Abstract:
Three wild stocks of Nile tilapia, Oreochromis niloticus were collected from three different agro-ecological zones in the Volta system in Ghana., A fourth stock of the same species was obtained from a farm at Nsawam. These were used in a study aimed at generating strain(s) of 0. niloticus with an improved growth rate or performance compared to the wild stocks. Equal aged brood stock were generated under similar environmental conditions from all four stocks and evaluated for growth and reproductive performance in monoculture and polyculture systems. Diallele crossing of the four stocks was conducted. The growth performance of progeny from the crosses were tested in three culture environments. Least square means of body weight and total body length at harvest were computed for each stock combination within the culture environments. Heterosis and breeding values (BVs) were estimated. A genetically mixed base population was established by creating a
selection line and a control line. Response to selection in the performance of progenies from the selection and control line was evaluated. The additive genetic variance (62 A), phenotypic variance (62 P) and heritability (h2) were estimated for the base population. Results mainly indicated the following: (i) Reference reproductive performance, the Yeji stock (Transitional zone) produced the highest number of seed (0.17 fry /g female /day) followed by the Nawuni stock (Guinea Savana zone) and then the Kpando stock (Semi-deciduous forest zone). The Farm stock produced the least value (0.10 fry /g female /day). (ii) Growth performance assessment showed that males were significantly heavier compared to females in all stocks. The ratio of the weight of females to males ranged from 0.61 - 0.70 for Yeji and Nawuni stocks respectively. (iii) Observed Sex ratio was skewed towards females, being 1: 1.8 and towards females, being 1:1.8' and 1 :2.2 in the extensive and semi-intensive culture environments respectively. (iv)With respect to growth performance of stocks, the Nawuni stock was superior to the other three stocks. It had the highest mean daily growth rate in almost all growth evaluation trials while the lowest growth rate occurred in the Yeji stock. (v) Expression of heterosis was negative for all crosses. (vi)The genotype-environment interaction was very low (0.1 %) suggesting that it
would not be necessary to develop specialized O. niloticus strains for different culture environments. (vii) A positive response to selection of over 10 % in improvement in growth rate of the selection line over the control line was observed. Recommendations for further investigations and implications of selecting appropriate stocks for improving the growth rate of O. niloticus in Ghana are discussed.
