Aerobic scope falls to nil at and anaerobic ATP production increases below in the tidepool sculpin, .

The critical oxygen tension of whole-animal oxygen uptake rate, or , has historically been defined as the oxygen partial pressure () at which aerobic scope falls to zero and further declines in require substrate-level phosphorylation to meet shortfalls in aerobic ATP production, thereby time-limiting survival. Despite the inclusion of aerobic scope and anaerobic ATP production in the definition, little effort has been made to verify that measurements, the vast majority of which are obtained using respirometry in resting animals, actually reflect the predictions of zero aerobic scope and a transition to increasing reliance on anaerobic ATP production. To test these predictions, we compared aerobic scope and levels of whole-body lactate at oxygen partial pressures (s) bracketing obtained in resting fish during progressive hypoxia in the tidepool sculpin, . We found that aerobic scope falls to zero at and, in resting fish exposed to s < , whole-body lactate accumulated pointing to an increased reliance on anaerobic ATP production. These results support the interpretation of as a key oxygen threshold at which aerobic scope falls to nil and, below , survival is time-limited based on anaerobic metabolic capacity.