Increased densities of Atlantic salmon smolts in the River Orkla, Norway, after regulation for hydropower production
Fisheries Bioengineering Symposium: American Fisheries Society Symposium 10
Colt J;White RJ;
American Fisheries Society
The River Orkla was regulated for hydropower production in 1983. Estimates of smolt densities of Atlantic salmon Salmo salar in the river were conducted from 1983 through 1988. Each year, 3,000-5,000 wild smolts were captured in three sections of the river and marked by fin clipping. Recaptures were made during the smolt run by two traps lowered from a bridge downstream from the area where smolts were tagged. After regulation of the river, the water discharge always exceeded 10 m3/s, thereby eliminating periods with low discharge (2 m3/s). Smolt densities appear to have increased gradually from 4.0 (95% confidence interval, 2.7-6.1) per 100 m2 in 1983 to 7.9 (6.2 to 10.1) per 100 m2 in 1987, but dropped to 5.1 in 1988. Because major changes in abiotic conditions are related to stabilized discharges, increase in production area may have been responsible for increased smolt production. High minimum discharge during winter is hypothized to increase winter survival of parr. The estimated increase in smolt densities appears to have resulted in recent higher catches of adult Atlantic salmon by anglers.