Effect of dietary protein level on growth performance, plasma biochemical indices and flesh quality of grouper (Epinephelus lanceolatus×E.fuscoguttatus) at two growth stages

An eight-week feeding trial was conducted to obtain the protein requirement of groupers (Epinephelus lanceolatus×E.fuscoguttatus) at two growth stages. Five isolipidic and isoenergetic diets containing 37%, 40%, 43%, 46% and 49% crude protein were fed to triplicate groups of 15 groupers average initial body weight :Ⅰ(83.93±0.30) g, Ⅱ(341.29±5.72) g.The results show that the weight gain rate (WGR) and specific growth rate (SGR) increased and then decreased with increasing dietary protein level, and no significant difference was found among treatments at the second stage (P>0.05). The SGR reached to the maximum at 43% protein level (P < 0.05) at the first stage. With increasing protein level, the feed conversion rate (FCR), hepatosomatic index (HSI), viscerosomatic index (VSI), serum cholesterol (GHO) and glucose (GLU) decreased significantly, while the total protein (TP) increased and then decreased at both stages (P < 0.05). The muscle ash increased at the first stage and then decreased at the second stage (P < 0.05). The flesh drip-loss rate and cooking loss rate decreased, while the pH value increased gradually, and the muscle hardness and springiness increased and then decreased with increasing protein level (P < 0.05). The results indicate that the protein requirement of the second stage was 40.94%, and that of the first stage was 43.44%. The flesh quality at the first stage was better than that at the second stage.