Smolting of the brown trout (Salmo trutta L.) in Lestijoki water

The brown trout (Salmo trutta L.) of the Lestijoki strain, reared to the pre-smolt stage in fish farm conditions, were tested for their smolting ability in water from Lestijoki, a river in western Finland, during final winter before smolting. Reference fish were reared in local ground water. During the pre-smolt winter the fish were analysed for their gill ATPase activity, condition factor (CF) and osmoregulatory ability after exposure in 30% balanced sea water solution. The osmoregulatory ability of the gills was severely altered by the water quality of Lestijoki water. The decrease of condition factor (CF) and changes in gill Na+, K+-ATPase activity revealed an attempt of smolting in fish reared in the river water. The results of this investigation questions the ability to give Lestijoki a reproductive brown trout stock by re-establishing the species.