| Summary: | peer reviewed The combined impact of disturbances on downstream migrating fish in large-sized rivers, such as a hydropower station associated with a navigation channel, has never been studied. From 2014 to 2016, we investigated the downstream migration of n=72 acoustic-tagged smolts in the Meuse river at a bifurcation zone between a hydropower station equipped with three Kaplan turbines and a navigation canal. In order to identify the effect of water flow directions and velocities on smolt behaviour, the hydrodynamic characteristics of the study site were notably modeled. Of the migrating smolts, 41.5 % performed back and forth movements before approaching the complex, sometimes over long distances and at a slow pace, leading to significant delays (3‒298 h). Only 38.6 % passed through the complex, of which 36.4 % migrated by the navigation canal and 63.6 % by the hydropower station, with a median research time of 04:44. The escapement rate at the end of the study site has only reached 2.9 % by the canal and 8.3 % by the Meuse river. This site turns out to be problematic for successful smolt downstream migration by highly impeding the longitudinal Meuse river connectivity. Possible remediations will be discussed in order to improve the crossing rate of this type of complex.
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