Effects of replacing fishmeal protein by hemoglobin powder protein on growth performance, food intake, feeding-related gene expression and gut histology of hybrid grouper (Epinephelus fuscoguttatus x Epinephelus lanceolatus) juveniles

An 8-week growth trial was conducted to evaluate effects of replacing fishmeal protein (FMP) by hemoglobin powder protein (HPP) on growth performance, food intake, feeding-related genes expression and gut histology of hybrid grouper (Epinephelusfuscoguttatus x Epinepheluslanceolatus) juveniles. Seven isoenergetic (340 kcal per 100 g dry matter), isoproteic (53.5% of dry matter) and isolipidic (7.5% of dry matter) experimental diets were formulated to replace 0%, 15%, 30%, 45%, 60%, 75% and 90% FMP with HPP, being abbreviated as FMP, HPP15, HPP30, HPP45, HPP60, HPP75 and HPP90, respectively. Each experimental diet was fed to triplicate groups of 30 hybrid grouper juveniles (average initial body weight: 5.27 +/- 0.05 g/fish) which were stocked into small floating cages (L 120 cm x W 70 cm x H 50 cm). Experimental cages were labeled and located in seven connective 6-m(3) indoor concrete tanks (L 3m x W 2m x H 1 m) with 3 cages occurring in each tank. Fish were fed twice daily (08: 00 and 16: 00) to apparent satiation. Water quality was monitored daily. Weight gain% (WG%), feed intake (FI), feed efficiency (FE), protein efficiency ratio (PER) and protein productive value (PPV) were significantly influenced by different dietary treatments. Analysis for the quadratic broken-line model showed that the maximum replacement level of HPP to FMP at the intercept value of WG% in diets for hybrid grouper was estimated to be 23.3%. The intercept is the value of the zero replacement. Based on the quadratic regression model at the maximum response of WG%, the replacement level of HPP to FMP was estimated to be 17.3%. The relative mRNA expression levels of agouti-related protein (AgRP), neuropeptide Y (NPY) as well as orexin genes in hypothalamus of fish were significantly decreased by high HPP replacements (45%-90%) to FM protein in diets. For gut histology, high replacement levels of HPP to FMP significantly reduced the fold height, enterocyte height as well as microvilli height of fish foregut and midgut. Generally, approximately 17.3%-23.3% FM protein in diets for hybrid grouper can be replaced by HPP without negatively affecting fish growth performance and gut development.