Multi-year analysis of the fish colonisation dynamic in three newly installed fishways in medium sized Belgian rivers

The temporal dynamic use of newly installed fishways after a reopening event is not well known as most studies are not performed just after the opening and are generally limited to a single season or year. We carried out monitoring of three fishways for several consecutive years on three rivers in Belgium from the date of their opening. To identify the colonisation dynamics of fish species, we analysed temporal patterns in specific diversity, abundance, biomass, and associated environmental conditions. We detected different capture peaks and the appearance of new species several years after opening the migratory axis (up to 8 years post-opening). The dynamic of colonization showed that the same species may migrate earlier or later depending on the river. The analysis of the periodicity of capture indicated that some species made movements throughout the year while others at more precise periods. Moreover, the periodicity of movements was either stable or fluctuating over the year of monitoring, depending on the species. Our results highlight the importance of long-term monitoring to detect temporal dynamics in fish colonisation, allowing to improve our understanding of the opening effect of a migratory axis.