Dietary Protein Requirement of Juvenile Black Sea Bream, Sparus macrocephalus

An 8-wk experiment was conducted to determine the optimal protein requirement of juvenile black sea bream, Sparus macrocephalus, (initial weight 13.13 +/- 0.21 g, mean +/- SD) in 18 300-L indoor flow-through circular fiberglass tanks provided with sand-filtered aerated seawater. Six isoenergetic diets were formulated to contain varying protein levels ranging from 31.95 to 48.53% at about 3% protein increments by substituting corn oil and alpha-starch for fish meal. Each diet was assigned to triplicate groups of 20 fish in a completely randomized design. Twenty uniform-sized fish were stocked in a 300-L indoor flow-through circular fiberglass tank. After the feeding trial, weight gain and specific growth rate (SGR) increased with increasing levels of dietary protein up to 41.8% (P < 0.05) and both showed a declining tendency thereafter. Survival could not be related to dietary treatments. Both condition factor and protein efficiency ratio declined while dietary protein levels increased (P < 0.05). The highest and the lowest values of hepatosomatic index were found in fish fed 31.9 and 41.8% protein diets (P < 0.05), respectively, and showed no significant difference among other treatments (P > 0.05). Feed efficiency ratio (FER) improved as dietary protein level increased, with the maximum FER in the 41.8% protein diet, although this was not significantly different from the 45.2% protein diets. Apparent digestibility coefficients (ADCs) of main ingredients of diets tended to improve as the protein level increased, although they decreased to a different extent at higher levels of dietary protein. The highest ADC values of organic matter and protein were both found for fish fed 41.8% protein diet, while lipid and energy were at 38.5 and 45.2% protein diet, respectively. The whole-body protein content was positively correlated with dietary protein levels, while lipid content was negatively correlated. The muscle crude protein and crude lipid contents were significantly affected (P < 0.05) by dietary protein level, while moisture and ash showed no significant differences (P > 0.05). Dietary protein levels had significant influences on plasma parameters (P < 0.05). The concentrations of total cholesterol, triacylglyceride, alanine transaminase, and aspartate transaminase significantly reduced with dietary protein level except plasma total protein contents which presented reverse trend. Analysis of dose (protein level)-response (SGR) with second-order polynomial regression suggested that the optimal dietary protein requirement was 41.4%.