Abundance of Juvenile Atlantic Salmon and Brown Trout in Relation to Stocking and Natural Reproduction in the River Laerdalselva, Western Norway

Abstract

Since the regulation of the river Lærdalselva in 1974 many juvenile Atlantic salmon Salmo salar and brown trout S. trutta have been stocked above the natural reach of anadromous fishes to compensate for possible loss in Atlantic salmon production. The parental material for the stocking was obtained from the lower parts of the river. In 1988 the numbers of fish stocked were drastically reduced. The density of Atlantic salmon (all year-classes) was 75–80% less in years of light stocking than in years of intensive stocking. For Atlantic salmon, a strongly significant (P < 0.001) correlation was found between the numbers of young-of-the-year parr (age-0 fish) stocked in the river and the corresponding density of age-0 fish. The reduction in Atlantic salmon density occurred even though the fish ladders allowed fish migration to the spawning grounds. However, few of the Atlantic salmon moving up through the fish ladders were females, so natural recruitment was severely limited.