Effects Of Dietary Protein Levels And EAA Supplementation On Growth, Body Composition And Immunity Of Largemouth Bass (Micropterus Salmoides)

An89-day growth trial was undertaken to evaluate effects of differentdietary protein levels and essential amino acid （EAA） supplementation to dietswith lower protein levels on growth performance, body composition andimmunity in largemouth bass （Micropterus salmoides）. Eight isoenergeticdiets were formulated according to the EAA profile of whole body oflargemouth bass. Five diets of them contained crude protein concentrations of50%,45%,40%,35%and30%, respectively （designated as50CP,45CP,40CP,35CP and30CP）. the other three diets （designated as40AA,35AA and30AA,respectively） were formulated by adding EAA to three lower-protein diets of40CP,35CP and30CP to make their EAA levels consistent with that of45CP（used as a control）. Triplicate groups of thirty juvenile largemouth bass withinitial body weight of10.14g were fed with each diet twice daily to apparentsatiation in an indoor water-circulating system. During the experimentalperiod, water temperature was maintained at （28±1）℃and the natural lightcycle was adopted.The results of this study were as follows: The growth and feed utilization of test fish were significantly affected bydietary protein levels and EAA supplementation. Weight gain （WG） andspecific growth rate （SGR） decreased significantly among fish fed the dietswith the reduction of dietary protein levels from50%to30%（P <0.05）. WGand SGR of35AA-and30AA-fed fish increased with EAA supplementation（P <0.05）, but still lower than those of45CP-fed fish （P <0.05）. Dietaryprotein levels and EAA supplementation had no effects on the feed intake ofeach treatment except that of50CP treatment. However, feed efficiency （FE）showed a significant downward tendency with dietary protein levelsdecreasing from50%to30%（P <0.05）, but there were not significantdifferences between fish fed50CP and45CP （P>0.05）. Compared with45CP-fed fish, FE of40AA-fed fish was not significantly different （P>0.05）,while that of fish fed35AA and30AA was significantly lower （P <0.05）.Protein retention efficiency （PRE） decreased significantly with a declinein dietary protein concentration （P <0.05）, while no significant differenceoccurred between PREs of45CP-and50CP-fed fish （P>0.05）. PREs in fishfed35AA and30AA were significantly higher than those of35CP-and30CP-fed fish, respectively （P <0.05）, but PREs with40AA and45CPtreatments were not significantly different （P>0.05）. The retention efficiencyof threonine, isoleucine, phenylalanine, histidine and arginine wassignificantly affected by dietary protein levels and EAA supplementation （P <0.05）, while the other EAAs’ retention efficiency was not significantly different （P>0.05）.Crude protein concentrations of whole-body, muscle and viscera of trialfish significantly decreased with dietary protein levels decreasing （P <0.05）,while whole-body crude lipid concentration of trial fish increased significantly（P <0.05）. Crude protein concentrations of Whole-body, muscle and viscerain fish fed40AA～30AA were all increased with EAA supplementation.Whole-body crude protein and lipid concentrations in fish fed30AA weresignificantly higher than those in30CP-fed fish （P <0.05）. Moisture, ash andcrude lipid concentrations in muscle of trial fish were not affected by dietaryprotein levels and EAA supplementation （P>0.05）. Condition factor （CF）showed a downward tendency with dietary protein levels varying from50%to30%. EAA supplementation led to a significant increase in CF of trial fish,especially in the30AA-fed fish （P <0.05）. Hepatosomatic index （HSI） andviscerosomatic index （VSI） showed a first downward but then upwardtendency with dietary protein levels decreasing from50%to30%，while thosewere not significantly affected by EAA supplementation （P>0.05）.The apparent digestibility coefficients of protein, lipid and energy in dietswere significantly affected by the dietary protein levels and EAAsupplementation （P <0.05）. The decrease in dietary protein levels from50%to30%caused a negative effect on those parameters in trial fish. With EAAsupplementation, those parameters became high to some extent, but there wereno significant differences occurred among those of40AA,35AA and45CP （P >0.05）.Red blood cell count and hemoglobin with trial fish showed a decliningtendency with the decreasing of dietary protein levels from50%to30%（P <0.05）, while EAA supplementation had no effects on the haematologicalparameters （P>0.05）. EAA supplementation enhanced immune parameters oftrial fish to some extent. Serum lysozyme activity, serum classicalcomplement pathway (CH₅₀) as well as respiratory burst activity of headkidney leukocytes in trial fish showed a significant downward tendency withdietary protein levels decreasing from50%to30%（P <0.05）. The respiratoryburst activities in fish fed35AA and30AA were significantly higher thanthose in fish fed35CP and30CP （P <0.05）, but significantly lower than thatin fish fed45CP （P <0.05）. The serum lysozyme activity and classicalcomplement pathway with45CP-fed fish were significantly higher than thosein30CP-fed fish （P <0.05）, but compared with40AA-fed fish, no significantdifference occurred （P>0.05）.This study indicates that in juvenile largemouth bass the optimal dietaryprotein levels are from45%to50%, and that supplementing crystalline EAAto lower protein diets can improve growth performance, feed efficiency andimmunity to some extent, but the diets only with the supplementation ofEAAs are still inferior to those with protein-bound EAAs in dietary overallefficiency, suggesting that dietary nonessential amino acids seem to play animportant role on improving biological value of dietary protein.