Evaluation of nitrogen and phosphorus wastes produced by Nile tilapia (<i>Oreochromis niloticus</i> L.) fed Azolla-diets in concrete tanks

Abstract

Nitrogenous (N) and phosphorus (P) wastes discharged into water by Nile tilapia Oreochromis niloticus L. (initial mean weight: 16.4 g) fed in tanks for 90 days was studied. Fish were fed with six isonitrogenous (29.2% crude protein) and isoenergetic (16.9 Kj g-1) diets A0, A10, A20, A30, A40 and A50, containing 0%, 10%, 20%, 30%, 40% and 50% of Azolla meal (AM) respectively, as partial fishmeal (FM) substitutes. Diet A0, without AM, served as a control. Growth performances were higher with the control diet, and a decreasing trend was found when AM level was higher than 10% in diets (P < 0.05). Identical values were recorded for crude protein and P content in fish. The total P (TP) discharged as waste show identical values (89.6-91.2 % supplied) for all experimental fish; while total N (TN) discharged (69.5%-80.7% supplied) increased significantly when AM level was greater than 20% in diets (P < 0.05). This study indicate that AM could be used up to 10% in diets without adverse effects on growth performances. High AM in diets affects growth but high amounts of undigested N from AM-protein is wasted and provided to water. However, the results suggest further investigations on the use of AM in diets that sustain growth and reduce P waste for eutrophication alleviation.

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Keywords: Azolla, fish meal replacement, Oreochromis niloticus, nitrogen, phosphorus, nutrient
balance.