Hypoxia tolerance thresholds for post-smolt Atlantic salmon: Dependency of temperature and hypoxia acclimation

Atlantic salmon (Salmo salar L.) experience periodic drops in dissolved oxygen (O-2) inside aquaculture sea cages. In order to evaluate whether fish function or welfare is compromised during such drops in O-2 (termed hypoxia), it is necessary to establish the limit for acceptable O-2 reductions; the hypoxia tolerance threshold. In the present study, effects of temperature (6, 12 and 18 degrees C) and hypoxia acclimation (33 days of hypoxic periods down to 50% O-2 occurring every 6 h at 16 degrees C) on the routine oxygen consumption rate ((M) over dotO(2)) and the limiting oxygen saturation (LOS, defined as the hypoxia tolerance threshold) were investigated in undisturbed, fed fish kept in groups with the aim of resembling commercial aquaculture conditions. (M) over dotO(2) was measured using open respirometry where a progressive decline in O-2 was caused by fish O-2 consumption during a period of low water turnover. LOS was defined as the O-2 below which fish were no longer able to uphold routine (M) over dotO(2). Both (M) over dotO(2) and LOS were found to increase exponentially with temperature (Q(10) = 2.7 for (M) over dotO(2) and 1.8 for LOS), but no effect of hypoxia acclimation was found. The mean (+/- SE) LOS at 6, 12, 16 and 18 degrees C was 30 +/- 1, 39 +/- 1, 47 +/- 1 and 55 +/- 2% of air saturation respectively. The variation in LOS within and across temperatures was to a large extent explained by variation in (M) over dotO(2) (R-2 = 0.93), suggesting that LOS increases with any factor that raises the metabolic rate of Atlantic salmon. This study provides aquaculturists and legislators with a lower limit for acceptable drops in O-2 at temperatures relevant for the on-growing phase in Atlantic salmon aquaculture, and discusses the variation in LOS that can be expected at a given temperature. (C) 2013 Elsevier B. V. All rights reserved.