Estimating maximum oxygen uptake of fishes during swimming and following exhaustive chase - different results, biological bases and applications.

The maximum rate at which animals take up oxygen from their environment (MO2,max) is a crucial aspect of their physiology and ecology. In fishes, MO2,max is commonly quantified by measuring oxygen uptake either during incremental swimming tests or during recovery from an exhaustive chase. In this Commentary, we compile recent studies that apply both techniques to the same fish and show that the two methods typically yield different mean estimates of MO2,max for a group of individuals. Furthermore, within a group of fish, estimates of MO2,max determined during swimming are poorly correlated with estimates determined during recovery from chasing (i.e. an individual's MO2,max is not repeatable across methods). One explanation for the lack of agreement is that these methods measure different physiological states, each with their own behavioural, anatomical and biochemical determinants. We propose that these methods are not directly interchangeable but, rather, each is suited to address different questions in fish biology. We suggest that researchers select the method that reflects the biological contexts of their study, and we advocate for the use of accurate terminology that acknowledges the technique used to elevate MO2 (e.g. peak MO2,swim or peak MO2,recovery). If the study's objective is to estimate the 'true' MO2,max of an individual or species, we recommend that pilot studies compare methods, preferably using repeated-measures designs. We hope that these recommendations contribute new insights into the causes and consequences of variation in MO2,max within and among fish species.

Method dependency of maximum oxygen uptake rate and its repeatability in the Gulf killifish, Fundulus grandis.

The maximum rate at which fish can take up oxygen from their environment to fuel aerobic metabolism is an important feature of their physiology and ecology. Methods to quantify maximum oxygen uptake rate (MO2), therefore, should reliably and reproducibly estimate the highest possible MO2 by an individual or species under a given set of conditions (peak MO2). This study determined peak MO2 and its repeatability in Gulf killifish, Fundulus grandis, subjected to three methods to elevate metabolism: swimming at increasing water speeds, during recovery after an exhaustive chase, and after ingestion of a large meal. Estimates of peak MO2 during swimming and after an exhaustive chase were repeatable across two trials, whereas peak MO2 after feeding was not. Peak MO2 determined by the three methods was significantly different from one another, being highest during swimming, lowest after an exhaustive chase, and intermediate after feeding. In addition, peak MO2 during recovery from an exhaustive chase depended on the length of time of recovery: in nearly 60% of the trials, values within the first hour of the chase were lower than those measured later. A novel and important finding was that an individual's peak MO2 was not repeatable when compared across methods. Therefore, the peak MO2 estimated for a group of fish, as well as the ranking of individual MO2 within that group, depends on the method used to elevate aerobic metabolism.