- generally defined as the  below which the animal can no longer maintain a stable rate of O consumption ( ), such that  becomes dependent upon  - provides a single number into which a vast amount of experimental effort has been invested. Here, with specific reference to water-breathers, I argue that this focus on the is not useful for six reasons: (1) calculation of usually involves selective data editing; (2) the value of depends greatly on the way it is determined; (3) there is no good theoretical justification for the concept; (4) is not the transition point from aerobic to anaerobic metabolism, and it disguises what is really going on; (5) is not a reliable index of hypoxia tolerance; and (6) carries minimal information content. Preferable alternatives are loss of equilibrium (LOE) tests for hypoxia tolerance, and experimental description of full  versus  profiles accompanied by measurements of ventilation, lactate appearance and metabolic rate by calorimetry. If the goal is to assess the ability of the animal to regulate  from this profile in a mathematical fashion, promising, more informative alternatives to are the regulation index and Michaelis-Menten or sigmoidal allosteric analyses.