Standardizing the determination and interpretation of P_crit in fishes

The critical oxygen tension (P_crit) for fishes is the oxygen level below which the rate of oxygen consumption (Ṁ_O ) becomes dependent upon ambient oxygen partial pressure (P_O ). We compare multiple curve-fitting approaches to estimate P_crit of the Gulf killifish, Fundulus grandis, during closed and intermittent-flow respirometry. Fitting two line segments of Ṁ_O  versus P_O  produced high and variable estimates of P_crit, as did nonlinear regression using a hyperbolic (Michaelis-Menten) function. Using nonlinear regression fit to an exponential (modified Weibull) function, or linear regression of Ṁ_O versus P_O  at low P_O , and determining P_crit as the P_O  when Ṁ_O equals standard metabolic rate (SMR) yielded values that were consistent across fish and among experimental trials. The magnitude of the difference in P_crit determined by alternative calculation methods exceeded the differences determined in closed and intermittent-flow respirometry, highlighting the need to standardize analytical as well as experimental approaches in determining P_crit.