Sammendrag
Under-water video was used to observe the behaviour of adult Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) on a spawning ground during ramping events in a hydro-peaked river. The water depth in the reach was continuously recorded at a stream gauge and water depths in front of the cameras were reconstructed by means of a hydrodynamic model. Fish species, size, spawning-related activity, chasing behaviour and a swimming-speed-related activity level were recorded for each camera in 10-min intervals. Decreases in flow occurring during down-ramping led to higher activity levels and spawning interruptions. Trout were observed in the reach down to 0.13 m and salmon down to 0.17 m. The salmon was observed preparing nests at a water level of 0.30 m. Salmon used on average 122 min after the beginning of an up-ramping event to return to the spawning site and start digging again.