Vegetable production in a recirculating aquaponic system using Nile tilapia (Oreochromis niloticus) with and without freshwater prawn (Macrobrachium rosenbergii).

Abstract

Two recirculating aquaponics systems were installed in a controlled environment greenhouse to study the growth and yield of lettuce (Lactuca sativa), Chinese cabbage (Brassica rapa pekinensis) and pac choi (Brassica rapa) using Nile tilapia (Oreochromis niloticus) culture with and without freshwater prawn (Macrobrachium rosenbergii). Culture water was lifted by a 40 W submersible pump from a 220 L bio-filtration tank to a 250 L fish tank, and allowed to flow by gravity to a 2.44 × 4.88 m raceway in a closed loop. Water was maintained at 20 cm depth permitting rafts to float. Hydroponically germinated seedlings in rockwool blocks were planted on the rafts at 15 cm spacing 30 days after stocking 22 kg of mixed-sex Nile tilapia in the fish tanks while 295 prawns were added in one of the raceways. Environmental conditions were maintained and water quality parameters were monitored in a compromise between the ideal requirements of fish, prawn and vegetables including the beneficial bacteria throughout the108-day culture period. Two sets of data for the three vegetables and one for tilapia and prawn were gathered after the two 35 day growing seasons of vegetables. Results showed that average dissolved oxygen of 5.6 ppm at 98% saturation and 21°C temperature, and a pH of 7.1‒7.7 were established by the systems that provided a favorable environment for tilapia, prawn and nitrifying bacteria. However, the pH was disadvantageous to vegetables. With a low concentration of total dissolved solids of less than 330 ppm which was far below the requirement, the high pH retarded the normal growth of the vegetables resulting in chlorotic and necrotic leaves. Results also revealed that the vegetables demonstrated significantly better growth in the system with prawns. Among the three vegetables, pac choi had the highest growth, yield, and productivity followed by Chinese cabbage and lettuce. It was determined that integrating prawn culture helped stabilize and diversify the system which aided in improving the harvest. It also confirmed that stocking density and component ratio were critical in designing an aquaponic system.