Factors influencing the spawning success of Oreochromis karongae (Trewavas) in ponds

Fry and fingerlings production of a pelagic tilapia from Lake Malawi, Oreochromis karongae subgenus Nyasalapia (Trewavas), was lower (4.07 ± 5.09, 10–21 fry female−1 month−1) than reported for other tilapias (about 135 fry female−1 month−1). Although seasonality was very pronounced in all experimental systems, the use of net bags (hapas) improved fry yield to 24.7 ± 27.90, 0–354 fry female−1 month−1. Multiple regression analysis demonstrated that the depth of ponds, solar radiation, rainfall and maximum temperature significantly influenced fry and fingerling production (adjusted R2= 0.75; P= 0.0024). Injection of exogenous hormones intramuscularly (pimozide, LH-RHa) induced spawning and significantly (P < 0.05) improved spawning success to a maximum of 998 fry female−1 month−1. As in other cultured tilapias, when placed in fine-meshed net bags, broodstock density was critical to fry production. A density of 1.0 fish m−2 for broodfish of 120–200 g produced significantly larger numbers of progeny in the hatchery system employed. Spawning was seasonal and restricted to November–February. Broodfish ponds given inputs of chicken layers' mash produced significantly (P < 0.05) higher numbers of fry and fingerlings than maize bran (4.17 vs. 1.22 fry m−2 female−1 month−1).