Take my breath away: Defining the oxygen saturation threshold for fish cortisol stress response.

Human activities are affecting the number, size and severity of hypoxic zones in aquatic ecosystems. As oxygen is one of the most important abiotic factors affecting fish biology, exposure to low levels of dissolved oxygen may lead to a reduction in fitness. In response to environmental changes that fish perceive as threatening, such as a decrease in dissolved oxygen (DO) concentration, fish initiate a stress response to cope with these changes through cortisol release into blood circulation. However, the DO concentration threshold triggering fish cortisol stress response remains unresolved. In the present study, we conducted a phylogenetic meta-analysis to (1) determine if it is possible to define an oxygen saturation threshold that triggers a sharp increase in cortisol in fish and (2) evaluate the role of hypoxia duration on measured blood cortisol levels. Our meta-analysis reveals a sharp blood cortisol increase at 9.3%-24% of air saturation (according to minimum and maximum threshold values predicted by different models used), which is generally considered as severe hypoxia. Also, we found that when fish were exposed for a short time to hypoxia, cortisol increased at low levels of DO, but this tended to revert when fish were exposed to prolonged hypoxia, suggesting habituation to sub-lethal low oxygen levels. While these findings define an overall critical threshold for hypoxia-induced stress in fishes, it is important to note that our study did not identify any individual or ecological traits influencing this threshold among the traits tested. Therefore, such a general threshold should be refined depending on species-specific characteristics and environmental factors.