Journal of Aquatic Ecology

 Rainbow trout (Oncorhynchus mykiss) is an exotic fish in the inland waters of Iran and little is known about this species in Iranian waters. In this study, some biological characteristics of rainbow trout were examined in 355 specimens collected monthly from the Beheshtabad River of Chaharmahal-va-Bakhtiari Province from April 2013 to March 2014. The total length of females and males ranged from 17.1 to 40.2 cm (26.05±4.54 SD) and from 15.5 to 40.3 cm (24.9±5.93), respectively. The age of male and female rainbow trout ranged between 1⁺ to 4⁺ and 2⁺ to 5⁺ years, respectively. The most frequent age groups were 1⁺ and 2⁺ in males and females, respectively. The sex ratio was 1.3F:1M. Based on macroscopic gonad observation, the maturation cycle consisted of five stages; immature, developing, spawning, regressing, and regeneration phases. The length-weight relationship was described as W= 0.0094L^3.063, (r²= 0.96) for males and W= 0.054L^3.087, (r²= 0.944) for females. This relationship showed that the growth was isometric for both sexes. The von Bertalanffy growth models were estimated as Lt=52.04[1-e^{-0.1582(t-1.85)}] and Lt=66.39[1-e^{-0.105(t-2.46)}] for males and females, respectively. The growth performance indices were estimated as 6.06 and 6.14 for males and females, respectively, indicating a faster growth rate for female individuals.