Historical evolution of fish biodiversity and capture periodicity in the River Meuse (Belgium) as revealed by 15 consecutive years of fishpass monitoring

In the River Meuse at 323km upstream from the North Sea, the hydroelectric dam of Lixhe (rated power of 5300KW) is equipped with two fishpasses: one pool and weir fishpass (in operation since 1980; 0.13m³.s-1 operating flow) and one vertical slot fishpass (in operation since 1998; 1m3.s-1 operating flow). Both fishpasses were scientifically monitored using capture traps during 15 consecutive years (from 1999 to 2014). The objective of the study is to analyse i) the evolution of fish stock and biodiversity over 15 years and ii) the capture periodicity of fish and it’s variation over years. During the considered period, we observed 36 species at adult stage (88597 individuals with 75% of cyprinids) and 13 species at juvenile (0+) stage (335067 individuals with 90% of cyprinids). Global results between 1999 and 2014 showed a drastic reduction in capture per year (>90%) for some species like Anguilla Anguilla, Alburnus alburnus, Blicca bjoerkna, Rutilus rutilus and Tinca tinca. Whereas other species have emerged like Aspius aspius and Silurus glanis. For the 0+, an important interannual variation is observed from few individuals to several tens of thousands per year. The comparison between both fishpasses indicated a greater use (>50%) of the vertical slot fishpass by rheophilic species and species that have good swimming ability. The analysis of capture periodicity for the most abundant species at adult stage demonstrated the presence of stable behaviours between years for the Abramis abrama and B. bjoerkna respectively a Q50 of the capture day of the year from 108th to 144th day at mean=17°C and from 118th to 155th day at mean=18°C. In contrast other species have patterns more variables (Barbus barbus: Q50 from 96th to 266th day; R. rutilus: from 10th and 309th day). Finally the 0+ exhibited a capture periodicity stable in time during the summer.