EFFECTS OF FISH DENSITIES ON GROWTH, SURVIVAL AND HEAVY METAL ACCUMMULATION  IN FISH AND PLANTS IN A RECIRCULATING AQUAPONIC SYSTEM

An experiment was conducted to determine the optimum fish density and investigate heavy metal accumulation in fish and plants grown in a recirculating aquaponic system. The aquaponic system consisted of 6 fish tanks and 6 plant tanks (80 x 40 x 40 cm) each containing 100 L of tap water. Based on the previous experiment that showed better growth and survival of fish at 20 fish per tank compared with 10 and 15 fish per tank, the present experiment tested higher densities of 20, 25 and 30 fish per tank. Seedlings of lettuce, beans, and the ornamental plant Petunia were planted on a styrofoam raft with holes for the roots to extract water and nutrients they need to grow. Tilapia (Oreochromis niloticus) with initial weight of 40 g were grown in combination with either lettuce and Petunia or lettuce and beans and were fed a commercial feed three times a day. Fish and plant sampling for heavy metals and measurement of fish weight and length were conducted every two weeks. Lettuce weight was measured during harvest at the end of the 6-week experiment. Growth and survival of fish at different densities were not significantly different but highest density of 30 fish/tank is recommended based on economic consideration.  Final weight of lettuce was highest among those grown in combination with 20 fish/ tank. Concentrations of heavy metals (lead, cadmium, arsenic and mercury) in fish were very much below the maximum limits and are therefore safe for human consumption. There was low concentration of lead (Pb) in all three plant species grown in combination with fish at different densities. Cadmium (Cd) concentration was low in the beans and lettuce, thus, they are safe for human consumption but the concentration of Cd in the ornamental plant Petunia was high. Mercury was not detected in the samples.