RECIRCULATING VERSUS NON-RECIRCULATING AQUAPONIC SYSTEM: WATER QUALITY AND GROWTH AND SURVIVAL OF TILAPIA AND LETTUCE  

Wenresti G. Gallardo*, Mohammed Al Mahfudhi and Mohammed Al Siyabi
Department of Marine Sciences and Fisheries
College of Agricultural and Marine Sciences
Sultan Qaboos University
Sultanate of Oman
gallardo@squ.edu.om

Aquaponics is considered to have great potential for growing fish and plants in an environment-friendly way but there are still some research gaps that need to be addressed. We conducted an experiment to compare the growth and survival of tilapia and lettuce in recirculating and non-recirculating aquaponic systems at different fish densities (10, 15 and 20 per tank). The system consisted of 12 units of rectangular tanks (80x40x40cm, with 100-l water), 3 of which were stocked with fish only (control), another set of 3 tanks with fish and lettuce grown on top (to save space) with Styrofoam platform (non-recirculating aquaponic system), another set of 3 tanks with fish only but these tanks were connected to another set of 3 tanks serving as filtration tanks in which lettuce were grown on top with Styrofoam platform (recirculating aquaponic system). All tanks had gravel on the bottom and the 3 filtration tanks had a vertical layer of gravel filter. Submersible pumps were used to bring water back to the tanks containing fish only and the water from the fish tanks went to the filtration tanks by gravity. Experiment was conducted for 6 week and sampling for growth was done every 2 weeks. Growth of fish was highest at the highest density (20/tank) in a recirculating system and lowest in tanks without plants (control). Survival was highest at 10 and 15 fish/tank in recirculating system. Survival was lowest in non-recirculating aquaponic system. Growth of lettuce was highest in recirculating system at 10 fish/tank and lowest in non-recirculating system at 20 fish/tank. There was no mortality among lettuce. In terms of maximizing production of both tilapia and lettuce, fish density of 15/tank may be used. During the 6-week experiment period, temperature, dissolved oxygen, pH, electrical conductivity, and salinity were measured weekly while water samples for analysis of ammonia, nitrite, nitrate and phosphate were collected at the start, middle and end of the experiment. Results showed that in the recirculating aquaponic system, there was less accumulation of nitrite, nitrate and phosphate, indicating that the biofilter in the system was effective in converting nitrite to nitrate; decrease in pH was not as much as in non-recirculating systems; there was lower electrical conductivity due to high uptake of nutrients (ions) by the plants; and no Staphylococcus and Vibrio which were detected in non-recirculating systems. Therefore the recirculating aquaponic system is the most suitable for growing tilapia and lettuce.