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Treebeard's Stumper Answer
20 September 2002

Stay in School

I was entranced by the huge schools of baitfish we saw while snorkeling at Catalina Island. Fish in a school act like a single organism with a purpose. They divide and regroup around us when we dive into them. They all change direction so abruptly that I'm sure they are not following a single leader like a line of pelicans. Flocks of starlings over local vineyards are similar. How do fish stay in school without a single leader to follow? Why do fish form schools at all? I was attracted to these fish, so aren't other predators also attracted to such an obvious target?

Jack Mackerel (Trachurus symmetricus) at Catalina.
Photo by Erik Erikson.
Baitfish school in Key Biscayne, Florida.
Note the threatening shark on the bottom.
Photo from National Parks photo gallery

Schooling fish are less interesting up close (left). It's the dynamics that are interesting (right). Note how the school stays together with a sharp boundry despite the threatening shark on the bottom. This is intelligent social behavior, but where is the intelligence? Wouldn't individual fish fare better on their own, so they wouldn't attract such large preditors?

I'm not sure what fish species we saw "in school" while diving at Catalina. There are many small schooling fish including sardines, anchovies, herring, mullet, salema, jack mackeral, blacksmith, topsmelt, grunion, flying fish (really!) and lots more. The nomenclature of these fish is confusing as some names are proper species and others are generic. To a fisherman, these are all baitfish, and that's good enough for me.

I can't take my digital camera in the water, and I had a surprisingly hard time finding photos of local fish schools that I could "borrow" with a clear conscience. I can share some links. Natural history photographer Phillip Colla has a remarkable collection of photos of schooling fish, including many taken at Catalina. There'a another beautiful gallery by Marc Shargel. These photos don't show the beautiful dynamics that make this a real stumper. There is a quicktime movie of schooling anchovies at the Monterey Bay Aquarium, but these are aquarium fish going in a circle. In the ocean, it's more complicated.

Charge into a group of kids, and you might not tag anyone in the confusion if every kid is trying to get away. That's how schools of fish work. Each fish follows its own rules to evade predators and maintain a certain distance and speed with its neighbors. Fish have special lateral line sense organs to "see" other fish. The resulting fish school is an emergent property of this individual behavior without any leader. It's an effective strategy in nature, but it goes terribly wrong when commercial fishermen use a purse seine net to tag the entire school at once.


Two facts struck me when I dived into the schools of baitfish at Santa Catalina Island: The schools stay together like a single organism with very few stragglers, and I never could touch a single fish even though they were all around me! If I'm the preditor, then this behavior works!

Schooling is a remarkable social adaptation that appears across the animal kingdom. Flocks of starlings and crows, and herds of bison and sheep are similar. There are many interesting names for congregations of animals such as a murder of crows and a murmuration of starlings and a pod of dolphins. There are fun word lists at the Collective Noun Page and this USGS site.

But not every group of critters is a school or flock in this narrow sense. A line of pelicans of a V-formation of geese is clearly following a leader A parade of people also has a designated leader, but maybe a human crowd is the same phenomenon that can have a mind of it's own. "A riot of people" makes good sense next to a "school of fish" and a "murder of crows".

Why the word "school"? I believe it's a corruption of the original word "shoal", and people still talk abut shoals of fish. Kids in school don't have to be all the same! This is from the Online Etymology Dictionary:

school - "group of fish," c.1400, from M.Du. schole "group of fish or other animals," cognate with O.E. scolu "band, troop, school of fish," from W.Gmc. *skulo- (see shoal).

shoal - "large number" (especially of fish), O.E. scolu "band, troop, school of fish," cognate with M.Du. schole, W.Fris. skoal, perhaps all originally meaning "division." Related to school.

The magic of fish schools is that there is no single leader. Craig Reynolds wrote the first convincing software simulation of his computer "Boids" based on these simple rules:

We can add two more simple rules: You can view the results in a JAVA program on Craig Reynolds' Boids page. He has links to many other software simulations. Of course a convincing simulation doesn't prove that real fish and birds are following the same rules!

The strength of the school is that every critter is looking for good and bad things and taking the appropriate action. And the rest follow, so it's good for all. The school or flock effectively has hundreds or thousands of eyes, and confusion is a great defence. I never touched a single fish.

This is an interesting social model that produces beneficial behavior for the group based on individual choices. I think of Adam Smith's "invisible hand" in The Wealth of Nations, book 4, chapter 2:

As every individual, therefore, endeavours as much as he can both to employ his capital in the support of domestic industry, and so to direct that industry that its produce may be of the greatest value; every individual necessarily labours to render the annual revenue of the society as great as he can.

He generally, indeed, neither intends to promote the public interest, nor knows how much he is promoting it.

He intends only his own gain, and he is in this, as in many other cases, led by an invisible hand to promote an end which was no part of his intention.

Nor is it always the worse for the society that it was no part of it. By pursuing his own interest he frequently promotes that of the society more effectually than when he really intends to promote it. I have never known much good done by those who affected to trade for the public good. It is an affectation, indeed, not very common among merchants, and very few words need be employed in dissuading them from it.

It's a fine libertarian ideal, but the evidence lately is that it just doesn't work when wealth and influence are concentrated in a few hands across the world. OK, it's still an open question. Maybe we should study these fish and birds and learn how they do it!

Here are a few Web links I found for more research on schools and flocks:

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