Effects of Dietary Rapeseed Meal on Growth Performance, Intestinal Structure, Gut Microbiota, and Related Gene Expression of Juvenile Largemouth Bass (Micropterus salmoides)

This study aimed to explore the effects of dietary rapeseed meal replacing fish meal on growth performance, intestinal structure, gut microbiota, and related gene expression of juvenile largemouth bass (<i>Micropterus salmoides</i>). Five isonitrogenous and isolipidic diets were designed, in which rapeseed meal replaced 0% (FM, control), 5% (RM5), 10% (RM10), 15% (RM15), and 25% (RM25) of fish meal. Then, largemouth bass (11.00 ± 0.20 g) were randomly and equally allocated to 15 experimental tanks (25 fish per tank) for an 8-week feeding trial. The results showed that growth performance declined as replacement levels increased to 25%. However, the RM5 group had the highest body crude protein, distal intestinal muscle layer thickness (MLT), and plica height (PH) and width (PW), which were significantly higher than those of the FM group. In addition, compared to the FM group, the RM15 and/or RM25 groups had higher levels of D-lactic acid, diamine oxidase, and lipopolysaccharide. Furthermore, the RM25 group exhibited higher abundances of <i>Lactococcus</i> and <i>Weissella</i> but lower levels of <i>Aeromonas</i> and <i>Staphylococcus</i> compared to the FM group. Intestinal transcriptome analysis revealed that the PI3K-Akt and NF-κB signaling pathways were significantly up-regulated when comparing the RM25 and FM groups. The results demonstrate that the replacement of 5% fish meal with rapeseed meal did not have a negative impact on the physiological status of largemouth bass. However, a replacement level of 25% reduced growth performance and damaged intestinal structure, potentially by altering the abundance of intestinal microbiota and up-regulating the PI3K-Akt and NF-κB signaling pathways.