Bernard B. Rees

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 ( Ṁ O 2 ), therefore, should reliably and reproducibly estimate the highest possible Ṁ O 2 by an individual or species under a given set of conditions (peak Ṁ O 2 ). This study determined peak Ṁ O 2 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 Ṁ O 2 during swimming and after an exhaustive chase were repeatable across two trials, whereas peak Ṁ O 2 after feeding was not. Peak Ṁ O 2 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 Ṁ O 2 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 Ṁ O 2 was not repeatable when compared across methods. Therefore, the peak Ṁ O 2 estimated for a group of fish, as well as the ranking of individual Ṁ O 2 within that group, depends on the method used to elevate aerobic metabolism.