Smoltification is physiological, biochemical, morphological and behavioral changes in salmo juvenile for migration from fresh water to brackish water. This prose's is gradual evaluation and started in fresh water juvenile (par) before complete migration to sea many times ago. Salmo trutta caspius is an important and economic fish in the Caspian Sea have same life cycle. This fish has several morphological and physiological changes during smoltification. In this study, Hormonal (T3, T4 & Cortisol) and ionic (Na+, Cl- & K+) changes in the serum were determined during the period of smoltification in 5, 10, 15 & 20 gr of hatchery reared salmo trutta caspius in different seasons (spring, summer, autumn & winter). Hormones were measured by Eliza and Radio Immuno Assay, Na+ and K+ using flame photometer, Cl- by colorimeter. T3 and T4 were quite high in spring, especially in the juvenile of 20 gr. Cortisol were quite low in spring and summer in all of weight groups. Ionic changes showed no significant differences with weight, but it was significantly different among seasons (P< 0/05). This result suggests that analyzing the plasma thyroid hormones and ionic provide useful information about the optimal time of transferring Caspian Sea trout from fresh water to sea water. It is concluded that the juvenile fish of 20 gr shows a better smoltification process in the spring.
