Abstract :
[en] Human activity has caused longitudinal fragmentation of many rivers. Fishways have been installed worldwide, but their successive use by potamodromous species remains poorly documented, particularly in large river ecosystems. Four vertical slot fishways were installed within a 32-km stretch of the lower Belgian Meuse River basin. From 2012 to 2016, n=532 individuals belonging to 11 potamodromous fish species (rheophilic, limnophilic and large carnivorous) were continuously captured in the most downstream fishway (M0), tagged with an RFID transponder and released upstream. These could be further detected in upstream river part within three fishways (M1 and M2 in the Meuse, and O1 in an important tributary, the Ourthe) that were equipped with RFID
detection stations. In the first downstream stretch (13 km from M0 to M1) we quantified an ascending rate until M1 (number of individuals detected in M1/ number of individuals tagged in M0) of 32.9% including all individuals tagged, with a maximum value of 67.2% for chub (Squalius cephalus), 40% for pike (Esox Lucius) and 35.5% for nase (Chondrostoma nasus), and a progression time M0 to M1 of 1.1 days per kilometre (d/km), with trout (Salmo trutta) as the fastest species (0.3 d/km). Upstream of the M1 fishway, many individuals of rheophilic species (trout and barbel, Barbus barbus) preferred to enter in the Ourthe tributary (detection at the Ourthe fishway) unlike the upstream Meuse (detection at the most upstream fishway in the Meuse), demonstrating a new accessibility to more adapted spawning sites. Most fish were present within fishways mainly in spring during the circum-spawning migration, and during summer and autumn for dispersal and/or seeking-refuge. The
diel activity cycle varied depending on the species, with detection during the entire diel cycle (e.g. chub and barbel), during the day (e.g. trout) and during the night (e.g. catfish, Silurus glanis). The installation of fishways in the degraded river Meuse can be considered adequate for the restoration of the free movement because potamodromous species demonstrated their ability to migrate over long distances (> 20 km) and to reach potential spawning habitats through the reopened access to a tributary.
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