Effect of water salinity on enzymatic and hormonal indices of (Oncorhynchus mykiss) fingerlings

Salinity is one of the stressful environmental factors, which affects life, metabolism, and distribution of aquatic animals. In this study, the effects of different levels of water salinity were investigated on serum biochemical parameters in rainbow trout (Oncorhynchus mykiss) fingerlings. For this, the fish (5.53±0.057 g) were exposed to 0, 5, 13, 20 or 30 ppt water salinities for 60 days; then blood samples were taken from all treatment for hormonal and enzymatic studies. All fish died at 30 ppt water salinity, during adaptation. The results showed that there was a tendency to triiodothyronine (T3) elevation along with water salinity levels, although the changes were not statistically significant (P>0.05). Increase in water salinity significantly increased serum thyroxine (T4), cortisol, alkaline phosphatase (ALP) and protease levels. Among the water salinities, 20 ppt led to a significant elevation in serum alanine aminotransferase (ALT) activity. There was no significant difference in serum glucose and aspartate aminotransferase (AST) levels in the fish reared at 5 and 13 ppt salinities; both treatments exhibited lower values compared to the 0 and 20 ppt salinities. Water salinity induced no significant changes in serum lipase activity. In conclusion, the results indicate that increase in water salinity causes elevation in cortisol and thyroid hormones, which are necessary for energy production and osmoregulation. Moreover, rainbow trout may face organ damage in saltwater, particularly at 20 ppt.