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

• This study provides aquaculturists with a limit for acceptable drops in O2 (hypoxia).
• The limiting O2 saturation (LOS) is defined as the threshold for hypoxia tolerance.
• LOS increases exponentially (30–55% O2) with temperature (6–18 °C).
• Acclimation to frequent, moderate reductions in O2 does not affect the LOS.
• MO2 is a reliable estimator of LOS, regardless of temperature and exp. conditions.

Atlantic salmon (Salmo salar L.) experience periodic drops in dissolved oxygen (O2) inside aquaculture sea cages. In order to evaluate whether fish function or welfare is compromised during such drops in O2 (termed hypoxia), it is necessary to establish the limit for acceptable O2 reductions; the hypoxia tolerance threshold. In the present study, effects of temperature (6, 12 and 18 °C) and hypoxia acclimation (33 days of hypoxic periods down to 50% O2 occurring every 6 h at 16 °C) on the routine oxygen consumption rate (ṀO2) 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. ṀO2 was measured using open respirometry where a progressive decline in O2 was caused by fish O2 consumption during a period of low water turnover. LOS was defined as the O2 below which fish were no longer able to uphold routine ṀO2. Both ṀO2 and LOS were found to increase exponentially with temperature (Q10 = 2.7 for ṀO2 and 1.8 for LOS), but no effect of hypoxia acclimation was found. The mean (± SE) LOS at 6, 12, 16 and 18 °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 ṀO2 (R2 = 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 O2 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.