Fish are propelled through the water by fins, body movement, or both. In general, the main moving force is the caudal fin, or tail, and the area immediately adjacent to it, known as the caudal peduncle. In swimming, the fins are put into action by muscles attached to the base of the fin spines and rays. Fish with a fairly rigid body, such as the tilefish, the trunkfish, the triggerfish, the manta, and skates, depend mostly on fin action for propulsion. Eels, in contrast, rely on extreme, serpentlike body undulations to swim, with fin movement assisting to a minor extent. Sailfish, marlin, and other big-game fish fold their fins into grooves (lessening water resistance) and rely mainly on their large, rigid tails to go forward. Trout, salmon, catfish, and others are well adapted for sudden turns and short, fast moves. When water is expelled suddenly over the gills in breathing, it acts like a jet stream and aids in a fast start forward.
A fish can swim even if its fins are removed, though it generally has difficulty with direction and balance. In some kinds, however, the fins are highly important in swimming. For example, the pectoral fins of a ray are broad "wings" with which the fish sweeps through the water almost as gracefully as a swallow does in the air. The sharks, which are close relatives of the rays, swim swiftly in a straight line but have great difficulty in stopping or turning because their fins have restricted movement.
Flyingfish glide above the surface of the water with their winglike pectoral fins extended. Sometimes they get additional power surges by dipping their tails into the water and vibrating them vigorously. This may enable them to remain airborne for as long as a quarter of a mile. Needlefish and halfbeaks skitter over the surface for long distances, the front halves of their bodies held stiffly out of the water while their still-submerged tails wag rapidly. An African catfish often swims upside down, and seahorses swim in a "standing up" position.
Many kinds of fish jump regularly. Those that take to the air when hooked give anglers the greatest thrills. Often, however, there is no easy explanation for why a fish jumps, other than the possibility that it derives pleasure from these momentary escapes from its watery world. The jump is made to
Caudal (Tail) Fin Types
Caudal (Tail) Fin Types
dislodge a hook or to escape a predator in close pursuit, or the fish may try to shake its body free of plaguing parasites. Some species make their jumps by surging at high speed from deep water. Others swim rapidly close to the surface, then suddenly turn their noses skyward and give a powerful thrust with their tails as they take to the air. Sailfish and other high jumpers may leap several feet into the air when hooked, often leaping as many as 20 times before being brought to the boat.
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