Sharptooth catfish of the Clariidae family ("walking catfishes") are an indispensable source of animal protein in many regions of the world. And they are in the meantime produced in Europe, too: an increasing number of countries farms the species in warm-water recirculating systems. A particular characteristic of the fishes in this family is a special respiratory organ which enables them to absorb oxygen from the air in addition to using their gills to breathe. This additional air-breathing organ (suprabranchial organ) is located in the upper part of the gill cavity. It consists of a "cauliflower-like" structure (a formation in the mucous membrane of the pharynx) that is well-supplied with blood and has tubular air sacs that run on both sides of the spine towards the body’s rear. Aerial respiration enables catfish to survive in extreme biotopes with low oxygen levels. They can even leave the water for a few hours to search for food on land near the shore, change their habitat, or look for suitable spawning grounds. During their land excursions which usually take place at night the catfish crawl forward by twisting their body from side to side. For this they use powerful bony spikes on the pectoral fins, rather like the ski-sticks used for Nordic walking. Injuries caused by these spikes can be very painful with persistent bleeding.
The most species-rich and economically most important group within the Clariidae family is the genus Clarias. According to FishBase it currently comprises 61 recognised species of which 34 occur naturally in Africa and 27 in Asia. Clarias catfishes are often the only fish to survive in water bodies that dry out almost completely during dry periods. It is their air-breathing ability that allows them to endure these critical phases during which the fishes often lie tightly packed together in the most confined spaces. This means that high stocking densities in aquaculture are not a problem for them. Clarias species are not particular about their environment. They are extremely robust and insensitive to external influences – with the exception, that is, of temperature. The addition "African" in the species name (which also applies to Clarias species from Asia!) already indicates that the fishes need relatively high temperatures. Apart from this, however, they are not demanding. Their air-breathing ability and their modest needs make them ideal candidates for aquaculture in the tropics where suitable water temperatures prevail all the year round. Today, however, African catfish are also produced outside their natural range in Europe, where they are farmed in warm-water recirculating systems.
Ideal for production in aquaculture
Pioneers of Clarias breeding in Europe were catfish farmers in the Netherlands but other countries have followed suit since then. More than one million tonnes of the species are probably produced worldwide today. Unfortunately there are large gaps in the FAO statistics for Clarias species. One reason for this is that African catfish are kept in various different systems, from tiny backyard ponds and irrigation ditches to regular aquaculture facilities. Due to their large number and spatial distribution they will probably never be recorded fully. The other reason is that some producing countries in Asia or Africa fail to record and report their aquaculture production in full. Since African catfish are easy to rear the actual production is likely to be higher than the officially declared quantities. These fishes probably make a larger contribution to the supply of the world population with animal protein than often assumed.
Something else that is not clear is which species are produced in aquaculture throughout the world. In many countries no exact distinction is made between different species. That is why the FAO statistics assign the largest part of production to Clarias spp. without differentiating them further. However, taking into account species-assignable aquaculture production, catch reports from the fishery sector, and also the size of the distribution area, Clarias gariepinus is probably the most important species. This assumption is also shared by the South African ichthyologist Paul Skelton, who describes the African sharptooth catfish as the second most important freshwater fish in Africa after tilapia.
Clarias gariepinus was first scientifically described by the British natural scientist William John Burchell (1782 - 1863) who is regarded as one of the most important explorers in Africa. Of course, the natives of this region had been familiar with the air breathing catfish for a long time already but it was Burchell who described the fish for the first time according to scientific criteria and gave it its name. The generic name Clarias which is derived from the Greek word "chlaros" meaning "particularly vigorous, lively" refers to the fish’s ability to survive out of water. The species name gariepinus is derived from the locality in which the catfish was found. Burchell found it in a river that the English called Orange River but in the native language it was called Gariep.
The African catfish C. gariepinus is fairly insensitive to diseases, has no demanding nutritional requirements, and is fairly easy to reproduce. And that is why it has been settled in many regions of the world. The fishes have spread widely in many places since then and have become a danger to the native fauna because they kill fish, amphibians, insects and even birds and have allegedly brought some species to the brink of extinction. Today, African catfish are regionally classified as invasive. This explains why Clarias catfish are strongly polarizing. On the one hand they are in great demand as a food fish and are produced in increasing quantities in aquaculture; but on the other hand they can become an ecological threat and a real plague in the waters they inhabit.
Aerial respiration allows habitats that are low in oxygen
Typical features of Clarias are the fish’s flexible eel-like body, its long dorsal and anal fins, and its flattened head with the helmet-shaped ossified skull. The smooth skin is covered by a thick layer of slime. The body colour varies from greenish-olive to shades of grey and deep black, the back and sides of the body are marbled. A particular characteristic of Clarius catfish is its four pairs of barbels, whose main function is the identification of prey and orientation in murky waters. As obligate air breathers Clarias gariepinus is even found in waters that are periodically dry or have an extremely low level of oxygen. During the dry season the fishes often remain in the mud of swampy ponds for weeks until the next rainy season. If food or spawning facilities are lacking the fishes will also go ashore in search of more suitable habitats. To spawn they migrate into the side arms of large rivers which overflow in the rainy season and flood adjacent flat areas.
African catfish were introduced in a lot of countries for aquaculture purposes. One of the largest producers of Clarias is China, where catfish are often kept in paddy field culture. In 1998, Bangladesh launched a national programme for rearing Clarias in tiny holes in the ground measuring only one square metre to provide farmers with regular additional income. The aim of this small-scale production was to "revolutionise" aquaculture, similar to the idea of keeping chickens in cages. Such projects entail a high risk, however, and several studies have confirmed that under particular site conditions catfish can become a danger to the native animal world. Once the fish has settled successfully it is difficult to keep it under control. In Central Europe, on the other hand, the risk of an uncontrolled spread of African catfish is low. One reason is that the animals are reared in isolated indoor warm-water recirculating systems which make escape into open waters practically impossible. And another is that the temperate climate in our latitudes ensures that any escapes would only survive for a few months at the most anyway. As soon as the temperature drops to near freezing point in winter the fish would have no chance of survival.
Gariepinus is a relatively poor swimmer. When looking for prey it proceeds purposefully and methodically and does not make any sudden attacks. The fishes are mainly night-active and fulfil approximately three quarters of their nutritional needs after nightfall. The barbels, that are equipped with taste-buds and touch-sensitive mechanoreceptors, play an important role when tracking down prey. There is in the meantime growing evidence that African catfish also have an "electrosensitivity" which they use during their search for food. Apparently they perceive electrical discharges that emanate from some prey animals. The gastrointestinal tract of Clarias catfish can absorb large amounts of food. After a copious meal the fish’s belly is often grotesquely rounded. Clarias is an opportunistic feeder that will not spurn any food. However, the fishes remain flexible throughout their lives so that even as adults if necessary they can still feed on zooplankton that they sieve from the water with their gill rakers.
Clarias catfish grow rapidly. In their first year growth is almost linear which leads to a considerable leap in size. Females grow faster than males at first but the growth rate slows down as from the third year so that at an advanced age males are almost always larger. The available data on the fish’s maximum age range from 8 to 15 years, the EU’s European Fish Farming Guide even states 30 years. Depending on their size and the nutritional situation the fishes become sexually mature after one or at the latest two years at which point the females measure between 40 and 45 cm and the males between 35 and 40 cm.
Fry requirements met by European hatcheries
Rapid advances in recirculation technology have made Clarias interesting for European aquaculture. As is often the case with new fish species the Dutch were among the first to bring catfish to Europe. In 1976 work began to develop a parent stock based on wild-caught fishes from the Central African Republic. Later, catfish strains from Israel and South Africa were added which led to the "Dutch breeding line" of African catfish. Commercial production of Clarias catfish began in specially developed warm-water RAS in about 1985. These enclosed systems offer the heat-loving African catfish the conditions that enable them to grow optimally. In intensive farming they reach the desired slaughter weight of 1.3 to 1.5 kg after 140 to 150 days with daily growth rates of 1.7 per cent. Good farms achieve FCR values (Feed Conversion Ratio) of 0.85 and at an average of 85 per cent the survival rate is comparatively high.
The fry required for stocking the RAS farms are today produced by European hatcheries, some of which also run breeding programmes. The main breeding goals are rapid growth and good feed conversion, perfect appearance, a high slaughter yield, and disease resistance of the fry. Probably the largest producer of Clarias fry in Europe is the Dutch company Fleuren & Nooijen, which produces over 2 million young fish a year. They are not only in demand in Dutch catfish farms, in the Ukraine and Costa Rica but are also sold in classic Clarias countries such as Nigeria and Israel. Clarias females experience seasonal gonadal maturation, the beginning of which is linked to the rainy season, changes in water temperatures and daylight hours. At temperatures above 22°C, a certain number of ovules in the ovaries are in the "pre-ripening" stage throughout the year so that the eggs can be stripped after special preparation. The females’ fertility is about 500 eggs per gram body weight. While ovulated eggs can be easily stripped, males must be operated on or killed for sperm collection.
The development time from fertilisation of the egg to hatching of the yolk sac larva depends on water temperature. As a rule, egg development in the hatchery takes no longer than one day. Two to three days after hatching, the larvae have eaten their yolk stocks and begin active feeding. Usually feeding starts with Artemia nauplii, which are administered ad libitum several times a day. In this phase the larvae should be offered as much as they want. At the same time, the amount of dry food fed to the larvae is gradually increased so that the animals get used to the smell and taste of this food until, after about a week, they are only fed "dry". The willingness of catfish larvae to eat can be seen from the circumference and colouring of their bellies: the abdomen of well-fed animals is roundish and yellowish-orange in colour.
Two breeding cycles per year possible in RAS
In order to avoid the emergence of cannibalism the larvae and young catfish must be regularly sorted by size. After about one month the offspring has reached a weight of 5 to 7 grams and can be transferred to the on-growing farms. This can take place in various production systems from simple, water-filled earth holes to high-tech, computer-controlled RAS. Each system has advantages and disadvantages, and choice will depend on local conditions as well as the technical and financial means of the plant operator. With the exception of warm geothermal water, which supplies individual ponds regionally, the construction of recirculating aquaculture facilities (RAS) is unavoidable under European climatic conditions. Given well-functioning operation, regular fish stocking and the use of high-quality feed, 700 to 1,000 kg of Clarias catfish can be produced per cubic metre tank volume and year in such plants with two annual breeding cycles. Such quantities are necessary because the high production cost in RAS requires a corresponding output if farming is to be a commercial success.
The processing depth of Clarias catfish varies greatly in the individual regions of the world. While fish in Africa and Asia, with the exception of traditional forms of preservation such as smoking, drying or salting, are usually traded live, more convenient products based mainly on fillets are preferred by buyers in Central Europe. Processing is often still by hand, but increasingly also with machines. The Brüder Pommerehne production facility in Altkalen (Germany), which has a processing line with a daily capacity of almost 2,000 kg of African catfish, is an example of machine processing. With a fillet yield of between 40 and 42% this corresponds to around 840 kg of fillet per day. But even this company cannot do without manual labour. Due to the special skeleton structure of Clarias catfish the BAADER filleting machine can separate the fillets to a large extent but the fine work has to be done afterwards by hand.
The reddish muscle flesh of African catfish is tender, boneless, relatively firm and has a medium fat content with a high proportion of healthy omega-3 fatty acids and thus a high nutritional value. The mild taste is typical of the species, discreet and by no means "fishy". Due to its rather neutral character catfish is suitable for a variety of products, both traditional and modern. The range of Clarias products on the market is already quite broad. In addition to fillets and calibrated fillet portions, which are available both fresh and frozen, there are smoked products, delicatessen products, canned foods and convenience products. Many of these catfish specialities were developed by small creative processing enterprises which are quick to pick up original ideas. The firm muscle flesh, fine fibre structure and mild taste of the African catfish open up a wide range of culinary possibilities and offer ideal conditions for value-adding.