Standardizing the determination and interpretation of Pcrit in fishes

For most fishes, there is an oxygen level, the critical oxygen tension (Pcrit), 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, during closed and intermittent-flow respirometry. The traditional approach fitting two line segments of MO2 versus PO2 produced high and variable estimates of Pcrit. Nonlinear regression using hyperbolic or Weibull functions resulted in either variable Pcrit estimates or, in some cases, failed to converge upon meaningful solutions. Pcrit determined as the PO2 when MO2 equals standard metabolic rate (SMR) based upon a linear relationship of MO2 and PO2 at low PO2 were consistent across fish and experimental trials. Therefore, we recommend that Pcrit specifically refer to the PO2 below which SMR cannot be maintained. Its determination, therefore, requires accurate measurement of SMR.