Effects of replacing fishmeal protein with poultry by-product meal protein and soybean meal protein on growth, feed intake, feed utilization, gut and liver histology of hybrid grouper (Epinephelus fuscoguttatus ♀ x Epinephelus lanceolatus ♂) juveniles

Two consecutive eight-week growth trials were undertaken to evaluate the effects of replacing fishmeal protein (FMP) with poultry by-product meal protein (PBMP) and soybean meal protein (SBMP) on growth, feed intake, feed utilization, gut and liver histology of hybrid grouper (Epinephelus fuscoguttatus female x Epinephelus lanceolatus male) juveniles. In trial 1, eight isoenergetic (340 kcal per 100 g dry matter) and isoproteic (53.5% of dry matter) experimental diets were formulated to replace 0%, 10%, 20%, 30%, 40%, 50%, 60% and 70% FMP with PBMP, being abbreviated as FMP, PBMP10, PBMP20, PBMP30, PBMP40, PBMP50, PBMP60 and PBMP70, respectively. Based on the results of trial 1, another six isoenergetic (340 kcal per 100 g dry matter) and isoproteic (53.5% of dry matter) experimental diets including a high FM (86.64%) diet (FMP) and five low-FM (25.99%) diets were formulated in trial 2, and in the low-FM diets, 0%, 7%, 14%, 21% and 21% PBMP was replaced with SBMP, being abbreviated as PBMP, SBMP7, SBMP14, SBMP21, SBMP28, respectively. The initial average body weights of experimental fish were 6.0 +/- 0.05 g in trial 1 and 6.8 +/- 0.08 g in trial 2 (means +/- S.D.), and fish were fed their described diets by hand to apparent satiation twice daily (08:00 and 16:30) at a density of 30 fish/cage (trial 1) or 15 fish/cage (trial 2). Experimental cages were labeled and located in connective 6-m3 indoor concrete tanks (L 3 m x W 2 m x H 1 m) with 3 cages occurring in each tank. Each treatment had three replicates both in trial 1 and in trial 2. Results showed that in trial 1, weight gain% (WG%), daily feed intake (DFI), feed conversion ratio (FCR), protein efficiency ratio (PER) as well as protein productive value (PPV) were not significantly affected by different FMP replacements with PBMP. Fish fed FMP, PBMP10 and PBMP20 had higher hepatosomatic index (HSI) and condition factor (CF) than fish fed PBMP30, PBMP40, PBMP50, PBMP60 and PBMP70, and intraperitoneal fat (IPF) ratio was increased as dietary PBM inclusion levels increased. Whole-body and muscle lipid contents of fish were elevated by the increments in PBM. High FMP replacements (50%-70%) by PBMP induced steatosis in hepatocytes. Different replacements of FMP by PBMP had no significant effects on gut morphology of experimental fish. In trial 2, there were also no remarkable variations in WG%, DFI, FCR and PER among various experimental treatments. Fish fed SBMP7, SBMP14, SBMP21 and SBMP28 showed similar fold height (hF), enterocyte height (hE) as well as microvilli height (hMV) in foregut and hF, hE in midgut in comparison to fish fed PBMP or the high FM, but for midgut hMV and hindgut hF, hE, hMV, fish fed the low-FM diets (PBMP, SBMP7, SBMP14, SBMP21 and SBMP28) displayed lower values of these parameters than fish fed the high-FM diet. Swelling in hepatocytes was observed in fish fed SBMP14, SBMP21 and SBMP28. Generally, replacing 70% FM protein or reducing the FM inclusion level to about 26% with PBMP or the combination of PBMP and SBMP at the 53.5% dietary crude protein level did not negatively affect growth, feed intake as well as feed utilization of hybrid grouper, but liver health and gut morphology of fish fed low FM diets need to be further focused on.