Effects of chronic salinity stress on growth performance, physiological response and intestinal microbiota of largemouth bass (Micropterus salmoides)

To address the critical shortage of freshwater in global fisheries, the cultivation of largemouth bass (Micropterus salmoides) using geographically widespread saline-alkaline water is a viable resource alternative. In the present study, we systematically investigated the effects of chronic salinity stress (180 days, 9 ppt) on the growth performance, osmoregulation, antioxidant capacity, digestion capacity, and intestinal microbiota of largemouth bass. The results showed that the growth performance (CF, ADG, and SGR) of largemouth bass in the saltwater group decreased significantly compared with the freshwater group after 180 days of culture. The osmoregulation capacity (serum osmolality, serum cortisol, NKA, and CMA) and antioxidant capacity (SOD, CAT, MDA, ACP, AKP, and GSH-Px) in the saltwater group were significantly higher than those in the freshwater group at 60d, 120d and 180d (P < 0.05). Histological analysis showed that chlorine cells and macrophages proliferated in the gill and liver of the saltwater group. Additionally, trypsin and pancreatic lipase levels in the saltwater group peaked on 180 days, but remained significantly lower than those in the freshwater group (P < 0.05). According to intestinal microbiota analysis, the relative abundance of Bacillus and Cetobacterium in the saltwater group was significantly higher than that in the freshwater group (P < 0.05). The above results indicated that largemouth bass showed excellent stress resistance in the face of chronic salinity stress, which could be used as a saline-alkali culture species. The work provides new insights for the study of salt tolerance in freshwater fish and promotes the utilization of saline-alkali water.