Swimming for long periods needs oxygen for the red fibres. The oxygen supply has to be constant because these fibres only operate aerobically. They are red because they have a rich blood supply, and they contain myoglobin. Myoglobin transports the oxygen to the oxidising systems. Red muscle gets its energy by oxidising fat, which weight for weight has twice as much energy as carbohydrate or protein. Using their red fibres, fish can keep up a speed of 35 lengths per second for long periods.