Effects of replacing fish meal with rendered animal protein blend on growth performance, hepatic steatosis and immune status in hybrid grouper (Epinephelus fuscoguttatus ♀ x Epinephelus lanceolatus ♂)

An 8-week growth trial was conducted to evaluate the effects of substitution of fish meal (FM) by rendered animal protein blend (APB) consisting of poultry by-product meal (PBM), shrimp meal (SM) and spray-dried blood meal (BM) in diets of juvenile hybrid grouper, Epinephelus fuscoguttatus female x Epinephelus lanceolatus male) (32.51 +/- 0.31 g). A basal diet (FM70) with FM as the sole protein source was compared to diets progressively replacing 20% (FM56), 40% (FM42), 60% (FM28) and 80% (FM14) of FM protein. No significant differences were found in final body weight (FBW) and weight gain (WG) of fish when up to 80% of FM protein was replaced by APB. The feed efficiency (FE) and protein efficiency ratio (PER) were markedly decreased when dietary APB inclusion levels exceeded 40%. The heaptosomatic index (HSI) and whole body crude lipid content increased as dietary APB levels increased. Plasma cholesterol (CHO), high-density lipoprotein cholesterol (HDL-C) and low-density lipoprotein cholesterol (LDL-C) contents of fish fed the APB containing diets were higher than the control group. Liver histological analysis showed the amounts of hepatic vacuoles and lipid droplets were significantly increased as dietary APB levels increased, which is a sign of hepatic steatosis. Moreover, dietary APB inclusion up-regulated the mRNA levels of lipid metabolism-related genes including peroxisome proliferator-activated receptor alpha (PPAR alpha), carnitine palmitoyltransferase 1 (CPT1), fatty acid synthase (FAS) and apolipoprotein AI (Apo-AI) while down-regulated lipoprotein lipase (LPL) levels. On the other hand, dietary APB inclusion also up-regulated the apoptosis-related genes including caspase-3, caspase-8, caspase-9 and p53 as well as inflammation-related genes including interleukin 8 (IL-8), interleukin 10 (IL-10) and transforming growth factor beta 1 (TGF-beta 1) in the liver of hybrid grouper. In conclusion, up to 80% FM can be replaced by APB protein in diets for hybrid grouper juveniles without compromising growth performance, and high levels of dietary APB could induce hepatic steatosis and thus impair the immune status of hybrid grouper.