The utility and determination of P_crit in fishes

The critical O₂ tension (P_crit) is the lowest P_O  at which an animal can maintain some benchmark rate of O₂ uptake (Ṁ_O ). This P_O  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_O  state for P_crit determination (Ṁ_O, the Ṁ_O  required to support standard metabolic rate); (2) reducing water P_O ; and (3) calculating P_crit from the Ṁ_O  versus P_O  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.