Effects of dietary protein levels on growth, intestinal structure and air exposure tolerance in juvenile largemouth bass （Micropterus salmoides）

【Objective】The purpose of this study was to investigate the effects of different dietary protein levels in feed on the growth performance, intestinal health and air exposure resistance of juvenile largemouth bass（Micropterus salmoides）.【Method】Six isoenergetic diets with actual protein contents of 45.02%, 48.14%, 52.12%, 55.54%, 56.26% and 58.17% were configured. A total of 2880 juvenile M. salmoides with an initial body weight of 0.07±0.00 g were randomly divided into 6 groups, which were fed with 6 kinds of isolipids and isoenergetic diets respectively, and were divided into groups T1, T2, T3, T4, T5 and T6 according to the dietary protein levels from low to high. The experiment lasted for 30 d. Before the experiment, 50 juvenile M. salmoides were randomly selected to determine the initial composition of the fish. After fasting for 24 h, 9 juvenile M. salmoides were randomly taken from each group to measure body length, weigh body mass and visceral mass weight to calculate body indexes. Thirty juvenile M. salmoides from each group were dissected on the ice, and the whole intestines and liver were taken to detect the activities of digestive enzymes and antioxidant enzymes. Two juvenile M. salmoides were dissected on ice at random in each group, and the intestines were isolated and fixed in 4% paraformaldehyde for the preparation of tissue sections. Twenty juvenile M. salmoides were randomly selected from each group to determine the composition of the fish body. Total RNA was extracted from muscle and intestinal tissues of M. salmoides by TRIzol method, and the relative expression of genes related to growth and antioxidant in M. salmoides larvae was detected by real-time fluorescence quantitative PCR.【Result】The results showed that with the increase of protein level in the diet, the weight gain rate, specific growth rate of juvenile M. salmoides showed a trend of first increasing and then decreasing, and feed conversion rate（FCR） was decreased first and then increased, among which the specific growth rate of T4 group were the significantly higher than those of T2, T3, T5 and T6 groups（P＜0.05, the same below）. The liver trypsin activity was the highest in T3 group, which was significantly higher than that in T6 group. The activities of catalase （CAT） and total superoxide dismutase （T-SOD） in the T4 group after air exposure were the highest, and malondialdehyde（MDA） content was the lowest. Real-time fluorescence quantitative PCR results showed that there were no significant differences in the relative expression levels of growth related genes IGF1, S6K, 4EBP1 and antioxidant related gene CAT1 among all groups（P＞0.05）. 【Conclusion】Appropriate dietary protein levels can improve the feed utilization efficiency of juvenile M. salmoides, increase their weight gain rate, specific growth rate, protein deposition rate and fat deposition rate, and thus achieve optimal growth performance. Using specific growth rate as the evaluation index, the broken line model analysis showed that the optimal requirement of juvenile M. salmoides for dietary protein level is 55.05%（dry matter）. At the same time, appropriate dietary protein levels can also improve the antioxidant capacity of juvenile M. salmoides, laying a foundation for them to cope with air exposure.