Safe two-way migration for salmonids and eel past hydropower structures in Europe: a review and recommendations for best-practice solutions

This review provides a summary of knowledge on two-way fish migration of salmonids and eels past hydroelectric plants in Europe. On the basis of a summary of international literature, general designs and recommendations for best practices for fish-pass facilities are provided. The review is part of the Norwegian SafePass project, which focuses on Atlantic salmon, brown trout, grayling and European eel. According to recent international recommendations, many existing European fishways for upstream migration do not have an optimal design. This is especially evident for denil and pool-and-weir fishways in inland areas with species such as grayling and brown trout. Based on the review, we generally recommend (1) using ramps, nature-like channels and vertical-slot fishways for these species and (2) reducing water drop between the pools in pool-and-weir fishways and reducing energy dissipation compared with the design of traditional Atlantic salmon ladders. There are few well-functioning passages for downstream migration of fish in Europe and significant progress has been made in the past decade to improve technology and knowledge. Several international studies have shown that physical structures, such as fine-mesh trash racks with alternative escape routes and bypass arrangements, provide >90% passage efficiency for downstream migration, especially for brown trout and salmon, and have, in recent years, shown good results also for silver eels.