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Brown trout Salmo trutta in a flume used cover more often and were more aggregated at low water temperatures in winter compared with summer. Cover combinations providing the most cover (overhead, visual and velocity cover) were preferred in winter whereas velocity refuges were preferred in late summer; and brown trout rarely used any of these cover structures in early summer. Flow affected cover use especially in winter, when most of the trout moved to shelters during the first high flow period. Movements by brown trout were most frequent in early summer. The much higher aggression rate in summer compared with winter decreased during the first period of high flow in both summer experiments. The results indicate seasonal changes in cover habitat and cover type preferences suggesting that habitat complexity is important, and availability of cover is particularly important during different seasons and …