Standardizing the determination and interpretation of Pcrit in fishes

The critical oxygen tension (Pcrit) for fishes is the oxygen level below which oxygen consumption (MO2) becomes dependent upon ambient oxygen partial pressure (PO2). We compare multiple curve-fitting approaches to estimate Pcrit of the Gulf killifish, Fundulus grandis Baird & Girard, 1853, during closed and intermittent-flow respirometry. Fitting two line segments of MO2 versus PO2 produced high and variable estimates of Pcrit, as did nonlinear regression using a hyperbolic (Michaelis-Menton) function. Using nonlinear regression fit to an exponential (modified Weibull) function, or linear regression of MO2 versus PO2 at low PO2, and determining Pcrit as the PO2 when MO2 equals standard metabolic rate (SMR) yielded values that were consistent across fish and among experimental trials. The magnitude of the difference in Pcrit 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 Pcrit.