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Swimming like a fish
"He swims like a fish!" It's an expression you might hear someone say while watching the people at a swimming pool or a beach. It’s a poor comparison. People swim very differently than fish. If a person could swim like a fish, logically, a fish could swim like a person and that just doesn’t happen.
How do fish swim at all and do they all swim alike? Basically, they undulate their bodies through the water in a snakelike motion. The undulations pass through the fish's muscles in waves and end with a brisk tail snap. Water is a much denser substance than air, and therefore much more difficult to move through. Fish have to be “hydrodynamically” streamlined (think aerodynamic - only underwater) in order to travel efficiently.
In accordance with Newton's Third Law, for every action there is an equal and opposite reaction, when a fish swims it’s body and fins pushes on the water (action) and the fish itself is moves forward as the water is pushed (reaction.) The rate at which these undulations pass through a fish's muscles and the power of the final flip of the tail have a direct relationship to a fish's speed.
For some species efficiency means speed; while for others, maneuverability and turning ability are critical. You can tell a lot about how a fish moves—and how it makes a living—simply by the shape of its body.
A long, skinny fish such as a pike or barracuda derives most of its forward motion from the muscle wave. Part of this is due to the fact that such a long fish can generate more waves in its body than a stubbier and thicker fish. Also note that a pike’s fins have a relatively small surface area compared with the length of the body, and the small tail fin provides a correspondingly smaller amount of forward motion during the tail snap.
On the other hand, a stockier species like the largemouth bass or grouper gets most of its speed from the snap of its relatively large tail, but not as much forward thrust from the muscle wave of its stubbier body.
What can be learned about how a fish lives by how it swims? Let's compare the aforementioned pike and bass. The pike’s long streamlined body is designed for speed. Its small fins tend to be farther back on its body, increasing its hydrodynamic efficiency and allowing it to "knife" through the water more easily. This is perfect for a pike-like fish which must be able to chase down and capture its prey. Pike almost exclusively feed on other fish as opposed to slower foods such as crayfish.
The largemouth bass, on the other hand, stockier in build and only able to achieve high speeds briefly and over short distances, is an ambush feeder - surprising and capturing its prey from relatively short range. The comparatively larger size of its fins, as well as their placement closer to the center of the body indicates for bass, maneuverability is more important than velocity. Instead of out-swimming prey over a distance, the bass is better able to turn and maneuver sharply enough to capture its food almost immediately once the bass gets close to it - or the prey gets close to the bass.
Bass are more likely to be a generalist predator, eating a broader range of prey species than the pike. They are certainly able to capture fish when the opportunity arises, but also regularly consume such prey as crayfish and frogs.
How fast can fish swim? Bass have been measured traveling at about 12 miles per hour and salmon slightly faster at 14. The real speed records, however, all go to saltwater fish. The barracuda - very similar in shape and feeding strategy to the pike - can move at about 28 mph. By far the fastest fish are the open ocean species. Bonitos and marlin have been estimated to reach 40 mph, while speeds of up to 60 mph have been attributed to swordfish and tunas.
An Olympic swimmer tops out about 6 mph. Even Michael Phelps can’t swim like a fish.