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Effect of dietary phenylalanine level on growth performance, body composition, and biochemical parameters in plasma of juvenile hybrid tilapia,
 Oreochromis niloticus
 ×
 Oreochromis aureus

An 8‐week feeding trial with juvenile hybrid tilapia (Oreochromis niloticus × Oreochromis aureus) was conducted to assess the effects of dietary phenylalanine on growth, body composition, and biochemical parameters in plasma. Six diets were formulated with graded levels of l‐phenylalanine (0.43, 0.74, 1.01, 1.30, 1.60, and 1.91% of the diet). Each diet was randomly assigned to triplicate groups with 20 fish per replicate (5.63 ± 0.04 g). Weight gain, thermal growth coefficient (TGC), and protein retention efficiency (PRE) increased with the increasing levels of dietary phenylalanine, up to 1.30%, and declined slightly at higher levels. The ratio of whole‐body protein and lipid was significantly higher in fish fed 1.30% phenylalanine compared with those fed 0.43% phenylalanine (p < .05), whereas whole‐body moisture contents showed a decreasing trend. Plasma lysozyme and catalase activities and total protein were improved by the phenylalanine supplementation, while alanine aminotransferase and aspartate aminotransferase activities showed a converse trend. Based on broken‐line regression analyses of TGC and PRE against different dietary phenylalanine levels, the value of optimal dietary phenylalanine requirement for juvenile hybrid tilapia might be estimated at 0.878%.