The utility and determination of in fishes.

The critical O tension () is the lowest  at which an animal can maintain some benchmark rate of O uptake ( ). This  has long served as a comparator of hypoxia tolerance in fishes and aquatic invertebrates, but its usefulness in this role, particularly when applied to fishes, has recently been questioned. We believe that remains a useful comparator of hypoxia tolerance provided it is determined using the proper methods and hypoxia tolerance is clearly defined. Here, we review the available methods for each of the three steps of determination: (1) measuring the most appropriate benchmark  state for determination (, the  required to support standard metabolic rate); (2) reducing water ; and (3) calculating from the  versus  curve. We make suggestions on best practices for each step and for how to report results to maximize their comparative value. We also discuss the concept of hypoxia tolerance and how relates to a fish's overall hypoxia tolerance. When appropriate methods are used, provides useful comparative physiological and ecological information about the aerobic contributions to a fish's hypoxic survival. When paired with other hypoxia-related physiological measurements (e.g. lactate accumulation, calorimetry-based measurements of metabolic depression, loss-of-equilibrium experiments), contributes to a comprehensive understanding of how a fish combines aerobic metabolism, anaerobic metabolism and metabolic depression in an overall strategy for hypoxia tolerance.