Effects of salinity on growth, physiology, biochemistry and gut microbiota of juvenile grass carp (Ctenopharyngodon idella)

•Grass carp could adapt to low salinity (2 ppt).•Grass carp maintained a stable physiological and immune status at low salinity.•Salinity altered the microbial composition in the gut of the grass carp.•High salinity induced a decrease in the proportion of probiotics and an increase in the proportion of harmful bacteria in the gut. Grass carp (Ctenopharyngodon idella) is among the most important freshwater fish species in China. However, it remained unclear how salinity could affect grass carp. Two experiments were performed. The first experiment was a 4-day acute salt tolerance experiment with six salinities (0, 4, 8, 12, 16, and 20 ppt). The second experiment was an 8-week chronic salt stress experiment with three salinities (0, 2 and 6 ppt). To investigate the intestinal bacterial community of grass carp from three salinities (0, 2, and 6 ppt), the 16S rDNA sequencing was performed. The results showed that grass carp exhibited great adaptability to low salinity (2 ppt), with no significant difference in growth and maintained stable physiological and immune status. However, exposed to high salinity (6 ppt) caused significant deleterious effects on grass carp, including growth inhibition as well as physiological and immune-related changes. The gut microbiota in grass carp changed with salinity. With the increase of salinity, the proportion of beneficial bacteria in the gut of grass carp gradually decreased, while some harmful bacteria gradually occupied the dominant position. Changes in gut microbial composition ultimately affected the growth of grass carp. This study helps further clarify the effects of salinity on grass carp.