More than 100 species of the genus Clarias have been described in Africa. A recent systematic revision based on morphological, anatomical and biogeographical studies has been carried out by Teugels (1984), who recognizes only 32 valid species. Of these C. gariepinus (Burchell, 1822), synonymous with C. lazera, is the most important for aquaculture. It has an almost pan-African distribution, from the Nile to West Africa and from Algeria to Southern Africa. Representatives also occur in Asia Minor (Israel, Syria and the south of Turkey). Of secondary importance is C. anguillaris. This species has a more restricted distribution, occurring in Mauritania, in most West Africa basins and in the Nile. Thus C. anguillaris lives in most river basins sympatrically with C. gariepinus. The African catfishes inhabit calm waters from lakes, streams and rivers to swamps, some of which are subject to seasonal drying.
Description of the genus and species
The catfish genus can be defined as displaying an anguilliform shape, having an elongated cylindrical body; dorsal and anal fins are extremely long, nearly reaching or reaching the caudal fin; both fins contain only soft fin rays. The outer pectoral ray is a spine. The pelvic fin normally has six soft rays. The head is flattened, highly ossified, and the body is covered with a smooth, scaleless skin. They have four pairs of unbranched barbels, one nasal, one maxillar (longest and most mobile) on the vomer and two mandibulars (inner and outer) on the jaw. Tooth plates are present on the jaws as well as on the vomer.
The swim bladder is small, bi-lobed and encapsuled by the extended transverse parapophyses of the fourth and fifth vertebrae. It apparently has little hydrostatic function. Buoyancy is controlled by air carried in the suprabranchial chamber.
Clarias species have a scaleless skin, which is darkly pigmented in the dorsal and lateral parts of the body. The colour is uniform or marbled and ranges from greyish olive to blackish according to the substrate. Exposed to light the colour becomes lighter.
1 Based on papers prepared by Mr J. Janssen, FAO Fish Culturist, ARAC, Port Harcourt, Nigeria
A suprabranchial or accessory respiratory organ, composed of a paired pear-shaped air-chamber containing two arborescent structures is generally present. These cauliflower-like structures situated on the second and forth branchial arcs, are supported by cartilage and covered by highly vascularized tissue resembling gill lamellae which can absorb oxygen from the air. The air-chamber communicates with the pharynx and with the gill-chamber. Access to the surface is a necessity for these catfishes especially when the oxygen content of the water is low. In air-saturated water catfish can survive without air breathing. The accessory air breathing organ allows the fish to survive during many hours out of the water or few weeks in muddy marshes. It also enables them to migrate over land. Reports of Clarias “walking” occur widely in literature.
C. gariepinus feed mainly on aquatic insects, fish and debris of higher plants. They also feed on terrestrial insects, molluscs and fruits. It may be considered as an omnivorous fish with a high tendency to predation. Slow, methodical searching is their normal predatory tactic. The catfish grasps its prey by suction. The necessary negative pressure is created by increasing the volume of the buccopharyngeal chamber.
The ovaries of C. gariepinus are paired elongated organs situated dorsally in the body cavity. Each ovary consists of a outer membrane with lamellae penetrating the central lumen. The lamellae contain oogonia and oocytes in follicles at different stages of development. Each ovary extends posteriorly into a distinct oviduct. The two oviducts fuse at their end and open into a urogenital papilla, situated just behind the anus. Mature females have very large ovaries which fill the body cavity and may constitute up to 25–30% of their total body weight. In nature the ovaries of a mature population usually represent from 7 to 12% of the total body weight of females (Micha, 1973; Bruton, 1979).
The testes are paired and connected by fused spermatic ducts which open into an elongated, posteriorly pointed, urogenital papilla. The testes are externally differentiated into two distinct regions: a milkish-white, opaque anterior part, the true testes (spermatogenesis and sperm storage) and a semi-transparent posterior part consisting of a series of finger-like lobes, the seminal vesicles (glandular function). During the non-breeding season the seminal vescicles regress and become inconspicuous. The function of the fluid secreted by this part of the testis is not yet clear. The testes only represent a maximum of 2–4% of the total body weight.
There is no prominent sexual dimorphism in C. gariepinus except that (i) males have a more pointed urogenital papilla whereas this papilla has the form of a longitudinal groove in females and (ii) mature females have a swollen soft abdomen, and eggs can easily be obtained by slight pressure on the abdominal part of the belly.
The median size of fish at first maturity shows a remarkable variation and ranges from 260 to 750 mm total length (TL). Females are slightly smaller than males. Micha (1973) reported that C. gariepinus in the Central African Republic reached maturity during the first year at a total weight of about 200 g (240–280 mm TL). In a more subtropical climate, Bruton (1979) found a median size at first maturity of 330 mm TL in females and 370 mm TL in males (about 300 g). Maturity was reached towards the end of the second year by most of the population. Under artificial conditions catfishes mature after about 6 months when they have attained a weight of about 200 g. Feeding and water temperature are major factors regulating the age/size at first maturity. For females, the gonado-somatic index is more affected by feeding than by temperature, whereas it is the opposite for males. A low feeding level has a positive effect on the development of the ovaries.
There is a great variation in the number of developing eggs in the ovary prior to spawning. Absolute fecundity of C. gariepinus is related exponentially to total length and linearly to weight (Micha, 1973 and Bruton, 1979), as is typical in fish. Hogendoorn (1979) found an inverse linear relation between the number of eggs produced after stripping and the weight of the females up to a weight of 1 500 g (about 600–700 mm TL). Larger females produced relatively fewer eggs. In nature fecundity of C. gariepinus ranges from 10 000 to 200 000 eggs/female (about 300–900 mm TL). A few authors found higher fecundities up to 400 000 eggs/female. According to Hogendoorn (1979), the number of eggs produced after stripping (“relative” fecundity) for females up to 1 500 g may be estimated using the equation: total number of eggs = 103 ± (27 × females weight in g).
Eggs of mature females are small (1.2–1.6 mm), slightly oblong and have a yellowish-green colour. The nucleus in the centre is clearly visible. One gramme of stripped eggs contains between 600 and 900 eggs.
Gonadal maturation, expressed as increase of the gonado-somatic index, is seasonal and often associated with the rainy season. Observations in tropical areas show mature eggs from April to December, with peak maturity during July to September. In a subtropical area in the southern hemisphere Bruton (1979) found maturation of catfish associated with increasing water temperature and photoperiod (July to September); spawning took place from September to March during the major rainy season.
Spawning takes place mostly at night in recently inundated marginal areas of a lake, stream or river. Most often catfish spawn on marginal and emergent aquatic, semi-aquatic or flooded terrestrial plants. There is a massive gathering of catfish before spawning and courtship is preceded by highly aggressive encounters between males. Courtship and mating take place between isolated pairs in shallow waters. The mating posture, when the male lies in a U-shape curved around the head of the female, is held for several seconds. A batch of milt and eggs are released followed by a vigorous swish of the female's tail to distribute the sexual products over a wide area. The pair usually rests after mating and then resume mating. Mating pairs are disturbed by intruding males. After spawning the shoal of catfish migrates back to deeper water.
The spawning runs of Clarias species are typically of short duration. Clarias populations which inhabit impoundments or lakes with a large inflowing river usually spawn once each reproductive season in water bodies fed by direct rainfall, seepage or small streams, several while runs may take place.
There is no parental care for ensuring the survival of the catfish offspring except by careful choice of a suitable site. Development of eggs and larvae is rapid and larvae are able to swim within 48–72 hours after fertilization.
Little is known of the environmental stimuli inducing spawning in catfish. It is expected that they rely on visual as well as on non-visual stimuli (temperature, photoperiod, rainfall, presence of opposite sex or pheromones, spawning ground with suitable environment, etc.). As spawning runs are performed at night, mechanical, chemical and auditory stimuli may be important. Temperature and flooding are probably the decisive stimuli for gonadal development and spawning respectively.
As spawning does not occur spontaneously in captivity, artificial methods are required in order to produce large numbers of fry and fingerlings for culture. At the beginning of the seventies many workers developed semi-artificial breeding methods following the techniques developed for other species. All these experiments were conducted during the breeding season. Mature females were selected on the basis of a swollen, soft abdomen. Attempts to collect milt by “stripping” remained unsuccessful, probably due to the structure of the seminal vesicles. Therefore, males were often chosen on the basis of their aggressiveness. Two males were placed in a large aquarium or concrete tank. The male which was less injured after 1–2 hours was chosen.
Natural propagation in ponds and tanks
Mature breeders are stocked in small ponds which have been kept dry for a few weeks and are partly refilled. Micha (1973) obtained his best results in a 400 m2 pond after stocking 6 females and 4 males per pond. A few hours later the decisive natural spawning condition was simulated by raising the water level up to about 0.5 m. Spawning occurred at night and the following morning the broodfish were removed. The percentage of succesful spawning was high but the number of fingerlings obtained after a nursing period of 6–8 weeks was low (one or two fingerlings per square metre). This technique of natural reproduction has also been applied successfully in concrete tanks.
Semi-artificial propagation in ponds and tanks
Induced propagation in ponds. The same method as described above is used, except that spawning is induced by injecting the females with 5 mg Deoxycorticosteroid Acetate (DOCA) per 100 g body weight. Simulating the decisive spawning condition by raising the water level is not necessary for successful spawning. As for the above described techniques low survival in nursery ponds is the limiting factor for mass production of fingerlings.
Induced propagation in concrete tanks. Pairs of breeders are separately placed in a 1 m3 spawning tank on the bottom of which a layer of clean gravel has been spread. Females are injected with 5 mg DOCA per 100 g body weight. The injection is given intra-peritonally or intra-muscularly in one single dose generally in the afternoon. Spawning then occurs at night. The broodfish are removed the following day. Fertilized eggs adhere to the gravel or any other substrate available. As in nature, spawning is often partial and not all the ripe eggs are released; up to a maximum of 10 000–15 000 eggs/female hatch. The number of fry which can be collected a few days later is still smaller due to high mortalities (fungus infection). Generally up to 5 000 3–4 day old fry are collected per female. The success of this method is often hampered by the difficulty of the choice of male and the partial release of gametes. Some disadvantages are: (i) breeders often injure each other, sometimes ending in the death of one of them: (ii) the number of fry obtained is relatively small.
Semi-artificial propagation as described above has not proved to be a reliable method for mass production of fry. Therefore, artificial propagation under controlled environmental conditions in a hatchery has become a necessity.
Artificial propagation through hormone treatment, artificial fertilization and incubation of fertilized eggs, and subsequent rearing up to fingerling size has several advantages:
Broodstock and hatchery management (Fig. 11.1)
Breeders can be collected either from nature or from fish ponds. The capture of wild broodfish is recommended during: (i) the breeding season when mature catfish aggregate in or towards the shallow spawning grounds or (ii) the dry season when they are relatively concentrated in their reduced natural habitats, especially small pools and streams.
An alternative to capturing broodfish from spawning grounds or natural habitats is the rearing of fingerlings up to maturity in fish ponds. At harvest, breeders are selected and transferred to the holding unit in the hatchery, or to a special broodfish pond.
Hatchery management is simplified if only two age-groups are kept in the hatchery, i.e., broodfish and one batch of juveniles up to 1 g at a time. One gramme fingerlings are the minimum size required for proper stocking of on-growing ponds. Broodstock maintained for at least one year under controlled conditions in a hatchery lose their seasonal reproductive cycle and mature breeders become available year round. Consequently two stocks of broodfish should be maintained in the hatchery, i.e., (i) actual broodstock for propagation and (ii) conditioning broodstock.
Individual broodfish of weight 0.5–1 kg are preferable. They have a substantial quantity of mature eggs and are easy to manipulate.
A moderate-size hatchery (annual production capacity of 500 000 fingerlings) requires a total incubation capacity of 800 g of fertilized eggs/batch. This amount of eggs can be produced by about 16 females of 500 g and be incubated in 4 incubation troughs.
The period of indoor rearing of fry up to early fingerlings of about 1 g varies from 6 to 8 weeks, depending on water temperature and feed quality. This would mean that artificial propagation should be carried out every 6–8 weeks. If nursing is in ponds, artificial breeding should be carried out once or twice every month in order to meet the annual production target of 500 000 fingerlings.
The same female broodfish can be induced to reproduce artificially every 4–6 weeks without affecting either the quality or quantity of eggs obtained after stripping.
(A) Hatchery (pond nursing)
(B) Hatchery (hatchery nursing)
Scale 1cm to 1m
Fig. 11.1 (A) Layout of a catfish hatchery where the fry are nursed in earthen ponds
(B) Layout of a catfish hatchery where the fry are nursed in the hatchery
The size of the male broodstock depends on the number of males required for each artificial propagation and the number of artificial reproductions per year. One, rarely two, males need to be dissected for the procurement of milt.
Taking into account the above and with a large safety margin, the following stock of broodfish should be maintained in separate tanks in the hatchery:
A reserve broodstock of about 150 males and 150 females should be maintained in one pond for safety reasons. In ponds, two sexes can be maintained together because they do not spawn in confined waters.
Qualitatively and quantitatively adequate sexual products are prerequisites for artificial propagation. Therefore, sexually mature and healthy breeders kept under suitable environmental conditions and with an adequate food supply are mandatory. Optimal hatchery management including the following factors is required to maintain broodfish under these optimal breeding conditions.
The breeders are kept in rectangular tanks of about 1 to 1.5 m3. The water inflow is at one end, while the water flows out at the other end through a turn-down pipe. Each tank may be stocked with 100–150 kg fish/m3. Oxygen concentration in the water should not fall below 3 mg/l. Approximate water flow rates and turnover times are given in Table 11.1.
The optimum temperature for keeping and conditioning broodfish for artificial propagation is 25°C. Such temperature with a minimum of fluctuation is a pre-requisite for adequate gonadal development year round. Although the temperature of the hatchery water fluctuates with the temperature of the water source (stream, reservoir or borehole), some simple techniques can help to increase or decrease the water temperature to maintain it as close as possible to the required optimum.
CALCULATED WATER SUPPLY (1 min) FOR ADULT BROODFISH
AND TURNOVER TIME (min) FOR 1 m3 BROODFISH DEVICE
FOR DIFFERENT BIOMASSES OF BREEDERS
|Biomass per tank|
Little is known about the influence of light on the gonadal development of C. gariepinus. Light periodicity does not seem to be a decisive stimulus for gonadal development. An illuminated environment may irritate catfish since their preferred habitat is in turbid waters. It is, therefore, recommended to cover three-quarters of the tanks' surface, starting from the inlet. An artificial light is placed on the open part of the tank (near the outlet), about 20–30 cm above the water level. This increases the difference between the dark part (for gathering of healthy fish) and the illuminated part (where sick fish gather). It facilitates monitoring the fish and the cleanliness of the tank.
COMPOSITION AND CALCULATED CHEMICAL COMPOSITION
OF ARTIFICIAL DIET USED FOR CLARIAS GARIEPINUS
BROODFISH AND YOUNG FISH
|cotton seed cake||25.0|
|sesame seed cake||10.0|
|Calculated chemical composition|
|crude protein, %||47.0|
|digestible energy, Kcal/kg||3 050|
|methionine and cystine||1.4|
1 Composition per kg: Vitamin A, 4 800 000 IU; Vitamin D3, 800 000 IU; Vitamin E, 4 800 mg; Vitamin K, 800 mg, Vitamin B1, 600 mg; Vitamin B2, 2 800 mg; Vitamin B3, 4 800 mg; Vitamin B6, 600 mg; Vitamin B12; 4 mg; folic acid, 200 mg; cobalt, 160 mg; copper, 1 200 mg; iron 9 000 mg; iodine, 480 mg; magnesium, 2 730 mg; manganese, 28 000 mg; zinc, 20 000 mg
Adequate food supply is also of foremost importance to broodfish. A well balanced compounded diet containing all the essential nutrient requirements, particularly amino acids, vitamins and minerals is a pre-requisite for proper gonadal development. Except for protein and lipid requirements, little is known about the nutrient requirements of C. gariepinus. Artificial diets can be made with locally available agricultural by-products. In most African countries, feed ingredients containing high amounts of animal protein such as fish and blood meal are scarce and costly. Therefore, it is easier to meet the high protein requirement by using ingredients containing large amounts of vegetable protein such as oil cakes and oil meals. These ingredients are more common, cheaper and generally available in large quantities. The composition of the artificial diet used for broodfish in Central African Republic is given in Table 11.2.
All the ingredients are pulverized until they have a particle size less than 1–1.5 mm, using a high speed hammermill. Then they are mixed thoroughly and compounded to pellets (diameter 4–5 mm). The pellets can be prepared following the “dry” or “moist” processing technique.
A feeding rate of 1% of the body weight should be applied for broodfish of 500 g or more. Careful hand feeding, avoiding stress and over-feeding is recommended. The feeds are given in three or even better four rations during day time near the inlet. Supplementary feeding with under-sized tilapias is recommended whenever they are available. Feeding must be stopped when catfish stop showing interest in the feed. This type of feeding allows control according to appetite. It also helps to monitor the condition of the broodstock. Automatic feeders, which generally result in a higher food intake, may be used, but the appetite and the health of the broodfish must be monitored daily for the above mentioned reasons.
Hormone treatment of broodfish
Final maturation followed by ovulation can easily be induced in C. gariepinus. The presence of a large number of ovocytes which have completed yolk accumulation, also called ovocytes in the “dormant” stage, is a pre-requisite for successful induced breeding.
The success of the artificial propagation (number of ovulated eggs obtained after stripping), depends on the number of such dormant ovocytes in the ovary; i.e., the size of the female gonad or gonadal maturity. Females selected for induced ovulation and spawning should show:
a well-rounded and soft abdomen which extends anteriorly past the pectoral fins to the urogenital papilla. Mature eggs, showing clearly the nucleus in the centre, can be obtained easily by slight pressure on the abdomen;
a genital opening which is swollen and sometimes reddish or pink in colour.
There are no clear external symptoms to indicate the maturity of the males. Some authors describe a more elongated, slightly swollen urogenital papilla for “ready to spawn” males.
The following hormones/compounds have been successfully used to induce artificial propagation with C. gariepinus:
The use of DOCA is not recommended since this compound only induces pre-ovulation or final maturation (migration of nucleus to micropyle; breakdown of germinal vesicle followed by first meiotic division). Ovulation itself (rupture of follicle and accumulation of ripe eggs in the ovary cavity) does not occur. The eggs have to be “ovulated” mechanically through stripping.
Hypophysation, using for example commercially available carp pituitary glands, is presently the most common technique for the artificial propagation of C. gariepinus; treatment with locally available catfish pituitary gland is less expensive, but certainly more laborious.
The most common procedure to inject catfish breeders is to select females of more or less equal size. In that case, all the females can be injected with the same quantity of pituitary gland solution. Otherwise each female should be weighed, after which the quantity of hormone solution must be calculated individually.
Generally the hormone solution is injected into the dorsal muscles above the lateral line, just below the anterior part of the dorsal fin, using a graduated syringe (2–5 ml). The needle is placed parallel to the fish, pointing posteriorly at an angle of approximately 30°. After injection, the injected area is rubbed with one finger to distribute the hormone suspension evenly throughout the muscles. The injection can also be given into the body cavity.
When more than 10 females are selected, it is advisable to separate them into two groups of equal numbers and to inject them with a time interval of about 30–60 minutes between groups. This will give the operator more time for stripping the females at the right moment. Females are generally injected in the evening. The injection time is calculated according to the water temperature and the desired time of stripping (see below).
Handling of breeders should be done with care using a wet towel. After injection, the females are gently replaced in their covered containers. There is no need to suture the genital orifice of catfish to prevent wastage of ovulated eggs, since the females do not scatter their eggs without the presence of a male.
Procurement of ripe eggs
If stripping is done too early the processes of final maturation and ovulation are not yet completed; if done too late the ovulated eggs have become overripe and partly resorbed. Incubation of such unripe or overripe eggs will result in relatively low hatching percentages. Therefore, it is essential to strip females as soon as the main bulk of their eggs are ovulated. The majority of eggs mature and ovulate at the same time, depending on the water temperature. Figure 11.2 shows the relation between water temperature and time interval between injection of a pituitary gland solution and ovulation of the eggs (latency time). When HCG is used, the latency time is slightly larger (about 14–16 hours at 25°C).
The time of injection and the exact time of stripping can easily be calculated using this figure as shown in Table 11.3. Eggs are stripped into plastic or enamel bowls in the usual way.
Fig. 11.2 Latency time of Clarias gariepinus injected with a pituitary gland suspension in relation to water temperature
EXAMPLES OF CALCULATION OF INJECTION
AND STRIPPING TIME
|Calculation of injection time|
|The fish farmer wants to strip his fish on Friday morning around 09.00 h|
|Water temperature (Thursday afternoon)||27°C|
|Expected temperature on Friday morning||24°–25°C|
|Expected mean temperature||25.5°–26°C|
|Expected latency time (Fig. 11.1)||± 11 hours|
|Injection timer (Friday 09.00 h minus 11 hours)||22.00 h (Thursday)|
|Calculation of stripping time|
|Water temperature (Friday 07.30 h)||24.5°C|
|Mean water temperature (24.5°C + 27°C)/2||25.8°C|
|Latency time (Fig. 11.1)||10 h 45 min|
|Stripping time (Thursday 22.00 h plus 10 hours 45 min)||08.45 h (Friday)|
The approximate time available for stripping to bring about an optimal hatching rate, is 20–30 min, 60–90 min and 120–240 min for an average water temperature during ovulation of 30°, 25° and 20° respectively.
The same female can be stripped every 6–8 weeks without affecting the quantity and quality of eggs obtained. This would mean that a “spent” catfish breeder kept under optimum farming conditions (especially optimum temperature and adequate feeding) will develop a new batch of dormant eggs within this short time interval.
Procurement of milt
Attempts to strip catfish males to collect milt have not been sucessful. This is probably due to the anatomical structure of the seminal vesicles. Therefore, milt is obtained by sacrificing one male and dissecting the testis. Some small incisions are made into the cream coloured lobes of the testis. Milt can then easily be squeezed out and collected into a vial or small bottle. In this way, several droplets of milt can be obtained, after which the milt is diluted with physiological salt solution (0.6–0.7 % NaCl). It is essential to avoid any contact with water, otherwise the sperm will lose its activity. The milt solution can be stored in a refrigerator for one or two days.
The suitability of a sperm solution may be tested by mixing a drop of this solution with a drop of water. After mixing, ripe sperm will become active and will move vigorously for a period of about 30–60 seconds. This motility can be monitored using a microscope (60 or 100 magnification).
Fertilization of eggs
After stripping one or several female spawners, a few drops of milt solution are added onto the eggs and the sexual products are mixed by gently shaking the bowl. Mixing may be facilitated by adding some physiological salt solution.
The eggs are then fertilized by adding approximately the same volume of clean water. The water and eggs mass are thoroughly mixed by gently shaking the bowl. After about 60 seconds, the sperm loses its activity and the micropyle closes. This stops all the fertilization process.
Incubation and hatching of eggs
Fertilized eggs are incubated in stagnant or running water in troughs containing small trays or boxes. These trays have a perforated bottom (diameter of the holes: 1.2–1.5 mm) which can also be made of mosquito netting. The incubator is filled with clean, well oxygenated water, free of plankton organisms. The eggs are spread homogeneously in one single layer in the incubation tray. These trays are made in such a way that the eggs are continuously oxygenated by circulating water (Fig. 11.3). About 100–150 g eggs can be incubated in a trough containing about 80–100 l of water. If no incubation trays are available, fertilized eggs can be placed directly on one half of the bottom of the trough towards the outlet.
Once the fertilized egg comes into contact with water, it starts to swell and becomes sticky. The sticky layer, situated as a disc around the micropyle, is composed of glucoprotein (a compound of sugar and protein). The stickiness is strongest after 30–60 sec and disappears with time towards the end of the incubation period. Therefore, incubation must be started promptly after fertilization, at the most 60 sec after adding water to the egg mass.
Fig. 11.3 Incubation of Clarias gariepinus eggs in troughs, in running or stagnant water
For normal, healthy development of the embryos, the eggs need highly oxygenated water (5–6 mg/l), preferably oxygen-saturated. This can be ensured by a water supply of 1–2 l/min for an 80–100 l incubation trough, or by using an air-lift in case of incubation in stagnant water as shown in Figure 11.3.
The removal of the stickiness is not a pre-requisite for incubation of catfish eggs as it is for some other fish species, and large numbers of viable larvae have been obtained with the two techniques described above.
The time interval between fertilization and hatching, also called incubation period, depends on water temperature. The incubation period decreases with increasing temperature as given in Figure 11.4. At 25°C hatching takes place 28–32 hours after fertilization.
Hatching is the mechanical and enzymatic process of breaking of the egg shell (chorion) and release of larvae. Compared to hatching in stagnant waters, hatching in running water is retarded due to the dilution of the hatching enzymes. Therefore, the technique of incubation in stagnant water is slightly preferable.
During the incubation period the fertilized eggs are treated twice with a fungicide bath, e.g., malachite green (5 ppm for 10 min). These treatments are applied just after eggs are introduced into the incubators and about 10–12 hours later. They reduce considerably the fungus (Saprolegnia sp.) proliferation from spoiled eggs.
Although the number of normal, healthy larvae is difficult to estimate, about 50–70% can be obtained with the two incubation techniques described above. This hatching rate may reach 80–90% when small numbers of eggs (about 100) are incubated in Petri-dishes, maintained under laboratory conditions. The total number of hatched larvae is slightly higher than this since about 10–15% of the larvae are deformed. These deformed larvae die within a few days after hatching.
The production capacity of an incubation trough can be summarized as follows:
|- weight of stripped eggs||150 g|
|- number of eggs (600–900/g)||90 000–135 000 eggs|
|- percentage of viable larvae (50%)||45 000–67 500 larvae|
Technology of larval rearing
In the case of incubation in perforated trays, the separation of healthy larvae from egg remnants and spoiled eggs takes place automatically. Only viable larvae, seeking shelter, pass through the perforated bottom of the tray by actively swimming, leaving behind deformed larvae, dead eggs and empty shells. The trays are removed as soon as hatching is complete and normal larvae have gathered under the incubation trays.
In the case of incubation in trays with mosquito netting, the separation is less complete because most of the deformed larvae fall into the trough through the big mesh of the mosquito netting. Even small fertilized eggs will pass through this type of tray. The crippled larvae should be siphoned off on the second day after hatching. The first day after hatching the “swimming” capacity of the larvae is not yet well developed, and viable larvae will be wasted by too early siphoning.
Fig. 11.4 Incubation period of Clarias gariepinus eggs in relation to water temperature (after Hogendoorn and Vismans, 1980)
Fig. 11.5 Nursery trough for Clarias gariepinus
When fertilized eggs have been placed directly on the bottom of the trough, separation is obtained by covering the egg-free part of the incubator. The healthy larvae will then swim into this darker part under cover and cluster at the edges of the tank. Egg shells, dead eggs, and deformed larvae are then removed by siphoning.
The technology used for mass rearing of larvae and fry in indoor facilities is the flow-through technique. This technique is based on the following principles:
The trough used for larval rearing is the same as that used for incubation, except that the incubation trays have been removed. A non-translucent cover, which avoids direct exposure of larvae to light, is placed on the upstream two-thirds of the container; the downstream part is illuminated in order to:
create a separation between healthy larvae in the dark part of the tank (inlet side) and weak or diseased larvae in the illuminated part (outlet side);
facilitate monitoring of fish health (timely identification of diseased fish).
The water level in the larval tank can be adjusted by changing the position of the stand pipe or turn-down pipe. A fine mesh (<1.0 mm) screen is placed diagonally just in front of the water outlet. The screen should be cleaned several times a day to prevent water over-flowing and loss of young fish. The screen can be cleaned automatically by installing an airstone under it (Fig. 11.5).
After hatching, the rearing trough may contain 45 000 to 70 000 larvae. The recommended water depth is 12–15 cm, which corresponds to about 100–120 l of water and a stocking density of about 375–700 larvae/l.
Larvae need a highly oxygenated environment, preferably air saturated. It is advisable that the dissolved oxygen level does not fall under 5 mg/l. This can generally be obtained with a water flow rate of about 3–5 l/min. Catfish larvae, which gather on the tank bottom, beat their tails unceasingly. This forces the water around their body to move in order to ensure sufficient oxygenation. A very high water flow rate, which may press the larvae against the filtering surface, should be avoided. The dissolved oxygen content of the outflowing water must be measured at least once every day.
The optimum temperature for rearing catfish larvae and young fish is about 30°C. Too low (<22°C) and too high (>36°C) temperatures will retard larval development considerably.
In addition to proper hygiene, prophylactic bath treatments to prevent disease outbreak should be applied once a day (Table 11.4). Expensive anti-biotics such as Oxytetracycline, Neomycine, Streptomycine, Chloramphenicol and other chemical bactericides such as Sulphonamides are not advisable for prophylactic use. They should only be used for therapeutic treatment.
Yolk sac oedema due to bacterial contamination of fertilized eggs may sometimes occur. In that case, bathing with 50 ppm Oxytetracycline for 1 hour should be done during 4 to 6 days. From then on, it is advisable to disinfect eggs before incubation with an iodophor solution (25 ppm Wescodyne or Betadine for 5 min) to prevent contamination of eggs from broodfish. Contamination by pathogens can also be avoided by using boiled water for the incubation of eggs, using the stagnant water technique.
DAILY PROPHYLACTIC TREATMENTS FOR FERTILIZED
EGGS AND JUVENILES
|Development stage||Compound||Dose||Exposure time (min)||Treatment|
|Malachite green||0.05 ppm||permanent||fungicide|
|Larvae||Malachite green||0.05 ppm||30–60||fungicide|
|Early fry||Malachite green||0.1 ppm||60||fungicide|
|Advanced fry||Furaltadone||10 ppm||60||fungicide|
Nursing of early fry
After 3–4 days, when about two-thirds of the yolk sac has been absorbed, the larvae (weighing about 2–3 mg) become early fry. This major turning point in catfish life occurs when the larvae begin vigorously swimming in a fish-like manner and searching for exogenous food items.
Once the yolk sac is fully absorbed, the fry must find adequate food to ensure proper development; failure will weaken them beyond recovery and will stimulate cannibalism. During the early fry stage the development of the main organs will be completed after 10–18 days when the accessory air-breathing organ has developed. Catfish fry (now weighing about 30–50 mg) frequently rise to the surface to breathe air. They become then advanced fry.
Hatchery nursing of early fry
Early fry are kept in the larval rearing troughs, and the rearing conditions remain similar to those for larvae.
Catfish fry have been nursed successfully with the following first feeds: (i) live or frozen zooplankton; (ii) live or frozen nauplii of brine shrimp Artemia salina; (iii) decapsulated Artemia eggs.
A variety of artificial dry feeds such as complete diets, commercial trout starters, microencapsulated egg diet, etc., have been tested for the first nursing of C. gariepinus. All of them had more or less the same result: the food intake was considerably reduced especially a few days after initial feeding; growth was poor and mortality high. Recently rather good growth and high survival have been obtained using an artificial dry feed (Uys and Hecht, 1985). This feed, containing 55.4% of crude protein, was mainly composed of dried torula yeast (Candida utilis) (about 70%) and fishmeal (about 23%). Unfortunately, this type of yeast is not available in most African countries.
Feeding live zooplankton from nearby fresh water fish ponds seems to be the most reliable technique for African countries, since importation of Artemia eggs is either difficult or prohibited. A hatchery producing about 500 000 fingerlings a year needs at least 1 ha of ponds to produce the required quantity of zooplankton. Large quantities of zooplankton must be collected daily using a 100–150 micron mesh plankton net.
However, it is relatively easier to produce large quantities of “first food” in the form of Artemia. The brine shrimp eggs should be decapsulated and preferably incubated for hatching.
Early fry must be fed up to satiation 6 times a day between 06.00 and 20.00 h. Feeding every 3–4 hours during 24 hours is even better. The water supply is stopped during feeding to avoid washing out of food items. During each feeding, the feed is administered in two or three portions. The next portion is only given when all food items have been consumed. The behaviour of the fry may also be used as an indication of the quantity of feed to be given. Hungry fry swim vigorously in the water column, whereas well-fed fry gather in clusters on the bottom of the tank and have a considerably swollen belly. The stomach contents of the fry can easily be monitored since their ventral sides are transparent. Thus fry fed on Artemia nauplii or decapsulated Artemia eggs have a distinct orange belly after feeding. Once the fry show the satiation behaviour, feeding can be stopped and the water supply resumed.
The dry weight of zooplankton and brine shrimp eggs required for nursing early fry is about 13.5 kg and 25 kg respectively for an annual production of 500 000 fingerlings.
Mortality during the early fry stage is negligible under optimum nursing management.
Hatchery nursing of advanced fry
The early fry stage ends when the fry fill up their supra-branchial air chamber with air. From this stage young fish (about 50 mg each) accept and grow well on artificial dry feeds. The advanced fry, which have become real small catfish, are less delicate but they still need careful nursing.
The advanced fry are transferred to nursery troughs. These troughs have the same length and width as the larval-rearing troughs, but the water depth is increased to about 0.5 m. Greater depths should be avoided in order to conserve swimming energy, since fry rise regularly to the surface to obtain air. Transferring of advanced fry is a delicate procedure, and must be done by carefully siphoning fry into a bucket. The contents of the bucket are then gently released into the nursing device. Each nursing trough, filled with 160–200 1 of water, may be stocked with 10 000 fry (50 to 65 fry/1).
The water supply must be adjusted once a day according to the dissolved oxygen content of the outflowing water. Compared to larvae and early fry, advanced fry are less vulnerable to dissolved oxygen shortage since their gills as well as their accessory air breathing organs have developed. The recommended minimum dissolved oxygen level for advanced fry nursing is 3 mg/l.
Feeding. There are several physical and chemical requirements for artificial dry feeds. The feed must have the correct particle size (0.35–0.50 mm for fry of 50–100 mg; 0.50–0.75 mm for 100–250 mg fish; 0.75–1.25 mm for 250 mg-1 g fish). The fry must be able to recognize the feed chemically and optically. The feed particles must be water-stable to restrict nutrient leaching. The feed must have a low moisture content (<10%) to allow good storage and the complete range of nutrients required for fry must be present in each particle.
The advanced fry require a balanced diet which has a high protein and energy content. An example of artificial diet suitable for advanced fry is given earlier in Table 11.2.
From 10 to 18 days after hatching the fry will accept artificial diets. The change from live food to artificial dry feed is a major turning point in the life of hatchery nursed catfish. This change should be gradual to allow the fish to recognize and to accept artificial feeds. Therefore, the amount of artificial feed is gradually increased during the first week while the feeding of live food is proportionally decreased and then stopped. During feeding artificial feed is given first, followed by the live food.
Artificial feed can be administered manually (6 times a day) or automatically with feeders. Feeds must always be administered at the same place near the water inlet. The recommended feeding rates are given in Table 11.5.
Over-feeding must be avoided since this is believed to be the main cause of disease outbreaks at this stage of development.
After 5 to 8 weeks, the advanced fry will weigh about 1 g. At this size, they can be harvested and transferred to fattening ponds. A survival of about 70 to 80% can be obtained under optimal husbandry management.
FEEDING RATE (% BODY WEIGHT/DAY) FOR C. GARIEPINUS
AT DIFFERENT WATER TEMPERATURES
|50 mg||0.25 g||0.5 g||1 g|
Pond nursing of catfish fry
Fry can be nursed in small earthen ponds for about one month up to the fingerling stage. For an annual production of 500 000 fingerlings, about 4 000 m2 of nursery ponds are needed. This calculation is based on an average production of 12 500 fingerlings/100 m2/year.
The size of the ponds may vary from 200 to 1 000 m2. Rectangular ponds ranging from 10 × 20 m to 25 × 40 m are advisable in order to facilitate seining. These ponds should have a water depth varying from 50 to 100 cm. Greater depths should be avoided in order to conserve energy, Clarias fry often swimming to the surface to breathe air.
To establish a good standing crop of zooplankton, the nursery ponds are filled with non-polluted, slightly alkaline water (pH 6.5–8) and well exposed to sunlight. A minimum water supply of 4–6 l/s is recommended for an area of 4 000 m2. The water supply has to (i) replace water losses due to evaporation, seepage or leakage, (ii) fill the nursery pond rapidly, (iii) exchange the water if oxygen depletion or chemical water pollution occur.
For proper nursery management, it is important that each nursery pond is equipped with a rather wide water supply pipe or channel as well as a draining structure. Installation of a concrete harvest pit is optional. Nursery ponds should be located near the hatchery in an area which is free from flooding.
It is necessary to protect the nursery pond against predators such as juvenile fish, frogs, toads and their eggs. Therefore, the pond should be fenced by a fine mesh netting or roofing sheets if this is cheaper. The fence, having a height of 1–1.5 m, should be embedded for about 10 cm. The inflowing water should be filtered through a screened box placed on the inlet pipe. Nursery ponds for catfish are prepared in much the same way as tilapia ponds by drying, cutting vegetation, liming, and organic and/or inorganic fertilization (see Chapter 10).
The appropriate moment for stocking fry is about 3 to 5 days after fertilization, when a good standing crop of zooplankton (mainly rotifers) has been established. The nursery ponds should be free of predators to ensure high survival. This means that, in areas where predators (especially tadpoles and frogs) keep entering the pond in spite of fencing, stocking must be done earlier, 1–2 days only after filling the nursery pond. It is essential to seine the nursery ponds before stocking to guarantee removal of all predators.
To improve survival, it is advisable to stock the pond either with 2–3 day-old fry (two-thirds of yolk has been absorbed) or with 6–7 day-old fry, fed previously with zooplankton or Artemia in the hatchery. In both cases, fry will have two food sources during the initial days in the pond, i.e., natural food (which may not yet be available in optimum quantities) and the remaining yolk or some reserves built up during the previous feeding period.
The fry are stocked at a density of 50–100/m2. The number of fingerlings harvested does not increases with higher stocking rates in ponds where some predators (frogs and tadpoles) remain. Higher stocking rates up to 500/m2 may be considered in “predator free” ponds.
It is of foremost importance that the preparation of the nursing ponds is synchronized with the artificial propagation, to ensure that both the fry and the nursery ponds are “ready” at the desired moment (Table 11.6).
SCHEDULE FOR SYNCHRONIZED ARTIFICIAL PROPOGATION
AND POND NURSING ACTIVITIES
|Day||Artificial propagation||Nursing pond|
|- 4||Injection of broodfish||-|
|- 3||Stripping and incubation||Cleaning ponds: cutting grass, removal of silt, etc.|
|- 2||Hatching, separation of normal larvae and spoiled eggs||Liming|
|- 1||Cleaning larval troughs||Water filling and fertilization|
|0||-||Stocking of fry|
|+ 3, 7, 10|
14, 17, 21
Once the fry are stocked, a high standing crop of zooplankton must be maintained in the nursery ponds by regular fertilization to ensure good growth and high survival. The following fertilization may be applied twice weekly: 5 kg manure, 0.1 kg nitrogen and 0.025 kg phosphorus per 100 m2 of pond.
Feeding is not necessary during the first week of nursing since the early fry do not accept artificial feedstuffs. However, after one week the catfish need to be fed with a finely ground and sieved (through 0.25–0.5 mm mesh), artificial feed. This feed can be composed of blood meal or fish meal (25%), brewers yeast (25%), oil cakes (heated soy/groundnut/cotton/sesame cake, 25%) and wheat or rice bran (25%). Feeds are distributed twice a day at a rate (per 100 m2) of 0.5 kg during the second week after stocking, 0.75 kg during the third week, and 1 kg during the fourth week. After a week of feeding, the size of the feed particles should be increased to 0.5–1 mm but the food composition remains the same.
After about one month, the fingerlings (weighing 2–5 g) are harvested from the nursing pond. The fingerlings are collected in a concrete or wooden harvest box fixed to the outlet pipe. After harvest, the fingerlings are sold immediately or stocked temporarily in small storage ponds up to a density of 100–200/m2.
Sorting of the fingerlings by size is advisable in order to supply homogeneous size fingerlings to the fish farmers. The different size classes can be temporarily stored in hapas placed in a pond or concrete tank.
Under proper management, the survival rate varies between 20 and 30%. If frogs and tadpoles are not controlled, survival rate may be between 0 and 10%. This would mean that a yearly harvest of 10 000 to 15 000 fingerlings/100 m2 can be obtained.
C. gariepinus are normally grown-on to market size in earthen ponds, either in monoculture or in polyculture, especially with tilapias.
Generally, catfish ponds are constructed in the same way as ponds for other fishes. Specific details on catfish culture given here are based on the results of the Dutch Cooperation/FAO project in Central African Republic and those of local fish farmers in that country supported by the extension service of this project. Semi-intensive polyculture and intensive monoculture are carried out in static ponds, using the double crop system.
Up to the present time, very little information is available on ponds larger than 0.1 ha as ponds of this size are scarce in Africa, particularly among private fish farmers. Hence, the recommended size of on-growing ponds is 400 to 1 000 m2. Rectangular ponds are most suitable for seining. The average water depth should be about 1 m. Shallower ponds should be avoided, as the water temperature in such ponds may become too high and they may easily be invaded by aquatic plants. The water supply should be about 10 l/s/ha with a minimum of 5 l/s/ha.
Semi-intensive polyculture of catfish and tilapia
One way to enhance the production of tilapia ponds is to stock them together with a predatory fish such as C. gariepinus, which controls the excessive reproduction (see Section 10.10).
Semi-intensive polyculture of catfish and tilapia in earthen ponds aims at minimizing inputs. It is based on fertilization and supplementary feeding. In such culture systems, catfish will thrive on zooplankton, benthos, tilapia offspring and supplementary feed, whereas tilapia will consume phyto-, zooplankton, benthos and supplementary feed, see also Section 10.9.
Preparation of ponds. Pond preparation is done in the usual way. In addition the pond should be equipped with at least one compost crib placed in a corner near the inlet and made of bamboo or wooden stakes. The radius of each compost crib ranges from 1.5 to 3 m for a 400 m2 and 1 000 m2 pond respectively. A square floating bamboo frame (2 to 3 m per side) is fixed at the feeding place in the pond. Branches or stakes may be placed across the entire pond bottom to prevent poaching with cast nets, seine nets or line fishing.
Pond stocking. Fingerlings are stocked as soon as the ponds are filled with water. The stocking rate depends on the marketable size. If this size is at the most 200 g and 100 g for catfish and tilapia respectively), catfish fingerlings of about 1 g (hatchery nursed) or 2–5 g (pond nursed) and tilapia fingerlings of 5–20 g are stocked at a rate of 2 fingerlings of each species per square metre. If higher marketable sizes are desired, catfish may be stocked at a lower rate, but not less than 0.5 fingerling per square metre. In polyculture with monosex tilapia (1 to 2/m2), a stocking rate of 0.5 to 1 fingerling catfish/m2 is recommended, depending on the quality of the supplementary feed. The tilapia fingerlings are produced by the fish farmer or may be obtained from a seed production centre. The initial weight of the two species does not appear to be important and may vary. But the initial size of catfish fingerlings should be preferably less than that of the tilapias to prevent predation on these by the catfish toward the end of the culture period.
Pond fertilization. After stocking, the natural production of the pond should be regularly maintained by adding mainly organic fertilizers. Inorganic fertilizers, generally more expensive, are usually not necessary and their use should be restricted to areas with very poor soils and/or acid water. The most economic way of fertilizing the pond is by or one or more compost cribs.
Prior to water filling, each compost crib is filled up with layers of organic materials such as animal wastes (manure, blood, cows rumen), agricultural wastes (grasses, vegetable wastes, cotton seeds, wet brewers yeast, brewery waste), household wastes (spoiled fruits, kitchen wastes, ashes) or ready-made compost. After stocking, the compost crib should be maintained by adding organic wastes at regular intervals, according to the degree of decomposition of the compost and plankton development (water colour and transparency).
As an alternative, the ponds can be regularly fertilized by keeping organic manures at the water's edge.
A second alternative is to integrate animal husbandry (see Section 10.9). The animals can be kept in an enclosure in the pond (ducks), above the pond (poultry), or in a sty (pigs) built on the crest of the pond's dike. In the first two systems spoiled feed particles (about 10% of the feeding rate) and dropping fall immediately into the pond and are directly available to the fish, whilst in the latter these wastes are washed out into the pond once or twice a day. In all cases the wastes are partly consumed as direct food, and/or they decompose as fertilizers.
Supplementary food distribution. While some production (up to about 30–50 kg/are/year) may be obtained only by relying on the natural pond production increased through fertilization, successful polyculture of catfish and tilapia should involve supplementary feeding. The most economical ingredients which are locally available in significant quantities should be used. If available and economically feasible, oil cakes, which are rich both in energy and protein are preferable. They should be ground before feeding. If no hammer mill is available, they can be first soaked in water for a few hours, after which they can easily be pulverized by hand.
For the consecutive six months the recommended feeding rates per 100 m2 of pond are 200, 300, 400, 500, 600 and 700–900 g/day respectively. These feeding rates tally with feeding levels starting from about 8.5% of the biomass down to 1.6% at harvest. The monthly feed requirements and the corresponding feeding rates expressed as percentages of the total fish biomass are given in Table 11.7. The initial feeding rate is probably higher than necessary but then the feed also acts as a fertilizer.
Feeding should always be done at the same place in the pond, within the floating bamboo frame. This frame prevents the (floating) feedstuffs from scattering over the entire pond surface. The fish will learn very soon to feed at this feeding spot. The daily feed ration should be given in two equal rations, early in the morning and in late afternoon. These rations can be measured with a balance or with a tin or bowl which contains a known weight of feedstuff.
FFEDING RATES (PERCENTAGE OF BIOMASS) AND
AMOUNTS OF FEED TO BE DISTRIBUTED DAILY
|Amount of feed|
The feeding rate is monitored by checking the appetite of the fish during feeding and by determining monthly the average body weight of the two species. Sampling can be done with a cast or seine net. Sampling with a cast net should generally suffice, this involves less disturbance than seining. These average body weights allow the farmer to evaluate growth rate and to estimate the total biomass (or standing crop) of each species, assuming certain survival rates. Such estimates of average body weight, survival rate and calculated biomass of a typical catfish/tilapia culture are given as an example in Table 11.8.
ESTIMATES OF AVERAGE WEIGHT,
SURVIVAL RATE AND BIOMASS OF CATFISH
AND TILAPIA REARED TOGETHER IN EARTHEN PONDS
According to the appetite and the average body weight, the feeding rate may be either slightly increased (especially during the last two months of the culture period) or decreased.
Harvesting polyculture ponds. After about 6 months, when fish have reached marketable size, the pond should be harvested. The time of harvest may be postponed up to about one month when one of the two species or both have not reached the desirable size.
The quantity of fish harvested depends on the size of the pond, its productivity and its management. Under proper management conditions, an average quantity of about 40 to 60 kg/100 m2/crop can be expected. According to the quantity of fish to be harvested and the capacity of the local market one of the following two harvesting techniques may be chosen:
total harvest: the pond water is drained completely and all the fish are harvested. One part is sold the same day and the remaining fish are temporarily stored for sale during the following days. This technique requires adequate pond storage facilities.
partial harvest: during a number of consecutive days a certain quantity of fish is seined and marketed. Seining may be facilitated by partial draining of the pond. The last day the pond is completely drained and the remaining fish harvested.
Intensive monoculture of catfish
The intensive monoculture of catfish aims at optimum production and optimum profit. High yields may be obtained since catfish can be stocked at relatively high densities due to its accessory air breathing organ. At these high densities, the natural food production of the pond cannot meet the food requirements. External food supply with a balanced complete feed becomes a necessity.
The specific prerequisites for intensive monoculture of catfish, based on high stocking densities and artificial feeding with a balanced diet, are:
The preparation of monoculture ponds is similar to that for polyculture ponds with the exception of compost cribs, which are not required in this case.
Stocking. The stocking rate, depending on the marketable size desired, may vary from 2 to 10 fingerlings/m2, corresponding to a marketable size of about 500 and 200 g respectively after 6 months. The highest yield will be obtained with the highest stocking density. Higher stocking rates are not recommended for static water ponds.
Feeding. A balanced compounded diet is a prerequisite for intensive catfish monoculture. Although the dietary requirements for C. gariepinus are not known with certainty, the dietary levels given in Table 11.9 may be used as a guideline. This table shows that the catfish should be fed with a diet containing 30–35% digestible protein (40–50% crude protein) and 2 500–3 500 kcal digestible energy/kg food (3 500–4 500 kcal crude energy/kg food). Recent experiments have shown that it is difficult to indicate the optimum protein and/or energy level in balanced catfish diets as these two levels are highly inter-related.
Growth rate increases with increasing protein levels at a fixed energy level and vice versa. Therefore, the recommended values indicated in Table 11.9 are those which have given acceptable growth rates and which can be reasonably obtained using local sources of feed ingredients.
RECOMMENDED NUTRIENT LEVELS FOR INTENSIVE
CLARIAS GARIEPINUS FARMING
(in % dry matter)
|Nutrient||Fry and fingerlings||Growers||Broodfish|
|Methionine + Cystine|
It is not possible to give a standard formulation for a catfish balanced diet since the composition of such diet depends on the availability and prices of local feedstuffs which vary considerably. However, the composition of some least cost diets used in CAR and their calculated chemical composition is given in Table 11.10.
The balanced diet is to be compounded into either “dry” pellets (using a pelleting machine) or “moist” pellets (using a mixer/mincer or meat chopper). Generally investment for a pelleting machine to produce dry pellets is too expensive for a small farmer. Thus, this type of compounded diet must be purchased from a commercial feed company which is costly; moist pellets can be more easily prepared on the farm.
The recommended feeding rates between 21° and 33°C, corresponding with maximum growth rates and optimum food conversion, have been determined by Hogendoorn et al. (1983) (Table 11.11). These feeding rates are based on laboratory results where fish were fed with a commercial trout diet (crude protein 50%; gross energy 5 200 kcal/kg food). In practice, it has been found that slightly higher feeding rates may be applied during the first month(s) of culture in order to acclimatize the fish to the feed and the feeding place, while lower feeding levels should be applied during the last two to three months due to deteriorating water quality in static ponds.
LEAST COST FEED FORMULATIONS (kg) FOR ON-GROWING
OF CLARIAS GARIEPINUS IN CENTRAL AFRICAN REPUBLIC
|Ingredients||Dry or moist pellets||Moist pellets|
|Wet brewery waste|
(25% dry matter)
|Dried brewers waste||15||10||-||-||-||-|
|Wet brewery yeast|
(15% dry matter)
|Cotton seed cake||25||25||25||25||25||25|
|Sesame seed cake||10||10||10||10||10||10|
|Vit./minerals mix 1||0.25||0.25||0.25||0.25||0.25||0.25|
|Gentian violet (g)||-||-||5||5||5||5|
|Total weight (kg)||100||100||156.45||168.25||143.95||156.75|
|Calculated chemical composition|
|Digestible energy, kcal/g||2.70||2.76||2.62||2.67||2.70||2.71|
|Methionine + cystine, %||1.0||1.1||1.0||1.0||1.1||1.1|
|Cost/kg diet (FCFA)||78||88||71||67||81||77|
1 Commercial preparation meant for supplementing poultry (layers) diets, containing the following vitamins and minerals per kg: Vit. A 4 800 000 IU; Vit. D3 800 000 IU; Vit. E 4 800 mg; Vit. D 800 mg; Vit B1 600 mg; Vit B2 2 800 mg; Vit. B3 4 800 mg; Vit B6 600 mg; Vit. B12 4 mg; folic acid 2002mg; cobalt 160 mg; copper 1 200 mg; iron 9 000 mg; iodine 480 mg; magnesium 2 730 mg; manganese 28 000 mg; zinc 20 000 mg
RECOMMENDED FEEDING RATES (% BODY WEIGHT/DAY) FOR
CLARIAS GARIEPINUS AT DIFFERENT TEMPERATURES
(Hogendoorn et al., 1983)
|Body weight (g)|
In practice, the amount of feed to be distributed is calculated for a period of two weeks and adjusted every four to six weeks after estimating the average body weight. A fish sample should be taken using a cast or seine net for this purpose. The biomass of the catfish and the daily amount of feed are calculated according to the recorded average body weight and estimated survival rate.
It is difficult to predict growth rates and survival as they depend on many factors such as density, feed quality, temperature, pond fertility, and management, which are site and operator specific. However, estimates of biological data including average body weight, survival and standing crop as well as the daily amount of feed for a typical catfish culture system (stocking density 10/m2; temperature 25°–27°C), are given in Table 11.12 as an example.
Equal rations of feed are distributed three times a day, e.g., at 07.00 h, 12.00 h and 17.00 h, from one fixed place in the pond. They are distributed over a surface area of about 2 × 2 m in order to reduce feed competition to a minimum. There are no direct indications that the fish effectively ingest the pellets, since pellets generally sink to the bottom. A high concentration of air bubbles is the only indirect sign that food intake occurs. About 30–60 minutes after feeding, when air bubbles are no longer observed, the pond bottom may be checked with a fine mesh dip net for remaining feed. If there is no feed left and if the water quality is good, the feeding rate may be increased. If there is excessive feed left, the feeding rate should be decreased the next time.
ESTIMATES OF BIOLOGICAL PARAMETERS
FOR A TYPICAL CATFISH MONOCULTURE
(density 10/m2; mean temperature 25°–27°C)
|Week||Mean body weight (g)||Survival|
(% body weight/day)
Harvesting: After about 6 months, when catfish have reached the desired marketable size (200–250 g at a stocking rate of 10/m2), the whole crop should be harvested.
As stated earlier, production is site and operator specific. However, under adequate management conditions, a net production of 160–240 kg/are/year (16–24 t/ha/year), and a food conversion rate between 1.5 and 2.5 may be obtained. The rather wide divergence in expected production is caused mainly by the difference in survival, which is above all management specific. At a stocking rate of 10/m2, survival ranging from 50 to 75% can be expected. There is a substantial decrease of survival rate with increasing stocking rate. For example, at 20/m2, survival may be as low as 40%, while at 2/m2 this may be as high as 90%.
Bruton, M.N. 1979 The breeding biology and early development of Clarias gariepinus (Pisces, Clariidae) in lake Sibaya, South Africa, with a review of breeding in species of the subgenus Clarias (Clarias). Trans.Zool.Soc. London, 35:1–45
Hogendoorn, H. 1979 Controlled propagation of the African catfish Clarias lazera (C. and V.). I. Reproductive biology and field experiments. Aquaculture, 17:323–33
Hogendoorn, H. and M. M. Vismans. 1980 Controlled propagation of the African catfish Clarias lazera (C. and V.). II. Artificial reproduction. Aquaculture, 21:39–53
Hogendoorn, H., J.A.J. Janssen, W.J. Koops, M.A.M. Machiels, P.H. van Ewyk and J.P. van Hees. 1983 Growth and production of the African catfish, Clarias lazera (C. and V.). II. Effects of body weight, temperature and feeding level in intensive tank culture. Aquaculture, 34:265–85
Micha, J.C. 1973 Etude des populations piscicoles de l'Ubangui et tentative de sélection et d'adaptation de quelques espèces à l'étang de pisciculture. Nogent-sur-Marne, Centre Technique Forestier Tropical, 100 p.
Teugels, G.G. 1984 The nomenclature of African Clarias species used in aquaculture. Aquaculture, 38:373–4
Uys, W. and T. Hecht. 1985 Evaluation and preparation of an optimal dry feed for the primary nursing of Clarias gariepinus larvae (Pisces; Clariidae). Aquaculture, 47:173–83