Fish Schooling

By Prentice K. Stout

Schooling of fish has very little to do with their education. It does have much to do with their ability to survive and reproduce in sufficient numbers. Schools are composed of many fish of the same species moving in more or less harmonious patterns throughout the oceans. A very prevalent behavior, schooling is exhibited by almost 80 percent of the more than 20,000 known fish species during some phase of their life cycle. Many of the world's fishing industries rely on this behavior pattern to increase their catch size, especially for species such as cod, tuna, mackerel, and menhaden.

Aristotle, over 2,400 years ago, observed this behavior in fish. Perhaps he sparked the interest humans have had in this fascinating trait of certain fishes.

Why school? For one, there are ecological advantages. Some species of fish secrete a "slime" that helps to reduce the friction of water over their bodies. Also the fish swim in fairly precise, staggered patterns when traveling in schools, and the "to-and-fro" motion of their tails produces tiny currents called "vortices" (swirling motions similar to little whirlpools). Each individual, in theory, can use the tiny whirlpool of its neighbor to assist in reducing the water's friction on its own body.

Another advantage is the safety factor against predators. A potential predator breeding hunting for a meal might become confused by the closely spaced school, which can give the impression of one vast and frightening fish. Additionally, there is the concept of "safety in numbers"—a predator cannot consume and unlimited quantity of prey. The sheer number of fish in a school allows species to hide behind each other, thus confusing a predator by the alteration of shapes and colors presented as the school swims along. Of course, those on the outside edges of the school are more likely to be eaten than those in the center. Predatory fish also gain from schooling because it gives them the ability to travel in large numbers in search of food. Bluefish (Pomatomus saltatrix) in pursuit of menhaden are a good case in point.

Schooling fish respond quickly to changes in the direction and speed of their neighbors. Anyone who has swum in a school of fish can attest to their ability to change direction swiftly while still retaining their closely knit swimming pattern. They can move from one configuration to another and then regroup almost as one unit.

When young, most fish species do not exhibit the schooling pattern. As they mature, they begin to swim in pairs and then in larger and larger clusters until they attain the classic parallel pattern. Thus, schooling can be said to be a formed behavior pattern imprinted on the genetic material. Research leads us to believe that as the sense organs of the young mature, their schooling behavior strengthens. The first sense used is that of sight, which begins to function immediately after birth to allow for feeding. Fish eyes cannot focus directly forward because they are located on the sides of the head. This placement does, however, permit the eyes to be especially sensitive to lateral movement—a very helpful attribute in schooling. The fish can see what other members of the school are doing in relationship to themselves and respond accordingly.

Of interest is the acoustico-lateralis, the much-studied lateral line system on the sides of some fish. This is a line of special neuromast cells that runs down either side of a fish body. The scientific name for these lines gives us a clue to their function: "acoustico" means sound, and sound waves produce pressures; and "lateralis" alludes to the sides of the fish's body. These two lateral lines are highly sensitive to movements and the displacement of water as the fish swims close to its neighbor. They aid in keeping the fish in a neat, orderly pattern. Some fish do not have lateral lines, nor the sensitive cells, and thus rely on their eyesight. Research suggests that if fish are blinded and their lateral lines cut, schooling does not take place; but if the lateral lines are left in place, the fish are still able to school. The lateral lines are especially important to fish living in the highly murky waters of the estuarine environment where sight is not particularly useful. The silver strip of the Atlantic silversides (Menidia menidia) affords us a good representation of the lateral lines of a typical species.

We are now ready to attempt a definition of schooling—no easy thing to arrive at. An accepted version could be a "grouping of fish based on mutual attraction and exhibiting a geometrical relationship." We say "mutual attraction," for fish of different species are almost never found intermingled. Fish stay with their own kind in a schooling configuration.

Schooling has some other interesting aspects. In the spring along the New England coast the alewife (Pomolobus pseudoharengus), in response to an ancient biological urge to reproduce, begins to form in large schools. They begin just where the rivers pour into estuaries and then, seemingly without a central signal, they migrate up the rushing currents. So large are their numbers that the bottom of the stream cannot be seen, and the whole picture is one of wriggling, bluish bodies swimming against the current. Alewives are "anadromous" fish that, much like salmon and shad, mature in salt water but spawn in fresh water. (Eels, on the other hand, are "catadromous," meaning just the reverse—they grow in fresh water and spawn in the sea.) the shad (Alosa sapidissima), first cousin to the alewife, also schools in large numbers in the spring. They are the source of the highly prized "shad roe," the millions of eggs that will not reach maturity because they are frying in our breakfast pans.

In many ways fish schools are much like herds of land animals or flocks of airborne birds. There is that undefined need to stay together. In some instances this herding has been the undoing of certain species. The now-extinct passenger pigeon flocked in such staggering numbers that it was rather a simple task in the predawn hours to take a club and sweep a branch of roosting birds into a sack for future eating. There are stories of the sky being darkened by the passage of these relatives of today's mourning doves. For centuries, wildebeest and antelope have formed huge herds that have crossed the endless African plains in search of greener pastures or to migrate to their ancestral breeding grounds. Indeed, if one looks at the huge cities of today's society one wonders if we humans are not prone to schooling. We live and move in vast numbers controlled by the technology of our society.