ABSTRACT
This study examined the effect of dissolved oxygen (DO) level on critical thermal maximum (CTMax) in diploid and triploid brook charr (Salvelinus fontinalis) exposed to a temperature increase of 3°C/h. Because gas solubility is inversely proportional to temperature, DO declines during standard CTMax tests. With this treatment as a baseline, oxygen or nitrogen injection was used to provide three other DO conditions during CTMax tests: two hyperoxic (maintenance at initial 10mg/L and increase from 10mg/L at 2mg/L/h) and one hypoxic (decrease from 10mg/L at 2mg/L/h). Hyperoxia had no effect on temperature at CTMax or time taken to reach CTMax. Hypoxia, on the other hand, resulted in a significantly lower CTMax and shorter time to CTMax than under standard or hyperoxic conditions, with both indices affected by triploidy but not in a consistent fashion: in one experiment triploids had a lower CTMax and shorter time to CTMax than diploids and in a second experiment they had a higher CTMax and longer time to CTMax than diploids. Indices of the secondary stress response (plasma glucose and ions) during CTMax tests under hypoxia responded as would be predicted for an acute stress, with no difference between triploids and diploids.