Estimation of digestible protein and energy requirements of yellowtail kingfish Seriola lalandi using a factorial approach

This study used a factorial approach to estimate the digestible protein (DP) and energy (DE) requirements of yellowtail kingfish Seriola lalandi weighing between 50 to 2000 g and reared at 20–25 °C. The factorial model was subsequently used to calculate theoretical changes in the optimum ratio of DP:DE (g DP MJ DE−1) and estimate changes in the theoretical feed conversion ratio (FCR) or feed intake with increasing fish size. Growth of S. lalandi was described by the function; growth (g fish−1 d−1) = 0.268 ± 0.33 BW (g)0.52 ± 0.2. Body weight exponents for protein or energy loss in S. lalandi were found to be statistically similar to 0.7 or 0.8, respectively. Maintenance requirements for DP were found to be 1.70 g DP BW (kg)−0.7 d−1 while those for DE were found to be 87.44 kJ BW (kg)−0.8 d−1. Utilisation coefficients for DP or DE estimated from linear regressions which incorporated data above and below maintenance ration were 0.51 and 0.65, respectively. However, regressions which excluded data below maintenance ration returned coefficients of 0.41 (DP) and 0.55 (DE). The partial energetic efficiencies of protein (KP) and lipid (KL) deposition as estimated by multiple linear regression were 0.61 and 0.83, respectively. Piecewise linear regression analysis was applied to the curvilinear response of DP:DE on increasing fish weight in order to differentiate key growth stages in the production of S. lalandi. Three stages were identified; a juvenile stage ≤ 200 g, an intermediate stage between 200 and 1000 g and a stage for fish ≥ 1000 g. These stages corresponded to DP:DE requirements of 38, 31 and 24 g DP MJ DE−1, respectively. Practical diet formulations that would best meet the DP:DE criteria for fish ≤ 200 g are those which contain approximately 456 g DP kg−1 and 12 MJ DE kg−1. For fish growing between 200 and 1000 g a diet containing 465 g DP kg−1 and 15 MJ DE kg−1 would be suitable while a diet containing 432 g DP kg−1 and 18 MJ DE kg−1 would be appropriate for fish ≥ 1000 g. Further evaluation of growth potential and nutrient requirements at different water temperatures will assist in improving the utility of this model.