Eyesight plays role in keeping fish in schools

One of my most memorable snorkeling experiences occurred several years ago in Tonga. The incident involved only one species, but that was the beauty of it. Thousands of identical, 6-inch-long, silver fish let me swim in their school.

Whichever way I turned, twisted or dived, the fish outlined my arms, legs and body perfectly, maintaining a precise space between each other and me. So many fish surrounded me that at times I couldn’t see the surface above or the seafloor below. For a glorious 20 minutes or so, I was one with the fish.

As I approached the dense school that morning, I expected its members (species unknown) to scatter and flee. However, those fish decided, as one entity, not only to stay together, but also to accept me in their midst.

Such are the mysteries of schooling.

Schooling is a social behavior in fish that has fascinated people for centuries. Aristotle noted it, thinking that this is what human society ought to strive for. Today a wide variety of people, from oceanographers to anglers, engineers to biologists, are interested in fish schools. All want to know how fish do it and why.

About 50 percent of fish spend part or all of their lives shoaling. A shoal is a group of fish hanging together in no particular pattern, whereas a school is a polarized, synchronized shoal.

Fish can move in and out of formation in a heartbeat, going from a classic school, where each fish swims in the same direction at the same speed, to a mass of individuals pointed every which way, to veering off and going it alone.

The shape-shifting depends on whether the fish are traveling, feeding, resting, fleeing or spawning. But when it comes to schooling, whatever their mission and however they communicate, all the fish are on the same page.

Given those bulging eyes on each side of the head, most fish have excellent vision and use sight to orient themselves in schools. Researchers know this because schools break up at night. Also, in lab experiments where workers temporarily blind fish, schools fall apart.

The other fish sense crucial to schooling is the lateral line, a streak of supersensitive cells, visible in some species, running the length of a fish on both sides. The cells detect tiny water turbulence made by neighboring fish and, in this way, help individuals keep their distance.

There are many unknowns about why fish school, but scientists have theories. Maintaining a precise position in a neighbor’s wake may increase swimming efficiency, like riding a bow wave or drafting a bicycle.

Schooling may be also be an advantage in finding food, since many eyes searching an area are better than two. The negative of group foraging is that the fish have to share, a drawback for the ones in back. This might be why schools are constantly changing leaders.

Schools also appear useful for confusing predators, finding mates and perhaps other payoffs known only to the fish.

I don’t know what the fish in Tonga were up to the day they let me in, but swirling inside that flowing mass felt like swimming in a lava lamp. It was the best school I’ve ever attended.

Reach Susan Scott at www.susanscott.net.