Ahmed AlSooti ^1* Marwa M Zaher ² Amr M Helal ² Dina Mansour ² Dina T Meshhal ² Afify D G Al-Afify ² Mohamed M A Rafaey ³ Abeer El-Saharty ² Ehab El-Haroun ⁴ Marian G Nassif ² Abdallah Tageldein Mansour ⁵ Mohamed Ashour ^2*

• ¹ Aquaculture Unit, Department of Marine Science and Fisheries, College of Agriculture and Marine Science, Sultan Qaboos University, Muscat, Oman
• ² National Institute of Oceanography and Fisheries (NIOF), Alexandria, Egypt
• ³ Department of Animal Production, Faculty of Agriculture, Mansoura universiy, Mansoura, Egypt
• ⁴ Department of Animal Production, Faculty of Agriculture, Cairo University, Giza, Giza, Egypt
• ⁵ Department of Fish and Animal Production, College of Agricultural and Food Sciences, King Faisal University, Al-Hassa, Eastern Province, Saudi Arabia

The present study conducted to investigate the interaction of biofloc water supplementations and potential zooplankton abundance and structure in Nile tilapia Oreochromis niloticus rearing systems on zootechnical performance and biochemical indicators. Nile tilapia juveniles (13.30gand9.50cm) were randomly distributed into 18 fiberglass tanks (500L/tank with a stocking density of40 fish/tank) to start the feeding experiment for 60 days. Fish weights were recorded weekly to adjust the feeding rate at 3% of their biomass using commercial diet. Compared to control group (T0,zero biofloc water supplementation), the influence of five biofloc supplementation levels were applied as follows:14.2,28.4,42.6,56.8,and71g L-1(T1,T2,T3,T4,andT5respectively).The biofloc was prepared in an external fermentor fiberglass tank (300 L) and added to the fish tanks to keep the biofloc levels constant during the experiment. After 30 and 60 days of the experiment, the number of zooplankton was 46,501 and 24,537 Ind. L -1 , respectively, which include four families (Rotifera, Copepoda, Cladocera, and free-living nematodes) with domination of family Rotifera as 81.65% and 93.89%, respectively. The water quality indicated was within the standard values recommended for fish culture.Compared to the control group, the values of growth performance, whole-body biochemical composition, and blood biochemical indicators were significantly higher in biofloc groups than in the control group. Group T 3 achieved the highest significant growth performance values. In comparison with the control group, T 3 achieved the lowest number of cultures and the abundance of small invertebrate prey after 60 days of culture.The fish reared in groups T 0 and T 1 showed the highest significant urea content and the highest concentrations of liver function enzyme activities. Interestingly, compared to all groups, T 3 achieved the best FCR value (1.68). Principal component analysis (PCA) and the Pearson correlation coefficient confidence (PCCC) clarified a closed positive relationship between T0 and T3 with the total individual, rotifera abundance and FCR.The highest PCCC value with T 0 was in group T 3 (0.947). In conclusion, biofloc supplementation (42.6 g L -1 ) shows a sustainable clean aqua diet strategy, significantly improved Nile tilapia growth and FCR, with regard to the culture of small prey invertebrates for 60 days.