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¹ Izmir Katip Celebi University, Faculty of Fisheries, Department of Aquaculture, Izmir, Turkey DOI : 10.4194/1303-2712-v16_3_11 Viewed : 4294 - Downloaded : 3641 The objective of the study was to investigate the effect of high temperature on sex ratios of early stage rainbow trout (Onchorhynchus mykiss) in commercially farmed populations from distinct hatcheries. Almost 32500 larvae from five different farms were used in experiments as thermal groups and their corresponding controls. Temperature treatments were conducted at 17.5°C for 40 days starting at day 10 after hatching. In only one population, the sex ratio was skewed significantly in favor of females in thermally treated group (67.5%), when compared to control group (52.1%) and there was deviation from the expected 1:1 ratio. In the treatment group the females grew 8.3% more than males. However there was no repeatiblity and any skewness in sex ratio of progenies from photoperiod induced spawning. Survival rates were not affected by the thermal treatments. The results showed that the increasing of femaleness of farmed stocks is possible by thermal treatment in rainbow trout and which depends on percentage of thermosensitive individuals rather than genetic background of population. Keywords : Onchorynchus mykiss, temperature, sex ratio, farmed stocks