Effects of dietary protein and lipid levels on growth, feed utilization, body and plasma biochemical compositions of hybrid grouper (Epinephelus lanceolatus ♂×Epinephelus fuscoguttatus ♀) juveniles

An 8-week growth trial was conducted to determine the effects of dietary protein and lipid levels on growth, feed utilization, body and plasma biochemical compositions of hybrid grouper (Epinephelus lanceolatus ♂ × Epinephelus fuscoguttatus ♀) juveniles. Twelve experimental diets with the same estimated gross energy (360 kcal per 100 g dry matter) were formulated to contain 40%, 45%, 50%, or 55% dietary crude protein (CP) level (dry-matter basis) together with either 7%, 10%, or 14% dietary crude lipid (CL) level, being abbreviated as P40/L7, P45/L7, P50/L7, P55/L7, P40/L10, P45/L10, P50/L10, P55/L10, P40/L13, P45/L13, P50/L13 and P55/L13, respectively. Juvenile fish (average initial weight of 4.68 ± 0.05 g/fish) were stocked as groups of 22 fish in floating cages (L 120 cm × W 70 cm × H 50 cm) placed in indoor 6-m3 concrete ponds with continuous sea water flow (salinity: 33.1 g/L). From week 1 to week 8, fish in triplicate cages were fed each dietary treatment at rates of 9%, 7%, 5%, 5%, 4%, 4%, 3% and 3% of body weight (BW)/day, respectively. Weight gain (WG) of hybrid grouper fed the diet with 40% crude protein was significantly lower (P < 0.05) than that of fish fed the diet with 45%, 50%, or 55% CP while among fish fed 45%, 50% and 55% dietary CP levels, WG did not display significant differences. Increasing dietary crude lipid from 7% to 13% had no significant influences (P ≥ 0.05) on growth performance of experimental fish. P40 fed fish exhibited significantly higher feed conversion ratio (FCR) than P45, P50 and P55 fed fish among which FCRs were not significantly different. Dietary CL levels did not affect FCR of experimental fish. Protein efficiency ratio (PER) was significantly decreased as dietary CP level increased. Fish fed 7% dietary CL had significantly higher PER than fish fed 10% or 13% dietary CL. Values of hepatosomatic index (HSI) of fish fed P40 and P45 diets were significantly higher than those of fish fed P50 and P55 diets. Dietary CL levels did not cause significant changes in HSI. Intraperitoneal fat (IPF) ratios of fish had a reducing trend of variation as dietary CP was increased, but they were significantly increased when dietary CL was increased from 7% to 10% or from 10% to 13%. Fish fed 13% or 10% lipid had significantly higher whole-body lipid content than fish fed 7% lipid. Muscle lipid content increased as dietary CP or CL increased. Liver protein content had an increasing trend as dietary CP increased. Plasma cholesterol and high-density lipoprotein (HDL-C) had an increasing trend as dietary CP or CL increased. Fish fed 7% lipid had significantly lower plasma low-density lipoprotein (LDL-C) than fish fed 10% or 13% lipid.