Movement behaviour and fishway performance for endemic and exotic species in a large anthropized river

Studies of fishway performance, at the multi-species level in large rivers, are scarce and this raises the question of the passage success of both endemic and exotic species in anthropised environments. The study was conducted in the downstream part of the River Meuse (average annual discharge = 400 m³.s^-1) on a 13 km transect between two successive fishways (M₀ and M₁). From 2015–2021, a total of 1065 adult individuals, representing 14 large potamodromous (including asp Aspius aspius and catfish Silurus glanis as exotics) and diadromous species, were captured at the trap of the Lixhe fishway (M₀), individually tagged (using biocompatible RFID tags-Radio Frequency Identification), and released upstream of M_0. To analyse the performance of the M₁ vertical slot fishway using standardised metrics, a RFID detection station was placed with one antenna at the entrance and one antenna at the most upstream pool of the M₁ fishway (5.7 m height 18 pools). With 456 individuals detected in the M₁ fishway, the ascending rate from M₀ to M₁ was 42.8% (the common bream, Abramis brama achieving the best performance with 85.7%); the exotic species (catfish and asp) reached 21.5% and 30.5%, respectively. The adjusted passage performance was the best for the exotic asp (94.9%) followed by the trout, Salmo trutta (90.0%). The median time to cross the M₁ fishway was shorter for the trout (median = 01h08) and longer for the eel Anguilla anguilla (median = 21h17); the exotic asp was also very fast (median = 1h31). The hourly passage time at M₁ was variable, with some species migrating during daylight, dark periods or the entire 24 h cycle. The multispecies vertical slot fishway studied presented the best performance, in terms of passage success, at an international level, associated with good transit times. It also allows the passage of exotic species, which will increase their expansion area.