Immunological and antioxidant defence indices at early development (fertilized eggs, eyed, hatch and swimming stages) in triploid hybrids between female rainbow trout ( Oncorhynchus mykiss ) and Caspian salmon ( Salmo trutta caspius ) were studied. Gametes were collected from 9 females and 6 male’s two groups of brood fishes. Before fertilization, the female and males gametes from each groups of fish were mixed to avoid individual differennces amoung brood fishes. Fertilization was carried out at same time and different groups including rainbow trout (diploid control), Caspian salmon (diploid control) and hybrids were produced. For triploid induction in hybrids, the fertilization eggs were shocked at 28?C for 10 min and 10 min post fertilization. The fertilization eggs were incubated at California troughs at the same ambient conditions. Sampling was done in triplicates at 3 hours post fertilization eyed, hatching and free swimming stages. The results showed that the survival rates at all studied stages were lower for triploid hybrids in comparison to the control parents. All glubolar dimentions (nuclear small axis) were increased significantly in triploid (P 0.05) status at the hybrids were also confirmed by flow cytometery analysis. In general, all studied immunological and antioxidant defence indices of triploid increased till eyed stages similar to the parents, in addition, the indices were decreased significantly and reached the lowest amoment at free swimming stages in comparison to the parents (P 0.05). The negative effects of thermal shock on the egg membrane or gene expression unbalance since inter species hybridization, can cause perturbation in immunological and antioxidant defence mechanisms in triploid hybrids. The significant changes in immunological and antioxidant defence idences in triploid hybrids in comparison to the parents can explain the low and abnormal survival rate of them especially at free swimming stage, reflecting the differences in suitable environmental culture condition. Keywords : antioxidant defense, triploid hybrid, immunological indices, Rainbow trout, Caspian salmon
