The food of Atlantic salmon Salmo salar L. and brown trout Salmo trutta L. smolts during migration in the Orkla river, Norway

Abstract

Stomach analyses were carried out on 286 Atlantic salmon smolts and 163 brown trout smolts during migration in the river Orkla Central Norway (63°N, 10°E), from 1982 to 1984. Empty stomachs comprised less than 18,4% for salmon and 25,4% for trout. The food composition of both species varied little from year to year. The dominant food items of Atlantic salmon smolts were Trichoptera larvae and nymphs of Plecoptera and Ephemeroptera, each comprising from 17,7 to 31,2 % of the stomach volume. These three insect orders were also the main food of brown trout smolts in addition to Diptera pupae, varying from 17,4% to 32,3% in volume for each food item. The volume of Trichoptera larvae in the smolt stomachs decreased during the migration, while the importance of Ephemeroptera and Plecoptera nymphs as food for the smolts increased. There was a significant overlap in diet between salmon and brown trout smolts both in the different years and during the migration period in 1984.