This study formulated experimental diets with three different phosphorus levels (6.3, 10.1 and 13.8 g kg-1diet) that were fed to tanks in a Recirculating Aquaculture System (RAS) and to Ponds. The diets contained respectively 1.6, 5.0 and 8.5 g bio-available phosphorous kg-1 dry matter and are referred to as s ufficient phosphorus “SP” (control), m oderate phosphorus “MP ” and d eficient phosphorus “DP ” diets . Genetically Improved Abbassa Nile Tilapia (GIANT) were stocked in the experimental units (RAS with 125- L tanks, each stocked with 15 fish, and 200 m2 ponds each stocked with 1000 fish ). The hypothesis was Nile tilapia grown in clear water tanks in RAS would perform best on the SP diet and worst on the DP diet, while in ponds Nile tilapia would reduce the dietary phosphorous deficiency by consuming natural food . In ponds, fish will perform better and show smaller differences in performance between the diets than in RAS. There were 5 replicates per treatment. Fish were fed restrictedly a daily ration of 18 and 20g/Kg 0.8 in ponds and aquaria, respectively.
The apparent digestibility was not different between fish fed the SP, MP and DP diets (P > 0.05; Table 1). In RAS, fish fed the SP diet reached a final weight of 110 g, compared to 105 and 90 g for fish fed the MP an DP diet, r espectively (P < 0.05). In ponds, fish fed the SP, MP and DP diet reached similar weights after 8 weeks of culture of 120, 112 and 111 g, respectively (P > 0.05) (data not shown) . T he diversity and count of phytoplankton was higher than for zooplankton in the ponds, but the phytoplankton and zooplankton abundance in the ponds was similar between diets. Nevertheless, the phytoplankton gut content in Nile tilapia was higher (P < 0.05) in ponds fed the DP diet (77*106 organisms L-1) than in ponds fed the MP (41*106 L-1 ) and SP (37*106 L-1 ) diet (Table 2; P > 0.05).
In conclusion, in absence of natural food, deficiency in dietary bio-available P reduces feed intake and performance in Nile tilapia , while in ponds, access to natural food supplements dietary deficiency in bioavailable P. A follow-up study covering a full culture cycle is advised, to check if the ponds would not become deficient in P towards the end of the production cycle.