EFFECTS OF STOCKING DENSITY ON GROWTH, WATER QUALITY AND ECONOMIC PERFORMANCE OF MONOSEX NILE TILAPIA (Oreochromis niloticus) REARED IN AQUAPONICS SYSTEM

Abstract

Aquaponic systems integrate the culture of both plants and fish in recirculating aquaculture units. The nutrients derived from the fish are utilized by the plants for their growth in the system. The current study determined the effect of fish stocking density on growth performance, water quality and economics in a Nile tilapia (Oreochromis niloticus) - Lettuce (Lactuca sativa) aquaponic system. The fish whose initial average weight was 17.9 ± 0.07 g were stocked at densities of 150, 300 and 450 fingerlings m-3 in the three treatments; D1, D2 and D3 respectively and raised for 56 days. The treatments were replicated five times. Each treatment was subjected to 16 lettuce plants m-2 as the planting density. With respect to increasing stocking density the water quality parameters ranged from dissolved oxygen: 4-7 mg L-1; pH = 6.3 to 7.3; alkalinity: 64-90 mg L-1; TAN: 0.32 to 0.57 mg L-1; NO3: 0.13 to 0.36 mg L-1 and NO2: 0.020 to 0.046 mg L-1. There were significant (p < 0.05) interactions between stocking density and the water quality parameters (dissolved oxygen, pH, Total Ammonium Nitrate (TAN), ammonia and NO2). The growth performance of fish in terms of final weight, Food Conversion Ratio (FCR) and percent survival decreased with increasing stocking density. The final fish weight was 42.6±3.1g, 32.0± 3.8g and 25.2± 4.2 g; the FCR was 1.45±0.13, 1.66±0.1 and 1.86±0.07, and % survival was 93.85±2.1%, 89.73±2.35% and 82.05±2.9% for stocking densities 150, 300 and 450 fish m-3 respectively. The SGR decreased with increasing stocking density giving mean values of 1.5630, 1.1613 and 0.6371 for stocking densities 150, 300 and 450 fish m-3 respectively. Lettuce yield increased with increasing stocking density giving final biomass of 166.4± 9.8 g, 276.8± 23.2 g and 304.6± 23.2 g for stocking densities 150, 300 and 450 fish m-3 respectively and the lettuce heights were 22.6± 1.2 cm, 26.9± 0.9 cm and 24.9 ± 0.9 cm for stocking densities 150, 300 and 450 fish m-3 respectively. Basing on the enterprise budgets, all the treatments posted positive returns to the risk (i=18 %) and were viable investments. The break-even prices were Kshs 196.7, 137.9 and 114.2 kg-1 for stocking densities 150, 300 and 450 fish m-3 respectively. The break-even prices for variable costs were able to cover the cost of fish and lettuce in the local market as they were below the selling price the farm was able to receive in the market. All the other stocking densities were profitable except at 150 fish m-3 and profitability increased with increased stocking density. Stocking density of 300 fish m-3 had the best-combined performance in terms of growth of fish and lettuce, water quality and profitability, hence recommended for small-scale aquaponic farmers