Smolting of two-year-old lower and upper size class of Saimaa landlocked salmon (Salmo salar m. sebago Girard) under fish farm conditions

Smolt development was assessed in two-year-old lower and upper size class of hatchery-reared Saimaa landlocked salmon during January–July and late April–July, respectively. The fish in both size classes showed increased gill Na+,K+-ATPase during the spring suggesting smolting of these fish. The fish in the lower size class showed also a smolting-associated decrease in lipid content of muscle as spring progressed whereas the lipid content in upper-size-class fish appeared to increase during late April–May. The differences in lipid content of muscle as well as in gill Na+,K+-ATPase activity between the groups may indicate the presence of non-smolting individuals among the fish sampled from upper size class. However, variation in these as well as other parameters was small. Thus, it is suggested that each class of juvenile Atlantic salmon shows a distinct developmental pattern of ionoregulation.