Anthropogenic activities are greatly altering the habitats of animals,

whereby fish are already encountering several stressors

simultaneously. The purpose of the current study was to investigate

the capacity of fish to respond to two different environmental stressors

(high temperature and overnight hypoxia) separately and together.

We found that acclimation to increased temperature (from 7.7±0.02°C

to 14.9±0.05°C) and overnight hypoxia (daily changes from normoxia

to 63–67% oxygen saturation), simulating climate change and

eutrophication, had both antagonistic and synergistic effects on the

capacity of fish to tolerate these stressors. The thermal tolerance of

Arctic char (Salvelinus alpinus) and landlocked salmon (Salmo salar

m. sebago) increased with warm acclimation by 1.3 and 2.2°C,

respectively, but decreased when warm temperature was combined

with overnight hypoxia (by 0.2 and 0.4°C, respectively). In contrast,

the combination of the stressors more than doubled hypoxia tolerance

in salmon and also increased hypoxia tolerance in char by 22%.

Salmon had 1.2°C higher thermal tolerance than char, but char

tolerated much lower oxygen levels than salmon at a given

temperature. The changes in hypoxia tolerance were connected to

the responses of the oxygen supply and delivery system. The relative

ventricle mass was higher in cold- than in warm-acclimated salmon

but the thickness of the compact layer of the ventricle increased with

the combination of warm and hypoxia acclimation in both species.

Char had also significantly larger hearts and thicker compact layers

than salmon. The results illustrate that while fish can have protective

responses when encountering a single environmental stressor, the

combination of stressors can have unexpected species-specific

effects that will influence their survival capacity.