Protein and energy utilization and the requirements for maintenance in juvenile mulloway (Argyrosomus japonicus).

This study describes the digestible protein (DP) and digestible energy (DE) utilization in juvenile mulloway, and determined the requirements for maintenance. This was achieved by feeding triplicate groups of fish weighing 40 or 129 g held at two temperatures (20 or 26 degrees C), on a commercial diet (21.4 g DP mJ DE(-1)) at four different ration levels ranging from 0.25% of its initial body weight to apparent satiation over 8 weeks. Weight gain and protein and energy retention increased linearly with increasing feed intake. However, energy retention efficiency (ERE) and protein retention efficiency (PRE) responses were curvilinear with optimal values, depending on fish size, approaching or occurring at satiated feeding levels. Maximum predicted PRE was affected by body size, but not temperature; PRE values were 0.50 and 0.50 for small mulloway, and 0.41 and 0.43 for large mulloway, at 20 and 26 degrees C respectively. ERE demonstrated a similar response, with values of 0.42 and 0.43 for small, and 0.32 and 0.34 for large mulloway at 20 and 26 degrees C respectively. Utilization efficiencies for growth based on linear regression for DP (0.58) and DE (0.60) were independent of fish size and temperature. The partial utilization efficiencies of DE for protein (k (p)) and lipid (k (l)) deposition estimated using a factorial multiple regression approach were 0.49 and 0.75 respectively. Maintenance requirements estimated using linear regression were independent of temperature for DP (0.47 g DP kg(-0.7) day(-1)) while maintenance requirements for DE increased with increasing temperature (44.2-49.6 kJ DE kg(-0.8) day(-1)). Relative feed intake was greatest for small mulloway fed to satiation at 26 degrees C and this corresponded to a greater increase in growth. Large mulloway fed to satiation ate significantly more at 26 degrees C, but did not perform better than the corresponding satiated group held at 20 degrees C. Mulloway should be fed to satiation to maximize growth potential if diets contain 21.4 g DP mJ DE(-1).