"It takes at least three fish to form a school. When formed, its members match and maintain speed and direction, and keep a preferred minimum distance from each other. They set that distance using their visual and pressure (lateral line) sense about equally. When schooling casually, the distance between fishes increases. They move slowly and meander. When a predator approaches the school aligns, tightens up, and usually increases swimming speed.
Half the 40,000 or so known species of fish, mostly small, school at least part of their lives. The survival value of schooling is at least two-fold. One: the probability of detection of groups is less, on a "per fish" basis, than that of scattered individuals. Two: the sensory confusion presented to the predator by the movements of the school far exceeds that of an individual fish.
Two Primary schooling tactics are used to evade a predator.
If the predator approaches slowly the school will open and form a surrounding pocket. The fish in the lead will swim outward and then rearward behind the predator which will quickly find the school entirely behind it. If the predator reverses direction, the school also turns and repeats the process.
If the predator strikes quickly, the school expands explosively outward from the intended point of impact, each fish accelerates to a speed of 10-20 body lengths per second in less than 1/50th of a seconc. It's all done by a single flick of the tail.
For more on schooling, read Brian Partridge's article "The Structure and Function of Fish Schools" in the June 1982 issue of "Scientific American".
- "The American Littoral Society Marine Naturalist"
by David K. Bulloch