Effects of stocking density on production and economics of Nile tilapia (Oreochromis niloticus) and freshwater prawn (Macrobrachium rosenbergii) polyculture in periphyton-based systems

The present research investigated the effect of stocking density on pond (75 m(2), depth 1.2 m) production of Nile tilapia (Oreochromis niloticus) and freshwater prawn (Macrobrachium rosenbergii) stocked at a fixed 3:1 tilapia:prawn ratio. Three stocking densities were tried in triplicate: 20 000 ha(-1) (treatment TP-20), 30 000 ha(-1) (TP-30) and 40 000 ha(-1) (TP-40). The ponds were provided with bamboo as substrate for periphyton development. Bamboo poles (mean diameter 5.5 cm and 5.0 poles m(-2)) were posted vertically into pond bottoms, resulting in 60% additional substrate area in each pond. On average, 43 genera of algae and 17 genera of zooplankton were identified from pond water, whereas 42 genera of algae and six genera of microfauna were attached to bamboo substrates. No differences were observed between treatments in the ash-free dry matter (AFDM), chlorophyll a and phaeophytin a content of periphyton (P > 0.05). Survival of tilapia and prawn and individual weight gain of tilapia were lower (P < 0.05) in treatment TP-40. The net yields were higher (P < 0.05) in treatments TP-30 (2209 and 163 kg ha(-1) 105 day(-1) of tilapia and prawn respectively) and TP-40 (2162 and 141 kg ha(-1) of tilapia and prawn respectively) than in treatment TP-20 (1505 and 136 kg ha(-1) of tilapia and prawn respectively). The net tilapia yields were quadratic correlated (R-2=0.92) with fish stocking density. The cost-benefit analysis shows that the net profit margin was highest in treatment TP-30 (69%), followed by TP-20 (50%) and TP-40 (44%).