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A 6-week growth trial was conducted to study the effects of dietary protein levels on growth, feed utilization, ammonia excretion and expression of growth related genes of juvenile giant grouper (Epinephelus lanceolatus). Seven experimental diets were formulated to contain 38% (P38), 42% (P42), 46% (P46), 50% (P50), 54% (P54), 58% (P58) or 62% (P62) dietary crude protein (CP) (dry-matter basis). Experimental fish (average initial body weight: 5.39 ± 0.05 g) were randomly distributed into 21 glass tanks (length 60 cm × width 45 cm × height 50 cm) at a density of 15 fish / tank and fed their prescribed diets to apparent satiation twice daily (08: 00 and 16:30) by hand. All tanks were connected to the mechanical and biological water filters as a recycling system, and each experimental treatment had three replicates. At the end of the trial, two fish per tank were collected for whole-body composition analysis, and other three fish per tank were individually weighed and dissected to obtain liver and intraperitoneal fat (IPF) weights. Total RNA was extracted from grouper liver and pituitary using Trizol Reagent. Results showed that weight gain% (WG%) of fish was remarkably increased as dietary CP levels increased from 38% to 54%, and when it continued to rise from 54% to 62%, there only had a slight increment in values of this parameter. Analysis of a nonlinear exponential model indicated that the optimal dietary protein requirement for maximum growth of giant grouper was estimated to be 56% of dry matter. Daily feed intake (DFI) and feed conversation ratio (FCR) were decreased with the increasing of dietary CP levels. Fish fed P38 and P42 showed lower lipid retention efficiency (LRE) and lower contents of liver lipid, whole-body protein as well as muscle protein than fish fed other dietary CP levels. The water ammonia production was linearly related to dietary CP levels. Expression of pituitary growth hormone (GH) and hepatic target of rapamycin (TOR), S6 kinase 1 (S6K1) as well as insulin growth factor I (IGF I) were lower in fish fed P38, P42 and P46 than those in fish fed P50, P54, P58 and P62. Generally, the optimal dietary crude protein requirement for maximum growth of giant grouper was estimated to be 56% of dry matter. |
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