The effect of stocking density on the growth and survival of improved and unimproved strains of Oreochromis shiranus

Growth performance and survival rate of improved strains (F5 and F6) and unimproved strain of Oreochromis shiranus were assessed. Three stocking densities (5, 7 and 9 fish/m3) were used to randomly allocate fingerlings, of mean weight 6 ± 0.6 g, into 27 hapas of 9 m3 each, fixed in a pond of 700 m2 at Bunda Fish Farm, Malawi. Stocking density significantly (P < 0.05) affected the growth of the 3 strains though there was no significant difference between stocking density of 5 and 7 fish/m3. The highest final weight was noted at a stocking density of 5 fish/m3 with an average weight of F6 being 28.1 g, followed by the F5 (24.9 g) and the unimproved (24.0 g) strain. The improved strains had a higher final mean weight (F6: 23.41 g, F5: 21.84 g) than the unimproved strain (18.70 g) but there was no significant difference between improved strains (P > 0.05). The apparent genetic gain due to selection between the unimproved strain and F5, F6 strains was estimated to be 16.8 and 25.2%, respectively. Based on this, farmers can be encouraged to use F6 strain at a stocking density of 5 fish/m3. The revelation that there was no difference between the improved strains has implications on the continuity of the selection program.