Physiological Tradeoffs Underlie the Evolution of Hypoxia Tolerance and Exercise Performance in Fish

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 O 2 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 O 2 tension (P crit ) and the O 2 tension at loss of equilibrium. The least hypoxia‐tolerant species had the highest critical swimming speed (U crit ) and maximal O 2 consumption rate during swimming, and suffered the greatest decrease in U crit 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)