Fish migration through ship locks at the Danube River, Austria

Longitudinal connectivity in the Austrian section of the Danube has improved over the last three decades thanks to the progressive installation of fish passage facilities at hydropower plants. However, navigation locks which are present at all barrages, remain poorly quantified with respect to their role in fish migration. This study used long-term PIT tag monitoring to evaluate the impact of navigation locks on the upstream migration of fish at the Aschach hydropower plant in Upper Austria over a 16-month period (March 2019 to June 2020). Additionally, indirect evidence of ship lock passage was obtained by analysing long-distance movements across dams lacking fishways. A substantial number of fish, representing a wide range of species and body sizes, were detected passing through ship locks. The repeated detection of individual fish and their passage through multiple consecutive locks indicates a consistent and deliberate pattern of use rather than an incidental occurrence. Lock passage showed pronounced seasonal peaks in spring and autumn, and detections were predominantly diurnal. These temporal patterns were not significantly correlated with the frequency of lock operation. Nevertheless, an effect of lock operations could not be ruled out. Our results demonstrate that navigation locks can effectively serve as supplementary migration routes in large, regulated rivers. They also provide novel insights into species composition and size structure, as well as seasonal and diurnal patterns of lock use.