Effect of dietary protein levels and feeding rates on the growth and health status of juvenile genetically improved farmed tilapia (<Emphasis Type="Italic">Oreochromis niloticus</Emphasis>)

A 2 × 3 factorial study was conducted to investigate the effects of dietary protein levels (DPLs) and feeding rates (FRs) on the growth and health status of juvenile genetically improved farmed tilapia (GIFT), Oreochromis niloticus. Triplicate tanks of fish (initial weight 15.87 ± 0.11 g) were fed diets containing 25 or 35% protein at rates of 3, 5, or 7% body weight per day (BW day^?1) for 8 weeks. At the end of the feeding trial, the results showed that fish growth (final mean weight 34.61–81.07 g) and condition factor (3.39–4.45 g cm^?3) increased with the DPLs and FRs. Feed efficiency (FE, 0.48–0.88) increased as DPLs increased but decreased as FRs increased; the opposite trend was observed for feed cost (FC, 3.24–5.82 CHN Yuan kg^?1) and hepatosomatic index (0.98–2.33%). Apparent protein retention efficiency (APRE, 23.92–38.78%) was reduced by high FR. A 35% protein diet resulted in higher (P < 0.05) FE and APRE, and lower (P < 0.05) FC at 5% BW day^?1 than those at 7% BW day^?1. As FRs increased, lipid contents of the hepatopancreas, viscera, muscle, and eviscerated body increased, while moisture contents of hepatopancreas and viscera decreased. All serum biochemical parameters, including glutamic-oxalacetic transaminase and glutamic-pyruvic transaminase activity and levels of creatinine, glucose, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, total cholesterol, triglyceride, and total protein were unaffected by DPL or FR (P > 0.05), except urea nitrogen levels, which were affected by DPLs (P < 0.05). Moreover, the size of hepatocytes and the area ratio of hepatocyte vacuoles were enlarged (P < 0.05), whereas the area ratios of the nucleus and cytoplasm were reduced (P < 0.05) with increasing FRs. These results suggested that the optimal feeding strategy for juvenile GIFT is 35% protein diet at 5% BW day^?1.