fisketrapp

Fisketrapp is a structure designed to enable migratory fish to bypass man-made barriers in rivers, such as dams, weirs, or rapids. It functions as a fish ladder or pass, consisting of ascending pools or steps connected by channels that convey water and fish upstream at a reduced velocity. Fish encounter a gentler slope than the barrier, allowing them to rest between short climbs and continue their migration.

Common designs include stepped ladders, where each pool provides a resting area, and smooth-sloped passes or bypass channels. Some installations also incorporate fish lifts that physically transport fish over the barrier when natural ascent is impractical. Materials range from concrete and masonry to natural stone, with baffles, weirs, and flow deflectors to manage velocity and direction.

Purpose and effects: The primary aim is to restore longitudinal connectivity of rivers, supporting anadromous and catadromous species such as salmon, trout, and eels, and helping maintain population genetics and access to spawning habitats. Effectiveness varies with design, barrier height, flow regime, and species behavior; some passes are more suited to large, powerful swimmers, others to smaller fish.

Maintenance and limitations: Regular inspection, debris removal, sediment management, and flow adjustment are required. Predation, entrainment of juveniles, and poor passage efficiency can occur. In some cases, barrier removal or river restoration is a more effective long-term solution.

See also: Fish ladder, fishway, river restoration.