Effects of dietary lipid levels on growth, body composition, digestive enzyme activities, serum biochemical indexes and antioxidant performance of Polyodon spathula

This study investigated the effects of dietary lipid levels on growth performance, body composition, digestive enzyme activities, serum biochemical indexes and antioxidant enzyme activities of paddlefish (Polyodon spathula). A total of 270 P.spathula with initial body weight of (81.38±0.14) g were randomly divided into five groups with 3 replicates per group and fed with five diets (dietary protein level 41.35%) containing different lipid levels (3.01%, 5.12%, 7.14%, 9.35% and 11.64%) for 8 weeks. With the increment of dietary lipid level, weight gain rate, specific growth rate, feed conversion ratio, protein efficiency ratio, hepatosomatic index and condition factor of P.spathula all showed a trend of first rising and then decreasing, and regression analysis determined that the specific growth rate and protein efficiency ratio reached the maximum when the dietary lipid level was 7.88% and 9.47%, respectively. The lipid content of muscle and liver increased with the increase of dietary lipid level, and the lipid content in 11.64% group was significantly higher than those in 3.01%, 5.12% and 7.14% groups. The total fish and muscle crude protein contents were related to dietary lipid level, and crude protein contents in total fish and muscle of 3.01% groups were the lowest, while 7.14% and 9.35% groups had no significant difference. Main fatty acid composition in the muscle and liver of the P.spathula were related to the fatty acid in the feed, with the increase of dietary lipid level, the ratio of n-6PUFA in the muscle and liver increased, the n-3/n-6 PUFA and SFA (C16:0, C18:0) in the liver decreased; MUFA C18:1n-9 first ascended, then decreased, and there was no significant difference in the proportion of n-3HUFA and n-3PUFA in the fatty acid composition of muscle and liver. The enteric protease activity of 7.14% group was significant higher than 9.35% group, α-amylase activity was more stable when lipid level reached 7.14%, and no significant difference between hepatic protease and lipase activity. The concentration of serum total protein, albumin and globulin increased with the increase of lipid level, the serum total cholesterol and high density cholesterol concentration in 11.64% group were significantly higher than 3.01% group, in addition, alanine aminotransferase and aspartate aminotransferase were not affected by dietary lipid level. The serum superoxide dismutase activity reached the maximum when dietary lipid level was 9.69%, and there was no significant difference in glutathione peroxidase activity and malondialdehyde content. The superoxide dismutase activity of liver in 11.64% group was significantly lower than other groups and glutathione peroxidase activity in 9.35% group was significantly higher than other groups except 11.64% group, whereas there was no significant difference in glutathione and malondialdehyde content. The comprehensive analysis of growth, feed utilization, fish quality, digestive enzyme activity and healthy condition showed that appropriate lipid level for P.spathula was 7.88%-9.69% under this trial condition.