The scaling of anaerobic metabolism and red muscle mass was examined in rainbow trout (Salmo gairdneri) ranging in size from 2 to 1200 g. The initial rate of white muscle lactate production during maximal burst activity was significantly higher in large (28.1 cm) than in small (8.0 cm) fish. ‘Resting’ lactate concentrations in anesthetized trout (approximately 30 s of stress) increased with fish size, also reflecting higher glycolytic potential for larger fish. Maximum muscle lactate concentrations following 6min of exhaustive exercise increased from approximately 25 to 45μmolg−1 with increased fish size (= L0.36, where L is fish length). Total white muscle lactate production, including changes in muscle mass, scaled as L3.52. A scaling comparison of total anaerobic capacity with theoretically predicted power requirements indicated decreased burst swimming performance with increased size. Red muscle mass increased from approximately 1 to 3 % of body mass with increased fish size. The positive allometry in red muscle mass (= L3.62) is greater than the scaling of power requirements during aerobic swimming predicted from hydrodynamic theory, and may provide compensation for decreased mass-specific power output with increased size.

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