The mobility pattern of brown trout was studied in two reaches along the Noguera-Pallaresa river (NE Spain). One reach was located downstream of a hydroelectric plant presenting an altered flow regime with fluctuating daily flows (i.e. hydropeaking). The second reach was located upstream the hydropower plant and was considered as the control. A radio-frequency transmitter was implanted to 34 trout and their movements were recorded by radio tracking. Daily mobility was monitored for 3 consecutive days, whereas breeding season mobility was recorded from September to December.

No significant changes in mobility were observed along the day. However, brown trout at the control reach tended to be more active during the night whereas trout at the downstream reach showed more activity during the morning, just when the daily peak flow occurred. No significant differences in mobility were observed before and after the peak flow, suggesting that the trout were able to stay in the area during the peak event and their movements were not affected by the increase in flow.

An increase in mobility was found during a few weeks before and after the spawning period (upstream and downstream movement respectively). No mobility was found during spawning. This pattern was found in both reaches.

Daily and breeding season mobility were significantly higher in the river reach affected by hydropeaking. This reach was hydrological less stable than the control, so trout have to move further to find different types of habitat conditions, especially looking for suitable spawning areas.