Fingerlings Production and Hatchery Management
1. Spawning and Fry Production
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. However, the catfish farming community in Africa is now using mainly 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. This is of particular importance in African countries where sophisticated chemicals are often expensive and difficult, if not impossible, to obtain.
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, or stored dry after acetone or alcohol impregnation for up to 18 months with no loss of efficacy.
Pituitaries can be taken from males or females. The pituitaries are homogenised in sterile water or pure rainwater and injected into the female. The dose is calculated on a 1.5:1 (donor: recipient weight basis).
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Females 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 fertilisation 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 fertilised.
To increase genetic variability a minimum of two males are used to fertilise batches of eggs. To obtain adequate quantities of sperm males are sacrificed and the testes removed. Fertilisation is best effected by first diluting the sperm in physiological saline where after the solution is mixed with the eggs.
Fertilisation can also be effected by squeeze the sperm (milt) directly onto the eggs which have been stripped into a bowl, adding some water (which activates the sperm) and then thoroughly mixed.
The fertilised eggs become sticky on contact with water and in commercial hatcheries are spread onto mosquito mesh screens, which are suspended slightly off the vertical axis in hatching troughs. 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 Clariid species. All commercial catfish hatcheries in Southern Africa work at 28oC at which the larvae hatch after 16-18 hours.
2. Hatching Nutrition and Feeding
After swim-up they flow into rearing tanks. Larval rearing is restricted to a 10-15 day period during which the fish are kept indoors under optimal conditions and fed on a complete dry feed every two hours. During the first 3-5 days they receive a supplement of Artemia nauplii three times a day. However, catfish larvae can be reared successfully without Artemia or other zooplankton supplementation.
After a 10 – 15 day intensive hatchery period they are transferred to nursery ponds (fertilized and filled two days prior to transfer), at a density of 2,000 fry/m2, or more. 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 fry are fed every4 hours, with a 38% protein diet. A water exchange rate of 0.5 l/min/m2 is maintained throughout the phase. At an average weight of 4-5g they are either sold to producers or put into the farms own production ponds. Average total survival rate from the time of hatching to the end of the nursery phase is 40%.
After yolk sac absorption, the larvae are fed on live zooplankton, usually caught from production ponds and are transferred to nursery ponds as early as two days after yolk sac absorption (approximately 5 days old) at an average weight of 5 mg. The rearing of the larvae to fingerling size under small-scale/subsistence conditions is usually achieved in organically fertilized ponds, filled two days prior to stocking, at a stocking density of 30 to 100 larvae/m2.
In some instances ponds are equipped with compost enclosures covering 10% to 25% of the surface area. The larval fish are fed on a supplementary basis with substances such as sun-dried brewery waste, rice and wheat bran or other agricultural by-products, if available. Fingerlings are harvested six weeks later at an average weight of 3 to 5 g.
3. Nutrition of Larvae and Early Juveniles
Due to the high densities at which catfish larvae are reared, it is essential that a reliable source of high quality larval feed, which satisfies all the nutritional requirements, is always and readily available. Live organism, particularly rotifers, cladocerans and Artemia nauplii have been used in the large scale rearing of African catfish larvae.
However, the collection of live food from ponds is cumbersome and only available on a seasonal basis and the cultivation of Artemia is expensive particularly for hatcheries in developing countries in Africa.
For this reason, some workers have successfully formulated an artificial dry-feed for larvae, based primarily on a single cell protein (SCP) Torula yeast and fishmeal. Studies on larval nutrition have however indicated that live-food is essential for the first few days after the start of exogenous feeding.
The minimum requirement is an Artemia nauplii supplement for the first 4 – 5 days after the start of exogenous feeding, during at least three of the 10 – 12 daily feeding periods. In semi-intensive and intensive hatcheries a number of different successful feeding strategies have been used.
In Southern Africa larvae are normally reared in the hatchery and fed to satiation at two hourly intervals. This continues for 10 – 14 days at temperatures of 280C at stocking densities of ca. 100 larvae / litres. This protocol results on an acceptable growth rate and survival (80%) at a low cost.
The larval period has been divided into an early phase, when a specific larval diet (consisting mostly of live food) is required, and a later period when the larvae and early juveniles are less dependent on live food.
Experience has shown that the earliest time that larvae should be weaned onto a dry feed is approximately 4 – 5 days after the start of exogenous feeding. Weaning onto a dry diet takes place gradually during the 10 – 15 day hatchery period.
The proximal composition of an optimized dry feed for early juveniles consists of 55% protein, 9% lipid (1:1 plant and fish oil mixture). 21% carbohydrate, with a methionine supplement of 150ug / g food. Vitamins and minerals are usually added to the diet as per the requirements of channel catfish, on the assumption that they are the same.
Work on the vitamin and mineral requirements of African catfish larvae and juveniles has only recently been initiated and much work remains to be done. It has been shown that larvae and early juveniles require at least 1500ug ascorbic acid / g dry weight of food and after approximately 6 weeks the gross dietary protein requirement decreases to between 38 – 42%.
4. Nutritional Requirements during the Grow-out Phase
From the age of six weeks the dietary requirements of African catfish do not seem to change, except that ration size decreases with increasing body size. As the fish grow larger their relative consumption rates decrease from approximately 10% of body weight per day (at 4 weeks) to around 2 – 4% of body weight per day (at 10 weeks and older).
Similarly growth rate decreases from 14% (at 4weeks) to 2% at 10 weeks and older. At this stage the best growth rates and feed conversion ratios are achieved with a diet containing 38 – 42% crude protein and an energy level of 12 kJ/g.
Programmed least-cost formulation is a widespread technique used in the animal feed industry. It is aimed at finding the cheapest way of combining a given set of ingredients with a known nutritional composition, while at the same time satisfying the requirements of the
animal concerned and obtaining maximum growth at least cost.
During the grow-out phase the fish should be fed twice a day. The recommended daily ration must be adjusted according to temperature and fish size.
Table 1.1 Approximate Minimum Dietary Requirements for African Catfish
| Feed | Percentage |
| Crude Protein | 40% |
| Total lipid | 11% |
| Digestible energy | 12 kJ/g |
| Calcium | 1.5% |
| Phosphorus | 0.5% |
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