The ever-expanding of aquaculture activities raised concerns about the unknown detrimental effects of high effluent loading on the aquatic ecosystems. Therefore, this study quantified the accumulation of nitrogen (N) and phosphorus (P) in different forms in the water, sludge and fry in rearing ponds. Four ponds were assigned in a Nile tilapia hatchery of private sector to study the nutrient mass balance and the chemical composition of the sludge, in two fry production cycles, each for 25 days divided into three periods, during April-June 2021.
The results showed that, every Kg feed (45% protein) produces 420 g sludge and 355.94 g suspended solids, 50% out of which settled at the bottom. Fry retained 27.6 % and 20.4 % of the applied dietary nitrogen and phosphorus, respectively. Accumulation of TN and TP in the sludge accounted for 16.6% of the dietary N and 50% of the dietary P. Effluent water contained 29.16 % and 25.8 % of introduced dietary nitrogen and phosphorus, respectively, distributed in different forms. The dietary nitrogen recovery in the present study was 73.38%, suggesting considerable role for ammonia volatilization, denitrification and anammox processes in the N-losing and almost no N-fixation occur in these ponds, while the phosphorus recovery reached 96%. The results also showed that the fry rearing ponds threaten the environment, as their effluent and sludge are rich in nutrients.
Thus, it is recommended to utilize the fry ponds effluent in microalgae production and recycle the sludge into vermicompost fertilizer.