Abstract
A 3 × 2 factorial experiment (protein levels, 42%, 46%, 50%; lipid levels, 9%, 12%) with three replicates was conducted in a circulating water system to investigate the effects of dietary protein and lipid levels on growth, feed utilization, body composition, and serum biochemical parameters of growing rockfish Sebastes schlegeli (initial weight, 29.98 ± 0.10 g). After an 8 weeks feeding trial, growth performance in terms of final body weight, percent weight gain, and specific growth rate increased with the increase of dietary protein level when fish fed diets containing a consistent level of dietary lipid. The feed conversion rate and daily feed intake were significantly affected by dietary protein and lipid levels, and decreased as dietary protein level increased from 42% to 46% or dietary lipid level increased from 9% to 12% (P < 0.05). Survival rate, viscerosomatic index, and hepatosomatic index were unaffected by dietary protein level (P > 0.05), but significantly increased with the increase of dietary lipid level (P < 0.05). On the contrary, condition factor was unaffected by dietary lipid level (P > 0.05), but significantly increased with dietary protein level increasing up to 46% (P < 0.05). The moisture contents of muscle and liver significantly decreased, but the whole-body crude lipid content, the crude protein and lipid contents of muscle increased as dietary protein or lipid level increased (P < 0.05). The contents of isoleucine, leucine, histidine, glycine, alanine of muscle, as well as the proportions of C14 : 0, C20 : 1, and C22 : 1n-9 in total fatty acids were higher in fish fed diets containing 12% lipid than those fed 9% lipid (P < 0.05), while C18 : 1n-9 and C18 : 2n-6 followed an opposite trend. The contents of phenylalanine, lysine, and tyrosine as well as the proportions of C18 : 0, C18 : 2n-6, C22 : 1n-9, and C22 : 6n-3 in total fatty acids decreased with the increase of dietary protein level (P < 0.05). Serum cholesterol and low-density lipoproteins increased significantly with dietary protein or lipid levels increasing, but TG concentration was elevated significantly in fish fed diets containing 12% lipid. Considering the present results in terms of growth and feed utilization, the suitable protein and lipid levels in diet for growing rockfish were 46% and 12%, respectively.