Migration and habitat use of the landlocked riverine Atlantic salmon Salmo salar småblank

Abstract

“Småblank” is the only population of Atlantic salmon in Europe living its entire life in the river habitat without performing migrations to the sea or lakes. Home range size and habitat use were compared between an unregulated tributary and hydro-power regulated parts of the main river during 2014–2017. In total, 140 småblank were tagged with radio transmitters and tracked for 4–5 months. Tagged fish tended to stay within small areas: the average 50% home range was 1123 m² and the average 95% home range was 4416 m². Mean distance between the furthermost positions for individual fish during tracking was 242 m, and the fish did not migrate between different stretches of the river. The small home range and non-migrating behaviour may explain why småblank are divided into several genetically distinct populations with limited gene flow. The results showed that småblank were far more stationary than what is known for other landlocked salmon populations. Use of moderate or high water velocities’ (> 50 s⁻¹) areas with coarse river substrates implies a vulnerability to anthropogenic effects such as water diversion, dams and weirs.