Figure 2.

Plasma cortisol and cortisol release rate after fixed and cyclic, low DO levels at high temperature (Experiment 2). This experiment aimed at mimicking DO levels measured in sea cages in fjords sheltered from waves, wind and strong currents. In these situations, cyclic drops in DO levels are frequently observed during slack water at tidal reverse and further decreased DO levels are observed during high temperatures. Four oxygen treatment regimes were initiated: fixed 50% or 85% DO levels, or 50% or 85% DO levels in two different 6 hour cycles (4:2 h at 85:50%; 1:5 h at 85:50%) at 16°C. Blood was sampled for plasma cortisol measurements from all treatment groups day 10 and 29, and from the fixed 85% and 50% DO groups between days 36-38. Cortisol levels in plasma at different time points after start of Experiment 2, assessing the impact of cyclic and fixed DO levels. Plasma cortisol samples below the limit of detection was removed and indicated in each bar (A). DO levels were not affected by sampling occasion (p = 0.572) but slightly influenced by DO treatment levels (p = 0.088). An interaction was observed (p = 0.029) and Bonferroni corrected pair-wise comparison of plasma cortisol levels revealed elevated cortisol levels in the 1:5 h at 85:50% DO group and 50% DO group compared to the 85% DO group (p = 0.002 and p = 0.012 respectively) at the first sampling after 10 days. Thereafter no differences could be observed. Overall, the cortisol release rate (B) reflected the plasma cortisol levels as there was a tendency toward lower cortisol release rate in the 85% DO group (p = 0.1). Further, plasma cortisol levels on day 29 appeared to be reflected in the cortisol release rate on day 28. All data are expressed as means ± SEM and p < 0.05 was regarded as significant and indicated as *, p < 0.01 as ** and p < 0.001 indicated as ***.