Long-term monitoring of European eels in the Belgian Meuse River basin. From the historical drastic decline to recent outcomes of restocking practices.

Long-term monitoring of yellow- and glass eel stages were performed in upland
areas (>300km from sea) in order (i) to quantify the decline of wild yellow eels
entering in the Belgian Meuse from the Netherlands; (ii) to follow their upstream
individual colonization; and (iii) to analyze the adaptation of young eels stocked as
glass eels imported from UK.
By monitoring a fish pass from 1992 to 2016, we quantified that the number of
ascending eels has declined from n=5613 in 1992 to n=21 in 2016 (3.99%/year) and
the mean length of eels has increased (4.1mm/year). During 6-years, upstream
individual colonization of eels (2010-2015, n=1371) was followed using fixed RFIDtracking
system. Few eels continued to migrate 4 years after tagging (<0.3%) and at
>20km upstream (3.7%); and velocity of eels varied between individuals (0.012-
3.1km/day). In a 4-year (2013-2016) monitoring study of restocked glass eels using
electrofishing and mobile RFID-tracking campaigns, we observed that eels grew
rapidly in upland small brooks and recruitment was better in rivers with high carrying
capacity (>15.8%, 2years post-stocking). Restocked eels dispersed in up- and
downstream directions with behaviors including sedentary, nomadic and
intermediate lifestyles. This suggests stocking as potential management measure to
enhance local eel stocks.