Rehabilitating the River Ätran for Diadromous Fish Species

The River Ätran is an important river for diadromous fish species on the Swedish west-coast. Wild populations of wild Atlantic salmon, trout, European eel and sea lamprey enter the river for spawning and growth. The conditions for migratory fish species are improving in the river since rehabilitation efforts now address the needs of all diadromous species and life-stages, and not only upstream moving salmonids. This presentation gives examples of rehabilitative measures and their feasibility for diadromous fish at two hydroelectric plants (HEPs 1-2) in the River Ätran.

Attempts to decrease the mortality among downstream moving fish are now being tested in the River Ätran. A surface spill gate at HEP1 was not an effective measure for downstream passage for silver eels and smolts, but instead they passed rapidly through the racks and turbines. Kelts spent a long time searching for a passageway downstream, and several of them were lost in doing so. By reducing the slope and the gap width of an intake rack at HEP2, the passage success for downstream moving silver eels was improved from 28% for the old rack to >90% for the new rack.

HEP1 in the River Ätran is passable for some upstream moving fish through a Denil fishway and a juvenile eel pass, whereas HEP2 is impassable for all upstream migrating fish. In the near future the spillways at HEP1 will be removed and replaced by two new overflowing sills and a large nature-like fishway. The aim of this nature-like fishway is to restore two-way longitudinal connectivity for all fish in the river. To determine the location and design for future rehabilitative measures targeting eels, new research activities will focus on gaining more knowledge on the existing and potential recruitment and production of eels.