The present study was conducted in a four-season period, from September 2016 to 2017, in Tonekabon River, northern Iran. A total of 91 brown trout fish were collected from five stations using electrofishing equipment. They composed 40 females, 41 males, and 10 of unknown sex. Their age ranged from 1 to 3 years, and they had a minimum and maximum length of 41 mm and 175 mm, respectively. The preys identified within their gastrointestinal tract included insect larvae of Ephemeroptera, Diptera, Liponeura, Simulium, Hydropsyche, Coleopteran, Trichoptera, flying insect, spawn, Odonata, Oligochaeta, Plecoptera taxonomic groups. The collected fish were classified into three age groups including 1- and <1-year, 2-years, and 3-years old as a scale to define their feeding intensity. The consumption percentage of Hydropsyche and Liponeura was significantly different among three classes. However, there was no significant difference in the consumption intensities of Ephemeroptera, Plecoptera, and Simulium. The reproduction season of brown trout fish is within fall. Maximum feeding intensity occurs in spring, whereas minimum feeding takes place within summer and fall seasons. Classes 1 and <1-year-old had higher feeding intensity than other classes (classes 2 and 3 years old). In general, Plecoptera, Ephemeroptera and Simulium constitute primary preys of the fish, while Liponeura, Hydropsyche, Dipteral, Trichoptera, and Oligochaete, Coleoptera, Oligochaeta, Odonatan, flying insects, and spawn are considered as the subordinate prey for the feeding regimen of brown trout. The highest amount of prey for stations 1, 2, 3, 4 and 5 included Simolium, Ephemeroptera, Simolium, Simolium and Liponeura, respectively. Station 2 had the highest glutting stomach index, followed by station 1, both of which were located on the Se-Hezar River.
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