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 O2 ) becomes dependent upon ambient oxygen partial pressure (P O2 ). 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 O2  versus P O2  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 O2 versus P O2  at low P O2 , and determining P crit as the P O2  when Ṁ O2 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.