Sex of fish larger than 17 cm can be determined externally by form of genital opening. If fish are bigger than 13 cm it is possible to distinguish by visual examination of young testes and ovaries after incision of belly.
The developing ovary is creamy white during accumulation of yolk which changes to greenish and reddish brown color later.
Clay (1979) separated the following six developing phases of ovary in Clarias gariepinus:
|Stage I||Immature ovary. Structure of gonad is ribbon like. Color is creamy white, translucent.|
|Stage II||Not ripe ovary. Small ova are visible through thick wall of gonad. Color of ovary is reddish brown. Ratio of gonad and body weight is less than 0.75%. Duration of this phase is 5–7 month.|
|Stage III||Almost ripe gonad. Color is reddish brown. Wall of ovary is thinner. Some large ova are visible. Weight of ovary is 5–7% of total weight of fish. Duration of this phase is 4–5 months.|
|Stage IV||Ripe ovary. Color of ovary is greenish or reddish - yellow. Ovary membrane is thin, large number of ova are visible. The blood vessels are visible well on surface of gonad. The gonad weight is about 7–15% of the body weight. No eggs can be pressed out with smooth pressing of belly. Duration of this stage is 2–3 months.|
|Stage V||Running ripe ovary. It is similar to previous stage. With moderate pressing some eggs would be released. Duration of this stage is two weeks.|
|Stage VI||Spent ovary. The ovary is loose. It soon transforms to Stage II.|
Sexual activity of African magur is cyclic in natural conditions (Clay, 1979). Female fish has a sexual resting time which lasts for 4–6 months. The period when running ripe fish remains dominant is approximately one month. Similarly, all matured male fish have a well developed testis and can produce viable sperm in the main season. Later ratio of male capable to produce sperm goes down and in off season all the testes are in inactive stage. Size of inactive testis is small and looks brown-reddish or translucent (Figure 3). It is impossible to distinguish between active and inactive males by external (visual) examination. As a result, selection of males available for reproduction in out of season period is difficult.
It has been observed that in African magur populations, the frequency of different diameter eggs has bimodal distribution in ovary. Majority of the eggs have diameters of 1.2 mm and 0.3 mm in the South African catfish. Similar distribution of eggs was detected in another (Nile) population of Clarias gariepinus with peaks at 0.1 and 0.84mm. Quick development of eggs would occur between these two phases in favorable environmental condition, so short time is sufficient between two reproductions of females (Clay, 1979; Clay and Clay, 1981). Accordingly, females kept in tanks and injected repeatedly one, two and three weeks after breeding can produce viable eggs after each injection, but quantity of stripped eggs gradually decreases (Hogendoorn and Vismans, 1980). However, Janssen (1987) found that if the fish are injected after every 6th–8th week, there is no reduction in the quantity of eggs. Moreover, he found that the fish can be breed troughout the year, if they are reared in tanks with constant quality water and feed supply.
Testis of African magur in inactive (1) and active (2) period (After Janssen, 1987)
Duration of embryonic development in water with normal oxygen content depends on water temperature. In Table 5 duration of incubation period at different temperature is shown. Different phases of embryonic development are shown in Figure 4.
Table 5 Length of incubation period at different temperatures (After Janssen, 1987)
Fertilized eggs are small lentil-shaped, with 1.5 mm diameter in size. Color of eggs is reddish-brown or brownish-green. Diameter of adhesive disc is similar to the diameter of eggs. It contains an elastic substance which sticks the eggs to substrate. Animal pole of egg is on opposite side to the disc.
Like other catfish, sensitivity of eggs is high to mechanical effect in first phases of embryonic development. Speed of embryonic development is high. Approximately, one hour after fertilization the fourth cleavage is completed. After nine hours the embryonic shield is developed at 25–27°C. Hatching starts 23–28 hours after fertilization at this temperature. It is completed within 2–3 hours, if tray method of incubation is carried out.
Newly hatched fry are 3.5–4.0 mm in length. They can swim actively immediately after hatching. Fry prefers the dark corners of incubation tanks. Though the larvae have a sucker on ventral part of their body, adhesiveness of newly hatched fry is not strong. Color of fry is reddish on first day after hatching and becomes dark by third day. Simultaneously the barbels start to develop and the fish scatter in tanks. The yolksack is absorbed within 4–5 days, but the fish can consume external food earlier, after partial absorption of yolksack on second-third day of hatching. Mouth of fry develops quickly. Only rotifer-size food (50–400 um) is accepted by fry on the day of feeding. They can consume much bigger food particle from second day.
Different stages of embryonic development of Clarias gariepinus (After Janssen, 1987)