Liming restores Atlantic salmon (Salmo salar) populations in acidified Norwegian rivers

Abstract

Acidification has exterminated or seriously reduced Atlantic salmon ( Salmo salar ) populations in 40–45 Norwegian rivers. As this problem still exists, liming to restore salmon has been necessary, which now involves 21 of these rivers. Thirteen rivers were electrofished 1 year before liming and again 1–12 years later. There was a significant effect both of time after liming and status (e.g., formerly lost and reduced stocks) on the densities of both fry (age 0+) and parr (age ≥ 1+). However, the rate of increase in densities of young salmon in these two status categories was not significantly different in either age group. The development in parr densities suggests that more than 20 years of liming is required to restore salmon in rivers with lost native populations. Stocked rivers and rivers unaltered by hydropower developments generally had higher fry densities and faster increase in parr densities. Annual rod catches of adult salmon increased significantly after liming started, reaching about 45 t after 10 years of treatment. This is 11%–12% of the current total catch of Atlantic salmon in all Norwegian rivers. Liming thus makes an important contribution to the restoration of salmon in formerly acidified rivers.