Two species of fish, the brown trout (Salmo trutta), and the goldfish (Carassius auratus), were exercised for 3 hours at various swimming speeds and the usage of glycogen and lipid stores was examined. In both species, the energy used increased with increasing swimming speed. Red muscle was active at the lowest speeds and all muscles were active at higher speeds. Pink muscle became active at ‘intermediate’ speeds.Salmorelied on lipid breakdown at all swimming speeds, mainly by breakdown in red muscle.Carassiusrelied on the metabolism of glycogen for energy.