Effects of cyclic environmental hypoxia on physiology and feed intake of post-smolt Atlantic salmon: Initial responses and acclimation

In order to investigate the effects of hypoxic periods on the feeding behaviour and physiology of Atlantic salmon (Salmo salar L.) post-smolts (237 +/- 7 g), fish were kept at constant 90% O-2 (control) or subjected to cyclic reductions in oxygen, from 90% O-2 (termed normoxia) to 40, 50, 60 or 70% O-2 (termed hypoxia) for 2 h every 6 h for 23 days at 16 degrees C (groups are denoted as 90:40, 90:50, 90:60, 90:70 and 90:90). Fish were fed to satiation three times per day, twice in hypoxia and once in normoxia. Blood samples were drawn during hypoxic periods on day 0, 7, 14 and 21 and analysed for haematocrit, haemoglobin concentration and plasma cortisol, chloride, lactate and glucose concentrations. During first exposure to hypoxia (day 0), plasma lactate was increased in 90:60, 90:50 and 90:40 groups, plasma cortisol was increased in 90:50 and 90:40 groups and plasma chloride was increased in the 90:40 group, indicating oxygen shortage and stress at oxygen saturations below 60% O-2. On day 7, 14 and 21, there were no signs of stress, but plasma lactate levels remained increased in hypoxic periods in 90:40, 90:50 (all the days) and 90:60 groups (day 7 and 14), suggesting that the stress response was down-regulated in spite of a persisting oxygen shortage. Feed intake was reduced according to hypoxia severity in groups subjected to 40-60% O-2 during the first meal of the day, and in all experimental groups (40-70% O-2) during the third meal of the day. The effect of hypoxia on feed intake persisted throughout the experiment, but total daily feed intake was substantially increased in 90:40 and 90:50 groups during weeks 2 and 3 due to compensatory feeding in the normoxic feeding period. Present results suggest that 70% O-2 may represent a threshold for reduced growth and that 60% O-2 represents a minimum O-2 saturation considering the welfare of Atlantic salmon post-smolts subjected to cyclic hypoxia at 16 degrees C. (C) 2011 Elsevier B.V. All rights reserved.