Low oxygen concentration events, known as hypoxic events, are naturally occurring but anticipated to become more frequent and severe in the future due to current global changes and anthropogenic pressures. While organisms may adjust their physiology and behaviour in response to hypoxia, better understanding of the link between physiology and behaviour is crucial for predicting the global response of an organism to environmental changes. Even if this link starts to be well known, there is a limited understanding of the genetic basis of it, as well as the potential impact of changing environments on it. Hypoxia, for instance, may modify the correlated evolution between physiology and behaviour by altering the genetic correlation among those traits, what could be important to predict. To address these questions, two populations of Gasterosteus aculeatus were collected from streams already exposed to strong hypoxic events or not. The collected individuals were bred, and half of the offspring were reared either in normoxia (>90% of O2 throughout the day) or hypoxia (>90% of O2 during the day and 30% of O2 during the night to mimic more natural conditions). The growth, swimming capacity, and behaviour (risk-taking and sociability) of the offspring were then measured, and the phenotypic and genetic correlations among the traits estimated. Unravelling the phenotypic and genetic correlations under fluctuating hypoxia can contribute to our understanding of the potential link and evolutionary paths of physiology and behavioural traits facing a changing environment, and the importance of considering that such link may be context dependent.