The Effect of Bacillus subtilis (IS02) Supplementation in the Diet of Juvenile Wild Common Carp, Cyprinus carpio, on Survival, Antioxidant and Ionic Characteristics, and Tissue Histology under Sudden Salinity Stress

This study was conducted to investigate the effect of adding Bacillus subtilis (IS02) to the diet on survival, biochemical indices, and histological alternations of juvenile wild common carp, Cyprinus carpio, exposed to salinity stress. Juvenile common carp (about 1.1 g) were fed diets containing 0 (control), 2.5 × 108 (pro-8), and 2.5 × 109 (pro-9) cfu/g probiotic for 15 days and then transferred directly to brackish water (13 g/L) and sampled after 3 and 10 days. Diet and sampling time had no significant effect on fish survival, which was above 96% in all treatments. Body moisture and potassium were only affected by sampling time, with moisture decreasing significantly after salinity stress while potassium increased significantly. Whole body sodium in pro-8 treatment did not change significantly during salinity stress, and probiotic treatments had lower chloride levels than the control before and after salinity stress. Adding probiotic to the diet increased glutathione reductase, glutathione peroxidase activity, and reduced glutathione concentration, while decreasing malondialdehyde concentration. Probiotics caused white blood cells infiltration in the kidney tissue. No significant tissue damage was observed in the gill and kidney, compared to the control after salinity stress. This study shows that juvenile wild common carp can tolerate direct transfer to the Caspian Sea without dietary probiotic supplementation. However, probiotic stimulates the kidney immunity, enhances ionoregulation, and increase antioxidant capacity in fish, which can have beneficial effects under field conditions.