Aspects of the biology of the Nile tilapia (oreochromis niloticus L.) in the Weija reservoir (Ghana)

Abstract

Studies on aspects of the biology of the Nile tilapia (Oreochromis niloticus L.) were undertaken in a man-made lake, the Weija reservoir in Ghana from September 2007 to August 2008. A total of 657 specimens of O. niloticus from commercial landings were examined of which 357 were males and 300 were females (sex ratio of 1: 0.84; P < 0.05). GSI was low in December 2007 and August 2008 and highest in March and May 2008. Fluctuations in the GSI, suggested that O. niloticus spawned throughout the year but major spawning occurred in March. Length at first sexual maturity (L50) was 13.49 cm for females and 16.33 cm for males. Observations on frequency distribution of ovum size which measured between 1.2 to 3.1 mm, showed two distinct peaks which were not completely separated from each other. This indicates protracted spawning in the population probably shedding eggs in batches. Fecundity ranged from 563 to 1542 ova for fish of total length 15.6 - 21.5 cm with mean of 851 ± 13.2 eggs. The relationships between fecundity (F) and total length (TL), and fecundity and body weight (BW) were: F = 12.36 TL + 628.5 and F = 0.519 BW + 785.4. The major reproductive activity of O. niloticus in the reservoir based on studies on monthly fluctuations in GSI and occurrence of ripe gonad, coincided with months with increasing total alkalinity, dissolved oxygen and reducing water temperature. The regression co-efficient of 3.1 for the length-weight relationship (for both females and males) was not significantly different from the expected value of 3.0 (P > 0.05), indicating isometric growth of the population. Collection of the species as brood stock or seed for stocking by fish farmers is recommended in March-April. Also appropriate management policies with periodic studies of the fishery and limnology of the reservoir are essential to sustain fishery production