School riddles - schooling fish - Cover Story

International Wildlife, March-April, 1995 by Kathryn Phillips

How many fish does it take to make a school? And why isn't the middle safer?

Biologist John Hunter leads the way to a dimly lit room at the National Marine Fisheries Service lab in La Jolla, California. In a tank that resembles an above-ground swimming pool from some suburban backyard, one-, two-, maybe three-hundred silvery anchovies swim in the same direction with a beat-and-glide motion, side-by-side and one above another. Watching the fish swim with each other like mirror images is hypnotic, casting a dreamy spell over observers. Then Hunter breaks the spell by drumming on the side of the pool.

"Let's see if we can intensify things a little bit," he says. Quicker than you can say June Taylor Dancers, the fish push away from Hunter's pounding hand, flee to the opposite side of the pool and press together into a tight mass. The fish now look like commuters rushing to squeeze into the subway.

As far as these fish are concerned, Hunter is some kind of suspicious predator. At first glance, grouping together makes no sense, seemingly making a bigger catch for a hunter. Yet most schooling fish under attack do just what these anchovies do.

And they're not so dumb after all. Laboratory studies have found that, indeed, if you increase the density of the group of fish, some predators hesitate more and their attack success goes down. A school can mean protection.

At least a fourth of the world's fish school. Some are hatchlings no larger than a paper clip, and others are adults the size of sofas. Fish school in freshwater and in the sea. Some school all their lives, and some only as juveniles or only as adults in mating season.

Exactly why fish who school do, and exactly how fish behave once they have joined a school, has kept researchers busy for decades. Scientists are looking at schools as defense alliances, as hunting parties and, in a few cases, as a way to find that special someone with whom to spawn. That research has become increasingly important as wildlife managers struggle to protect certain schooling species from being fished into near-extinction.

School Starts

One unresolved question about schools concerns definition. "Obviously one fish is not a school; is two fish a school?" asks Julia K. Parrish, a University of Washington zoologist who has studied schooling in various ocean species. "I, personally, do not believe that two fish is a school, but there are definitely people who put two fish together and say they're studying schooling behavior."

"I think of a school as enough individuals to form a three-dimensional structure," she says, "so we're in the high tens to low hundreds as a minimum number." Some schools have millions of members and span an area as large as a small town.

A number of researchers have avoided the difficulty by using the term "shoal" for all groups of fish and then classifying them as either simple aggregations or schools. "Schools would be what you probably think of as a group of fish traveling together that turn with an impressive synchrony...rather like coordinated airplanes at air shows," says Tony Pitcher, director of the University of British Columbia Fisheries Center. True schools have distinct boundaries, Parrish notes. "There are lots of fish and then no fish. It's not that they sort of peter out."

Swimming in the middle of such a gathering is a surreal experience. "It's like being in an episode of Star Trek," adds Pitcher, who has snorkeled with herring. "You don't see the outside world, where you've been or where you're going. And you're surrounded by quivering, moving silver bodies."

To move in such unison, fish use vision, smell and sensors along the sides of their bodies. The sensors detect vibrations from neighboring fish.

Safety Questions

Researchers know that fish swimming in the open ocean and living in the upper levels of the water column, such as herring, sardines and anchovies, are more likely to school than species on the ocean bottom or in streams. The reason relates to hiding from predators.

For little fish living close to the bottom, finding safety is easy. They can dart into some algae, next to a rock or inside a crack in the coral. But as fish range farther into open water, they become more vulnerable.

For self-defense, venturesome fish depend on several strategies: tiny or transparent bodies that are hard to spot, huge bodies that daunt all but the boldest predators, flesh that tastes bad or skin that's hard to bite through. The ocean sunfish, for example, is a tough-skinned fish that lives as a loner. Schooling is right up there in the list of defenses. "If you school," Parrish explains, "the general thought is: Get the other guy, not me. Safety in numbers."

Hunting Parties

To avoid getting eaten is not the only reason to school. In some cases, it may be a good tactic for eating somebody else.

Around coral reefs it is not uncommon to find schools of fish numbering fewer than a hundred but including four or five different species. By cruising in a group, they overcome the territorial defenses of resident fish and get feeding opportunities they couldn't as individuals. These multi-species schools often hang together just for food, not defense. When a predator comes along, the group breaks up.