The aim of this study was to investigate the effects of environmental salinity (2.5, 5, and 7.5‰), and cortisol pretreatment (0.01, 0.1, and 1 μg l−1) on gill Na+/K+-ATPase activity and survival and growth rates in a common carp, Cyprinus carpio (L.). Cortisol pretreated fish in water (0‰), showed a significant increase in gill Na+/K+-ATPase activity after 3 days of exposure when compared to that of the control groups. In contrast, cortisol pretreated fish exposed to various salinities (2.5, 5, and 7.5‰) showed a low level of gill Na+/K+-ATPase activity during the study period of 14 days. However, the gill Na+/K+-ATPase activity on day 7th and 14th day was found to be not significant (except 0.01 μg l−1 cortisol treatment at 5‰, and 7.5‰, respectively). We observed that the fish showed a great adaptation to all salinities and a higher survival rate during the study period. However, a long-term exposure study is needed to assess the effects of environmental salinity and cortisol pretreatments on these aspects for a desirable development in aquaculture practices. Keywords: Salinity, Hormone, Enzyme, Fish
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