Decoupled FLOCponics systems as an alternative approach to reduce the protein level of tilapia juveniles' diet in integrated agri-aquaculture production

Decoupled FLOCponics (DFP) is a promising aquaponics approach which takes advantage of the nutritional benefits of biofloc technology (BFT). Enabling the use of less protein in the fish diets is one of the benefits of BFT. The effect of the reduction of protein content, and consequently the input of nitrogen, on fish and plant production in DFP systems has not yet been investigated. This study was designed to investigate and evaluate the production of lettuce and tilapia juveniles in a DFP system using different levels of crude protein (CP) in the fish diets. The zootechnical performance of tilapia juveniles and lettuce growth in the DFP system were evaluated, using different diets containing 24, 28, 32, and 36% CP. Fish production in DFP systems was compared to those reared in traditional decoupled aquaponics systems (DAPS) and in biofloc-based systems (BFT), both fed with 32% CP diet. The experimental period of tilapia juvenile production lasted 56 days. Lettuce production in two cycles was also performed in DFP systems with different CP levels and their growth was compared to those in DAPS and hydroponics systems, as control treatments. In Cycle 1, the seedling phase was evaluated in a 14-day trial. In Cycle 2, the final production phase was performed for 21 days until harvest. The physical-chemical parameters of the water were monitored in the aquaculture and hydroponic subsystems. High mortality of fish occurred in DFP-36 in the middle of the experiment, thus this treatment was discontinued. The results showed that tilapia reared in DFP and fed with 24 and 28% CP (DFP-24 and DFP-28) grew similarly to those in DAPS fed with 32% CP diet. Fish in DFP-32 and BFT fed with 32% CP diet grew similarly and above the other treatments. Additionally, plant growth results showed no differences in both cycles among all treatments. With respect to water parameters, even though more nutrients were inputted in the treatments with high CP content in the aquaculture subsystem, the mean values of nitrogen compounds and orthophosphate were similar in all treatments. For water parameters in the hydroponics subsystems, only the mean values of pH in Cycle 1 were statistically different in the plant treatments. The results obtained in this study indicate that using less CP in fish diets to produce lettuce and tilapia juveniles is technically possible and feasible in a decoupled FLOCponics system.