a. Systematic Position:
More than 100 species of the genus Clarias have been described all over the world. In Bangladesh the family Clariidae has only one native species the Clarias batrachus (L.); and the African magur Clarias gariepinus (Burchell 1822) (Synonyms: Clarias lazera and Clarias mossambicus) has recently been introduced in this country.
b. Geographical distribution
Clarias gariepinus locally called African magur, is an indigenous species in Africa where it is widely distributed. It naturally inhabits tropical swamps, lakes, rivers and floodplains some of which are subject to seasonal drying. In recent years the species has been introduced in Europe, Asia and South-America. The species easily adapts to environments, where the water temperature is higher than 20° C.
c. Biological description/Diagnostic Characteristics
The Clariid species display an anguilliform shape, having an elongated cylindrical body.
African magur has a scaleless slimy skin, which is darkly pigmented in the dorsal and lateral parts of the body. The fish turn lighter in color when exposed to light. During stress they show a mosaic-like pattern of dark and light spots.
The head is flattened, highly ossified, the skull bones (above and on the sides) forming a casque. The length of head is 30–35 % of body length.
Around the mouth, 4 pairs of barbels (Fig.1A) can be distinguished (nasal, maxillary, the longest and most mobile, outer mandibular and inner mandibular). The magur can move the maxillary barbels independently of its mouth. The barbels serve as tentacles. Close to the nasal barbels, two olfactory organs are located (Fig.1A). Magur recognizes its prey mainly by touch and smell. This is of relevance during feeding at night and in highly turbid or muddy waters where visibility is less.
African magurs with their wide mouth are able to feed on a variety of food items ranging from minute zooplankton to large fish. They are able to suck benthos from the bottom, tear pieces of cadavers with the small teeth on its jaws and to swallow prey such as a whole fish.
The mouth circumference is about 25% of its total length and it determines the maximum size of its prey. A magur of 30 cm (approx. 200 g) has a mouth circumference of about 7.5 cm.
The distribution of the gills and the arborescent organs over the five branchial arches is given in Fig.1B. They can be observed by cutting away the operculum. For the purpose of respiration, water is taken into the mouth, passed over the gills for gaseous exchange and is then expelled through the opercular opening. Especially when the dissolved oxygen content of ambient water is depleted or if the fish is out of the water, air is periodically gulped in through the mouth. Gaseous exchange takes place via the arborescent organs in air chambers above the gills. The air is also expelled through the opercular openings. Due to its ability to breath out of water, the African magur is capable of existing in mud during the dry season even for few weeks.
Along the concave anterior border of the branchial arches of C. gariepinus, long slender gill rakers are present. These mainly serve as filters for feeding on smaller vegetable matters and invertebrates (Fig. 1B).
The unpaired fins of African magur consist of a dorsal, a caudal and an anal fin, while the paired fins consist of the pectoral and ventral fins (Fig. 1C). The pectoral fins have developed strong spines (Fig. 1C) which have locomotory and protective functions. The fish is capable of migrating over land by sculling with its tail as it elbows along on its spines. The sharp spines are not poisonous. A land migration of magur, as long as 180 m, from a marsh to a river, has been reported in Africa.
Fig 1. External features of C. gariepinus (Adopted from Viveen et al. 1986).
d. Water quality requirements
The water quality requirements of the African magur are not fully known. It is noteworthy, however, that the species can tolerate more unfavorable conditions than carps and other cultured species. Water quality requirements of C. gariepinus as described by Janssen (1987) are shown in Table I.
Table I: Water quality requirements of African Magur
|Eggs, larvae early fry||Advanced fry (Tolerance)||Fingerlings/adults|
|O2||80–100% saturation||3–5 ppm||>3 ppm|
|Temperature||opt. 30°C||opt. 30°C||opt. 26–28°C|
|NH3-N||0.1 ppm (1.0 ppm)|
|CO2-C||6 ppm (10–15 ppm)|
|Salinity||10 ppt. (15–16 ppt)|
In winter months (mid December to mid February) when water temperature goes below 20°C (in Bangladesh) the appetite and growth of magur are reduced.
e. Natural behavior, food and feeding
As previously mentioned C. gariepinus prefers calm and warm water of floodplain swamps and pools.
Several studies have been made on the natural food of C. gariepinus in Africa. Micha, (1973) considered African magur as an omnivorous fish with a high tendency for predation. Different kinds of food items were found by different authors in the stomach of the African magur captured from natural waters. The food items reported are: aquatic and terrestrial insects, fish, molluscs, fruits, diatoms, arachnids, plant debris, seeds, detritus, bird eggs, young birds, droppings of poultry and zooplankton. In zooplankton-rich fish pond, African magur often join other carp species to graze zooplankton on the surface.
Four modes of feeding behavior have been observed in C. gariepinus (Bruton, 1976):
Feeding by grasping from surface in group or individually (e.g. magur in poultry-fish integrated ponds)
Group feeding by forming a circle (e.g. zooplankton grazing)
Individual foraging of food
Individual shovelling of feed
Slow, methodical searching for food is their normal tactic. When a suitable food item is found, magur grasps it often by suction. A strong negative pressure (suction) is created by increasing the volume of the buccopharyngeal chamber.
Many authors concluded that the African magur is an omnivorous slow moving predatory fish which feeds on a wide variety of food items from zooplankton to fishes of half of its own length (Janssen 1987).
Predation by African Magur
In general, predation needs much more energy than swallowing food items. Thus, where abundant food items, other than fish, are available, the magur feeds almost entirely on these items. However, in juvenile stages, cannibalism does occur, mainly in high density pond culture. After reaching a certain size (50–100 g), in properly fed fish ponds, cannibalism practically stops.