Effects of digestible protein and lipid levels in practical diets on growth, protein utilization and body composition of juvenile rockfish (Sebastes schlegeli)

A feeding trial of three dietary digestible protein (DP) levels (37%, 42% and 47%) and two dietary digestible lipid (DL) levels (7% and 14%) factorial design with three replications was conducted to investigate the proper dietary protein and lipid (energy) levels for the growth of juvenile rockfish. Fish, average initial weight 21.9 g, were fed the experimental diets for 20 weeks. Weight gain improved with increasing dietary DP and DL levels, Weight gain of fish fed the diet containing 47% DP was significantly higher (P<0.05) than that of fish fed the diet containing 37% DP, and was not different from that of fish fed the diet containing 42% DP at the 14% DL level. However, weight gain was not significantly affected by dietary DP level at the 7% DL level, Feed efficiency ratio of fish was improved as dietary DP and DL levels increased, whereas there was no significant difference between those of fish fed the diets containing 42% or 47% DP at the same dietary DL level. Feed efficiency ratio of fish fed the 14% DL diets was significantly higher (P<0.05) than that of fish fed the 7% DL diets at the same DP level. Hepatosomatic index (HSI), visceralsomatic index (VSI), protein efficiency ratio, and protein retention were significantly influenced by dietary DL level, but not by dietary DP level. Daily feed intake tended to decrease with increasing DP and DL levels. Lipid contents of whole body, liver and viscera increased with increasing DP and DL levels, whereas dorsal muscle lipid was not affected by dietary DP and DL levels. The apparent digestibility coefficients (ADC) of protein and nitrogen-free extract (NFE) were not significantly affected by dietary DP and DL, while ADC of dry matter, lipid and energy increased with dietary DP and DL. The results of this study indicate that an increase of dietary lipid level has protein-sparing effect, and the diet containing 42% DP and 14% DL (4.2 kcal DE/g diet) is optimal for growth and effective protein utilization of juvenile rockfish. (C) 2002 Elsevier Science B.V. All rights reserved.