Comparative study on dietary protein requirements for juvenile and pre-adult gibel carp (Carassius auratus gibeliovar. CAS III)

Two 8-week growth trials were conducted in indoor recirculation system to evaluate the protein requirements for juvenile (3.70 +/- 0.20g) and pre-adult (85.2 +/- 0.70g) 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 250-450gkg(-1) in Trial 1 and 200-450gkg(-1) 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 330-450gkg(-1) in Trial 1 (P>0.05), while decreased after the maximum value in 350gkg(-1) in Trial 2 (P<0.05). Productive protein values (PPVs) were lower in 370-450gkg(-1) in Trial 1 and 400-450gkg(-1) 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 370-450gkg(-1) in Trial 1 and 250-450gkg(-1) in Trial 2 (P<0.05). Trypsin activity significantly increased in 370-450gkg(-1) 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 400gkg(-1) (P<0.05). Based on quadratic regression of WG, it was estimated that dietary protein requirement for maximum growth was 414gkg(-1) (digestible protein of 376gkg(-1)) and 365gkg(-1) (digestible protein of 324gkg(-1)) for juvenile (3.70g) and pre-adult gibel carp (85.2g).