Effects of salinity acclimation on physiology and biochemistry and antioxidant immunity of juvenile Luciobarbus capito

In saltwater fish culture, directly releasing freshwater-cultured fry into medium to high salinity water typically results in high mortality due to environmental adaptation challenges. However, prior adaptive acclimation in low to medium salinity environments can enhance fry resistance and improve survival rates. This study investigated the physiological, biochemical, and immune stress responses of Luciobarbus capito following salinity acclimation. Juvenile fish, averaging (15.09±3.54) cm in length and (13.66±1.26) g in weight, were divided into three groups: a control group maintained in freshwater, an acclimated group preconditioned at 4 g/L salinity for 168 hours before exposure to 8 g/L saltwater, and a non-acclimated group directly introduced in 8 g/L saltwater. Physiological, biochemical, and immune stress indicators were measured at 0, 6, 12, 24, 48, 96 and 168 h in all groups. The results indicated that, (i) the blood routine indexes (WBC, Lymph, Mon, Gran, RBC, HGB contents), plasma physiological and biochemical indexes (UREA concentration, GLU-G and ALB contents, plasma osmotic pressure), ACP, AKP, LDH in gill tissue and antioxidant indexes (SOD, CAT, GSH-Px, MDA content) in liver tissue of acclimated group and non-acclimated group all showed an increasing first and then decreasing with the extension of stress time. (ii) In the early stage of salinity stress, the biochemical indexes of juveniles in both acclimated and non-acclimated groups reached their peak at the same time. However, the amplitude and peak size of the physiological and biochemical indexes (WBC, Lymph, Gran, Mon, Urea, GLU-G, ALB, osmotic pressure), transaminases (AST, ALT) and antioxidant enzymes (SOD, CAT, GSH-Px, MDA) of liver tissue were significantly lower than those of non-acclimated group (P<0.05). (iii) After 7 days of 8 g/L NaCl salinity stress, both the acclimated and non-acclimated could recover to the level of the control group, but the plasma osmotic pressure and plasma ALB content of the L. capito juvenile in the acclimated group recovered to the level of control group earlier than those in the non-acclimated group. The study's findings suggest that L. capito juveniles acclimated at 4 g/L salinity exhibit superior physiological and biochemical self-regulation and recovery ability when subjected to 8 g/L salinity stress.