eggs become sticky on contact with water and in hatcheries are spread onto mosquito mesh screens, which are suspended slightly off the vertical axis in hatching troughs. The stickiness of the eggs is brought about by the adhesive disc on the egg. The disc is in the shape of a doughnut. The micropyle, through which the sperm enters the egg, is situated in the middle of the doughnut hole on the surface of the egg. If screens are not available, the eggs can also be adhered to the roots of floating aquatic plants (such as water hyacinth) during the incubation period.
Development time is temperature-dependent. Once hatching occurs, the free embryos fall to the bottom of the tank while the egg envelope remains adhered to the screen. Once a few embryos have hatched, the rest follow suit very rapidly. In fact, the hatching rate of African catfish embryos is quicker than in most Clarias species. All commercial catfish hatcheries in Nigeria work at 26oC-28oC, at which the larvae hatch after 18-24 hours.
After swim-up, they flow into the rearing tanks. Larval rearing is restricted to 10-15 days during which the fish are kept indoors under optimal conditions and fed on a complete dry feed every two hours. During the first 3-10 days, they receive a supplement of Artemia nauplii three times a day. Artemia is obtained from local feed stores under different propriety name. Also, other zooplankton, such as Daphnia, can be obtained in stagnant water drainage channels. After a 10-15 day intensive hatchery period, they are transferred to nursery tanks, at a density of 2000 fries/m2, or more.
After yolk-sac fry are transferred to rearing troughs or tanks, remove eggshells and dead fries from hatching troughs with a small mesh net or siphon hose. Newly hatched yolk-sac fry can be transferred by siphoning them into a bucket using a ½ – inch-diameter (1.3 cm) clear plastic tube. Newly hatched yolk-sac fry average 1,200 to 1,500 per fluid ounce (40 to 50/ml) and are placed in rearing troughs at the rate of approximately 15,000 per 100 gallons (380 liters) of water. The rearing troughs are usually equipped with an agitator or three or four 8-inch (20.3 cm) airstones for aeration and water movement. Airstones should be slightly elevated from the tank bottom by placing rubber or plastic bands around each end. This prevents yolk-sac fry from being pinched under the stones. A hatchery with 40 to 50 troughs can produce 20 to 30 million fries per season.
During the following 4-6 weeks, the juvenile fish are graded into three size classes at least two times. The smallest size class is usually discarded each time. The fries are fed every 4 hours, with a 38% protein diet.
A water exchange rate of 0.5 liter/minute/m2 is maintained throughout the nursery pond phase. The average total survival rate from the time of hatching to the end of the nursery phase is 40%. Fingerlings are harvested six weeks later at an average weight of 4 to 5 grams, which are either sold to other farmers or are stocked into their own farm. These fingerlings can be further fed for another 2-3 weeks to get to juvenile of average weight of 8-10 grams, which are stocked directly or are sold to farmers whose preference are the juveniles because of the risk associated with rearing fingerling to the juvenile stage.