The utility and determination of P crit in fishes.

The critical O2 tension (P crit) is the lowest P O2  at which an animal can maintain some benchmark rate of O2 uptake (Ṁ O2 ). This P O2  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 P crit 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 P crit determination: (1) measuring the most appropriate benchmark Ṁ O2  state for P crit determination (Ṁ O2,std, the Ṁ O2  required to support standard metabolic rate); (2) reducing water P O2 ; and (3) calculating P crit from the Ṁ O2  versus P O2  curve. We make suggestions on best practices for each step and for how to report P crit results to maximize their comparative value. We also discuss the concept of hypoxia tolerance and how P crit relates to a fish's overall hypoxia tolerance. When appropriate methods are used, P crit 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), P crit contributes to a comprehensive understanding of how a fish combines aerobic metabolism, anaerobic metabolism and metabolic depression in an overall strategy for hypoxia tolerance.