Feeding habits of Rainbow Trout (Oncorhynchus mykiss) in Beheshtabad River of Chaharmahal & Bakhtiari Province

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 (Oncorhynchus mykiss) as an exotic fish were examined in 355 specimens collected monthly from Beheshtabad River of Chaharmahal & Bakhtiari Province during April 2013 to March 2014. Total length of females and males ranged from 17.1 to 40.2 cm (26.05±4.54SD) and from 15.5 to 40.3 cm (24.9±5.93), respectively. Among all the examined stomachs, 39 were empty (Vacuity index, Vi= 11%). This variation was not significantly diffrent (P˃0.05) over the year and the maximum number of empty stomachs was recorded during the spring and the minimum during the autumn. The diet of the rainbow trout was composed of 45 different prey taxa, belonging to 10 main classes. The most important prey items were Annelida (Oligochaeta and Hirudinea), Platyhelminthes (Turbellaria), Arthropoda (Crustacea, Insecta and Archanida), Mollusca (Gastropoda and Lamellibranchiata), Coelenterata (Cnidaria) and Teleostei. Hirudinea, Crustacea, Insecta, Gastropoda, Teleostei and Lamellibranchiata were the most important prey items in all seasons. Crustacea, Insecta, Gastropoda and Teleostei were the most important prey items in all months. Sex did not appear to have any significant effects on the diet composition and on the main prey items consumed by the fish. Fish size showed significant effects on diet composition and the average size of prey increased with fish size. The rainbow trout could be considered a carnivorous fish, feeding on a wide range of prey items.