River continuum disruptions in a highly altered system: The perspective of potamodromous fish

In this study, we explored how barriers such as dams have affected the longitudinal connectivity of riverine habitats from the perspective of potamodromous fish. For this purpose, connectivity changes are investigated in the central part of the Austrian Danube system, where the national reporting for the EU Water Framework Directive provides detailed information on the position and characteristics of barriers as well as the distribution range of native fish species. This assessment is based on an estimation regarding the quantitative upstream and downstream passability of individual barriers, where we further investigate three different passability scenarios to account for uncertainties. We then apply several combinations of passability scenarios and assumptions on dispersal distances to calculate a series of network-based reach and catchment connectivity indices. On average, the estimation of barrier passability indicated a high downstream passability, while upstream passability was substantially lower across scenarios. Furthermore, existing fish passes were estimated to have increased passability on average between 20 % and 24 %. Overall, the results indicated a strong effect of barriers on the longitudinal connectivity of the investigated river network. Catchment scale indices revealed a loss of connectivity, which increased with dispersal distance. Reach connectivity indices displayed a strong disruption of the natural connectivity gradient along the river network and indicated that individual river reaches have, on average, become more isolated in addition to the overall decrease in connectivity. The average loss of connectivity across scenarios was estimated at 72 % (SD = 16 %) when taking into account all connections to other reaches and 66 % (SD = 7 %) when only connections to upstream reaches were considered. We conclude that longitudinal connectivity in the Austrian Danube system is still severely compromised, making it increasingly challenging for potamodromous fish species to complete their life cycle. This issue is further amplified by the severe loss of fish habitats as a consequence of river engineering.