Two 8-week growth trials were conducted in indoor recirculation system to evaluate the protein requirements for juvenile (3.70 AE 0.20 g) and pre-adult (85.2 AE 0.70 g) gibel carp, Carassius auratus gibelio var. CAS III. Six isoenergetic diets were formulated for each trial using fish meal and casein as protein sources, and protein level was 250-450 g kg À1 in Trial 1 and 200-450 g kg À1 in Trial 2. With the increasing dietary protein, feeding rate (FR) and feed conversion ratio (FCR) significantly decreased (P < 0.05). Weight gain (WG) increased first and then reached a plateau in 330-450 g kg À1 in Trial 1 (P > 0.05), while decreased after the maximum value in 350 g kg À1 in Trial 2 (P < 0.05). Productive protein values (PPVs) were lower in 370-450 g kg À1 in Trial 1 and 400-450 g kg À1 in Trial 2 (P < 0.05). Increasing dietary protein level increased protein content and decreased lipid content in whole fish body and white muscle (P < 0.05). Apparent digestibility coefficient of dry matters (ADC d ) decreased, while apparent digestibility coefficient of protein (ADC p ) increased in 370-450 g kg À1 in Trial 1 and 250-450 g kg À1 in Trial 2 (P < 0.05). Trypsin activity significantly increased in 370-450 g kg À1 in Trial 1 (P < 0.05) and was not affected in Trial 2 (P > 0.05). Hepatic alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities in both trials increased when dietary protein was above 400 g kg À1 (P < 0.05). Based on quadratic regression of WG, it was estimated that dietary protein requirement for maximum growth was 414 g kg À1 (digestible protein of 376 g kg À1 ) and 365 g kg À1 (digestible protein of 324 g kg À1 ) for juvenile (3.70 g) and pre-adult gibel carp (85.2 g).

Two trials were conducted to investigate protein requirements of juvenile (3.18 g in Trial 1) and on-growing (87.

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