To establish good ecological status in European rivers, firstly there is the question of how to re-establish unhindered migration for fish at barriers. This article documents a project to re-establish longitudinal connectivity at a large epipotamal river at the Schwabeck Hydro Power Plant, Carinthia/Austria, from the selection of an appropriate fish pass system to the final function control. Instead of a standard vertical slot, the innovative enature^® fish pass shape with a significant reduction of flow, velocities, energy dissipation rate, and turbulences, but with a clear enhancement of fish passage capability, was chosen. Using 2D hydraulic modelling and a statistical evaluation of fish passage, physical and ecological effects were reviewed, with the clear result that there is no identifiable, positive ecological effect on the number of fish migrating with an increase of concurrent flow in the fish pass. Passability and findability were monitored with the new FishCam, an automatic, precise, and constant (24/7 24 h a day, seven days a week) collection and pre-evaluation field data survey method which does not involve trapping of, contact with, or stress for fish. It was shown that the enature^® fish pass enables an unhindered migration for all available fish species. As >99% of fish migrate from April to November, there is no ecological need to operate a fully functional fish pass year-round. Combining all the individual factors together, the fish pass at the Schwabeck Hydro Power Plant is an almost exemplary solution for a fully functioning restauration of the continuum with a minimized loss of generation of electricity.

Restoring the longitudinal connectivity of rivers through fish passes is of great importance for achieving good ecological status of surface waters. However, the key stimuli determining the findability of the entrance of these structures is still subject to debate. In this article, the influence of water temperature, light and acoustic stimuli in addition to flow parameters on fish movement is assessed. Analyses are based on a comprehensive dataset of 40,000 fish migrations covering time accurate recording of ascending individuals during the observation periods from 2015 to 2020 in various fish passes on the river Drava in Carinthia, Austria. The data are assessed by technical, fish ecological and statistical methods. Results indicate that the effect of the water temperature gradient between the fish pass and the main river has most impact on the ascent rates, whereas the two factors of light and acoustic, as well as most of the flow parameters, have no or negligible effect on the findability. A favourable thermal environment can be important to ensure efficient upstream migration and thus facilitate the findability of fish passes.