In the present study, some hematological indices were assayed after salinity challenge in smolts obtained from wild populations of Caspian brown trout. The experiment was designed into three treatments and one control group containing 90 fish per treatment. In experimental treatment, fish were exposed to salinity challenge (12 mg/l salinity) for 48 h. Fish of control group were not challenged and held in freshwater with 0.37 g/l salinity. After salinity challenge in seawater, the higher values of plasma cortisol and glucose were found in fish acclimated to seawater compared to control group (P < 0.05). No significant differences were found between control group and fish before salinity challenge in terms of plasma glucose and cortisol levels (P > 0.05). Plasma potassium and sodium concentrations were statistically similar between experimental groups and control group (P > 0.05). Also, plasma protein levels were lower in fish exposed to salinity challenge compared to control (P < 0.05). The values of red blood cells (RBC), white blood cells (WBC), and hemoglobin (Hb) increased significantly after salinity challenge (P < 0.05). In addition, there were no significant differences between control group and fish before salinity challenge in terms of plasma RBC, WBC, and Hb (P > 0.05). In conclusion, the results of the present study confirm the ability of the Caspian brown trout smolts in controlling osmoregulation by alternation of some hematological parameters.

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