Map of the study area in the International and Belgian Meuse River basins (a), and location of the catch and release site in the Meuse (at R–Lixhe), the first upstream detection station in the Meuse (A–Monsin), the upstream migration route 1 in the upper Meuse (B–Yvoz-Ramet), the upstream migration route 2 in the Ourthe tributary (C–Angleur), and the alternative migration routes of yellow-phase eels (the navigable canals of Albert and Ourthe, and the non-navigable Liège Canal) (b).