Fish collection and experimental set-up
The experiment was conducted at the Bunda fish farm in 9 m3 hapas (3 × 3 × 1 m). The area was chosen because it has ponds suitable for the layout of the hapas, security and that the offspring of the improved strains and unimproved strain of O. shiranus were readily available. Juveniles of the improved F5 and F6O. shiranus strain and its unimproved strain counterpart were collected using a hand net. The juveniles of improved strains (F5 and F6) were collected from broodstock belonging to F4 and F5 respectively and were then kept in the tanks. These fingerlings were stocked at densities of 5, 7 and 9 fish/m3 at an average weight of 6 ± 0.6 g. Prior to sampling, the fish were not fed for 24 h. A total of 1701 juveniles (567 from each treatment population) were randomly allocated to 27 experimental hapas. A completely randomized design (CRD) was used to allocate three treatments replicated three times at three levels of stocking density (5, 7 and 9 fish)/m3.