Assessment of Fish Abundance, Biodiversity and Movement Periodicity Changes in a Large River over a 20-Year Period

A river is an ecosystem where fish fauna represents important structural element. To re-establish connectivity, it imperative to allow movement between functional habitats. Due the hydromorphological complexity of large anthropized rivers and lack study techniques that can be used in such environments, relevant data with regard ecology are scarce. On River Meuse, Belgium, at a point 323 km upstream from North Sea, Lixhe hydroelectric dam equipped two fishways. Both were continuously monitored using capture traps for 20 consecutive years (from 1999 2018), representing 4151 monitoring events. The objectives present describe overall abundance indicators mainly holobiotic potamodromous species analyse their temporal evolution. We captured 388,631 individuals (n = 35 species) during fishway monitoring; 22.7% adults (>75% which cyprinids), 83.3% juveniles (>90% cyprinids). From 2018, results showed drastic reduction yearly captures some native as well apparent emergence non-native (e.g., Silurus glanis) reintroduced Salmo salar). annual periodicities associated environmental factors clearly defined mostly related spring spawning migration adult stage. This long-term demonstrated how fishways by whole community allowed better understanding lowland river. appearance decline common widespread European should prompt managers adopt conservation measures.