Fish are exposed to large oxygen fluctuations in their aquatic environment since the inherent properties of water can result in marked spatial and temporal differences in the concentration of oxygen (see
oxygenation and
underwater). Fish respond to
hypoxia with varied behavioral, physiological, and cellular responses in order to maintain
homeostasis and organism function in an oxygen-depleted environment. The biggest challenge fish face when exposed to low oxygen conditions is maintaining metabolic energy balance, as 95% of the oxygen consumed by fish is used for
ATP production through the
electron transport chain. Therefore, hypoxia survival requires a coordinated response to secure more oxygen from the depleted environment and counteract the metabolic consequences of decreased ATP production at the mitochondria. This article is a review of the effects of hypoxia on all aspects of fish, ranging from behavior down to genes.