page_title: Hypoxia in fish
section_title: Hypoxia tolerance
hierarchical_section_title: Hypoxia in fish / Hypoxia tolerance
caption_attribution_description: English: Oxygen consumption rate decreasses with decreasing environmental oxygen tension(PO2) when PO2&lt;Pcrit. On the other hand, oxygen consumption rate is unaffected by the changes in oxygen tension when PO2&gt;Pcrit.
context_page_description: 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. 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 releasing the chemical energy of O₂ through the mitochondrial 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.
context_section_description: A fish's hypoxia tolerance can be represented in different ways. A commonly used representation is the critical O₂ tension (Pcrit), which is the lowest water O₂ tension (PO₂) at which a fish can maintain a stable O₂ consumption rate (MO₂). A fish with a lower Pcrit is therefore thought to be more hypoxia-tolerant than a fish with a higher Pcrit. But while Pcrit is often used to represent hypoxia tolerance, it more accurately represents the ability to take up environmental O₂  at hypoxic PO₂s and does not incorporate the significant contributions of anaerobic glycolysis and metabolic suppression to hypoxia tolerance (see below). Pcrit is nevertheless closely tied to a fish's hypoxia tolerance, in part because some fish prioritize their use of aerobic metabolism over anaerobic metabolism and metabolic suppression. It therefore remains a widely used hypoxia tolerance metric.
  A fish's hypoxia tolerance can also be represented as the amount of time it can spend at a particular hypoxic PO₂ before it loses dorsal-ventral equilibrium (called time-to-LOE), or the PO₂ at which it loses equilibrium when PO₂ is decreased from normoxia to anoxia at some set rate (called PO₂-of-LOE). A higher time-to-LOE value or a lower PO₂-of-LOE value therefore imply enhanced hypoxia tolerances. In either case, LOE is a more holistic representation of overall hypoxia tolerance because it incorporates all contributors to hypoxia tolerance, including aerobic metabolism, anaerobic metabolism and metabolic suppression.
caption: Oxygen consumption rate decreases with decreasing environmental oxygen tension (PO2) when PO2< Pcrit. On the other hand, oxygen consumption rate is unaffected by the changes in oxygen tension when PO2 > Pcrit
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