peer reviewed ; In the course of the 'Meuse Salmon 2000' programme aiming at the restoration of the Atlantic salmon Salmo salar and sea trout Salmo trutta in the River Meuse Basin, most large dams are progressively equipped with fishways to restore the free circulation of spawners between the North Sea and the first major spawning streams, the River Ourthe and its tributaries. Spawners entering the River Ourthe would still be confronted to so-called minor obstacles, aiming at water regulation for tourism purposes but of which the actual impact on fish migration has never been investigated. In order to test for the actual free-circulation of salmonid spawners in the upper River Ourthe and to locate potential spawning grounds, a probe fish (489 mm FL male sea trout) was tagged with an intraperitoneally implanted radio-transmitter. From the 18th of ovember 1995 onwards, the trout was tracked in a part of the river (44 km upstream of the confluence) which was thought to be devoid of any major obstacle to fish migration. Three days after its release, the trout had migrated over 6 km up to a small weir (1.8 m high). During four consecutive days, the trout was consistently located downstream of the weir but no successful climbing was observed, reflecting the poor efficiency of the central fishpass under dry weather conditions during summer and autumn. The trout then settled in a deep run habitat, 150 m downstream of the weir and no upstream excursion was recorded until the first major rise of water level, four weeks later, even when the weir was opened for water regulation purposes. When the water level was maximum (24th of December), the trout moved upstream of the weir and migrated over 28 km during the next 72 hours up to a spot identified as a potential spawning redd from habitat features, and where it was consistently located till the 31st of December. These results, though most preliminary, clearly indicate that even minor obstacles may cause a substantial lag in trout migration of which the impact on spawning ...