A preliminary telemetry investigation on the obstacles to anadromous Salmonids migration in spawning streams of the Belgian Ardennes (river Meuse bassin)

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 success remains to be determined. Since
similar minor man-made obstacles are most frequent in the salmonid spawning streams of the Belgian Ardennes,
it is thus uncertain that migratory trout having successfully climbed the major obstacles since the North Sea
would find their way to the spawning redds. As a corollary, it is suggested that more detailed case studies should
be undertaken, ideally via the use of telemetered probe-fish, in order to provide management policies that would
represent a suitable compromise between users of water resources with apparently conflicting interests (water
regulation, tourism, nature conservancy).