An overview of potamodromous fish upstream movements in medium-sized rivers, by means of fish passes monitoring

This study aims to investigate the upstream movement patterns of potamodromous fish species using multi-annual monitoring of modern multi-species fish passes in two medium-sized Belgian rivers: the Berwinne and the AmblSve. During a 6-year monitoring period including a frequency of 1-5 times per week, the captured individuals were identified, measured and weighed to determine species abundance, and the periodicity of their seasonal movement patterns was investigated in relation to environmental factors. In the AmblSve, 22 different fish species were monitored (n = 1513 individuals; biomass of 154 kg) and 14 species in the Berwinne (n = 3720; 408 kg). In both fish passes, inter-annual differences were observed, but the first year of monitoring was the best in terms of biomass, indicating the existence of an opening effect just after the fish passes opening, which allowed new migration routes. Salmonids and rheophilic cyprinids were predominant in terms of biomass, while minnow and spirlin were predominant in terms of number of fishes. The diversity in size ranged from 46 to 760 mm demonstrating the importance of movements in various life stages. The capture periodicity showed different mobility patterns between the adult and juvenile stages in most species. Movements occurred frequently during the circum-reproduction period for some species, but many species moved also apart from spawning period. This study highlights the omnipresence of patrimonial holobiotic potamodromous fish at different life stages that reflect their biological needs to move throughout the annual cycle. They must be considered as target species in river continuity restoration programmes and fish-pass design.