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 estimatePcritof the Gulf killifish,Fundulus grandis, during closed and intermittent-flow respirometry. The traditional approach fitting two line segments ofMO2versusPO2produced high and variable estimates ofPcrit. Nonlinear regression using hyperbolic or Weibull functions resulted in either variablePcritestimates or, in some cases, failed to converge upon meaningful solutions.Pcritdetermined as thePO2whenMO2equals standard metabolic rate (SMR) based upon a linear relationship ofMO2andPO2at lowPO2were consistent across fish and experimental trials. Therefore, we recommend that Pcrit specifically refer to thePO2below which SMR cannot be maintained. Its determination, therefore, requires accurate measurement of SMR.