Effect Of Dietary Protein Level On The Growth Performance, Physiological Responses And Digestive Enzymes Activities In Wuchang Bream,Megalobramaamblycephala Fry Under Acute High Temperature Challenge

Megalobrama amblycephala is a typical herbivorous freshwater fish native to China. It has been introduced to North America, Africa, Europe and other Asian countries. This species is a good candidate of freshwater fish intensive culture; because of its fast growth rate, use of natural food, high larval survival rate, tender flesh, high disease resistance and so on. The purpose of this study was to determine the optimum dietary protein level of M. amblycephala fry and investigate the effect of dietary protein on growth performance, feed utilization, blood physiology and digestive enzymes activities under acute high temperature challenge. A10-weeks feeding trial was conducted with five isoenergetic (15.72kJ/g, DM) and isolipidic (6.17%, DM) experimental diets (28%,30%,32%,34%and36%crude protein, in dry matter (DM)) were fed to the fish to establish the dietary protein level effect. Triplicate groups of25fish (initial weight16.08±0.03g) were stocked in15tanks (300-L cylindrical fiberglass) in a closed water recirculation system and fed formulated diets using casein, gelatin, and fish meal as protein sources, three times a day to near satiation. This study includes three aspects:1. Effect of dietary protein level on the growth performance and muscle composition in M. amblycephala fry:The results showed that final body weight, weight gain rate (WGR) and specific growth rate (SGR) increased with increasing the dietary protein levels up to34%(P<0.05); however, with further increase in dietary protein level, no longer increased. Likewise, feed conversion ratios (FCR) reduced significantly (P<0.05) with increasing dietary protein level up to34%. Protein efficiency ratio (PER), hepatosomatic index (HSI), viscerosomatic index (VSI) and Fulton condition factor (K factor) were significant (P<0.05) influenced by the dietary protein levels, whereas no significance was observed on survival rate among the groups. A positive trend was observed between K factor and dietary protein level (P<0.05), while the reverse was observed on PER, HSI and VSI values (P<0.05). Increased dietary protein level resulted in increased muscle protein content, but decreased in lipid content (P<0.05). The results of the present study indicated that the dietary protein requirement of M. amblycephala fry on the bases of the broken-line regression analysis of final body weight, WGR, SGR and FCR against dietary protein level was estimated to be between32and33%.2. Effect of dietary protein level on the blood physiology and digestive enzymes activities in M. amblycephala fry:Dietary protein level significantly (P<0.05) affected red blood cell (RBC) count, hemoglobin (Hb) and hematocrit (Ht) counts; however, white blood cell (WBC) and platelets (Pt) counts were not significantly affected. Elevated dietary protein level significantly raised RBC count, Hb and Ht concentrations (P<0.05). Significant effect of dietary protein level was observed on serum total protein (TP) content, total cholesterol (TC), aspartate aminotransferase (AST), thyroxine (T4) and triiodothyronine (T3)(P<0.05). However, no significant effect was observed on serum glucose, triacyglycerol (TG) and alanine aminotransferas (ALT). In contrast, serum TP, T4and T3significantly (P<0.05) raised as dietary protein level elevated, while the reverse was observed on serum TC and AST (P<0.05). Serum TP of the fish fed34%dietary protein level was significantly (P<0.05) higher than28%, but not significantly different from the other treatments. Furthermore, protease and amylase activities significantly increased as dietary protein level increased; while, the reverse was observed on the lipase activity (P<0.05). The result indicated that physiological responses and digestive enzymes activities were significantly affected by dietary protein levels and could be used as a metabolic tool for assessing the proper level of dietary protein in the feeding of M. amblycephala.3. Effect of dietary protein level on the physiological responses and digestive enzymes activities in M. amblycephala fry under acute high temperature challenge:After10weeks rearing period, fish were challenged with acute high temperature for7more days. As the results indicated:Before challenge:elevated dietary protein level significantly (P<0.05) raised the RBC, Hb and Ht concentrations, while the WBC and Pt were not significantly influenced. Meanwhile, significant (P<0.05) effect of dietary protein level was observed on serum TP, TC, cortisol, AST, T4, T3and concentration of complement C3and C4; whereas serum glucose, ALT and TG content not significantly affected. Protease and amylase activities significantly increased as dietary protein level increased; while, the reverse was observed on lipase activity (P<0.05).After challenge:the blood hematology in all groups remained higher and were significantly (P<0.05) higher than before challenge. Serum TP and TC contents are more affected by dietary protein level than water temperature, while the reverse was observed on TG content.7d after challenge, the serum T4and T3concentration in most groups remained similar to the before challenge. Meanwhile, the serum cortisol, glucose and ALT activity in most groups returned similar to the before challenge. However, serum AST activity in most groups was significantly (P<0.05) lower than before challenge. Besides, serum cortisol content, AST and ALT activities were significantly (P<0.05) influenced by both dietary protein level and water temperature, whereas serum glucose was influenced by water temperature only. Moreover, serum AST and ALT at Od, and serum cortisol and glucose levels at0d-2d after challenges, in most groups remained higher and were significantly (P<0.05) higher than before challenge, and thereafter tended to decrease. Likewise, at Od after challenge the serum complement C3and C4were significantly (P<0.05) higher than before challenge and thereafter tended to inhibit.7d after challenge, serum complement C3and C4concentrations in most groups returned similar to the level before challenge. However, the WBC count in all groups remained higher and was significantly (P<0.05) higher than the level before challenge. After challenge, digestive enzymes activities were significantly affected by both dietary protein levels and acute high water temperature (P<0.05).7d after challenge, the activities of protease in all groups remained higher and were significantly (P<0.05) higher than the level before challenge; whereas the activities of lipase and amylase returned similar to before challenge.The overall results indicate that the optimum dietary protein level would have considerable effect to increase protein synthesis, reduce blood cortisol level, enhance immune responses and improve resistance to high temperature stress in M. amblycephala.