Efficiency of fishways and impact of dams on the migration of grayling and brown trout in the Glomma river system, south‐eastern Norway

Abstract

AbstractThis study was carried out to assess the efficiency of eight fishways constructed in connection with hydroelectric dams in the Glomma river system in south‐eastern Norway. The fishways were of the pool and weir type, except for one which was of the counter current (Denil) type, and had a discharge of 0.5–1.0 m3/s. During 1985–8 3077 grayling and 2995 brown trout were captured in traps in the fishways, measured and floy‐tagged before being released in the river upstream of the fishways. The number of fish captured in two of the fishways were compared with estimated fish stocks in the areas around the fishways. Migrations began in May or June when the water temperature was > 5–6°C. Spawning migration of grayling occurred in May. During May more than 200 grayling may enter a single fishway within a week. In other fishways grayling occur only sporadically before late June or early July, when they migrate upstream to feed. Brown trout migrate during late spring, summer and autumn. The spawning migration takes place more or less during the whole summer, but mostly in late July and August. Immature brown trout also pass through the fishways, with a peak in October in three fishways. This migration is probably an ascending emigration from recruitment areas located downstream of these fishways. The efficiency of these fishways appears low, as the number of fish ascending was less than 2% of the estimated stock within the stretches where 90% of the recaptures occurred. The discharge in the fishway relative to the total discharge seemed to be of great importance, and to achieve efficient fishways they should be constructed for higher discharges, compared with river discharge, than the fishways in this area. The longest migration of grayling was 100 km, whereas the longest migration of brown trout was 122 km. The fish seemed to exploit shorter river stretches in areas influenced by several dams/fishways than fish in an area influenced by only one dam.