BACK TO SCHOOL
Roy S. Houston, Ph.D.
Have you ever noticed while strolling on the beach or just gazing out at the sea, you notice a tremendous number of birds feeding in the area of strange ripples on the water? Most likely you have encountered schools of anchovies, herrings or some other type of silverside. These fish are famous for their schooling behavior. Schools are quite common in fish and certain invertebrates such as squid, and can be defined as an aggregation of individuals due to mutual attraction. Generally a school of fish consists of a single species; however mixed schools can occur during feeding periods known as bait balls. The spacing of individual fish within a school is maintained through action of the lateral line system. Along the lateral line there are special “hair cells” that monitor changes in water pressure. This mechanism along with vision allows the fish to keep tabs on the location of adjacent individuals in the school. It was long thought that schooling was an innate or instinctual behavior; however, research has shown that is a learned phenomenon. Experiments were conducted where hatchlings were isolated for a period of time and then reintroduced into the population. Their swimming behavior was totally out of sync with the other individuals. They were observed bumping into others and swimming “out of step” with the group! In time, however, they learned the drill and became part of the school.
Schooling behavior increases the survival value for fish in providing “safety in numbers”. Fishes that wander from the school are more vulnerable to being eaten. In addition, schools may confuse predators by appearing as a super large organism and fishes may encircle a predator or swim in a wavy pattern. Most schooling fishes have silvery sides which help to reflect light to blind or startle intruders. In many cases fishes like anchovies or herring aggregate in spherical schools known as “bait balls” which reduce the surface area of the school to predators.
Schooling also helps minimizes the effect of drag. This greatly reduces the amount of energy for swimming and movement in fish, especially those individuals at the rear of a tightly packed school. It is kind of like when some people tailgate big rigs on highways to save gas! During the reproductive season some species will form schools to increase the probability of mating encounter and fertilization. This is especially the case in fishes that broadcast (release) their eggs and sperm into the water. I have observed Gulf grunion swimming in large schools just twenty feet from shore just prior to their beach spawning.
Predators such as jacks and mackerel occur in mixed schools while they are hunting and feeding on small schooling fishes. The feeding behavior of these predators is extremely interesting and can be observed in four phases. First they swim in loosely aggregated schools while patrolling for food. When these predators encounter a school of prey, they penetrate the school by swimming in zigzag movements. This behavior is called quartering and helps to break up the school of prey. Next is the hunting phase when predators identify individuals that have become separated from the school. The fourth event is the attack and final kill. As we have seen schooling behavior serves many functions for both predator and prey. So don’t get left behind for whatever reason!