Breeding season, maturation and fecundity of Oreochromis niloticus L. (Pisces: Cichlidae) in Lake Chamo, Ethiopia

The reproductive biology of Oreochromis niloticus in Lake Chamo, Ethiopia was studied from samples collected every month for a period of thirteen months between March 1996 and March 1997. A total of 1349 fish ranging in size from 125 to 610 mm total length (tl) were caught using gill nets (120, 140, 200 and 240 mm, stretched mesh). The mean monthly gonadosomatic index (gsi) of the males ranged from 0.21 to 0.61 whereas that of females ranged from 0.75 to 1.61. Higher gsi values were observed between March and June for females and between March and May for males. The pattern of gonad development for both sexes was more or less similar. Ripe males occurred at high frequency (ranged: 39—60%) between March and May whereas high frequency of ripe females (ranged: 47—53%) occurred between March and June. Thus, it was concluded that O. niloticus in Lake Chamo spawns throughout the year with peak breeding activity between March and June. The smallest sexually mature female was 385 mm tl, whereas the male was 395 mm tl. However, the 50% maturity length was about 420 mm tl for both sexes. O niloticus were all mature above 500 mm tl. Mean fecundity of O. niloticus was 2493 ± 300, N = 209 (ranged: 390—540 mm tl and 1300—2650 g total weight). The smallest count was 1047 (fish length = 420 mm tl) whereas the largest was 4590 (fish length = 510 mm tl). In general, fecundity of O. niloticus in Lake Chamo was linearly related to total length (F = 201.85tl — 6801), total weight (F = 1.91tw — 1197.8) and gonad weight (F = 55.59gw + 467.2). SINET: Ethiopian Journal of Science Volume 24, No. 2 (December 2001), pp. 255-264 Key words/phrases: Ethiopia, Lake Chamo, Oreochromis niloticus, reproductio