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How do fish use body waves and tail movements to swim forward?

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Least tern, Sternula antillarum, flying with wings raised and its yellow-tipped black bill pointed forward. — Locomotion These birds largely reside in coastal areas of North America, although some populations live in the continent's interior along large rivers.

Fish achieve forward movement through undulatory swimming, a process roughly analogous to using an oar at the stern of a boat to push water backward. This locomotion is driven by the rhythmic, side-to-side contraction of axial muscles along the body and the base of the tail.123

Mechanics of Body Waves

In many species, these contractions create sinuous waves that travel from the head toward the tail. As these waves pass backward, each segment of the fish's body oscillates laterally across the axis of movement. While the entire body contributes to thrust, the amplitude and speed of these waves increase as they move toward the rear, resulting in the greatest propulsive force being generated in the caudal (tail) region.2

The Role of the Tail

For many fishes, the undulating flexures of the body culminate in a powerful back-and-forth sweeping of the caudal fin, which can produce up to 85 percent of the total thrust required for swimming. The orientation and speed of each body segment as it crosses the locomotor axis are critical, as these factors determine the efficiency and magnitude of the thrust produced.32