Studying some reproduction characteristics of rainbow trout in Beheshtabad River of Chaharmahal and Bakhtiari Province

Rainbow trout (Oncorhynchus mykiss) is an exotic fish in the inland waters of Iran and little is known about this species in Iranian natural waters. In this study, some biological characteristics of rainbow trout such as GSI and absolute and relative fecundity were examined in 355 specimens collected monthly from Beheshtabad River of Chaharmahal and Bakhtiari Province during April 2013 to March 2014. 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. Sex ratio was 1M:1.3F. Based on macroscopic gonad observation, maturation cycle consisted of five stages; immature, developing, spawning, regressing and regeneration phases. The gonadosomatic index indicated that reproduction of rainbow trout in the Beheshtabad River occurred in October and Junary for males and in September and Jaunary for the females, with the highest  mean value of 3.28 and 4 in November and Jaunary for males and 4.22 and 3.48 in September and Jaunary for females. Oocyte diameters ranged from 0.125 to 3.5 mm. Absolute fecundity ranged between 680 and 3890 (1860±170) eggs. Relative fecundity ranged between 1900 and 9000 (4600±360) eggs/Kg body weight. It seems that this species has been able to adopt very well to the river conditions.