Estimation of protein and fat requirements for greater amberjack (Seriola dumerili) juveniles

The farming of greater amberjack (Seriola dumerili) in Europe is limited by the lack of suitable diets, requiring an optimal protein-to-energy balance for its sustainable production. Thus, this study investigates optimal dietary protein and fat levels in greater amberjack juveniles by testing six experimental diets formulated according to a bifactorial arrangement combining three crude protein (50%, 55%, 60% as fed) and two crude fat levels (14%, 17% as fed). Trials were conducted at ICTA facilities in RAS systems (temperature 22±1.6 °C; DO 7.5±0.5 mg/L; salinity 28±1.8 g/L; pH 8.1±1.2; NO3− 32±9.4 mg/L; NO2− and NH4+ not detectable) with fish initially stocked at 1.3 kg/m3 and fed to visual apparent satiation. Digestibility and postprandial nitrogen excretion assays followed a Latin square experimental design (5 replicates per diet) by feeding 20 fish (65 g on average) randomly distributed in 5 tanks for 90 days. The growth trial used 720 fish (58 g on average) randomly allocated in 18 tanks (3 replicates per diet) and fed experimental diets for 84 days. Data were analysed using ANOVA with initial weight as a covariate and differences were tested via Newman-Keuls test (p<0.05). Apparent digestibility coefficients of protein and fat increased as dietary CP decreased from 60% to 55% and 50% (+8% for both nutrients) and when CF decreased from 17% to 14% (+8% and +6%, respectively for protein and fat; p<0.05). No differences were observed in hourly nitrogen excretion either related to CP or CF level. Daily nitrogen excretion decreased (−49%) as CP decreased from 60% to 55% and 50% (p<0.05), whereas no differences were observed respect to CF level. Survival rate was lower in fish fed diets with 60% CP compared to those fed diets with 50% (52% vs. 83%, respectively) and in fish fed diets with 17% CF compared to 14% (57% vs. 79%; p<0.05). Final weight and specific growth rate were lower (−17% for both variables; p<0.05) in fish fed the diet formulated with 60% CP and 17% CF compared to other diets. No differences were found in body composition and somatic indexes of fish according to the dietary CP or CF levels. Overall, diets exceeding 55% CP and 14% CF impaired productive performance, likely due to surplus protein being metabolised for energy and surplus fat limiting nutrient utilisation, which increased nitrogen excretion. Future studies aimed at optimising protein-to-energy ratios are needed for eco-friendly and cost-effective greater amberjack aquaculture. Research supported by PLANASER 2.0 project “Plan Nacional de Consolidación del Cultivo de Seriola (Seriola dumerili)” financed by the MAPA, Spain. The PhD grant of Cecilia Fanizza was funded by PON project (MUR; CUP: C95F21007670007).