Tradeoffs between hypoxia tolerance and aerobic exercise performance appear to exist in some fish taxa, even though both of these traits are often associated with a high O 2 transport capacity. We examined the physiological basis for this potential tradeoff in four species of sunfish from the family Centrarchidae. Hypoxia tolerance was greatest in rock bass, intermediate in pumpkinseed and bluegill and lowest in largemouth bass, based on measurements of critical O 2 tension (P crit ) and O 2 tension at loss of equilibrium (P O2 at LOE). Consistent with there being a tradeoff between hypoxia tolerance and aerobic exercise capacity, the least hypoxia-tolerant species had the highest critical swimming speed (U crit ) during normoxia and suffered the greatest decrease in U crit in hypoxia. There was also a positive correlation between U crit in normoxia and P O2 at LOE, which remained significant after accounting for phylogeny using phylogenetically independent contrasts. Several sub-organismal traits appeared to contribute to both hypoxia tolerance and aerobic exercise capacity (reflected by traits that were highest in both rock bass and largemouth bass), such as the gas-exchange surface area of the gills, the pH sensitivity of haemoglobin-O 2 affinity, and the activities of lactate dehydrogenase and the gluconeogenic enzyme phosphoenolpyruvate carboxykinase in the liver. Some other suborganismal traits were uniquely associated with either hypoxia tolerance (low sensitivity of haemoglobin-O 2 affinity to organic phosphates, high pyruvate kinase and lactate dehydrogenase activities in the heart) or aerobic exercise capacity (capillarity and fibre size of the axial swimming muscle). Therefore, the cumulative influence of a variety of respiratory and metabolic traits can result in physiological tradeoffs associated with the evolution of hypoxia tolerance and aerobic exercise performance in fish.

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There is a frequently observed tradeoff between hypoxia tolerance and exercise performance in fish, even though both of these traits are often associated with a high O2 transport capacity. We examined the physiological basis for this tradeoff in four species of bass and sunfish from the family Centrarchidae. Hypoxia tolerance was greatest in rock bass, intermediate in pumpkinseed sunfish and bluegill sunfish, and lowest in largemouth bass, based on measurements of critical O2 tension (Pcrit) and the O2 tension at loss of equilibrium. The least hypoxia‐tolerant species had the highest critical swimming speed (Ucrit) and maximal O2 consumption rate during swimming, and suffered the greatest decrease in Ucrit during exercise in hypoxia, consistent with there being a tradeoff between hypoxia tolerance and exercise performance. Some traits appeared to contribute to both hypoxia tolerance and exercise performance, as reflected by the traits that were highest in both rock bass and largemouth bass, such as the gas‐exchange surface area of the gills and the enzyme activities of lactate dehydrogenase and the gluconeogenic enzyme phosphoenolpyruvate carboxykinase in the liver. Some other traits were uniquely associated with hypoxia tolerance (pyruvate kinase and lactate dehydrogenase activities in the heart) or exercise performance (capillarity and fibre size of the axial swimming muscle). Therefore, the cumulative influence of a variety of respiratory and metabolic traits likely underlies the physiological tradeoffs associated with the evolution of hypoxia tolerance and exercise performance in fish. (Supported by NSERC of Canada)