• fish migration;
  • migration barriers;
  • bypass channels


In most cases, the design of currently deployed migration aids is based on technical concepts, e.g. fish ladders, fish lifts, etc. These systems meet the swimming and migratory requirements of commercially interesting species such as salmonids, but not of smaller fish species and/or juvenile stages. In this context, bypass channels designed to resemble natural stream channels—with a comparatively flat gradient and a high morphological, current and substrate diversity—are a viable alternative. The utility of such systems, in compensating for the interrupted river course and in supporting migrations of various rhithral fish species, has been demonstrated in an investigation of a bypass channel on the Mur River in Styria, Austria. This 200 m long system with a total of 30 pools and riffle-like connections was successfully navigated by all seven existing fish species and was additionally used by juvenile fish as habitat. During the main spawning periods April–June and September–December, a total of 3658 fish ascended the bypass, 94% of which did so in the spring. In the 5.5 km stretch of river below the weir, the grayling (Thymallus thymallus) was the dominant species. From an estimated population of 13 300 adults, 17% migrated through the pass.