Smolting and seasonal variation in the smolt characteristics of one- and two-year-old Saimaa landlocked salmon under fish farm conditions

Silvering of the skin, reduced condition factor, elevated gill Na⁺, K⁺-ATPase activity and well-developed capacity to regulate the osmotic and ionic balance in sea water were observed in 1 and 2 year old hatchery-reared Saimaa landlocked salmon Salmo salar m. sebago during April-June. Loss of hypoosmoregulatory ability and gill Na⁺K⁺-ATPase activity was observed earlier in 2 year than 1 year old fish. Coincident with changes associated with smolting both age groups showed diminished osmoregulatory capacity in fresh water. Slow growth during May-June may also be attributed to osmoregulatory difficulties in fresh water. These results support the suggestion that the developmental changes at smolting are seasonal and unrelated to any salinity changes and the idea of smolting as evidence of maladaptation of the fish to fresh water.