Standardizing the determination and interpretation of in fishes.

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