The changes of cortisol and gonadosomatic index (GSI) in male and female Caspian brown trout reared under confinement condition in the hatchery were determined by radioimmunoassay. When the gonadosomatic index reached to its maxima, the concentration of cortisol showed a clear peak both in males and females. The results suggest that the elevated plasma levels of cortisol may not just be due to stresses during the breeding season but also may be due to its physiological functions in the reproduction of Caspian brown trout.
