Relationship between the drift of macroinvertebrates and the activity of brown trout in a small stream

Brown trout Salmo trutta were most active in a small stream at night, dusk and dawn when drift rate was highest, but correlations between hourly drift rates and the trout's activity varied substantially between individuals, between different dates for a single individual, and between different periods of the daily cycle. On some occasions, the trout were responsive to the total drift rate, either at night or during the day, and on others to the largest drifting organisms only (terrestrial organisms, adults of Ephemeroptera, Diptera and Trichoptera). The study supports the idea that trout adapt their activity pattern to the abundance of drifting prey, either as generalists towards any organism, or as specialists towards the largest ones.