2. BIOLOGICAL CHARACTERISTICS OF AFRICAN MAGUR

The skin of African magur is scaleless, similar to that of other catfish species. It is generally dark at dorsal and lateral surface of body and grayish-white at ventral side. Light (color of water, turbidity) has great effect on the coloration of the fish. During stress, black spots develop on the body surface of African magur (Viveen et.al., 1986).

The number of its barbels is eight. The fins have similar shape and location like local magur, Clarias batrachus. The first ray of pectoral fins is strong, but less sharper than that of C.batrachus, and not covered by poisonous mucus.

Figure 1.

Occipital process of Clarias macrocephalus (1), Clarias batrachus (2) and Clarias gariepinus (3)
(After Srisuwahtach and Tangtrongpiros, 1985; Viveen et al., 1985)

Its accessory breathing organs are located under the operculum. They are pear-shaped organs with a special cauliflower like structure inside, covered with epithelium facilitating gas transfer. The base of these accessory breathing organs are the gill rakers of second and fourth branchial arches. This organ can absorb total oxygen requirement of fish from air. However, in oxygen rich water the catfish does not use it for breathing. The accessory breathing organ is not active in fish younger than 12–14 days.

Ideal temperature for development of this catfish is in the range of 25–30°C. The lowest temperature required for artificial reproduction is approximately 21–22°C, for growth should be higher than 18–20°C. The fish can not survive long in water temperature lower than 9–10°C.

The maximum salinity tolerance of fingerlings is 9–9.5 ppt (Chervinski, 1984). The adults can live in slightly brackish water.

The fish can survive extremely harsh environmental conditions after development of accessory respiratory organ.

The African catfish can grow up to one meter body length in 8–10 years in its natural habitat. A fish of such size is approximately 5 kg (Van der Waal and Schoonbee, 1975). A fish with 1.45 m body length and 16 kg weight was recorded by Clay (1979). Body lenght-weight relationship of fish are shown in figure 2. In fish farms of Bangladesh market-size magur (300–350 g body weight) can be produced in 4–4.5 months (including the nursing period). Fish fed very well for 3 months may grow up to this size.

Like other catfish species, the African magur is also carnivorous with extra wide food- spectrum which facilitates adaptability of the species to different environment. Main food items of African magur are the aquatic and terrestrial insects and their larvae, plankton organisms of different size, other aquatic invertebrates and small-sized vertebrates including fish. In addition to this, the species can utilize plant materials and detritus also (Clay, 1981). Stomach content of fish where food supply was inadequate was found to contain pond soil in significant percentage. The species is strongly cannibalistic in habit, especially at fingerling stage. Frequency of fish consuming different food items at three different habitats is shown in Table 1.

Table 1 Frequency of individuals (in percentage of total examined fish) consuming different food items in three African magur populations (After Groenewald, 1963)

I : Fertile lake
II : Stream heavily polluted with organic materials
III : Riverain environment

For satisfactory production (FCR<1), animal protein rich feeds are necessary in intensive systems (digestible protein content 35–38%) (Janssen, 1987; Peteri et al., 1989). In ponds, where natural animal protein sources are available, artificial food with lower animal protein content is suitable for feeding of market-size fish.

Depending on average temperature and food supply, African magur matures at 1–3 year age in their natural habitat. In fish ponds of Bangladesh, a certain percentage of fish was found mature at the age of 7–8 month and all the fish reached maturity at the age of 9–10 month. (Usually quality of eggs produced by artificial reproduction of too young fish is not satisfactory.) Gonad weight of ripe females of natural water origin is 10%, and of pond origin 25– 30% of the total body weight. Size of testis is smaller, only 2–4% of the total body weight in active condition (Janssen, 1987).

This fish can spawn one or two times a year in the natural environment, mainly in rainy season. But in pond and tank conditions African magur can be forced for more frequent spawning. The two sexes can easily be distinguished by shape of belly and by the form of genital papilla. The belly of female is round and soft. Genital opening of females is round, slightly bulky and pinkish in mature fish. Males have long pointed genital papilla.

Figure 2.

Length-weight relation of African magur (After Clay and Clay, 1981.)

The African catfish lays eggs on aquatic plants, inundated grass and on detritus at natural spawning. Continuous high temperature and rains are usually the triggering factors of natural spawning. Large groups of fish gather at spawning grounds, but spawning itself is carried out only by one female and one male. Thus spawning of catfish is not similar to mass spawning of carps.) Males are extremely aggressive at spawning. Quantity of spawned eggs is 10,000–200,000 per fish of 30–90 cm body length (Janssen, 1987).

The ovulated eggs have oval shape. A disc is developed at basal part of eggs for fixing it to the substrate. Newly hatched fry has a sucker at ventral part of its body. This sucker is active only for a short period (Greenwood, 1955).