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

Abstract

Growth performance and survival rate of improved strains (F 5 and F 6) and unimproved strain of Oreochromis shiranus were assessed. Three stocking densities (5, 7 and 9fish/m 3) were used to randomly allocate fingerlings, of mean weight 6±0.6g, into 27 hapas of 9m 3 each, fixed in a pond of 700m 2 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 7fish/m 3. The highest final weight was noted at a stocking density of 5fish/m 3 with an average weight of F 6 being 28.1g, followed by the F 5 (24.9g) and the unimproved (24.0g) strain. The improved strains had a higher final mean weight (F 6 : 23.41g, F 5 : 21.84g) than the unimproved strain (18.70g) but there was no significant difference between improved strains (P>0.05). The apparent genetic gain due to selection between the unimproved strain and F 5, F 6 strains was estimated to be 16.8 and 25.2%, respectively. Based on this, farmers can be encouraged to use F 6 strain at a stocking density of 5fish/m 3. The revelation that there was no difference between the improved strains has implications on the continuity of the selection program.