Effects of dietary n-3 LC-PUFA levels on the growth, immunity, and lipid metabolism of freshwater carnivorous teleost largemouth bass (Micropterus salmoides) juveniles

The n-3 long chain polyunsaturated fatty acids (n-3 LC-PUFA) play important roles in the normal growth and metabolism of fish. In order to explore the appropriate requiremnts of n-3 LC-PUFA in juvenile largemouth bass Micropterus salmoides, a freshwater carnivorous teleost, four isoproteic (50 %) and isolipidic (11 %) diets were formulated. The control diet used fish oil (FO) as the sole lipid source, and the other three diets used mixed vegetable oil as the main lipid source and supplemented with 0.50 %, 0.80 % and 1.10 % n-3 LC-PUFA, respectively. After the juvenile largemouth bass (initial weight 12.11 +/- 0.01 g) fed with the four diets for 75 days, the growth, serum and heptic biochemical indices, as well as hepatic lipid metabolism were investigated. The results showed that the growth performance of 0.50 %-1.10 % groups were high that of control group; comparing to the 1.10 % group, high weight gain, specific growth rate and daily gain, as well as muscle hardness, chewiness and gumminess were detected in the 0.80 % group (P < 0.05). With the increase of dietary n-3 LC-PUFA levels, the serum biochemical indices (total cholesterol, triglyceride, and aspartate aminotransferase activity) and hepatic triglyceride were significantly decreased, while the serum acid phosphatase and alkaline phosphatase, as well as hepatic and muscle n-3 LC-PUFA contents were significantly increased (P < 0.05). Compared with the 0.50 % group, high hepatic mRNA expression levels of anti-inflammatory genes (il-10, tgf-ss) were observed in the 0.80 % group (P < 0.05); and the relatively high transcript levels of genes related to lipid transport and catabolism (hsl, ppara, fatp and mtp) and low transcript levels of genes related to lipid anabolism (acca and ppar gamma) were detected in the 0.80 % and 1.10 % groups, as well as high mRNA levels of acca and ppar gamma were measured in the 0.80 % group (P < 0.05). The results indicated that 0.80 % dietary n-3 LC-PUFA level is suitable for largemouth bass concerning the growth performance, muscle quality and hepatic lipid metabolism. When subjected the weight gain to the second-order polynomial regression analysis, the maximum value was achieved at 0.76 % n-3 LC-PUFA level, which provide a basis for the development of formula feed for largemouth bass.