The African catfish (Clarias gariepinus)

In 1976, the Netherlands became the first country in Europe to import African catfish. 40 African catfish imported from the Central African Republic to the Netherlands served as the initial broodstock. In recirculating aquaculture systems, commercial cultivation of this species began around 1985. (RAS).

Israel and South African catfish strains were imported much later. The “Dutch strain” of African catfish, which is now being cultivated, was created by crossing these strains in order to produce fingerlings.

On a farmer’s level, choosing brood stock is often done in a rather simple manner. It is highly expensive to maintain and strictly separate large populations of several African catfish strains over extended periods of time. If you are successful, the effects become apparent after a few years. For the commercial farmer, the next challenge is to maintain this edge for himself.

The need to demonstrate to his client the advantages of a superior strain is another challenge for the farmer raising African catfish fingerlings. In the eyes of a researcher, increases in growth and feed conversion ratio of, say, 5% are major accomplishments, but for a farmer, these improvements represent modest variations that are likely to go unnoticed without rigorous farm records.

Practically speaking, pricing is the primary factor influencing the market for fingerling African catfish. The availability, size, consistency, and health state of the African catfish are all priorities for hatcheries.

The majority of African catfish farmers (in the tropics and in Europe) are small to medium sized, poorly organized, and lack the resources and skills necessary to do meaningful research on genetic enhancements of their stocks. Universities have conducted the scant genetic study on African catfish, but it has never been taken seriously in reality.

Selection of African catfish broodstock

The production of African catfish is a relatively recent business. It began in Europe in the early 1980s, and at that time, only few farmers in Africa were engaged in widespread farming activities.

Picture of a ready-to-spawn African catfish broodstock

The initial broodstock originated in the wild. For instance, wild African catfish were imported into the Netherlands from Central Africa, and the first selection was made primarily on size. In my perspective, we were just selecting fish that could adapt to the intense farming methods used in warm water recirculation systems. Fish were brought in from Israel and RSA not long after the arrival of African catfish from Central Africa. The “Dutch African catfish” is a product of three strains since the strains were not maintained distinct in the Netherlands.

The processing business places a great deal of importance on the quality of the meat and the proportion of dress out. The strains from various parts of Africa differ from one another. Because fish is consumed whole in the tropics—no filets are removed from the fish, and heads and guts are not wasted—the element of flesh quality and dressing proportion is less significant.

Africa catfish hybridization
Heterobranchus and Clarias species interbred to create intergeneric hybrids, which are animals that belong to the Siluriformes order of animals (Ref.). The so-called “Hetero-clarias” is a well-known intergeneric hybrid created by mating a male Heterobranchus longifillis with a female Clarias gariepinus (see figure 5). The intergeneric hybrids exhibit traits from both parental species as a result of rearranging genetic material from both parents in the child.

For a group of farmers connected to a processing factory, we are raising this hybrid in our hatchery in the Netherlands. This group is particularly committed to raising and selling this kind of fish. Heteroclarias filets are white in color as opposed to Clarias gariepinus filets, which are pink or reddish, and have 30% more fat, which enhances the flavor. In hybrids, the gonads are essentially nonexistent and inactive. Because of this, the dressing % is higher than it would be for Clarias gariepinus. White filets from marine fish species can be substituted with the Heteroclarias fish filet.

Hybrid Heteroclarias juvenile showing characteristics of both Clarias (skin colour and body shape) and Heterobranchus (adipose fin, see detail picture below)
Detailed picture of the adipose fin (not connected as it is with Clarias gariepinus
A market size Heteroclarias is shown on the picture
A close up of a Heterobranchus longifilis fingerling. Please note the adipose fin (as shown in the hybrid)

benefits of hybridizing catfish
As a producer of Heteroclaria fingerlings, we see significant behavioral variations from the common African catfish. When frequent grading is ignored, the Heteroclarias fingerlings exhibit extreme cannibalism and exhibit a broad range of development. The Heteroclarias is readily strained in comparison to this. Well-graded fish exhibit highly equitable development and low mortality to the point of harvest at 1.4 kg at 12 weeks of age.

In the tropics, Heteroclarias is regarded as a better growing fish and is preferred over Clarias gariepinus for pond cultivation.

A producer of fingerlings will greatly benefit from hybridization. Because the hybrids are sterile, buyers cannot use those fish to continue reproducing. Pure parent stock is always kept on-site and never distributed to other farms. As compared to Clarias gariepinus, the pure Heterobranchus longifillis strain develops at a fairly late age of 2 years (1 year).

Upkeep of the African catfish broodstock
To obtain high-quality eggs and sperm, good brood stock maintenance is required. To have traceability for each batch of progeny and the potential for a breeding program, the broodstock should be individually branded. With the use of individual tagging, it is feasible to keep accurate records and ensure that the broodstock is given adequate time to recuperate after spawning.

For obvious reasons, broodstock should be kept apart from other farm activities:

  • To protect them from stress and illnesses
  • To achieve a year-round reproductive cycle, they must optimize their environment through a consistent light regime and consistent water quality.

We prefer recirculation systems in a confined area with temperature control if necessary. In the tropics flow through systems can work too if flushed with good quality borehole water.

Productivity of female broodstock

In our farm, the productivity of the female (fecundity) expressed as a percentage of the body weight is between 5-15%. The egg size tends to increase with the size of the female. In larger fish, the number of eggs per gram of eggs is lower than in smaller broodstock. On average, we count 500 eggs per gram. The average females we are using in our farm have a weight of 6 kg and produce 300-600 grams of eggs per female which equals to 150.000-300.000 eggs.

Egg production of African catfish

A female fish is selected from one of the broodstock fish tanks. With a small tube, it is possible to sample the eggs out of the ovaries to check if the nucleus has migrated to the side and if the egg size has a diameter of 1 mm or above. Most farmers do not perform this check as they just select by eye.

In captivity, females do not perform the final ripening of the eggs without hormonal treatment. The injection of gonadotropin-releasing hormone in the form of natural pituitary glands or synthetic products like OVAPRIM induces the final step of ripening called vitellogenesis. In this final step the eggs are provided with yolk and take up water.

The period between injection with hormones and stripping of the eggs depends on the temperature of the water and the type of hormone used. Through trial and error, the perfect time for stripping can be determined. Too early or to late stripping results in bad egg quality and thus poor spawning results.

  • Too early stripping results in a very dry egg mass. It is difficult to completely strip the female because the eggs do not flow out. The female often dies because of stress and internal injuries.
  • Too late stripping results in a fluid egg mass. Often the female already released a lot of eggs in the preparation tank but stripping is very easy.

Stripping an African catfish female

Several hours after stripping we perform final stripping to remove all ripe eggs from the ovaries of the female. This is to prevent that these ripe eggs die inside the ovaries and start to deteriorate. This will harm the fish and can cause death of the broodstock female. The females have to stay in recovery for a couple of days before putting them back to the broodstock tank.

Stripping an African catfish (Clarias gariepinus) female

Productivity of male broodstock

Sperm Harvesting

Male African catfish do not release sperm after treatment with hormones like many other fish species. The males have ripe sperm all year round. The fish should be at least 1 year old to have ripe gonads. A lot of variation can be seen between males of the same age in the ripeness and size of the testis. In practice, farmers sacrifice males in order to dissect the testis out of the abdomen. By making incisions in the testis tissue the sperm can be collected. As a consequence new male broodstock needs to be added to the broodstock population sacrificing the males is a big constraint on genetic programs.

Some farmers are using operations on the males. After tranquilizing the fish, a small incision is made in the belly of the fish, and with a syringe with needle, some sperm is taken out of the testis. Finally, the incision in the belly is stitched using veterinary stitching material and the male is able to recover in 1 to 2 weeks. During this time the wound closes completely.

Unlike the sperm of mammals sperm of fish is not active, but will become active as soon as it is in the water. The sperm is active for less than a minute, so it has to find an egg quickly before all the energy has gone. This is the reason that during the process of sperm collection all materials, hands, and fish should be dry. Although these precautions are taken, the sperm can be activated accidentally. The sperm concentration of good males is more than a million per ml!

Fertilization of African catfish

The eggs and sperm are collected in a dry glass or porcelain bowl and a dry small glass or porcelain cup respectively. A simple but effective way of fertilization is to bring the eggs and sperm together in the egg collection bowl and mix it gently before adding water.

In literature adding certain fertilizing solutions during fertilization is reported as being very beneficial because they are thought to extend the life of the sperm in order to improve the fertilization rate. I prefer not to use any extra fluids because any extra handling can give a problem too.

Artificial propagation of African catfish is a relatively simple procedure and many farmers are very skillful in doing it. Millions of larvae are hatched weekly in a country like Nigeria but until today there still is a shortage of good-quality fingerlings and juveniles. Reproduction of African catfish is following a certain procedure, but farming the larvae for 1 or 2 months to the juvenile stage comes down to the capabilities of the individual farmer.

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