Goldfish (Carassius auratus) were exercised continuously for periods of 28 days at swimming speeds of 1.5, 3.0 and 4.5 body lengths per second and their rates of growth were determined. Changes in muscle fibre size were examined, as were changes which occurred in the concentrations of the major chemical constituents of these cells. These fish, typical of the carp family in that they are found only in still or slowly moving water, did not adapt well to the flowing water environment at any swimming speed. They often grew less than the controls, although consuming much more food. Changes in the composition of the muscle fibres indicated that excess food was not being stored, and also indicated that the major fuel for swimming at all speeds was glycogen. The fish survived well at high speeds and it was suggested that this was due to the ability of the species to metabolize glycogen anaerobically without the production of lactic acid.

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