Over 100 years of longitudinal connectivity changes from the perspective of a migratory fish species

Disruptions of the longitudinal river continuum by artificial barriers have been widely recognized to impact aquatic biodiversity. However, while consequential connectivity changes have been researched extensively, the temporal component has been largely disregarded. Therefore, we investigate how almost 500 large artificial barriers, and later fish migration aids (FMAs), have affected longitudinal connectivity, specifically the connections between suitable habitats, along an Austrian river network since the beginning of the 20th century. For this purpose, a graph theoretic approach is deployed that integrates structural and functional aspects, illustrating connectivity changes from the perspective of a characteristic migratory fish species Chondrostoma nasus (Linnaeus 1758). Furthermore, different barrier passability scenarios, partly based on empirical findings, are assessed to account for uncertainties and identify to what extent passability must be restored to reach a proposed restoration goal. Our results illustrate that the most significant loss of connectivity has occurred after the beginning of the 20th century. Particularly, barriers constructed in the main stem of the river network, the Danube, have been identified as drivers of connectivity loss and, hence, as bottlenecks for connectivity restoration. While FMAs have partially restored connectivity, the remaining loss has been estimated between 31.7% and 60.7% across barrier passability scenarios. Contrastingly, despite the construction of roughly 350 artificial barriers, connectivity loss at the beginning of the 20th century ranged only between 6.8% and 8.3%. Therefore, since a full restoration must be deemed aspirational, we suggest restoring connectivity to an extent similar to the beginning of the 20th century. To reach this restoration goal, our results suggest that even the passability of barriers equipped with FMAs must be increased, and if this cannot be achieved, barrier decommissioning needs to be considered. Finally, we identify a need for a transparent assessment of quantitative barrier passability and conclude that the construction of further barriers must be considered unsustainable as long as previous connectivity and habitat losses are not compensated. • Danube hydropower plants are a major driver of longitudinal connectivity loss. • Loss of longitudinal connectivity already at the beginning of the 20th century. • Fish migration aids need to be highly functional to restore longitudinal connectivity. • Information on quantitative barrier passability needed to restore longitudinal connectivity.