Migratory behaviour of adult fast‐growing brown trout ( Salmo trutta, L.) In relation to water flow in a regulated Norwegian river

The migration pattern of spawners of brown trout (Salmo trutta) in relation to water flow was analysed by radio-tracking in the regulated River Gudbrandsdalslagen, Norway. During the upstream spawning migration in the high flow period, trout (2–12 kg) were caught, tagged and released 15 km downstream of a fish ladder at the Hunderfossen waterfall. The released fish displayed a systematic and directional upstream movement to the outlet of the tunnel from the hydroelectricity plant. In autumn and winter there is a minimum water discharge of 20–2 m3/s on p.s. in the river between the dam and the outlet of the tunnel, which has a discharge of 200–300 m3/s. When the water flowing over the Hunderfossen dam decreased to 20 m3/s, the ascent of brown trout up the river stopped and fish periodically entered the power plant tunnel. To determine the flow necessary to attract fish into the spawning reach above the tunnel outlet, two experiments were undertaken using 12 and 17 radio-tagged trout. In the first experiment, 60 m3/s of water released for 24 h resulted in the migration of 50% of the trout up the river. The second experiment, releasing 60 m3/s for 24 h, followed by 30 m3/s for 24 h two days later, resulted in the migration of 60% of the trout. Only one fish ascended the river at a flow of 30 m3/s. It is recommended that a repeated release of water at 60 m3/s is made in periods of minimum water discharge to save the spawning migration. The results demonstrate the advantage of using radio-tracking in experiments dealing with fish migration in relation to water-flow management.