Effects of replacing soybean meal with fermented rapeseed meal in low-fish-meal feed on the growth, immunity, and gut microbiota of juvenile white shrimp, Litopenaeus vannamei

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This experiment was conducted over a period of 8 weeks to investigate the effects of replacing soybean meal with fermented rapeseed meal (RM) on the juvenile white shrimp Litopenaeus vannamei. Six diets were prepared on an isonitrogenous and isolipidic basis. The normal fish meal (FM) group was used as a positive control, and the high soybean meal (SM) group was used as a negative control. The experimental groups used RM to replace 25 % (RM25), 50 % (RM50), 75 % (RM75) and 100 % (RM100) of the soybean meal in the low-fish-meal diet. All six diets were set up with four replicate groups stocking 25 shrimp (1.04 ± 0.04 g, N = 24) in each replicate tank. The results showed that weight gain was negatively correlated with RM addition. Compared with that in the FM group, the malondialdehyde content in the hepatopancreas was significantly lower in the RM25, RM50 and RM100 groups. Superoxide dismutase activity was positively correlated with the amount of substitution. The catalase activity and bacteriolytic activity were significantly lower in the RM50, RM75 and RM100 groups than in the SM group. The serum phenoloxidase activity in the RM50 group was not significantly different from that in the FM group. Based on the transcriptome data, the high RM level diet had a significant effect on lipid metabolism pathways in the hepatopancreas and resulted in the downregulation of transcripts related to “oxidoreductase activity”. Increased RM levels lead to separation of the intestinal mucosa from the base. High concentrations of RM can also increase the relative abundance of pathogenic bacteria (Nautella, Acinetobacter). The relative abundance of Proteobacteria in the RM100 group was also the highest of several groups. In summary, RM can replace 25 % of soybean meal in low-fish-meal feeds without affecting the growth performance of L. vannamei. Higher levels of fermented rapeseed meal in the feed (>50 %) can suppress the immunity of shrimp, affect lipid metabolism and cause intestinal flora imbalance.