Fisheries Biology: How fish swim
"He swims like a fish!" It's an expression you might have heard during last year's Summer Olympics. But how, exactly, do fish swim? Water is a much denser substance than air, and therefore much more difficult to move through. Fish have to be hydrodynamically streamlined (think aerodynamically, only underwater) in order to travel efficiently. For some species speed is most important, while for others maneuverability and turning ability are critical. In fact, you can tell a lot about how a fish moves—and how it makes a living—simply by the shape of its body.
First, how do fish swim at all? 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.
Forward motion is provided by both the pushing of the fish's body against the water, and by the final tail snap. The water is actually pushed backward and—in accordance with Newton's Third Law that for every action there is an equal and opposite reaction—the fish itself is thrust forward as this water is pushed off the fish's body and tail. The rate at which these undulations pass through a fish's muscles have a direct relationship to a fish's speed. In some ways, this thrust is similar to that produced by the propeller on a boat motor.
A long, skinny fish such as a pickerel 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 pickerel's fins have a relatively small surface area compared with the length of the body, and the small caudal (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 gets most of its speed from the tail snap of its large caudal fin, but not as much forward thrust from the muscle wave of its stubbier body. As the illustration shows, a bass can't generate as many waves in its shorter and stiffer body as a long supple pickerel can. For nearly all fish, though, both are important in providing thrust.
What can we learn about how a fish lives by how it swims? Once again, let's compare the pickerel and the bass. The pickerel'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. The pickerel can therefore be assumed to be a high speed feeder, able to run down and capture its prey. You can also assume that if this is true, then the pickerel must feed on fast-moving prey organisms such as fish as opposed to slower foods such as crayfish. And this is exactly the case.
The largemouth bass, on the other hand, is stockier in build and probably only able to achieve high speeds briefly and over short distances. Thus, a largemouth 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, also indicate that for a bass maneuverability is more important than velocity. Rather than running down prey over distance, the bass should be able to turn and maneuver sharply enough to capture its food almost immediately once it gets close to it. In addition, rather than concentrating its diet strictly on fast-moving prey, the bass is more likely to be a generalist predator, eating a broader range of prey species than the pickerel (able to capture fish when the opportunity arises, but also consuming such prey as crayfish and frogs). Again, this proves to be the case.
How fast can fish swim? As one can imagine, clocking fish underwater is not easy! However, bass have been measured traveling at about 12 miles per hour (mph), 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 pickerel—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.
To contact the Florida Freshwater Angler, email John Cimbaro, john.cimbaro@myfwc.com. FFWCC
