Response of fish species to river restoration – the role of species traits

Species are known to respond differently to restoration efforts, but we still lack a clear conceptual understanding of these differences. We analyzed the development of an entire fish community as well as the relationship between multi-metric response patterns of fish species and their ecological species traits at a comprehensively monitored river restoration project, the Lippe River in Germany. Using electrofishing data from 21 consecutive years (4 years pre- and 17 years post-restoration) from multiple restored and unrestored control reaches, we demonstrated that this restoration fully reached its targets, approximately doubling both species richness and abundance. Species richness continuously increased while fish density exhibited an overshooting response in the first years post restoration. Both richness and abundances stabilized approximately seven years after the restoration, although interannual variability remained considerable. The response of each species to the restoration was characterized using a set of six parameters. Relating the dissimilarity in species response to their ecological dissimilarity, based on 13 species traits, we found life-history and reproduction-related traits were the most important for species’ responses to restoration. Short-lived species with early female maturity and multiple spawning runs per year exhibited the strongest response, reflecting the ability of fast reproducers to rapidly colonize new habitats. Fusiform-bodied species also responded more positively than deep-bodied species, reflecting the success of this restoration to reform appropriate hydromorphological conditions (riffles and shallow bays), for which these species depend. Our results demonstrate that repeated sampling over periods longer than seven years are necessary to reliably assess river restoration outcomes. Furthermore, this study emphasizes the utility of species traits for examining restoration outcomes, particularly the metapopulation and metacommunity processes driving recovery dynamics. Focusing on species traits instead of species identity also allows for easier transfer of knowledge to other biogeographic areas and promotes coupling to functional ecology.