<p>In respect to protect and foster fish populations in the Danube catchment a workshop “ecology and hydropower” was held in 2025. One aspect was downstream migration and the impact of this migration on fish population. So far no technical solutions for large hydropower plant do exist to guarantee a balance between CO₂ free energy production and possible effects on fish populations. Since there is no state of the art for fish descent at the facility itself, it is important to classify which existing migration routes are being used, which facility-specific residual mortalities need to be compensated, and to what extent river restoration improve the local status of fish populations. All possible downstream passage facilities are used by the fish and especially fish bypass systems which are designed for the upstream migration are used for downstream migration. If these facilities include spawning areas and juvenile habitats these which may play a key role for the local habitat status and reduce the possible contact with a turbines. To pursue the common goal, to reach the good environmental potential (EU water framework directive), different approaches and concepts are followed. To offer river specific solutions to reach the good ecological potential a “best environmental option” should be implemented and chosen according to our experiences. Almost all discussed projects are in heavily modified water bodies (HMWB).</p>