Tetraploid lines are important for the mass production of strile tripliods as well as other kinds of polyploids such as penta-, hexa-, hepta and octaploids. The effective usage of tetraploid fish such as rainbow trout (Oncorhynchus mykiss) is compeletley depended on teteraploid induction and the ability of fish to survive and produce viable gametes. The first and most important factors affect tetraploid production is shock optimization for different fish stocks. In this study the effectiveness of different heat shock temperatures, 28, 30 and 32° C, shock durations, 1, 5 and 10 minutes as well as starting times, 45, 55, 65, 75 and 85 degreehours after fertilization on two egg diameter >groups, intact and physical shock group for each egg diameter batch were used in three replicates so, totally 276 experimental groups were used to find the optimum treatment of tetraploidy induction in rainbow trout egg. Survival rates up to eyed, hatching and swim up stages were compared between and within treated groups. Tetraploid fish were detected based on red blood cell dimensions as well as NORs analysis of fish at swim up stage. The results showed significant differences in survival rates from fertilization to eyed stages and from hatching to swim up stages between as well as within groups (p<0.05), but in both egg diameter >as well as NORs analysis, the range of tetraploid individuals were 0- 67 and 0-75 percent in two different egg diameters groups, upper and lower than 5 mm respectively. The highest percentage of tetraploids were achieved in groups of eggs which had diameters higher than 5 mm at 28° C, 65 degree-hour for 10 minutes (67%) and 5 minutes (60%). While in groups which included lower egg size, the highest rate of tetraploids (75%) was measured at 28° C, 65 degree-hour for 5 minutes. Comparsion of tetraploid yields between different groups showed that the thermal shock at 28° C, starting at 65 degree-hour and lasting for 10 minutes was the most effective shocks to produce tetraploid individuals for egg diameters upper than 5 mm and could increase the tetraploidy yields up to 12% (p <0.05).

Key words: Tetraploid, Rainbow trout Oncorhynchus mykiss, Optimal thermal shock, Egg

diameter.