Reproductve biology of Nile Tilapia, Oreochromis niloticus (L.) Pisces: Cichlidae in the Nyanza gulf, lake Victoria

Abstract

The reproductive biology of the Nile tilapia, Oreochromis niloticus (L.) from the Nyanza Gulf of Lake Victoria was studied between May 1998 and June 1999. The annual gonadal and reproductive patterns, using changes in gonad weight, condition, and size at maturity, sex ratio, occurrence of various maturity stages and fecundity were investigated. The effects of various macroclimatic factors (light, temperature and rainfall) on gonadal and reproductive patterns were also assessed. Fish samples were obtained monthly from bottom trawls, gillnets and beach seines. A total of 1711 fish ranging from 13 to 56 em total length (TL) were examined in the study. For each fish total length (em) and total weight (g) were measured. They were then dissected, sexed, and maturity stage assigned macroscopically. From the length weight-relationship, 9. niloticus exhibited an allometric growth. The population was in good condition throughout the year with the relative condition factor ranging from 0.96 to 1.18. Changes in the relative condition factor were not found to be a good indicator of breeding. Seasonal occurrence of maturity stages and seasonal variations in gonadosomatic index (GSI) showed two ~9rtant periods in the breeding cycle of 0. niloticus; the post spawning period (July to October) and the protracted breeding period (December to June). The size at first maturity (Lmso) was estimated to be 31 em TL for females and 34