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We studied the effects of dietary protein levels and water temperatures on growth performance, body composition, serum biochemistry, and digestive gland antioxidant enzyme activities of juvenile hybrid abalones Haliotis discus hannai♀ × H. fulgens♂ (1.47 ± 0.03 g ; 20.73 ± 0.16 mm). A 3 × 6 factorial design feeding trial was conducted with three water temperatures (19°C, 23°C, and 27°C) and six protein levels (152.5, 202.5, 252.6, 302.6, 352.7, and 402.7 g/kg) for 90 days. Dietary protein levels and temperature significantly affected the growth performance of the hybrid abalones, but there was no interaction effect except for daily increment in shell length. Body moisture decreased with increasing water temperature, and crude protein showed a contrary tendency. Crude lipid and ash were not affected by dietary protein level or water temperature. The alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase activities and the triglyceride content in serum increased with increasing water temperature, whereas the opposite was true for albumin activity and total protein and glucose contents. Total cholesterol contents decreased with increasing dietary protein levels. Dietary protein levels and water temperature did not affect contents of high-density lipoprotein cholesterol and low-density lipoprotein cholesterol (P > 0.05). A significant interaction was observed between dietary protein levels and water temperature in all antioxidation parameters (reactive oxygen species, total antioxidant capacity, protein carbonyl, superoxide dismutase, catalase, glutathione s-transferase, and glutathione peroxidase except for malondialdehyde content). Based on specific growth rate, the optimal dietary protein levels for juvenile hybrid abalones were 333.1, 318.6, and 306.3 g protein/kg diet at 19°C, 23°C, and 27°C, respectively. These findings will help to develop multidiet feeding strategies at different water temperatures throughout the culture period of juvenile hybrid abalones. [ABSTRACT FROM AUTHOR]