A Maximum 34% Substitution of Fish Meal by Soybean Meal Is Recommended for Rhynchocypris lagowskii Dybowski Cultured in Biofloc Systems: Evidence From Growth and Intestinal Barrier Function.

In this study, glucose was used as the carbon source for the bioflocs in each group, with a carbon-to-nitrogen ratio (C/N) of 20 : 1. Juvenile Rhynchocypris lagowskii Dybowski (6.89 ± 0.02 g) were used as the experimental animals. Soybean meal (SBM) protein replaced 0.0% (CK), 17.0% (B17), 34.0% (B34), 51.0% (B51), and 68.0% (B68) of fish meal protein to formulate five isonitrogenous and isolipidic experimental diets for an 8-week feeding trial. Under biofloc technology (BFT) conditions, the B68 group exhibited significantly lower growth performance (final weight [FW], weight gain rate [WGR], specific growth rate [SGR], feed efficiency ratio [FER], protein efficiency ratio [PER]) and muscle crude protein content compared to the CK group (p < 0.05). The B51 group showed significantly reduced activities of hepatopancreatic and intestinal protease (PRS), as well as of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-PX), and total antioxidant capacity (T-AOC) in the distal intestine (DI). Meanwhile, the malondialdehyde (MDA) content in the DI was significantly increased in both the B51 and B68 groups (p < 0.05). The B68 group showed significantly increased serum GPT, GOT, and immunoglobulin M (IgM) levels, and significantly decreased hepatopancreatic GPT and GOT activities as well as serum complement 3 (C3), complement 4 (C4), and LSZ levels (p < 0.05). In the B68 group, the expression levels of multiple genes were significantly downregulated (p < 0.05): IGF-1 in the mid intestine (MI) and DI, S6K in the hepatopancreas and DI, and ZO-1, mTOR, IL-10, TGF-β, Nrf2, Maf, HO-1, CAT, GSH-PX, and occludin-a in the DI. In contrast, significant up-regulation was observed for TNF-α (in MI and DI), as well as for 4E-BP, NF-κB, IL-1β, IL-8, and Keap1 (p < 0.05). In the BFT, SBM substitution (replacing ≥51% of fish meal protein) disrupted intestinal morphological integrity. This led to reduced muscularis thickness and fold height, increased lamina propria width, villus breakage, altered gut permeability, and elevated serum endothelin-1 (ET-1), D-lactic acid (D-LA), 5-hydroxytryptamine (5-HT), and diamine oxidase (DAO) levels. These changes are accompanied by changes in the structure of dominant intestinal microflora. Concluded, the proportion of SBM replacing fish meal should be lower than 34% under BFT system.