Increased winter temperature did not affect completion of smolting in Atlantic salmon

Four groups of 1+ year-old Atlantic salmon, Salmo salar, pre-smolts were reared under various temperature regimes: constant 10 °C from November onwards; ambient temperature until either 15 December or 1 February, then 10 °C; or ambient temperature throughout (control; 2–3 °C November–March). From 20 May onwards, temperature in all groups was ambient, increasing from 10 °C to 17 °C in late July. Rearing temperature had no significant effect on either the timing of completion of smolting, or the duration of the ‘smolt-window’. Mean gill Na⁺-K⁺ ATPase activity was not significantly affected by temperature regime; it increased in all groups from < 2.0 μmol Pi mg protein⁻¹ h⁻¹ (units) in January to 5–7 units in mid-April, then back to < 2.0 units in July. Survival in 96 h, 37‰) salinity (S) tolerance tests was similar in all groups, increasing from < 10% in early March, to > 90% from mid-April to mid-June, then decreasing to < 20% by early July. Increased winter temperature significantly increased growth and condition factor compared with the control, but during April–May all four groups exhibited similar temporary decreases in condition factor in association with the completion of smolting.