When a fish swims leisurely through the water, it propels itself chiefly with its back and tail muscles. These muscles consist mostly of slow-contraction muscle fiber because the work is performed with relatively slow movements over long durations. The large amount of myoglobin in the tissue around these slow-contraction fibers turns the flesh brownish red. The muscles in the other areas of a fish's anatomy are typically lighter-hued (usually off-white) because they predominantly consist of fast-contraction fibers, which don't require the presence of the oxygen-rich myoglobin. These pale-hued muscles are generally reserved for quick-movement activities, such as chasing a prey or escaping a predator.
