en aquaculture aquaculture پژوهشي Orginal research papers In this study, the effects of diets (D_1-3) including fish meal (FM), hazelnut meal (HM), soybean meal (SM), and phytase enzyme [- (0),+(1000 FTU)]&nbsp; in varying rates (D_1-, D₁₊; D_2-, D₂₊; D_3-, D₃₊)&nbsp; were examined on nitrogen and phosphorus discharge based on the nutrition of rainbow trout (<em>Oncorhynchus mykiss</em>) juveniles. The study was conducted in tanks with 3 replicates for each group. In this trial, it was found that the differences among total-particle nitrogen, particle phosphorus, and total solid waste values released from the rainbow trout groups were insignificant; however, the differences between the values of total-dissolved phosphorus and dissolved nitrogen released were found significant (<em>p</em><0.05).The interactions among the factors were determined as insignificant (<em>p</em>>0.05). Moreover, although there were increases in the total and dissolved phosphorus and dissolved nitrogen discharge in all groups fed with diets including phytase, the particle phosphorus discharge decreased (<em>p</em><0.05). Based on the decrease in the amounts of hazelnut meal and soybean meal used in the diets, it was determined that there was a decrease in the amount of dissolved phosphorus released to the environment; besides, it was determined that it led to an increase in the amount of released dissolved phosphorus in all groups fed with diets containing phytase enzyme (<em>p</em><0.05). &nbsp;&nbsp; Diets protein, Waste, Dissolve, Enzyme, Fish 2089 2105 http://jifro.ir/browse.php?a_code=A-10-3136-1&slc_lang=en&sid=1 O. Demir orhandemir@sdu.edu.tr 100319475328460035748 100319475328460035748 Yes Aquaculture and Fish Feeding A. Yilayaz 100319475328460035749 100319475328460035749 No Mediterranean Fisheries Research Institute,Ministry of Food, Agriculture and Livestock, Antalya, Turkey