A seasonal survey on feeding diet, breadth and feeding niche overlap of the native brown trout (Salmo trutta) and the exotic rainbow trout (Oncorhynchus mykiss) in Haraz River, in the southern Caspian Sea basin

Diet composition and feeding strategies of two native and non-native trout species i.e. the brown trout(Salmo trutta) and rainbow trout (Oncorhynchusmykiss)were investigated from autumn 2018 until summer 2019 in the uppermost of Emamzadeh Ali Lake (Haraz River, North of Iran). A total of 79 individuals including 59 brown trouts and 20 rainbow trouts were caught and studied. Digestive contents of feeding indices including CV (coefficient of vacuity), RLG (relative length of gut) and IF (intensity of feeding) revealed that both trout species were voracious carnivores, and showed a desirable nutritional status at all sampling seasons. Rainbow trout had a wider niche breadth than the brown trout in autumn, winter, and summer, except for spring. Feeding niche of the both trouts was highly overlapped in autumn and winter i.e. feedingon the same common resources. Percentage of overlap of both trouts was the most in autumn and winter and similar with the results of Morisita′s overlap index.