| To study the effect of dietary CHO level on immunity in the southern catfish Silurus meridionalis Chen, two experiments were conducted in this study. Experiments 1: the serum lysozyme activity, antibacterial activity and total serum protein concentration were measured in the fish fed with the control, middle level and high level carbohydrate (CHO) diets containing 0%, 15% and 30% gelatinized corn starch, respectively. The tested fish (20.7±0.5g) were acclimated with the control diet for 15d at 27.5±0.2℃. The feeding growth experiment lasted for 0, 2, 4, 8, 12 and 16 weeks in the recirculated water system. The lysozyme activity in the fish at middle CHO level was significantly lower than that in the control group only at the 4th and the 12th week (p＜0.05), and lysozyme activity in the fish at high CHO level was significantly lower than that in the control group from the 2nd week to 12th week (p＜0.05), while no significant difference was found between the two groups at the 16th week. Antibacterial activity in the fish at either middle or high CHO level was significantly lower than the control from the 4th to 16th week (p<0.05), but there was no significant difference between the former two groups. The total serum protein concentration in the fish at high CHO level and the middle CHO level were significantly higher than the control only at 16th week (p＜0.05), and there was no significant difference among the three groups at other sampling time. Although negative influence of dietary CHO of 15% on immunity was observed in the southern catfish, growth performance was nor affected. And the negative influence increased with increasing CHO level. It can be concluded that the CHO is an important factor that restrains the immunity in the southern catfish. Accelerating synthesis of lysozyme protein should be one of the adaptive mechanisms in the fish under a long term of dietary CHO stress, which resulted in the increased serum protein. The results indicated that an optimum carbohydrate diet should be evaluated by examining its influence on both growth and immunity in this fish.Experiment 2: the serum lysozyme activity, complement 3 (C3), total serum protein, serum albumin, serum globulin and maloaldehyde concentration were measured in the fish fed with the control and high level CHO diets at 20℃, 25℃, and 30℃, respectively. To test if the effect of dietary CHO on immunity in this fish would change in intensity with temperature. The result showed that lysozyme activity in the control and high CHO groups increased with the increasing temperature, and the activity in the high CHO group was significantly lower than that in the control group (P＜0.05) irrespective of temperature. The total serum protein and globulin concentration in high CHO level was significantly higher than that in the control group only at 25℃. The serum albumin concentration was not significantly different between the two groups, and among temperatures. The serum maloaldehyde concentration in the control group increased with the increasing temperature, and the peak appeared at 30℃, while the peak of high CHO group was at 25℃. Maloaldehyde in the high CHO group was significantly higher than that in control only at 25℃. No significant difference was found in C3 among temperatures and between groups. The results showed that temperature is an importment factor on immunity. The maloaldehyde concentration increased with the increase of the temperature, which could explain the enhance of immunity level. Total serum protein, serum globulin and maloaldehyde concentration in the control and high CHO group appeared varied trend with the increasing temperature, which indicated that the depression of the dietary CHO on the immunity in fish was correlated with temperature.
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